This special looked at accidents caused by seemingly minor defects or errors.
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00:00In the ocean near Los Angeles, on a lonely mountain in Japan, off the east coast of Canada,
00:11a plane crash can reduce an enormous jet plane to mangled pieces.
00:16The cause is buried somewhere in the wreckage.
00:19In the business we refer to often finding the golden nugget.
00:25A modern jet plane is made up of hundreds of thousands of parts.
00:29A failure in any one of them can lead to disaster.
00:33A missing screw can jeopardize the safety of flight.
00:37Constant checks keep planes flying and passengers safe.
00:42A single oversight can end in tragedy.
00:51Something exploded.
00:54Oh my god!
00:57Help me! Help me! Hold it! Help me! Hold it!
01:24A hot summer night in Phoenix, Arizona.
01:28It's 11 o'clock, but the maintenance workers at Southwest Airlines are just getting started.
01:36Tonight they're going to open up a state-of-the-art Boeing 737-700.
01:42Almost 40 inspectors and mechanics are going to spend the night making sure the plane is fit to fly.
01:50Without proper maintenance, airplanes don't fly.
01:53Pilots are usually the focus for the operation of the airplane,
01:57but maintenance has an equally high priority role in the safe operation of any aircraft.
02:04To keep airplanes in peak condition, they get more health checks than most passengers.
02:09It is a very intricately weaved web between the operation of the airplane
02:15and the maintenance of the airplane and the management of the airplane.
02:19Passenger planes are examined every time they come to a stop.
02:23This is the A check.
02:25A brief walk-around inspection turns up the most obvious problems.
02:30The more intensive work is done at set intervals.
02:33These are the B and C checks.
02:39Tonight, workers are performing a C check.
02:43From start to finish, it can require hundreds of man-hours of work.
02:50It all has to get finished tonight, so the plane is back flying in the morning.
02:58It's a massive challenge, because modern jets are made of hundreds of thousands of individual pieces.
03:06In 1903, when the Wright brothers took their historic first flight near Kitty Hawk, North Carolina,
03:13their plane had some 1,500 parts.
03:16A 737 has more than 360,000.
03:21You have to ensure that every one of those components is doing its respective job.
03:27It doesn't matter how big the part is,
03:30a missing screw can jeopardize the safety of flight.
03:34It's a lesson the aviation industry has learned the hard way.
03:40January 31, 2000.
03:45On board Alaska Airlines Flight 261, the situation is desperate.
03:50Operating a damaged plane, the captain is trying to land as safely as possible.
03:56Operating a damaged plane, the captain is trying to land at Los Angeles Airport,
04:01but the aircraft is not responding to controls.
04:04The MD-83 is plunging towards the Pacific Ocean.
04:10Other pilots flying nearby report the nightmare scene back to LA Air Traffic Control.
04:26Plane inverted, sir.
04:27Okay.
04:28Yeah, he's inverted.
04:29Push the blue side up!
04:34Here we go.
04:42And he just hit the water.
04:45Yes, sir, he hit the water. He's down.
04:50Flight 261 crashed off the coast of California at over 400 kilometers an hour.
04:57All 88 passengers and crew are killed.
05:10Investigators from the National Transportation Safety Board begin their work quickly.
05:15The cockpit voice recordings provide some of the earliest clues.
05:19We have a jammed stabilizer, and we're maintaining altitude with difficulty.
05:24We immediately suspected some problem in the tail of the airplane,
05:28which is where the controls are.
05:30Something was wrong back there.
05:35Investigators examine the MD-83's horizontal stabilizer.
05:39The stabilizer controls the plane's pitch, its ability to tilt up and down.
05:45As the stabilizer moves up, the plane's nose tilts down.
05:49As the stabilizer moves down, the nose moves up.
05:55In the MD-83, a motorized jack screw on the tail moves the stabilizer up and down.
06:02When investigators recover the tail from the crash site, they make a puzzling discovery.
06:08The jack screw wasn't mated with the nut that it screws into.
06:13It was just by itself.
06:15And the nut was found in another piece of structure, a few feet away from where the jack screw was.
06:20To have a screw separate itself from a nut,
06:23it was necessary to have a jack screw on the tail.
06:26A few feet away from where the jack screw was.
06:28To have a screw separate itself from a nut with very thick threads surprised us.
