Mayday (2013) s04e05 Episode Script
Hidden Danger
The Boeing 737 is one of the most popular passenger jets in the world.
Around the globe, the plane has carried more than 12 billion passengers.
It's the backbone of the aviation industry.
SCREAMING But in 1991, something happened onboard a 737 that sent shudders through the world of aviation.
SCREAMING BOOM! Oh, my lord! A deadly crash has investigators scrambling.
There was a time when I had doubts that we'd be able to solve it.
It was like he was tracking a serial killer.
The hunt for answers will take 10 long and gruelling years.
The fate of the airline industry hangs in the balance.
And the mystery isn't solved until more than 150 people are dead.
9:40am, March 3, 1991.
After a short 17-minute trip from Denver, United Airlines Flight 585 is on final approach into Colorado Springs.
It looks like a perfect day for flying.
But there's trouble in the air.
Nice looking day.
Hard to believe the skies are unfriendly.
There's been heavy turbulence during the flight and violent gusts of wind are forecast over Colorado Springs.
Never driven to Colorado Springs and not gotten sick.
(Laughs) At the controls is 52-year-old Captain Harold Green, a pilot with 20 years experience and a sterling reputation.
Green's copilot is Patricia Eidson.
At 42, she's one of the first female flight officers in United's history.
GREEN: Flight attendants, prepare for landing.
At Colorado Springs Municipal Airport, air traffic controller James Rayfield is ready to bring Flight 585 in.
United 585, report the airport in sight.
Got it? Yep.
Airport in sight.
United 585.
Lower landing gear.
United 585 is cleared for a visual approach to runway 35.
Weather conditions - wind 320 degrees at 16 gusting at 29.
As its speed decreases, flight 585 becomes more vulnerable to the turbulence.
Eidson wants to know what other planes have experienced on landing.
Any reports lately of any loss or gain of air speed? Yes, ma'am.
500 feet, a 50-knot loss.
At 400 feet, a 50-knot gain.
And at 150 feet, a gain of 20 knots.
Sound adventurous.
Thank you.
Starting on down.
Less than 3km from the airport, retired policeman Harold Darnell is on his way to a local flea market.
A kilometre overhead, Green and Eidson focus on keeping their speed constant as they descend.
We had a 10-knot change here.
Yeah, I know.
Awful lot of power to hold that air speed.
As United 585 approaches the runway, Darnell feels something strange.
Whoa! What the heck was that? Out of nowhere, a powerful gust of wind strikes his vehicle, almost blowing him off the road.
Another 10-knot gain.
30 flaps.
From the control tower, James Rayfield can now see flight 585's final approach.
As the aircraft closes in on the airport, the ride gets even bumpier.
Wow.
We're at 1,000 feet.
Then, without warning, the 737 starts to spin out of control.
Oh, God! (Passengers scream) - 15 collapsed! - 15! - What's going on?! - Oh, my God! - Oh, my God! - Whoa! - No! No! - Oh, my God! (Both scream) BOOM! Oh, my lord! Crash! Crash! Rescue workers arrive within minutes, but there's almost no sign of the 737.
The shattered remains of the 38-tonne jet lie buried in a fire-blackened impact crater.
The plane didn't it didn't skate or bounce, you know, like when a plane comes in normally and lands.
It just nosed right in and where it hit is where it stayed.
And I came down here and this is when I saw all the It was horrible.
There are no survivors.
All 20 passengers and five crew members are killed instantly by the high-speed impact and exploding jet fuel.
In 10 violent seconds, Colorado Springs has become the site of one of the most mysterious air crashes in aviation history.
By nightfall, investigators from the National Transportation Safety Board descend on Colorado Springs.
Known to insiders as 'tin-kickers', NTSB investigators examine over 2,000 aviation accidents a year, at times by picking through the metal debris of fallen aircraft.
While coroners mark the location of human remains in red, NTSB investigators mark scraps of metal in yellow, looking for clues to help them solve the mystery of flight 585.
Like investigating a mass murder, it's a tough job walking onto a crash site.
Among the investigators assigned to the case is Malcolm Brenner, a specialist in human performance.
His job will be to find out if the crash was caused by pilot error.
The area was cordoned off by police and there were Salvation Army trucks.
I got a cup of coffee - a cup of hot chocolate or something, and I thanked them for it and they said, "No, no.
Thank you.
" And they had this look in their eyes like, "My God.
You have to go into this site.
" Clues to the fate of United 585 lie mangled in a deep, black hole.
The fuselage is crushed like an accordion in the impact crater.
The rest of the plane is in pieces spread over an area smaller than a football field.
There was a lot of fire damage.
There'd been a fire afterwards.
And it was all contained in a relatively small area, which - just initial impression - it can be a sign that the airplane was intact.
If there was a midair explosion or something came off the airplane you'd expect that to be a much larger site.
My first sense that it was going to take some time to investigate the accident was the damage that we saw in the parts.
When they're burnt and broken, the process always takes longer.
But with their work just beginning, members of the NTSB have no clue that the case of United 585 will become the longest crash investigation in aviation history.
(SCREAMING) - 15 flaps! - 15! A clear late winter day in 1991 turned deadly in just 10 short seconds.
Moments from landing in Colorado Springs, United Airlines Flight 585 fell out of the sky at 370km/h.
Crash! Crash! All 20 passengers and 5 crew members died.
The National Transportation Safety Board begins a painstaking investigation into the crash.
Engine turbines, hydraulic pressure gauges, the cockpit voice recorder and in-flight data recorder are all carefully extracted from the site, photographed and sent to the lab for analysis.
An important first step in the investigation is the analysis of the cockpit voice recorder.
With pilot error a factor in 70% of air disasters, Malcolm Brenner's job is to see what role pilots Green and Eidson played in the crash.
This crew was - and I felt this at the time - was one of the more impressive crews I'd ever dealt with.
There was a little bit of tension release, a little bit of humour - the captain said Never driven to Colorado Springs and not gotten sick.
The first officer suggested that they add extra speed as a safety margin and the captain agreed with it.
It was good interaction.
- EIDSON: Got it? - GREEN: Yep.
Airport in sight.
United 585.
Lower landing gear.
This is the sense of an excellent crew .
.
caught randomly, if anything.
So, again, that was my first impression is that this would be consistent more with a hardware situation.
As more about flight 585 becomes known, mechanical failure becomes a serious suspect.
Just seconds before it crashed, the plane rolled onto its back and spun wildly out of control.
