- 30/5/2025
This special looked at accidents caused by pitch issues.
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TVTranscripción
00:00Okay, it really wants to pitch down.
00:04The pilots of an MD-83 suddenly lose control.
00:08They need to find a way to control the pitch of the jet.
00:14Without warning, an ATR-72 rolls to the right.
00:19That airplane can fall out from under them in the blink of an eye.
00:23Two Boeing 737s suffer the same deadly fate after going into sudden dives.
00:31If a pilot cannot control the pitch system, literally that means the pilot cannot control the plane.
00:39These are the toughest cases to solve.
00:42Speeding planes dive nose first into the ground.
00:45There was very little left of the airplane.
00:48The plane pitched, full nose trimmed down.
00:50Investigators in three cases hunt for answers when pilots lose control of their plane's pitch.
00:57It just jammed.
00:59It stopped working completely.
01:01Take a look.
01:03I never allowed myself to think this investigation could go undetermined.
01:08MD, MD.
01:09Alaska Airlines Flight 261 climbs into sunny skies over Puerto Vallarta, Mexico,
01:36carrying 83 passengers and five crew.
01:40Gear up.
01:44Gear's up.
01:45Captain Ted Thompson is an Air Force veteran.
01:49Thank you, sir.
01:51First Officer Bill Tansky has been flying for almost four decades.
01:56The pilots on 261 were highly regarded.
02:01These were really top-notch aviators.
02:04The MD-83 will be stopping over in San Francisco before heading to its final destination, Seattle, Washington.
02:15But 15 minutes after taking off, there's a problem.
02:19It's pushing down.
02:20The nose of the plane is dropping.
02:24And First Officer Tansky must wrestle with the control column to keep the plane climbing.
02:30Increasingly, there's more and more pressure that he's having to hold.
02:34Well, we're still climbing.
02:3621 minutes into the flight, they level off at 31,000 feet.
02:41But it appears the stabilizer is jammed.
02:44No dice.
02:46Let's try it on autopilot.
02:47The plane is steady.
02:52The autopilot is keeping the plane at the correct altitude.
02:55But Captain Thompson has no idea how long that will continue.
03:00All right.
03:01I'm thinking we get this plane on the ground sooner rather than later.
03:05He's worried about the stabilizer, so decides to divert to Los Angeles Airport.
03:10Dispatch 261, requesting a diversion to LAX.
03:15Our longitudinal trim system is inoperative.
03:19Thompson contacts Alaska Airlines Dispatch in Seattle.
03:24Dispatch 261, copy that.
03:28This will click the autopilot off.
03:31Stabilizer motion.
03:34Stabilizer motion.
03:37Holy crap.
03:38Flight 261 goes into a dive.
03:43It got worse.
03:47Center, Alaska 261.
03:49We are in a dive here.
03:52I've lost control of vertical pitch.
03:55Alaska 261, say again, sir.
03:57We are in a vertical dive at 26,000.
04:02Speed brakes.
04:04The speed brakes are movable surfaces on the wings that increase drag.
04:12The maneuver works.
04:15Once we get the speed slowed, maybe we'll be okay.
04:19The plane has dropped 8,000 feet in just 80 seconds.
04:23We've got it back.
04:28Under control here.
04:31No, we don't.
04:33It still requires all their strength to keep the nose of the plane level.
04:39Okay.
04:40It really wants to pitch down.
04:42They're in a fight for control of the airplane.
04:50The passengers' anxiety grows.
04:53We better talk to the people back there.
04:56Yeah.
04:57I know.
04:58Folks.
04:59We've had a flight control problem.
05:02We're intending to go to Los Angeles.
05:06We're working on it, and I don't anticipate any problems once we get a couple systems back on the line.
05:13As they near Los Angeles, the pilots prepare for an emergency landing.
05:19Okay, we're pretty stable here.
05:22But we've got to get down to 180 knots.
05:24They need to find a way to control the pitch of the jet.
05:30If it's controllable, we ought to just try and land it.
05:34They would not be able to pull the plane up and do a go-round.
05:39They had one shot to get it on the ground, or all would be lost.
05:46You feel that?
05:48Yeah.
05:48But at that moment, disaster strikes.
05:53The airplane pitched down and rolled.