06:35Without the jack screw, the stabilizer was beyond control.
06:39Without the stabilizer, the plane was doomed.
06:47The investigators very quickly figured out how the accident happened.
06:51Now they want to know why.
06:53The answer is tragically simple.
06:56There was no lubrication or visible grease on the working area of the screw.
07:02That was surprising and strange.
07:09The Federal Aviation Administration orders an immediate check on all MD-80s in the USA.
07:17At Alaska Airlines, the jack screws on six of its fleet of 34 MD-80s fail inspection.
07:27Investigators discover even more alarming evidence as they go through the carrier's maintenance records.
07:33Mechanics at Alaska Airlines report that they are under tremendous pressure to cut corners to keep the planes flying.
07:41We interviewed all the mechanics who had worked on these airplanes.
07:45We knew that they had been falsifying records or not doing the work they had indicated.
07:52To survive an economic recession in the 1990s, Alaska Airlines slashed their maintenance regime.
07:59With air carriers, especially those that may be economically strapped, they're going to stretch inspection cycles to the maximum.
08:08The FARs, the Federal Aviation Regulations, set a minimum level of safety.
08:13Now, if you're going to operate on a shoestring, you're only going to meet that minimum level of safety.
08:19If I'm a good carrier or I want to be a good carrier and I want to show that we're going to operate at the highest levels of safety,
08:26I'm going to typically exceed the minimums.
08:29It's going to cost more, but I'm going to exceed it.
08:32A lot of companies that say, wait, the regulations only say I only have to go to here, that's what I'm going to do.
08:41Jack screws in the company's fleet had been inspected every 500 to 700 flight hours.
08:47But in 1996, to cut costs, Alaska Airlines began checking the jack screws every 2,500 hours.
08:54At the same time, they doubled the average daily use of their fleet.
08:59If you had 600 hours between inspection points and greasing points, we have no chance of ever having a metal-to-metal contact situation.
09:08But if you put that out to 2,000 hours or 2,500 hours, now what you do is eat into some of these protective stages,
09:15these barriers that we have towards catastrophic failure.
09:20Proper maintenance becomes even more critical when there's no backup to a component.
09:25On the MD-83, there was no alternative if the jack screw failed.
09:30So proper maintenance was a matter of life and death.
09:34But in the aviation industry, it's also a matter of dollars and cents.
09:39There's a lot of pressure in the airline industry.
09:41When you look at it, whether you're hauling boxes or hauling people, the fact of the matter is that competition is stiff.
09:50And how do you get the competitive advantage against the next guy?
09:55How am I going to get more for less?
09:57And a lot of times it's labor, the other times it's maintenance.
10:01If I can stretch the inspection to 500 hours instead of 400 hours, that saves me a lot of money.
10:09To stay afloat financially, Alaska Airlines put countless lives at risk.
10:16But disaster can erupt even when an airline doesn't cut back on its maintenance regime.
10:30It's past midnight in Phoenix, Arizona.
10:33A maintenance crew works through a 737-700.
10:38They're performing a so-called C-check, one of the most detailed inspections any plane can go through.
10:46We work overnight because that's when nobody flies.
10:50It's better for the airline to keep their men in the ground overnight to fix them up.
10:53Tonight, 339 individual inspections are set to be made.
10:57Each one of these is tracked by computer.
11:00Anything that comes up yellow is an unscheduled procedure, a problem that's just been spotted.
11:05Unscheduled maintenance are those kinds of things typically that people will experience with their car
11:10where they're driving down the highway and all of a sudden the air conditioner doesn't work.
11:14Well, the same with an airplane.
11:16Tonight, the inspectors discover a tire on one of the main landing gears is worn out.
11:21They add it to the list of unscheduled maintenance items.
11:24It has to be replaced before the plane goes back into service.
11:28Obviously, the stakes are extremely high.
11:31Every night we come to work and try to do our best job possible.
11:34Make sure everything's in working order so people get to where they need to go.
11:39But sometimes, despite all the maintenance, the worst-case scenario comes true.
11:44A simple repair can unexpectedly lead to disaster.
11:48August the 13th, 1985.
11:51Mount Osutaka, Japan.
11:55This is the wreckage from the deadliest single-plane disaster in aviation history.