Investigators wonder if the sudden motion was caused by the plane losing an engine or a wing.
From the state of the aircraft on site, it's clear that it was intact at the time of the crash.
What investigators don't know is if the engines were still working at impact.
Technicians examine the engine turbines.
They discover dirt has been drawn deeply into every crevice.
These blades were clearly spinning and sucking in air at the time of impact.
The engines may have been running but technicians aren't sure how well.
The plane's hydraulic pressure dials are destroyed, their glass covers broken.
The indicator needles have been snapped off by the force of the impact.
But even these ravaged dials tell a tale.
On close inspection, investigators find a critical mark.
At the moment of impact, a dent was made on the faceplate by the jarred indicator needle.
It proves that when United 585 crashed, its engines were running normally.
SCREAMING With engine failure ruled out, there seems to be only one other mechanical explanation for why 585 suddenly rolled over and then fell out of the sky.
It appears increasingly likely that the plane had suffered a catastrophic problem with its flight controls.
Investigators quickly become suspicious of the rudder at the back of the tail.
We focused in after eliminating other flight control services that we thought could contribute to the roll.
We started looking at the rudder.
Bring that up so I can take a look at it.
Investigators begin their examination of the rudder on site.
But the violence of the crash makes the job extremely difficult.
Almost nothing left.
Most of the plane's parts are too crushed or burned for testing but a vital component is still reasonably intact - the power control unit, or PCU.
Used constantly during flight, especially during landings, the PCU performs like a car's power steering.
When the pilot pushes on a rudder pedal, the PCU uses hydraulic fluid to convert gentle movements of a pilot's foot into the pressure needed to move the 737's enormous rudder.
The heart of the PCU is something called the dual servo valve.
Shaped like a soda can, it has two slides which glide past one another, directing the flow of pressurised hydraulic fluid that moves the rudder.
The servo valve is very unique that it is in effect two valves in one and that that creates a whole range of interactions that don't occur in a more conventional hydraulic valve.
When a technician opens the power control unit, chips of metal are found floating in the hydraulic fluid.
It's a disturbing find.
These particles could cause the servo valve to jam, making it impossible to work the plane's rudder.
It's a chilling prospect.
Could a microscopic fault bring down a 38-tonne jet? It's difficult for Phillips to tell.
While more intact than much of the wreck, the PCU and dual servo valve are both damaged.
Er, the airplane crashed and burned in a pretty confined area and there was a lot of damage to the flight control components and the things we were testing - needed to test and look at.
And the more damaged the components are, the harder it is to take measurements and do functional testing.
To test what he does have, Phillips travels to California, to the labs of Parker Hannifin - where the rudder control unit is made.
The curious metal chips floating in the PCU's chambers are dismissed.
Phillips is told that filters keep them out of the delicate servo valve that directs fluid and moves the rudder.
Nothing else is found that could explain any sudden movement of the rudder on Flight 585.
We didn't have any absolute indication or information that we could point to that said the rudder, power control unit, the servo valve or any part of that flight control system caused that accident.
Phillips still suspects a mechanical problem.
But with no conclusive evidence that the PCU or servo valve caused the crash, he's forced to sign off on the tests.
It's a pass.
With Phillips at a dead end, only bad mountain weather remains as a primary suspect.
An expert on weather-related aviation accidents, Greg Salatollo is trying to determine if heavy winds on the day of the crash were a factor.
SALOTOLLO: There is a history of events where there've been airplane accidents attributed to mountain waves and rotors.
In 1966, a BOAC 707 near Mount Fuji .
.
er, disintegrated in a mountain wave and rotor.
High winds crashing over mountain peaks leave so-called wind rotors in the leeside - invisible, highly turbulent down draughts that come plunging down with devastating power and are extremely dangerous to aircraft.
We found a great deal of evidence, er .
.
looking at the surface upper air data and talking to witnesses in the area that rotors were a possibility.
.
.
explosion.
It was right over there.
Salatollo hears several eyewitness reports of bizarre mountain weather on the day of the crash.
One of the most intriguing is from Harold Darnell - TYRES SCREECH - Whoa.
What the heck was that? .
.
whose truck was struck by a powerful gust of wind just moments before 585 crashed.
But as Greg Salatollo combs through his evidence, the theory that a wind rotor knocked the plane from the sky is getting less and less likely.
Nice-looking day.
Hard to believe the skies are unfriendly.
Wind rotors are areas of extremely low barometric pressure.
So if Flight 585 did pass through one, its altimeter reading would've spiked as the plane was blown suddenly upwards.
There was no evidence that we saw that on the flight data recorder of 585.
What the flight recorder did show was a fast and deadly drop in altitude, as the plane fell to earth.
(SCREAMING) It's been 21 months since the investigation into the crash of United Flight 585 began - almost two years, in which the NTSB has studied the crew, the weather, the rudder and thousands of other pieces of evidence.
They've come up empty-handed.
For only the fourth time in its history, the NTSB releases a report which doesn't reach a conclusion.
The cause of the crash of Flight 585 is undetermined.
We had put a lot of time and effort in, into the investigation, and we just weren't sure what had happened.
It was like he was tracking a serial killer.
He was frustrated that they had not solved 585.
He did not want that to happen again.
But almost two years after the report on 585 is released, the killer strikes again.
At 7pm on a clear, windless day, USAir Flight 427 is nearing Pittsburgh.
Captain Peter Germano and First Officer Chuck Emmett are getting ready for their final approach.
Folks, from the flight deck, we should be on the ground in about 10 more minutes.
Sunny skies.
A little hazy.
Flight attendants, please prepare for landing.
I ask you to check the security of your seatbelt.
Thank you.
MAN OVER RADIO: USAir at 309, descend and maintain 6,000.
As they close in on the airport, the pilots are on the lookout for another flight, about 10km ahead of them.
Looking for the traffic.
Turning 100.
USAir 427.
I see the jet stream.
As they pass through the turbulence left behind by the other flight, their jet suddenly and alarmingly rolls left.
- WARNING TONES BEEP - Hold on, hold on.
- Hold on.
- Shoot! Nothing the pilots do seems to have any effect.
SCREAMING What the hell is this?! - What the hell? - Shoot! SCREAMING 427 - emergency! SCREAMING Oh, shoot.
- Oh! Oh! - Oh, God.
- Oh, God, no! No! - Noooo! SCREAMING Rescue crews arrive quickly but the fate of Flight 427 is tragically clear.