05:56This is pilot's nightmare.
05:58Mayday!
06:03Push and roll!
06:05Push and roll!
06:08Flight 261 continues to roll left and dive.
06:11And this is like an air show maneuver.
06:19Okay, we are inverted.
06:23Now we've got to get it up.
06:26They've got to get the airplane back right side up.
06:30Their one hope is if they could kick that rudder, they could flip the plane back over.
06:36Okay, let's kick rudder.
06:39Left rudder.
06:41Left rudder.
06:42The pilots are kind of hanging upside down like bats.
06:45And it was very difficult to reach the rudder pedal.
06:49I can't reach it.
06:50Okay.
06:52We've got to get it over again.
06:57Speed brakes?
06:59They were giving everything they had.
07:01The windscreen's full of the ocean.
07:03They're not going to solve this.
07:05Rescuers search for survivors.
07:25They find none.
07:29All 88 passengers and crew on board Flight 261 are dead.
07:35Investigators from the National Transportation Safety Board, the NTSB, are assigned to find an explanation for the crash.
07:47LAX is saying the pilots reported a jammed stabilizer.
07:49We knew quite a lot just from the transmissions between the flight crew and Los Angeles.
07:58The plane pitched.
07:59Full nose trimmed down.
08:00We immediately began to research the horizontal stabilizer trim system.
08:09The leading edge of the horizontal stabilizer is raised or lowered by a jack screw.
08:14The jack screw changes the stabilizer's angle as it moves through an acme nut.
08:26We need to see the jack screw assembly.
08:29Investigators wonder how the horizontal trim system could have failed.
08:33Recovering parts of the stabilizer system could give investigators important clues.
08:42But the wreckage field lies on the bottom of the ocean.
08:48Beyond the reach of divers.
08:50Nine days into the investigation, remotely operated vehicles equipped with robotic arms
08:58retrieve the parts investigators are looking for.
09:03We got the jack screw.
09:06With the brain power we had, we had a fighting chance to find out exactly what went wrong during this flight.
09:12Systems investigator Jeff Gazzetti immediately spots a problem with the jack screw assembly.
09:23Why isn't the nut attached to the jack screw?
09:26When we looked at that, we asked ourselves, how can that happen?
09:31Threads inside the nut should hold the jack screw securely in place.
09:36But these threads failed.
09:39Incredible.
09:40It's completely stripped.
09:42No one thought that you could get those thick threads to rip out.
09:50Metallurgist Joe Epperson examines the stripped nut and the jack screw.
09:59There's some grease here on the bottom.
10:03The way to prevent wear is by lubrication.
10:07With grease.
10:08In flight, the jack screw rotates inside the nut.
10:13To prevent wear, it needs to be lubricated frequently.
10:18In the case of an extreme wear event, you want to look at, is there grease where it's supposed to be?
10:28There should be more grease here in the middle.
10:30There was very minimal signs that there was any grease on it at all.
10:37During maintenance, grease is applied to the interior of the nut through a small valve called a zerk fitting.
10:44But the valve is blocked.
10:45So, the zerk fitting is this fitting right here.
10:51And what mechanics have to do is put a grease gun hose in here and then squirt grease into this little grease fitting that goes inside this passage.
11:02That zerk fitting should have preserved and kept the remnants of whatever grease it saw last.
11:08It's packed with dried grease.
11:11How long has it been clogged like this?
11:16At least a year, maybe more.
11:20I've been around grease long enough to know that if you don't replenish it, and if you just leave it, eventually it dries up and gets hard.
11:30And it basically says that it had not been replenished.
11:35NTSB investigators track down the mechanic who serviced the jack screw and acme nut on the Alaska Airlines MD-83 four months before it crashed.
11:52Could you walk us through how you lubricate the jack screw assembly?
11:57We were very suspicious about how well this component was lubricated.
12:03We used a grease gun to the zerk fitting.
12:07Well, how do you know whether the lubrication has been done properly and when to stop pumping the grease gun?
12:13I don't.
12:15He couldn't have known that that zerk fitting has taken grease.
12:18You have no idea that it's clogged.
12:20But it's not just the way the jack screw assembly was lubricated that bothers investigators.