12:01JAL Flight 123 crashed the night before, killing 520 passengers and crew.
12:09Only four people survived.
12:11Because the 747 jet was built in the United States,
12:14the National Transportation Safety Board joins the investigation.
12:19When I arrived in Tokyo, the atmosphere in Japan was extremely stressful.
12:24The news media were everywhere. There was a tremendous amount of anger.
12:31Soon after the crash, the National Transportation Safety Board
12:35Soon after the crash, experts get a helping hand from an amateur photographer.
12:40He managed to take a picture of the 747 minutes before it crashed.
12:48The picture reveals that JAL Flight 123 was flying without its massive tail fin.
12:54The tail fin houses critical control surfaces, like the rudder,
12:58as well as tubes that carry the aircraft.
13:01What force could be strong enough to tear off the tail fin?
13:07Digging through the 747's maintenance history,
13:10investigators discover that seven years earlier,
13:13the jet had landed with its nose too high.
13:18The tail hit the ground and scraped along the runway.
13:21The rear part of the plane had to be repaired, including the pressure bulkhead.
13:28Japan Airlines called in Boeing technicians to help repair the cracked bulkhead.
13:37After this unscheduled maintenance, the 747 was sent back to Tokyo,
13:41where it would be reassembled.
13:43It was time to repair the cracked bulkhead.
13:49After this unscheduled maintenance, the 747 was given a clean bill of health
13:53and flew for another seven years.
13:57But this bulkhead becomes a prime suspect for the investigators.
14:02We had an idea that we wanted to find the rear pressure bulkhead
14:06because we had a flight attendant who had been interviewed
14:09that described an explosion in the back of the airplane
14:12and she could see out, so we wanted to focus on the bulkhead.
14:18During his investigation, Schlied finds a piece of the panel
14:21that had been spliced into the bulkhead seven years before.
14:26The mystery of Flight 123 is solved.
14:29The 747 went down because of a faulty repair.
14:34The repair had in fact not been done correctly.
14:36There was only one row of rivets holding that joint together
14:41where there should have been two rows of rivets holding the joint together.
14:45With only one row of rivets straining to hold the repaired panel in place,
14:49this was a disaster waiting to happen,
14:52especially because this was such a busy jet.
14:55This particular airplane was used in Japan on a domestic operation,
15:00so it made multiple take-offs and landings on domestic operations,
15:05unlike most 747s that make long-range hauls.
15:09So this was considered a high-cycle airplane.
15:14Investigators calculate that with the repair job,
15:17the bulkhead would survive approximately 10,000 flights or cycles.
15:23But on the day of the crash, the 747 had already racked up over 12,000 cycles.
15:33On 747 jets, the cabin is pressurized but not the tail.
15:40During flight, the pressurized cabin air presses against the repaired bulkhead.
15:44After some 12,000 cycles, this pressure stretched the faulty repair to breaking point.
15:54The highly pressurized air blasted into the hollow tail fin and blew it off.
16:01Losing part of the tail crippled the plane's hydraulic systems.
16:06The Boeing 747 had four independent hydraulic systems to power its systems,
16:12so it had quadruple redundancy.
16:14Unfortunately, these four lines came together in the lower part of the spar,
16:19and when it separated, it sheared those four lines.
16:22All four hydraulic systems were depleted.
16:25For some 30 minutes, the crew tried to fly their 747 using only thrust.
16:31This is like trying to drive a car using only the accelerator, no steering wheel, no brakes.
16:39Despite their heroic efforts, it was a losing battle.
16:54All this death and destruction boils down to a missing row of rivets.
16:59Why had the growing metal fatigue in the bulkhead remained undetected
17:03through seven years of scheduled maintenance and inspections?
17:08The primary inspection method for the bulkhead area and the seams was a visual inspection.
17:14And at heavy maintenance periods, when they would take the insulation out,
17:19off the walls and everything, and off the bulkhead,
17:22they would do a detailed visual inspection.
17:25And during subsequent maintenance checks, the faulty repair was never found.
17:36Two decades after JAL Flight 123,
17:40airlines are constantly looking for hidden flaws that aren't visible from the outside.
17:47Back at the southwest maintenance hangar, inspectors are using a boroscope.
17:52A tiny flexible camera to inspect the engines.
17:57Engines are the heart of passenger planes.