There's no hope for the 132 passengers and crew.
The human carnage is so bad, authorities declare the crash site a biohazard.
USAir 427 accident was the first US accident where biohazard suits were used.
And it made it more difficult - they were uncomfortable, they were hot.
To this day, when I put on a pair of rubber gloves for any reason, I'm instantly transformed back to the site in Pittsburgh.
Captain John Cox - a 737 pilot and a flight systems specialist with the Airline Pilots Association - is asked to join the team investigating the crash of Flight 427 .
.
as coroners attempt to collect human remains.
NTSB lead investigator Tom Haueter already knows his hunt for clues will be long and painstaking.
When we first arrived at the crash site, well, first of all, there was no aircraft there.
There were only bits and pieces of the airplane.
It wasn't really recognisable as an airplane.
With the help of eyewitnesses, information from the flight data recorder and the cockpit voice recorder, investigators begin to quickly see some striking similarities between 427 and the unsolved case of United 585.
In fact, they seem to be mirror images of each other.
On final approach, United 585 rolls right while USAir 427 rolls to the left.
- Oh, God! - Hang on, hang on.
Both crews are caught by surprise and after just a few terrifying seconds, both aircraft plummet straight into the ground.
Certainly, the whole team was aware of the previous accident with United 585 in Colorado Springs.
We tried to keep that in the back of our minds and take a look at this one, as to what it presents.
As the investigation continues, the list of similarities grows.
427's engines were also attached and functioning at the time of impact.
But for all the similarities, there's one important difference.
Unlike United 585, as USAir 427 approached Pittsburgh, weather conditions were dead calm.
Folks, from the flight deck, we should be on the ground in about 10 more minutes.
Sunny skies.
A little hazy.
Taking a look at the flight data recorder information, the aircraft's approaching Pittsburgh.
It's an extremely smooth night.
Er, there's just no turbulence at all.
The pilots are relaxed, they're talking about the landing.
Flight attendants, please prepare for landing.
I ask you to check the security of your seatbelts.
Thank you.
As he did in the case of United 585, Greg Phillips will once again lead the investigation into the mechanical aspects of the crash.
Almost immediately, he makes a promising discovery.
Miraculously, much of USAir 427's tail and rudder appear intact.
The hydraulic devices inside the tail have also sustained very little damage.
Phillips and Haueter prepare to send the parts to the manufacturer, Parker Hannifin, for testing as soon as possible.
They need answers.
Pressure on the NTSB to solve the accident is growing quickly.
Not only are the crashes of flights 585 and 427 disturbingly similar, both of them involved the same kind of airplane - a Boeing 737.
But with serious questions being raised about the plane's safety, billions of dollars and perhaps the airline industry itself are at risk.
We couldn't live with the fact, as investigators, of having two unsolved 737 accidents.
The airplane is in too much use, too wide a use around the world.
It carries too many people every day.
Unsolved was not an acceptable answer.
If 427 was an undetermined accident, we could not find the cause of this accident, there was a great chance if there was a third accident with a 737 fleet, under similar circumstances, that the 737 fleet would have been grounded.
Careful with that.
The investigation into the crash of Flight 427 may be the most important in the history of the NTSB, but it won't be easy.
Answers are still years away.
In just three years, two Boeing 737s have crashed in the United States with no survivors.
Airport in sight, United 585.
In both cases, the planes were moments from landing (OVER PA) Prepare for landing.
.
.
and in both cases, the planes were under full power.
But the pilots were unable to control their jets.
In the crash of United Flight 585, NTSB investigators were unable to find a cause.
Now they're searching for clues into the second crash - USAir Flight 427.
As the investigator in charge, I never allowed myself to think this investigation could go undetermined.
We kept on pushing and kept on researching.
As long as we had things to research, we kept on going.
To find their killer, the NTSB can't afford to rule anything out - from the possibility that a collision with birds brought 427 down, to strange, even bizarre, theories.
They looked at electromagnetic interference, they got calls from people saying it might be Russian death rays, they considered everything.
There were a couple of witnesses who gave reports of the aircraft suddenly descending and hovering before it blew up.
We discounted those.
But the investigation's primary suspect is the dual servo valve, part of the power control unit that moves the 737's rudder, and a suspect in the crash of United 585.
Parker Hannifin made the valve.
At its lab in California, investigators look inside the main cavity of the USAir power control unit.
Just like in the earlier crash, they find tiny chips of metal floating in the hydraulic fluid.
But once again, Parker and Boeing repeat their claim - filters designed to stop any debris from interfering with the delicate metal slides have done their job.
Investigator Greg Phillips wants to be absolutely sure.
If the chips were blocking the slides, they would have left tiny scratch marks behind where they rubbed against the metal.
But Phillips can't find any.
Another pass.
OK.
Phillips has technicians put the servo valve from Flight 427 through as many tests as he can think of, trying to find a weakness.
If he can find one, it could explain why two planes were ripped from the sky.
But he comes up empty.
That unit passed all its operational tests.
There wasn't any indication that it had failed and it operated within the parameters we expected it to.
Once again, the investigators are forced to shift their focus back to the pilots.
By studying the plane's flight data recorder, investigators know the jet's rudder was deployed fully to one side, what's called rudder hardover.
We were definitely focused on rudder, on hardover rudder, full rudder input for about 20 seconds.
It can be caused either by hardware, something unknown in the hardware, or it can be caused by pilot input.
First Officer Chuck Emmett, who was flying 427, did indeed step down hard on his rudder and then held it there while the plane plummeted towards the earth.
It raised a grisly question - was he trying to fly the plane into the ground? In looking at this, being a pilot myself, this doesn't seem like rational behaviour.
What the hell is this? Human performance specialist Malcolm Brenner listens closely for evidence on the cockpit voice recorder.
What the hell?! In this case they had microphones right by their mouths and you can hear as well as in real life or better.
You can hear breathing sounds.
Hmm, yes, I see the jet stream.
The cockpit recordings indicate that Flight 427's troubles began at the moment it flew through the jet wake of a Delta Airlines 727 that had just passed in front of them.
Both pilots are startled by the wake.
I see the jet stream.
The first officer breaks off at the end of a sentence, "I see the jet stream," and there's no more discussion of the jet stream or anything else.
They both focus - something happened here.
The captain says, "Jeez!" Jeez.