12:31The last time the jack screw was lubricated was about four months before the accident.
12:38And before that?
12:39January 1999.
12:41They're doing it every 2,500 hours.
12:49They discover longer and longer periods between lubrications.
12:54Check it out.
12:56The airline made multiple requests to extend the intervals on the lubrications.
13:01In 1987, the interval between lubrications was 500 hours.
13:14Those intervals are measured by the number of hours the plane is in the air.
13:18In 1991, it goes up to 1,200 hours.
13:26By 1996, it increases all the way up to 2,500 hours.
13:35If you're going to extend these lubrication functions, then you better be doing something to make sure that what you're doing is correct.
13:46But even if the lubrication wasn't being done properly, regular inspections should have caught the wear on that Acme nut.
13:54Was the jack screw assembly on Flight 261 inspected when and how it should have been?
14:00We looked at the maintenance records for information about the last check.
14:07Yeah, this doesn't look right to me.
14:11The team finds paperwork showing that a mechanic determined the Acme nut was badly worn more than two years before the crash.
14:23You sure the rating is 0.040?
14:27The lead mechanic ordered the nut to be replaced.
14:30This was evidence that someone had caught the fact that this Acme nut was worn out.
14:37Most of the alliance, the decision would be, we will get the piece to the airplane as soon as possible and replace it.
14:45Do you remember inspecting an MD-83 on September 27, 1997?
14:50I remember it well.
14:53Investigators contact John Leartine, the mechanic who reported the worn Acme nut on Flight 261.
14:59I wrote up the evaluation.
15:04The nut is worn down.
15:07Replace it.
15:08But when investigators probe further, they learned that another mechanic cancelled the order to replace the nut.
15:18That mechanic determined that it was still within safe limits and cleared the plane to fly.
15:24How could a maintenance facility allow the airplane to be put back into revenue service with the wear that it found on the Acme nut?
15:34This accident could have been prevented.
15:41Literally 50 cents worth of grease cost a plane load of people their lives.
15:45In the wake of the investigation, federal regulators reduced the interval between jackscrew lubrications at Alaska Airlines from 2,500 hours to 650 hours.
15:58Looking back, it makes no sense at all.
16:01They decided you could put off maintenance, put off maintenance, and put off maintenance with the approval, I might add, of the authorities and the airline who allowed greasing their parts to be extended almost indefinitely.
16:14Poor maintenance can play havoc with even the best-designed aircraft, but sometimes, flaws can develop while a plane is still on the drawing board.
16:28There are 64 passengers on board American Eagle Flight 4184, as it approaches Chicago's O'Hare Airport.
16:41Chicago Center, we have discretion down to 10,000.
16:46We're on our way down now.
16:48Eagle 184, right?
16:50Fog and reduced visibility at O'Hare are jamming up traffic and creating delays.
16:57Eagle 184.
16:58Hold southeast on Vector 7.
17:01Chicago Center, roger.
17:03Hold southeast on Vector 7.
17:05Eagle 184.
17:06Captain Orlando Aguiar is in command of the ATR-72.
17:11He programs the autopilot to keep the plane circling at 10,000 feet.
17:16This thing gets a real high-tech angle in these turns.
17:19Yeah.
17:20The circling turn keeps the plane's nose pitched up.
17:24You want flaps 15?
17:26I'll bring the nose down.
17:27First officer Jeff Galliano extends the flaps to 15 degrees, and the aircraft levels out.
17:33That's much nicer.
17:36Well, flaps 15.
17:38Sure, once they let us out of the hold and forget they're down, we'll get the overspeed warning.
17:41A half hour later, air traffic control directs the flight to a lower altitude.
17:51Eagle flight 184, descend and maintain 8,000.
17:56Down 8,000, Eagle 184.
17:59Eagle flight 184 should be about 10 minutes until you're cleared in.
18:09Are we out of the hold?
18:10No, just going down to 8,000.
18:17I knew we'd do that.
18:19The overspeed warning sounds.
18:22I was trying to keep it at 180.
18:24The plane is going too fast to fly with extended flaps.
18:29Galliano retracts them.
18:30Suddenly, the plane banks sharply to the right.
18:40The control column is stuck.
18:53American Eagle flight 4184 is in an uncontrolled dive.