18:00If they stop working, pilots don't have the option of pulling over to the side of the road.
18:05Yeah, there we go.
18:08In this area, we're looking for cracks, looking at the blades, the rotor blades.
18:14We're looking for missing material off of them.
18:16You know, any hot spots that have worn through the metal.
18:19You know, any hot spots that have worn through the metal.
18:22Cracks, radial and axial cracks.
18:28Any kind of crack or trace of metal fatigue in any of the fan blades could spell disaster.
18:43August the 21st, 1995.
18:47Atlantic Southeast Airlines Flight 529, an Embraer Brasilia,
18:52is about to take off with 29 people on board.
18:56It's bound for Gulfport, Mississippi.
18:59It was, at the time, the fastest, sleekest turboprop around.
19:07Before the plane even reaches its cruising altitude, something seems to explode outside.
19:16The sound of that was tremendous.
19:19It was as if someone had taken a baseball bat and hit an aluminum garbage can as hard as they could.
19:26It was just a gigantic crashing sound.
19:30An airplane immediately lurched to the left.
19:35No matter what the flight crew tries to do, the plane pulls violently to the left.
19:40Autopilot engine control.
19:45Help me hold it. Help me hold it.
19:47Over there.
19:52The captain and co-pilot are pushed to the brink of their experience.
19:57Help me. Help me. Help me hold it. Help me hold it!
20:11Atlantic Southeast Flight 529 crashes near the small farming community of Carrollton, Georgia.
20:18Emergency?
20:19Yes, we have a plane crashed in our backyard.
20:22A plane crashed?
20:24All 29 people survive the violent landing.
20:31No!
20:34But ten passengers eventually die from their injuries.
20:51Called into action, the NTSB creates teams to examine various parts of the plane.
20:57Jim Hookey, an aerospace engineer, is in charge of the propeller maintenance group.
21:03We came along a lot of pieces of the wing, came along the propeller assembly that was missing one part of the blade.
21:12The blade broke in a very specific fashion, leaving behind all the tell-tale signs of a fatigued plane.
21:19A fatigued fracture tends to be a very flat fracture.
21:23It also has what we call beach marks radiating out from the origin.
21:27So you see these radiating concentric rings coming from the origin of the crack.
21:35Hookey had good reason to focus on the broken propeller blade.
21:3917 years after the crash, Hookey and his team were able to repair the broken propeller blade.
21:4517 months before ASA 529, identical propeller blades broke on separate flights over Canada and Brazil.
21:58Fortunately, in both cases, the aircraft managed to land safely.
22:04The manufacturer of the propeller was Hamilton Standard.
22:08Hookey and his team start combing through Hamilton Standard's materials.
22:12Hookey and his team start combing through Hamilton Standard's maintenance records.
22:17They're looking for anything out of the ordinary.
22:20It's whatever's abnormal. You really don't know what you're looking for until you find it.
22:24But you just go through and there's a lot of routine maintenance that's done, regular inspections, A checks, B checks, C checks.
22:32And then there's the non-routine maintenance that occurs if something is broken or a truck hits the airplane or they have a bird strike or something like that.
22:40And it's those that you look for.
22:44The maintenance records reveal that the broken propeller blade had earlier problems.
22:49We found out that that propeller blade had actually been removed from service once already for a crack indication.
22:56And that became the first clue about there may be a problem with that propeller blade and those inspections.
23:04Deep inside the hollow propeller, investigators find what they're looking for.
23:11In the hollow interior, or taper bore, weights are inserted to balance the prop.
23:17They're kept in place by a cork. This simple cork was the trigger in a deadly chain of events.
23:24About 95% of the cork that's produced in the world is used by the medical industry.
23:30And for aesthetic purposes and for sterilization, they like to have the light color.
23:37So the cork is bleached with chlorine.
23:41The NTSB discovers that moisture inside the propeller caused the chlorine in the cork to leach out and corrode the propeller's aluminum alloy.
23:52They also notice something else on the broken blade.
23:56On the inner surface, extending about four centimeters from the fracture, there are a series of sanding marks.
24:03Going through the blade's repair records, Hookey notices the initials CSB.
24:09Christopher Scott Bender.
24:11This technician worked at a Hamilton Standard repair facility.