It was such a smooth flight that it was a momentary jolt that they just hadn't anticipated, and with that, the pilots got on the controls and immediately, you know, put in a rudder input.
Jeez.
The cockpit recorder even records the thumping sound of the jet stream turbulence as 427 flies through it.
As Flight 427 encounters the turbulence, Brenner hears something unusual.
First Officer Emmett begins to grunt.
The grunting is unusual.
The controls are designed so that pilots don't need to grunt.
They're especially designed around human capabilities.
So to have someone grunting is typically a sign of an emergency.
- INSTRUMENTS BEEP - (Grunts) By matching data from the flight recorder with the crew's voices, Brenner is able to confirm that Emmett's grunts begin a split second after he pushed down on the rudder pedal and three to four seconds after the wake turbulence affected Flight 427.
What the On their own, the cockpit voice recordings prove very little, but it seems clear that the crew weren't trying to crash their plane.
Something happened which took them by surprise.
They reacted as quickly as they could but nothing they did seemed to help.
What the hell is this? It's been almost two years since the crash of Flight 427 and the investigation has stalled.
Now two 737s have gone down in startlingly similar ways and investigators still don't know why.
We were all frustrated as months wore into years.
What were we missing? It definitely took a toll on their personal lives.
They worked incredibly long hours.
They never stopped thinking about it.
We were going up against an aircraft that had an incredible safety history.
It was really everything You could see for 30 years this had been a great airplane.
We weretrying to prove that there was something wrong with the straight-A student.
It clearly was on his mind.
He, at one point, had a nightmare about it where he dreamed that he was in front of a congressional committee that was grilling him on now there had been a third 737 crash, and in Tom's dream, all the cameras were pointing to him and a congressman asked, "Why didn't you ground the fleet?" Unsure of where to look next, and with the trail of evidence getting colder, investigators need a break in the case .
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and fast.
CONTROL TOWER: Eastwinds 517, you're clear for landing.
On June 9, 1996, they finally get the break they've been looking for.
Captain Brian Bishop is on final approach to Richmond, Virginia, when, without warning, his Eastwinds jet rolls sharply to the right.
We didn't know to what extent, but we knew we had a rudder problem.
I turned the yoke the opposite direction and stood on the opposite rudder pedal but the pedal didn't move for me.
For over 30 seconds, the 737 flies in a precarious right bank as Bishop fights to keep it from rolling over.
Then suddenly, the unknown forces holding the jet let go .
.
snapping the wings back to horizontal.
In a matter of seconds, it released itself and went back to normal.
We had started the checklist.
Almost before I could finish the sentence, all of a sudden there was a just a WHAM! For a second time, the 737 is pushed onto its side.
For 30 harrowing seconds, the 737 takes on a life of its own.
Then once again, as quickly as it began, the rollover stops.
After the second time, I said to the first officer, "Declare an emergency.
" Tell the controller we have flight control problems.
SIRENS WAIL As they slow down to land, the risks increase.
October 15, 1996.
For the last five years, the National Transportation Safety Board has been struggling to crack its toughest case.
Two completely separate but seemingly linked accidents The crash of United 585 killed 25 passengers and crew.
Three years later, the USAir Flight airline disaster took another 132 lives.
Now, after examining hundreds of clues, investigators have made a surprising discovery.
A new test has revealed that under the right circumstances, the hydraulic valve that moves the 737's rudder can jam.
It just jammed.
But the surprises aren't over.
The most important breakthrough came when a Boeing engineer examining the data from that test discovered some numbers that indicated the valve at that point had actually reversed.
Whoa! Jeez.
It's a stunning revelation.
Not only can the servo valve jam but it can then function in reverse.
It means that any time a pilot tries to correct a rollover by pushing on the rudder, the rudder might turn in the opposite direction, causing a fatal accident.
And the reversal is like driving your car - you turn to the right, it goes to the left.
You're not gonna figure out this failure mode until you go off the road and in these cases that's what the pilots were faced with - something so unusual that they didn't understand what was happening.
What the hell is this? They had evidence now that the valve was unique, that the valve not only could jam but would reverse.
427 emergency! That would explain why the first officer, Chuck Emmett, would keep his foot on the rudder pedal, because he's thinking, "Why isn't the plane going right?" And he's feeling the plane go to the left.
To the very end, Chuck Emmett pushes hard, hoping his rudder will help him pull out of his deadly spiral.
Tragically, he has no way of knowing that he is steering the aircraft straight into the ground.
Noooo! MAN: (OVER RADIO) Never driven to Colorado Springs and not gotten sick.
Flight attendants, prepare for landing.
Satisfied that they've determined the cause of the crash of USAir 427, the NTSB turns its attention to the unsolved case of United Flight 585.
Another 10.
Going back to Colorado Springs, you could follow a progression of what the captain was doing.
He's close to the ground and suddenly under rudder reversal, he puts on a little bit of pedal.
The pedal violently pushes his leg back.
- Oh, God! - 15 flap! Rudder reversal certainly fits what I know about this crew and how it fits.
We were able to show the failure mode.
It matched the flight data recorder from each aircraft.
It fit like a glove.
So we now had a lot more information we could apply to United 585 and based on that, we redid the accident report.
- Oh, my God! - Oh, no! From rudder reversal to impact took less than 10 seconds.
585's flight crew had no chance to save their plane or passengers.
In 1991, something happened on board a 737 that sent shudders through the world of aviation.
In the aftermath of the investigation, sweeping changes were made to improve the safety of the 737 and the entire aviation industry.
New training protocols were designed to help pilots react to unusual in-flight events.
In the 737 fleet, pilots are now trained on how to react to both rudder hardovers and reversals.
Whoa! The scenario of the USAir 427 accident If the crew had the information that we have today, I believe they would've landed safely in Pittsburgh that evening.
The FAA also directed Boeing to redesign the rudder's dual servo valve to eliminate the potential for reversal.
Boeing spent hundreds of millions of dollars to replace the valves on thousands of 737s around the world.
One thing we don't like at the Safety Board is to have an undetermined accident because then we can't make a change to improve safety.
So out of USAir 427, United 585, we have a much safer 737 fleet.
It took NTSB tin kickers 10 years to solve the mysterious crashes of flight 585 and 472 - the longest investigation in aviation history.
There are still some people in aviation who don't think the NTSB got it right.
But I became convinced after talking to very, very, very many people - pilots, engineers, people at Boeing - and spending a lot of time with the investigators, that they did get it right.