18:57The pilot's fight to pull out of it and level off.
19:06That's it.
19:08Pull up.
19:08Nice and easy.
19:13Pull up.
19:14Pull up.
19:16Pull up.
19:18Pull up.
19:18Pull up.
19:18Flight 4184 crashes in a field just outside Roselawn, Indiana.
19:32All 68 people on board are killed.
19:39The next morning, an NTSB team is on the scene.
19:43It was one of those types of accidents where there was very little left of the airplane.
19:57Amidst the destruction, the airplane's two flight recorders are found intact.
20:02While the voice and data recordings are analyzed, investigators focus on the holding pattern the flight was put into by air traffic control.
20:15Eagle 184 holds southeast on Victor 7.
20:19Well, folks, I do regret to inform you that air traffic control has put us into a holding pattern up here.
20:24When we looked at the air traffic control records, we found that the flow rate, that is the number of airplanes that were going into Chicago O'Hare at the time,
20:33had to be reduced because of the weather moving through the area.
20:37Chief investigator Greg Fythe wants to know more about the weather during the hold.
20:45I want weather data, pilot reports, atmospheric conditions, Doppler rate, or anything you can find.
20:51Pilots of other planes circling O'Hare Airport that day point investigators towards an important clue.
21:01They all experienced icing, some as much as three-quarters of an inch.
21:07Icing is a well-known threat to all aircraft.
21:10It creates drag, it reduces lift, it causes performance and handling problems.
21:16Examining weather records from the day of the crash, investigators discover that during the 39-minute holding pattern,
21:24flight 4184 repeatedly passed through cloud banks.
21:294184 we knew had been holding in an icing event.
21:32There were layers of clouds that they had been flying in and out of.
21:36But did the pilots know they were flying into icy conditions?
21:40The cockpit voice recorder provides the answer.
21:45That's much nicer now, flaps 15.
21:47Yeah.
21:50I'm throwing some ice now.
21:53Nine minutes before the crash, they noticed their plane was picking up ice.
21:59Fythe and his team wonder what the crew did about it.
22:01They turned to the flight data recorder.
22:09It indicates that almost 17 minutes before the crash, the master caution warning went off.
22:15In response, Captain Aguiar turned the plane's de-icing system to its maximum setting.
22:25But was the de-icing system working?
22:27Normally, a rubber boot inflates to break ice that coats the surface of the leading edge of the wing.
22:37The key component to the de-icing system is probably not the black rubber boot that you see on the airplane,
22:43but the timer and the valves that control those boots.
22:48The force of the crash has left markings that show the position of the valves at the time of impact.
22:54Those markings indicate the plane's de-icing system was working properly.
23:02I'm throwing some ice now.
23:05So if there wasn't a problem with the de-icing system, what else could have caused the crash?
23:14Other pilots believe they know the answer and decide to act.
23:18I distributed a brochure which essentially warned people about the problems with the ATR aircraft.
23:27The brochure says the ATR-72 fleet operated by American Eagle is dangerous in icing conditions,
23:34and the airline is doing nothing about it.
23:38A month after the crash, pilot frustration boils over.
23:42A group of a dozen pilots refuses to fly the ATR, citing risky weather.
23:49The public protest leads investigators to take a closer look at the aircraft's vulnerability to ice.
23:56The NTSB's Charlie Pereira visits the ATR's French manufacturer, Aerospatiale.
24:01We embarked on a research effort to try to identify all previous ATR-42 and 72 role control incidents.
24:15He finds a disturbing pattern.
24:19And about five of those were found to be similar after the fact in our review to the Rose Long case.
24:28In December 1988, an American Eagle ATR-42 preparing to land in icy conditions suddenly pitched down and stalled without warning.
24:42The pilots recovered by boosting engine power.
24:47But other flights end differently.
24:51In October 1987, an ATR-42 en route to Germany crashed.
24:58Killing all 37 people on board.
25:08The investigation found that ice was a factor, but placed the blame on pilot procedures.
25:15For investigators, it's really important to look back at historical information about any airplane.
25:21Did any of your studies uncover a wing design flaw?
25:26Charlie Pereira wants to know if the engineers who built the ATR series can explain why the plane would go out of control.