24:16When Christopher Bender watches news of the accident, he learns that the investigators are examining the Hamilton propeller.
24:24And as soon as I heard that, my heart just sank.
24:26I was like, you know, I think I might have even cried a little bit.
24:29I was just, you know, just emotionally overwhelmed that, you know, something I had put my hands on, a procedure that somebody trusted me to do, failed.
24:36And because of that, somebody had died.
24:40After discovering that it was Bender who last worked on the deadly propeller blade,
24:45the NTSB now has to find out how the blade had passed inspection at Hamilton Standard.
24:51Investigators ask Bender to perform his standard maintenance technique on the propeller.
24:57He demonstrated how he would go down into the barrel of the taperbore with a fiber-optic borescope and look for cracks.
25:07And therein lied one of the primary problems.
25:10The borescope that he was using had a bright white light that would illuminate the barrel.
25:17The borescope that had a bright white light that would put a lot of glare back into the inspector's eyes
25:24really did not lend itself to the inspection that was required.
25:31And investigators also find a gap in Bender's training.
25:35He had never been shown what a crack would look like.
25:39He was just told to find a crack, and he would look for a crack.
25:44When he was examining the propeller blade, Bender had been unable to detect any evidence of corrosion.
25:51He then did what he'd been told to do, polish the inside of the blade.
25:56He was given a directive to use a repair to blend out the inside of the taperbore.
26:05He blended it out, he did an inspection, and the blending that he had done had roughened the surface,
26:10so it actually masked the indication of the crack in the subsequent inspection.
26:15And the blade was returned to service where the crack continued to propagate
26:20until it ultimately reached critical length and separated.
26:24The draft accident report we present to you today involves Atlantic Southeast Airlines Flight 529.
26:30According to the NTSB, by polishing the blade, Hamilton Standard had unwittingly removed all traces of the crack.
26:37Even a later, more thorough ultrasound examination could not detect it.
26:43The company that manufactured Flight 529's propeller is now renamed Hamilton Sunstrand.
26:50Its inspection and repair process was made more stringent, in some cases exceeding FAA requirements.
26:59Flight 529 was the last time one of its propellers failed in flight.
27:08You know, I wish this had never happened.
27:11I wish I could go back in time and fix it and take care of it, that it didn't happen.
27:17Out of the thousands of parts on board an Embraer Brasilia,
27:21a simple small cork was the key to a horrific accident.
27:26ASA Flight 529 underlines the critical need for proper maintenance.
27:31But sometimes maintenance can create the potential for disaster,
27:35when a new component is installed into an older airplane.
27:51It's early morning in Phoenix, Arizona.
27:54Southwest Airlines engineers are continuing their scheduled maintenance.
27:58Southwest is unique among larger airlines.
28:01It flies just one kind of plane, the 737.
28:05Tonight, engineers are working on a 700 model, one of the newest 737s.
28:11But the company's very first 300 model, bought in the mid-1980s, is still flying.
28:18You can still operate an old airplane as long as you have the right equipment.
28:22When you look at some of the cargo carriers,
28:25they're operating airplanes that are 30 and 40 and even 50 years old.
28:29They're still reliable airplanes.
28:32They've been maintained, they've been retrofitted with modern-day equipment.
28:36Updating older planes is a standard part of maintenance.
28:40But sometimes installing a new component in an older plane can lead to tragedy.
28:44Inside this makeshift lab are the shattered remains of Swiss Air Flight 111.
28:50On September 2nd, 1998, the passenger jet crashed off the coast of Nova Scotia, Canada,
28:56killing everyone on board.
29:00Recovered from the seabed, the debris is overwhelming.
29:04It's the first time in the history of aviation that a plane has crashed.
29:08It's the first time in the history of aviation that a plane has crashed.
29:11Recovered from the seabed, the debris is overwhelming.
29:17There is almost 250 kilometers of wiring alone.
29:21In Swiss Air, we've had about 2 million pieces of airplane,
29:25and we pretty much almost had to look at them all.
29:28In the business, we refer to often finding the golden nugget.
29:32That's saying, aha, there's the cause of the accident.
29:36Somewhere in this wreckage, investigators hope to find that golden nugget.
29:41The one piece that will reveal the reason why Swiss Air 111 crashed into the Atlantic Ocean.
29:53The cockpit voice recorder gives investigators their first critical clues.