Since the replacement of the 737 servo valves, there hasn't been a similar crash of the most popular, most profitable plane in the world.
Around the globe, the plane has carried more than 12 billion passengers.
It's the backbone of the aviation industry.
SCREAMING But in 1991, something happened onboard a 737 that sent shudders through the world of aviation.
SCREAMING BOOM! Oh, my lord! A deadly crash has investigators scrambling.
There was a time when I had doubts that we'd be able to solve it.
It was like he was tracking a serial killer.
The hunt for answers will take 10 long and gruelling years.
The fate of the airline industry hangs in the balance.
And the mystery isn't solved until more than 150 people are dead.
9:40am, March 3, 1991.
After a short 17-minute trip from Denver, United Airlines Flight 585 is on final approach into Colorado Springs.
It looks like a perfect day for flying.
But there's trouble in the air.
Nice looking day.
Hard to believe the skies are unfriendly.
There's been heavy turbulence during the flight and violent gusts of wind are forecast over Colorado Springs.
Never driven to Colorado Springs and not gotten sick.
(Laughs) At the controls is 52-year-old Captain Harold Green, a pilot with 20 years experience and a sterling reputation.
Green's copilot is Patricia Eidson.
At 42, she's one of the first female flight officers in United's history.
GREEN: Flight attendants, prepare for landing.
At Colorado Springs Municipal Airport, air traffic controller James Rayfield is ready to bring Flight 585 in.
United 585, report the airport in sight.
Got it? Yep.
Airport in sight.
United 585.
Lower landing gear.
United 585 is cleared for a visual approach to runway 35.
Weather conditions - wind 320 degrees at 16 gusting at 29.
As its speed decreases, flight 585 becomes more vulnerable to the turbulence.
Eidson wants to know what other planes have experienced on landing.
Any reports lately of any loss or gain of air speed? Yes, ma'am.
500 feet, a 50-knot loss.
At 400 feet, a 50-knot gain.
And at 150 feet, a gain of 20 knots.
Sound adventurous.
Thank you.
Starting on down.
Less than 3km from the airport, retired policeman Harold Darnell is on his way to a local flea market.
A kilometre overhead, Green and Eidson focus on keeping their speed constant as they descend.
We had a 10-knot change here.
Yeah, I know.
Awful lot of power to hold that air speed.
As United 585 approaches the runway, Darnell feels something strange.
Whoa! What the heck was that? Out of nowhere, a powerful gust of wind strikes his vehicle, almost blowing him off the road.
Another 10-knot gain.
30 flaps.
From the control tower, James Rayfield can now see flight 585's final approach.
As the aircraft closes in on the airport, the ride gets even bumpier.
Wow.
We're at 1,000 feet.
Then, without warning, the 737 starts to spin out of control.
Oh, God! (Passengers scream) - 15 collapsed! - 15! - What's going on?! - Oh, my God! - Oh, my God! - Whoa! - No! No! - Oh, my God! (Both scream) BOOM! Oh, my lord! Crash! Crash! Rescue workers arrive within minutes, but there's almost no sign of the 737.
The shattered remains of the 38-tonne jet lie buried in a fire-blackened impact crater.
The plane didn't it didn't skate or bounce, you know, like when a plane comes in normally and lands.
It just nosed right in and where it hit is where it stayed.
And I came down here and this is when I saw all the It was horrible.
There are no survivors.
All 20 passengers and five crew members are killed instantly by the high-speed impact and exploding jet fuel.
In 10 violent seconds, Colorado Springs has become the site of one of the most mysterious air crashes in aviation history.
By nightfall, investigators from the National Transportation Safety Board descend on Colorado Springs.
Known to insiders as 'tin-kickers', NTSB investigators examine over 2,000 aviation accidents a year, at times by picking through the metal debris of fallen aircraft.
While coroners mark the location of human remains in red, NTSB investigators mark scraps of metal in yellow, looking for clues to help them solve the mystery of flight 585.
Like investigating a mass murder, it's a tough job walking onto a crash site.
Among the investigators assigned to the case is Malcolm Brenner, a specialist in human performance.
His job will be to find out if the crash was caused by pilot error.
The area was cordoned off by police and there were Salvation Army trucks.
I got a cup of coffee - a cup of hot chocolate or something, and I thanked them for it and they said, "No, no.
Thank you.
" And they had this look in their eyes like, "My God.
You have to go into this site.
" Clues to the fate of United 585 lie mangled in a deep, black hole.
The fuselage is crushed like an accordion in the impact crater.
The rest of the plane is in pieces spread over an area smaller than a football field.
There was a lot of fire damage.
There'd been a fire afterwards.
And it was all contained in a relatively small area, which - just initial impression - it can be a sign that the airplane was intact.
If there was a midair explosion or something came off the airplane you'd expect that to be a much larger site.
My first sense that it was going to take some time to investigate the accident was the damage that we saw in the parts.
When they're burnt and broken, the process always takes longer.
But with their work just beginning, members of the NTSB have no clue that the case of United 585 will become the longest crash investigation in aviation history.
(SCREAMING) - 15 flaps! - 15! A clear late winter day in 1991 turned deadly in just 10 short seconds.
Moments from landing in Colorado Springs, United Airlines Flight 585 fell out of the sky at 370km/h.
Crash! Crash! All 20 passengers and 5 crew members died.
The National Transportation Safety Board begins a painstaking investigation into the crash.
Engine turbines, hydraulic pressure gauges, the cockpit voice recorder and in-flight data recorder are all carefully extracted from the site, photographed and sent to the lab for analysis.
An important first step in the investigation is the analysis of the cockpit voice recorder.
With pilot error a factor in 70% of air disasters, Malcolm Brenner's job is to see what role pilots Green and Eidson played in the crash.
This crew was - and I felt this at the time - was one of the more impressive crews I'd ever dealt with.
There was a little bit of tension release, a little bit of humour - the captain said Never driven to Colorado Springs and not gotten sick.
The first officer suggested that they add extra speed as a safety margin and the captain agreed with it.
It was good interaction.
- EIDSON: Got it? - GREEN: Yep.
Airport in sight.
United 585.
Lower landing gear.
This is the sense of an excellent crew .
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caught randomly, if anything.
So, again, that was my first impression is that this would be consistent more with a hardware situation.
As more about flight 585 becomes known, mechanical failure becomes a serious suspect.
Just seconds before it crashed, the plane rolled onto its back and spun wildly out of control.