25:33What he's told...
25:37Still got ice.
25:38...will shock the airline industry.
25:40NTSB investigator Charlie Pereira presses engineers for the reasons behind the ATR's tragic record.
25:51When I asked one of their oldest, most senior aerodynamicists what he thought would cause the behavior that we saw on the flight recorder for the Rose Long airplane,
26:07He drew up a little shape just behind the de-ice boots.
26:12After the de-icing boot.
26:14If ice builds up behind the boot, the crew has no way to clear it.
26:29Still got ice.
26:30But how could ice form in that location?
26:33Additional weather analysis shows Flight 4184 flew into something much more dangerous than freezing rain.
26:48Super-cooled large droplets, or SLD.
26:52Raindrops the size of a human hair that stay in liquid form even when the temperature is below freezing.
26:59They only freeze when they contact a solid.
27:01On every lap of its holding pattern, Flight 4184 flew through layers of cloud that contained the...
27:19...couraged by some wind shear near the cloud top.
27:22They just put that poor guy in the wrong place at the wrong time.
27:25Investigators learned that while ordinary ice accumulates on the leading edge of the wing, where de-icing boots can get rid of it,
27:36SLDs slide back over the boots before freezing and form a ridge out of reach of the de-icing system and beyond the view of the flight crew.
27:46Eagle Flight 184 descent and maintain 8,000.
27:52Down to 8,000, Eagle 184.
27:55Investigators conclude that the ice build-up had no effect on Flight 4184 until the plane began its descent to 8,000 feet.
28:03I knew we'd do that.
28:05The descent increased the plane's airspeed and triggered a warning from the flight computer,
28:11telling the pilots they were flying too fast with the flight...
28:16...action had a dramatic effect on the plane's aerodynamics.
28:26When they retracted the flaps as they started their descent down to 8,000 feet,
28:31the airplane pitched back up into a very nose-high attitude.
28:37When the nose lifted, airflow over the ice-laden wing was disrupted.
28:42This created powerful turbulence, lifting the aileron on the right wing and causing the plane to roll.
28:50The forces were determined to be about 250 pounds,
28:54so for the pilot, it would have been trying to roll against 250 pounds of aerodynamic force.
29:00The NTSB has uncovered a deadly flaw in a popular airplane.
29:10The ATR's wing design makes it vulnerable to icing.
29:17In response, the plane's French builder, Aerospatial, makes the de-icing boots wider,
29:24so they cover a larger part of the wing.
29:27Several things had to go wrong for this to happen.
29:30It was only because of the very detailed investigation in these accidents
29:35that these were able to be resolved so that it would not happen again.
29:39It took more than a year to wrap up the investigation into Flight 4184.
29:44But the factors involved in other planes' pitch problems can be so rare and unusual
29:49that it takes several years to solve the mystery.
29:54United Airlines Flight 585 is on final approach into Colorado Springs
29:59with 20 passengers and five crew on board.
30:04The skies are clear, but heavy turbulence is rocking the flight.
30:09Nice-looking day. Hard to believe the skies are unfriendly.
30:13At the controls is 52-year-old Captain Harold Green.
30:21Green's co-pilot is Patricia Ideson.
30:27Flight attendants, prepare for landing.
30:28At Colorado Springs Municipal Airport,
30:32controllers are ready to bring Flight 585 in.
30:35United 585 is cleared for a visual approach to Runway 35.
30:41Starting on down.
30:46Suddenly, the 737 spins out of control.
30:50What's going on?
30:52Oh, my God!
30:52Flight 585 is in a 230-mile-an-hour dive.
31:06It crashes less than four miles from the runway.
31:13The shattered remains of United Airlines Flight 585 lie buried in a fire-blackened impact crater.
31:20Rescue workers arrive within minutes.
31:23None of the 25 people on board have survived.
31:29By nightfall, NTSB investigators are on site.
31:33My first sense that it was going to take some time to investigate the accident was the damage that we saw in the parts.
31:44When they're burnt and broken, the process always takes longer.
31:49The black boxes are extracted from the site and sent to the lab in Washington.
31:56While pouring over the voice recordings, human performance specialist Malcolm Brenner hears nothing to indicate that the pilots struggled with the approach.