Investigators wonder if the sudden motion was caused by the plane losing an engine or a wing.
From the state of the aircraft on site, it's clear that it was intact at the time of the crash.
What investigators don't know is if the engines were still working at impact.
Technicians examine the engine turbines.
They discover dirt has been drawn deeply into every crevice.
These blades were clearly spinning and sucking in air at the time of impact.
The engines may have been running but technicians aren't sure how well.
The plane's hydraulic pressure dials are destroyed, their glass covers broken.
The indicator needles have been snapped off by the force of the impact.
But even these ravaged dials tell a tale.
On close inspection, investigators find a critical mark.
At the moment of impact, a dent was made on the faceplate by the jarred indicator needle.
It proves that when United 585 crashed, its engines were running normally.
SCREAMING With engine failure ruled out, there seems to be only one other mechanical explanation for why 585 suddenly rolled over and then fell out of the sky.
It appears increasingly likely that the plane had suffered a catastrophic problem with its flight controls.
Investigators quickly become suspicious of the rudder at the back of the tail.
We focused in after eliminating other flight control services that we thought could contribute to the roll.
We started looking at the rudder.
Bring that up so I can take a look at it.
Investigators begin their examination of the rudder on site.
But the violence of the crash makes the job extremely difficult.
Almost nothing left.
Most of the plane's parts are too crushed or burned for testing but a vital component is still reasonably intact - the power control unit, or PCU.
Used constantly during flight, especially during landings, the PCU performs like a car's power steering.
When the pilot pushes on a rudder pedal, the PCU uses hydraulic fluid to convert gentle movements of a pilot's foot into the pressure needed to move the 737's enormous rudder.
The heart of the PCU is something called the dual servo valve.
Shaped like a soda can, it has two slides which glide past one another, directing the flow of pressurised hydraulic fluid that moves the rudder.
The servo valve is very unique that it is in effect two valves in one and that that creates a whole range of interactions that don't occur in a more conventional hydraulic valve.
When a technician opens the power control unit, chips of metal are found floating in the hydraulic fluid.
It's a disturbing find.
These particles could cause the servo valve to jam, making it impossible to work the plane's rudder.
It's a chilling prospect.
Could a microscopic fault bring down a 38-tonne jet? It's difficult for Phillips to tell.
While more intact than much of the wreck, the PCU and dual servo valve are both damaged.
Er, the airplane crashed and burned in a pretty confined area and there was a lot of damage to the flight control components and the things we were testing - needed to test and look at.
And the more damaged the components are, the harder it is to take measurements and do functional testing.
To test what he does have, Phillips travels to California, to the labs of Parker Hannifin - where the rudder control unit is made.
The curious metal chips floating in the PCU's chambers are dismissed.
Phillips is told that filters keep them out of the delicate servo valve that directs fluid and moves the rudder.
Nothing else is found that could explain any sudden movement of the rudder on Flight 585.
We didn't have any absolute indication or information that we could point to that said the rudder, power control unit, the servo valve or any part of that flight control system caused that accident.
Phillips still suspects a mechanical problem.
But with no conclusive evidence that the PCU or servo valve caused the crash, he's forced to sign off on the tests.
It's a pass.
With Phillips at a dead end, only bad mountain weather remains as a primary suspect.
An expert on weather-related aviation accidents, Greg Salatollo is trying to determine if heavy winds on the day of the crash were a factor.
SALOTOLLO: There is a history of events where there've been airplane accidents attributed to mountain waves and rotors.
In 1966, a BOAC 707 near Mount Fuji .
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er, disintegrated in a mountain wave and rotor.
High winds crashing over mountain peaks leave so-called wind rotors in the leeside - invisible, highly turbulent down draughts that come plunging down with devastating power and are extremely dangerous to aircraft.
We found a great deal of evidence, er .
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looking at the surface upper air data and talking to witnesses in the area that rotors were a possibility.
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explosion.
It was right over there.
Salatollo hears several eyewitness reports of bizarre mountain weather on the day of the crash.
One of the most intriguing is from Harold Darnell - TYRES SCREECH - Whoa.
What the heck was that? .
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whose truck was struck by a powerful gust of wind just moments before 585 crashed.
But as Greg Salatollo combs through his evidence, the theory that a wind rotor knocked the plane from the sky is getting less and less likely.
Nice-looking day.
Hard to believe the skies are unfriendly.
Wind rotors are areas of extremely low barometric pressure.
So if Flight 585 did pass through one, its altimeter reading would've spiked as the plane was blown suddenly upwards.
There was no evidence that we saw that on the flight data recorder of 585.
What the flight recorder did show was a fast and deadly drop in altitude, as the plane fell to earth.
(SCREAMING) It's been 21 months since the investigation into the crash of United Flight 585 began - almost two years, in which the NTSB has studied the crew, the weather, the rudder and thousands of other pieces of evidence.
They've come up empty-handed.
For only the fourth time in its history, the NTSB releases a report which doesn't reach a conclusion.
The cause of the crash of Flight 585 is undetermined.
We had put a lot of time and effort in, into the investigation, and we just weren't sure what had happened.
It was like he was tracking a serial killer.
He was frustrated that they had not solved 585.
He did not want that to happen again.
But almost two years after the report on 585 is released, the killer strikes again.
At 7pm on a clear, windless day, USAir Flight 427 is nearing Pittsburgh.
Captain Peter Germano and First Officer Chuck Emmett are getting ready for their final approach.
Folks, from the flight deck, we should be on the ground in about 10 more minutes.
Sunny skies.
A little hazy.
Flight attendants, please prepare for landing.
I ask you to check the security of your seatbelt.
Thank you.
MAN OVER RADIO: USAir at 309, descend and maintain 6,000.
As they close in on the airport, the pilots are on the lookout for another flight, about 10km ahead of them.
Looking for the traffic.
Turning 100.
USAir 427.
I see the jet stream.
As they pass through the turbulence left behind by the other flight, their jet suddenly and alarmingly rolls left.
- WARNING TONES BEEP - Hold on, hold on.
- Hold on.
- Shoot! Nothing the pilots do seems to have any effect.
SCREAMING What the hell is this?! - What the hell? - Shoot! SCREAMING 427 - emergency! SCREAMING Oh, shoot.
- Oh! Oh! - Oh, God.
- Oh, God, no! No! - Noooo! SCREAMING Rescue crews arrive quickly but the fate of Flight 427 is tragically clear.
There's no hope for the 132 passengers and crew.
The human carnage is so bad, authorities declare the crash site a biohazard.
USAir 427 accident was the first US accident where biohazard suits were used.
And it made it more difficult - they were uncomfortable, they were hot.
To this day, when I put on a pair of rubber gloves for any reason, I'm instantly transformed back to the site in Pittsburgh.
Captain John Cox - a 737 pilot and a flight systems specialist with the Airline Pilots Association - is asked to join the team investigating the crash of Flight 427 .
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as coroners attempt to collect human remains.
NTSB lead investigator Tom Haueter already knows his hunt for clues will be long and painstaking.
When we first arrived at the crash site, well, first of all, there was no aircraft there.
There were only bits and pieces of the airplane.
It wasn't really recognisable as an airplane.
With the help of eyewitnesses, information from the flight data recorder and the cockpit voice recorder, investigators begin to quickly see some striking similarities between 427 and the unsolved case of United 585.
In fact, they seem to be mirror images of each other.
On final approach, United 585 rolls right while USAir 427 rolls to the left.
- Oh, God! - Hang on, hang on.
Both crews are caught by surprise and after just a few terrifying seconds, both aircraft plummet straight into the ground.
Certainly, the whole team was aware of the previous accident with United 585 in Colorado Springs.
We tried to keep that in the back of our minds and take a look at this one, as to what it presents.
As the investigation continues, the list of similarities grows.
427's engines were also attached and functioning at the time of impact.
But for all the similarities, there's one important difference.
Unlike United 585, as USAir 427 approached Pittsburgh, weather conditions were dead calm.
Folks, from the flight deck, we should be on the ground in about 10 more minutes.
Sunny skies.
A little hazy.
Taking a look at the flight data recorder information, the aircraft's approaching Pittsburgh.
It's an extremely smooth night.
Er, there's just no turbulence at all.
The pilots are relaxed, they're talking about the landing.
Flight attendants, please prepare for landing.
I ask you to check the security of your seatbelts.
Thank you.
As he did in the case of United 585, Greg Phillips will once again lead the investigation into the mechanical aspects of the crash.
Almost immediately, he makes a promising discovery.
Miraculously, much of USAir 427's tail and rudder appear intact.
The hydraulic devices inside the tail have also sustained very little damage.
Phillips and Haueter prepare to send the parts to the manufacturer, Parker Hannifin, for testing as soon as possible.
They need answers.
Pressure on the NTSB to solve the accident is growing quickly.
Not only are the crashes of flights 585 and 427 disturbingly similar, both of them involved the same kind of airplane - a Boeing 737.
But with serious questions being raised about the plane's safety, billions of dollars and perhaps the airline industry itself are at risk.
We couldn't live with the fact, as investigators, of having two unsolved 737 accidents.
The airplane is in too much use, too wide a use around the world.
It carries too many people every day.
Unsolved was not an acceptable answer.
If 427 was an undetermined accident, we could not find the cause of this accident, there was a great chance if there was a third accident with a 737 fleet, under similar circumstances, that the 737 fleet would have been grounded.
Careful with that.
The investigation into the crash of Flight 427 may be the most important in the history of the NTSB, but it won't be easy.
Answers are still years away.
In just three years, two Boeing 737s have crashed in the United States with no survivors.
Airport in sight, United 585.
In both cases, the planes were moments from landing (OVER PA) Prepare for landing.
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and in both cases, the planes were under full power.
But the pilots were unable to control their jets.
In the crash of United Flight 585, NTSB investigators were unable to find a cause.
Now they're searching for clues into the second crash - USAir Flight 427.
As the investigator in charge, I never allowed myself to think this investigation could go undetermined.
We kept on pushing and kept on researching.
As long as we had things to research, we kept on going.
To find their killer, the NTSB can't afford to rule anything out - from the possibility that a collision with birds brought 427 down, to strange, even bizarre, theories.
They looked at electromagnetic interference, they got calls from people saying it might be Russian death rays, they considered everything.
There were a couple of witnesses who gave reports of the aircraft suddenly descending and hovering before it blew up.
We discounted those.
But the investigation's primary suspect is the dual servo valve, part of the power control unit that moves the 737's rudder, and a suspect in the crash of United 585.
Parker Hannifin made the valve.
At its lab in California, investigators look inside the main cavity of the USAir power control unit.
Just like in the earlier crash, they find tiny chips of metal floating in the hydraulic fluid.
But once again, Parker and Boeing repeat their claim - filters designed to stop any debris from interfering with the delicate metal slides have done their job.
Investigator Greg Phillips wants to be absolutely sure.
If the chips were blocking the slides, they would have left tiny scratch marks behind where they rubbed against the metal.
But Phillips can't find any.
Another pass.
OK.
Phillips has technicians put the servo valve from Flight 427 through as many tests as he can think of, trying to find a weakness.
If he can find one, it could explain why two planes were ripped from the sky.
But he comes up empty.
That unit passed all its operational tests.
There wasn't any indication that it had failed and it operated within the parameters we expected it to.
Once again, the investigators are forced to shift their focus back to the pilots.
By studying the plane's flight data recorder, investigators know the jet's rudder was deployed fully to one side, what's called rudder hardover.
We were definitely focused on rudder, on hardover rudder, full rudder input for about 20 seconds.
It can be caused either by hardware, something unknown in the hardware, or it can be caused by pilot input.
First Officer Chuck Emmett, who was flying 427, did indeed step down hard on his rudder and then held it there while the plane plummeted towards the earth.
It raised a grisly question - was he trying to fly the plane into the ground? In looking at this, being a pilot myself, this doesn't seem like rational behaviour.
What the hell is this? Human performance specialist Malcolm Brenner listens closely for evidence on the cockpit voice recorder.
What the hell?! In this case they had microphones right by their mouths and you can hear as well as in real life or better.
You can hear breathing sounds.
Hmm, yes, I see the jet stream.
The cockpit recordings indicate that Flight 427's troubles began at the moment it flew through the jet wake of a Delta Airlines 727 that had just passed in front of them.
Both pilots are startled by the wake.
I see the jet stream.
The first officer breaks off at the end of a sentence, "I see the jet stream," and there's no more discussion of the jet stream or anything else.
They both focus - something happened here.
The captain says, "Jeez!" Jeez.
It was such a smooth flight that it was a momentary jolt that they just hadn't anticipated, and with that, the pilots got on the controls and immediately, you know, put in a rudder input.