32:05This is a sense of an excellent crew caught randomly, if anything.
32:12So, again, that was my first impression, is that this would be consistent more with a hardware situation.
32:20Investigators examined the plane's wings and tail assembly.
32:23After eliminating other flight control surfaces that we thought could contribute to the role, we started looking at the rudder.
32:34The violence of the crash makes recovering these parts a daunting challenge.
32:39Almost nothing left.
32:41But a vital component of the tail assembly is mostly intact.
32:50The power control unit, or PCU.
32:56When a pilot pushes on a rudder pedal, the PCU uses hydraulic fluid to convert movements into the pressure needed to move the 737's rudder.
33:06Inside the PCU, a dual servo valve directs the flow of pressurized hydraulic fluid that moves the rudder.
33:15When a technician opens the power control unit, chips of metal are found floating in the hydraulic fluid.
33:23It's an eye-opening find.
33:26Could these particles have caused the servo valve to jam?
33:31Testing produces no evidence that they led to the loss of control.
33:35We 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.
33:47Over the next 21 months, investigators rule out pilot error, mechanical failure, and weather.
33:55Then, for only the fourth time in its history, the NTSB releases a report which doesn't identify a cause.
34:02We had put a lot of time and effort into the investigation, and we just weren't sure what had happened.
34:09Two years later, U.S. Air Flight 427 approaches Pittsburgh.
34:19It hits a patch of turbulence.
34:21Jesus.
34:23The 737 suddenly rolls left.
34:27Hold on, hold on.
34:30Nothing the pilots do can pull the plane out of the dive.
34:33What the hell is this?
34:42Shoot!
34:45427! Emergency!
34:49Oh, shoot.
34:51Oh!
34:52Oh, God!
34:52Oh!
34:55Oh, God, no!
34:56First responders arrive quickly, but there's no hope for the 132 passengers and crew.
35:14NTSB investigators arrive at the site of a second Boeing 737 crash.
35:20Oh, God!
35:21Oh, God!
35:22Oh, God!
35:23Oh, God!
35:24Oh, God!
35:25Oh, God!
35:26There was no aircraft there.
35:28There were only bits and pieces of the airplane.
35:30It wasn't really recognizable as an airplane.
35:35Eyewitness accounts, along with voice and data from the flight recorders, indicate Flight 427 suddenly rolled.
35:44And then dived.
35:46It matches what happened aboard United 585.
35:50Certainly the whole team was aware of the previous accident with United 585 in Colorado Springs.
35:57We try to keep that in the back of our minds and take a look at this one as to what it presents to us.
36:02Almost immediately, investigators make a promising discovery.
36:06Much of U.S. Air 427's tail and rudder appear intact.
36:13The hydraulic devices inside the tail have also sustained very little damage.
36:18The primary suspect is, again, the dual-servo valve.
36:34The dual-servo valve.
36:34Just like in the earlier crash, tiny chips of metal are found floating in the hydraulic fluid.
36:46But once again, there's no evidence the debris interfered with the delicate valve.
36:51There wasn't any indication that it had failed, and it operated within the parameters we expected it to.
36:59We were all frustrated as months wore into years.
37:04What were we missing?
37:07Investigators need a break in the case.
37:10On June the 9th, 1996, they get one.
37:14An Eastwind 737 is on final approach to Richmond, Virginia, when without warning, it rolls sharply to the right.
37:24We knew we had a problem with the rudder.
37:25I turned the yoke the opposite direction, stood on the opposite rudder pedal.
37:30The pedal didn't move for me.
37:32Then, suddenly, the unknown forces holding the jet let go.
37:36In a matter of seconds, it released itself, went back to normal.
37:44The Eastwind 737 lands safely.
37:52The next day, the NTSB team arrives in Richmond.
38:03The airplane literally didn't move.
38:05It stayed at its location in the airport until we got down there.
38:08It gave the NTSB a tremendous break, because suddenly they had a 737 that had had a rudder incident that was intact,
38:17and they had a pilot who was alive and who could talk about it.
38:20NTSB investigators quickly determined that what happened on board Eastwind Flight 517
38:30is alarmingly similar to events on flights 427 and 585.
38:37They questioned the Eastwind pilots.
38:39And when we said, well, what happened?