Jeez.
The cockpit recorder even records the thumping sound of the jet stream turbulence as 427 flies through it.
As Flight 427 encounters the turbulence, Brenner hears something unusual.
First Officer Emmett begins to grunt.
The grunting is unusual.
The controls are designed so that pilots don't need to grunt.
They're especially designed around human capabilities.
So to have someone grunting is typically a sign of an emergency.
- INSTRUMENTS BEEP - (Grunts) By matching data from the flight recorder with the crew's voices, Brenner is able to confirm that Emmett's grunts begin a split second after he pushed down on the rudder pedal and three to four seconds after the wake turbulence affected Flight 427.
What the On their own, the cockpit voice recordings prove very little, but it seems clear that the crew weren't trying to crash their plane.
Something happened which took them by surprise.
They reacted as quickly as they could but nothing they did seemed to help.
What the hell is this? It's been almost two years since the crash of Flight 427 and the investigation has stalled.
Now two 737s have gone down in startlingly similar ways and investigators still don't know why.
We were all frustrated as months wore into years.
What were we missing? It definitely took a toll on their personal lives.
They worked incredibly long hours.
They never stopped thinking about it.
We were going up against an aircraft that had an incredible safety history.
It was really everything You could see for 30 years this had been a great airplane.
We weretrying to prove that there was something wrong with the straight-A student.
It clearly was on his mind.
He, at one point, had a nightmare about it where he dreamed that he was in front of a congressional committee that was grilling him on now there had been a third 737 crash, and in Tom's dream, all the cameras were pointing to him and a congressman asked, "Why didn't you ground the fleet?" Unsure of where to look next, and with the trail of evidence getting colder, investigators need a break in the case .
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and fast.
CONTROL TOWER: Eastwinds 517, you're clear for landing.
On June 9, 1996, they finally get the break they've been looking for.
Captain Brian Bishop is on final approach to Richmond, Virginia, when, without warning, his Eastwinds jet rolls sharply to the right.
We didn't know to what extent, but we knew we had a rudder problem.
I turned the yoke the opposite direction and stood on the opposite rudder pedal but the pedal didn't move for me.
For over 30 seconds, the 737 flies in a precarious right bank as Bishop fights to keep it from rolling over.
Then suddenly, the unknown forces holding the jet let go .
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snapping the wings back to horizontal.
In a matter of seconds, it released itself and went back to normal.
We had started the checklist.
Almost before I could finish the sentence, all of a sudden there was a just a WHAM! For a second time, the 737 is pushed onto its side.
For 30 harrowing seconds, the 737 takes on a life of its own.
Then once again, as quickly as it began, the rollover stops.
After the second time, I said to the first officer, "Declare an emergency.
" Tell the controller we have flight control problems.
SIRENS WAIL As they slow down to land, the risks increase.
October 15, 1996.
For the last five years, the National Transportation Safety Board has been struggling to crack its toughest case.
Two completely separate but seemingly linked accidents The crash of United 585 killed 25 passengers and crew.
Three years later, the USAir Flight airline disaster took another 132 lives.
Now, after examining hundreds of clues, investigators have made a surprising discovery.
A new test has revealed that under the right circumstances, the hydraulic valve that moves the 737's rudder can jam.
It just jammed.
But the surprises aren't over.
The most important breakthrough came when a Boeing engineer examining the data from that test discovered some numbers that indicated the valve at that point had actually reversed.
Whoa! Jeez.
It's a stunning revelation.
Not only can the servo valve jam but it can then function in reverse.
It means that any time a pilot tries to correct a rollover by pushing on the rudder, the rudder might turn in the opposite direction, causing a fatal accident.
And the reversal is like driving your car - you turn to the right, it goes to the left.
You're not gonna figure out this failure mode until you go off the road and in these cases that's what the pilots were faced with - something so unusual that they didn't understand what was happening.
What the hell is this? They had evidence now that the valve was unique, that the valve not only could jam but would reverse.
427 emergency! That would explain why the first officer, Chuck Emmett, would keep his foot on the rudder pedal, because he's thinking, "Why isn't the plane going right?" And he's feeling the plane go to the left.
To the very end, Chuck Emmett pushes hard, hoping his rudder will help him pull out of his deadly spiral.
Tragically, he has no way of knowing that he is steering the aircraft straight into the ground.
Noooo! MAN: (OVER RADIO) Never driven to Colorado Springs and not gotten sick.
Flight attendants, prepare for landing.
Satisfied that they've determined the cause of the crash of USAir 427, the NTSB turns its attention to the unsolved case of United Flight 585.
Another 10.
Going back to Colorado Springs, you could follow a progression of what the captain was doing.
He's close to the ground and suddenly under rudder reversal, he puts on a little bit of pedal.
The pedal violently pushes his leg back.
- Oh, God! - 15 flap! Rudder reversal certainly fits what I know about this crew and how it fits.
We were able to show the failure mode.
It matched the flight data recorder from each aircraft.
It fit like a glove.
So we now had a lot more information we could apply to United 585 and based on that, we redid the accident report.
- Oh, my God! - Oh, no! From rudder reversal to impact took less than 10 seconds.
585's flight crew had no chance to save their plane or passengers.
In 1991, something happened on board a 737 that sent shudders through the world of aviation.
In the aftermath of the investigation, sweeping changes were made to improve the safety of the 737 and the entire aviation industry.
New training protocols were designed to help pilots react to unusual in-flight events.
In the 737 fleet, pilots are now trained on how to react to both rudder hardovers and reversals.
Whoa! The scenario of the USAir 427 accident If the crew had the information that we have today, I believe they would've landed safely in Pittsburgh that evening.
The FAA also directed Boeing to redesign the rudder's dual servo valve to eliminate the potential for reversal.
Boeing spent hundreds of millions of dollars to replace the valves on thousands of 737s around the world.
One thing we don't like at the Safety Board is to have an undetermined accident because then we can't make a change to improve safety.
So out of USAir 427, United 585, we have a much safer 737 fleet.
It took NTSB tin kickers 10 years to solve the mysterious crashes of flight 585 and 472 - the longest investigation in aviation history.
There are still some people in aviation who don't think the NTSB got it right.
But I became convinced after talking to very, very, very many people - pilots, engineers, people at Boeing - and spending a lot of time with the investigators, that they did get it right.
Since the replacement of the 737 servo valves, there hasn't been a similar crash of the most popular, most profitable plane in the world.