38:41They said, there was something wrong with the rudder pedal.
38:44The pedal wouldn't go down.
38:46I was standing on the rudder pedal, and I couldn't get it to go down.
38:52My God.
38:53Investigators immediately focus their attention on Eastwind's rudder controls.
38:58The PCU is removed, inspected, and tested again and again.
39:03To everyone's frustration, the unit performs perfectly.
39:07We tested that aircraft as is.
39:09It was intact.
39:10We went through it completely, did flight tests with it, and it passed all tests.
39:18Investigators go back to the control unit from U.S. Air Flight 427
39:22and put it through more extreme testing.
39:25One fellow mentioned a test they had done in the military of a thermal shock
39:30where if you had the actuator being very cold
39:35and put in very hot hydraulic fluid, it would cause it to react in strange ways.
39:43So we put together a thermal shock test.
39:47The PCU is supercooled to simulate the minus 40 degree temperatures at 30,000 feet.
39:54Then it's quickly injected with superheated hydraulic fluid.
40:07As we were standing there listening to the actuator move left and right, left and right,
40:14it stopped.
40:18And it was not commanded to stop.
40:20It just jammed.
40:22Stopped working completely.
40:24It appears that the small hydraulic valve that controls the rudder of the Boeing 737
40:30can jam in certain circumstances.
40:34But that's not all.
40:36The most important breakthrough came when a Boeing engineer,
40:42examining the data from that test,
40:45discovered some numbers that indicated the valve at that point had actually reversed.
40:52It's a stunning revelation.
40:55Not only can the dual servo valve jam,
40:59but it can also cause the rudder to reverse.
41:01You turn to the right, it goes left.
41:05The pilots were faced with something so unusual that they didn't understand what was happening.
41:11To the very end, the pilots of flight 427 had pushed hard,
41:15hoping the rudder would help pull them out of a deadly spiral.
41:19But they had no way of knowing they were making the plane's drastic roll even worse.
41:25Flight attendants prepare for landing.
41:28Re-evaluating the data of United Flight 585,
41:34investigators conclude its flight crew fell victim to the same malfunction.
41:38Going back to Colorado Springs, you could follow a progression of what the captain was doing.
41:45He's close to the ground, and suddenly, under rudder reversal,
41:49he puts in a little bit of pedal.
41:50The pedal violently pushes his leg back.
41:53Oh, God!
41:54Flip!
41:5515, Bob!
41:5615!
41:59Rudder reversal certainly fits what I know about this crew.
42:02We were able to show the failure mode.
42:05It matched the flight data recorder from each aircraft.
42:09It felt like a glove.
42:10A move!
42:12It took less than 10 seconds to go from rudder reversal to impact.
42:23In the aftermath of the investigation,
42:26sweeping changes are made to improve the safety of the Boeing 737.
42:30The federal regulator directs Boeing to redesign the rudder's dual-servo valve
42:37to eliminate the potential for reversal
42:39and to replace those valves on thousands of 737s around the world.
42:46One thing we don't like of the safety board is to have an undetermined accident
42:50because then we can't make a change to improve safety.
42:53So out of US Air 427, United 585, we have a much safer 737 fleet.
43:00It took the NTSB 10 years to solve the mysterious crashes of flights 585 and 427,
43:09the longest investigation in its history.
43:14You know, the first thing people say is,
43:17oh, who would have thought this could happen?
43:18But when they go back and look at the data, often it's there.
43:21It was a problem waiting to happen.
43:23A pitch problem can cause catastrophe so total that evidence is scarce.
43:30But by revealing deeply hidden flaws,
43:33these three gruelling investigations changed aviation and made the skies safer.
43:39The biggest lesson to take away for the industry and for the public at large
43:44is that in a complex accident such as this,
43:47the answers may not come in a few months or even a few years.
43:50Sometimes it would take several events
43:53in order to piece together a pattern that reveals the answer.
43:57Based in perspective, from a strict level during the journey today is
44:05that everybody is cient- desert.
44:06And I've never had to take their end and 될 the question andбbeshe
44:09that people have the potential
44:18that a result of the things that somebody wants,
44:20does not necessarily just weet a thing,
44:22and tries to take it from the feet to get there.
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