29:59Do you smell something?
30:01Yeah, what is that?
30:02Go have a look, I'll take the controls.
30:04Roger, you have control.
30:08The first officer checks the area around the air conditioning vent.
30:12Nothing seems wrong.
30:16I don't see anything, Gert.
30:18There's nothing up there now.
30:22Captain Zimmerman is troubled by the smell of smoke.
30:25Here it is again.
30:27He starts to divert the plane to the nearest airport.
30:29Find the closest place to land, Stefan.
30:31He radios air traffic control in Moncton, New Brunswick.
30:35Moncton center, Swiss Air 111 heavy is declaring pan, pan, pan.
30:39We have smoke in the cockpit.
30:41Pan, pan, pan is an international term used to notify air traffic control of an urgent situation.
30:47It's one step below declaring mayday.
30:50Swiss Air 111 is directed to Halifax and starts its descent.
30:58The pilots appear calm and in control.
31:01Halifax is just 20 minutes away.
31:04They want us to turn to the south.
31:07At that point, everything was normal.
31:10I gave the pilot a signal to turn to the south.
31:13I gave the pilot a signal to turn to the south.
31:16I gave the pilot a signal to turn to the south.
31:19I gave the pilot an initial descent.
31:21And he requested to level off at an intermediate altitude to get the cabin in order for the landing,
31:27which took to mean that they needed to pack away dinner trays and things like that.
31:32Cabin bus off.
31:34Cabin bus off, roger.
31:37But the seemingly controlled situation on board Swiss Air Flight 111 escalates into a full-scale emergency.
31:45Autopilot disconnect.
31:50We are declaring emergency now, Swiss Air 111, at time 0-1-2-4.
31:59All my screens are down. I'm flying my standby instruments, maintaining 300.
32:05Shortly after declaring an emergency, the plane goes silent.
32:09It was probably one of the most helpless feelings that any individual can have,
32:14not being able to do anything but just sit and watch the target
32:18and hope that it would turn back toward the airport.
32:21And of course it didn't.
32:27At 10.31pm, Swiss Air 111 takes off.
32:31And of course it didn't.
32:36At 10.31pm Atlantic time, residents of Peggy's Cove hear a devastating explosion.
32:53From the cockpit voice recorder, investigators know they're dealing with a fire and not a plane that was malfunctioning.
33:00We found no anomalies or no problems in any of that flight data that suggested there was a problem with the aircraft.
33:09Investigators work their way through the hangar of wreckage recovered from the Atlantic Ocean.
33:21Finally, they find scorch marks, which reveal that the source of the fire was in the back of the cockpit.
33:27Directly behind the first officer.
33:32Following this trail leads the team to an unlikely suspect, the entertainment system in first class.
33:42The Swiss Air MD-11s provided first class with one of the world's most sophisticated entertainment systems.
33:49Passengers in first class could choose their own movies, access the internet and even gamble.
33:55This entertainment system was not part of the original MD-11 design.
34:01The system had some major deficiencies.
34:04It was getting very hot.
34:06It drew a lot of power.
34:09And thereby, for example, raising the cabin temperature considerably because it was always running.
34:19They did not install a simple off switch.
34:21Nor did they install appropriate cooling systems.
34:26Anytime you have an electrical system or you're putting an aftermarket install into an airplane,
34:32you run the risk of compromising the integrity of the aircraft itself as it was originally designed.
34:40When informed about the flaw in the wiring, Swiss Air immediately disabled the entertainment system on the rest of its fleet.
34:47But investigators discover that another piece of the jet had helped the fire spread with alarming speed.
34:56And in this instance, we did discover a wire that arced in that way.
35:03And right next to it was some very flammable material called metallized polyethylene terephthalate covering material
35:12that covers the insulation blankets.
35:16This polyethylene insulate was common on commercial airlines around the world.
35:21It had somehow passed the industry's flammability tests,
35:25which require materials to self-extinguish after a reasonable period of time.
35:34This thermal acoustical material that was in this aircraft,
35:37was very flammable even though it passed a test.
35:41It does sustain and it does propagate flame.
35:46The fire spread quickly from the cockpit back into the first class galleys.
35:51Some metals showed heat damage from temperatures as high as 1,100 degrees Fahrenheit.
35:58Less than 12 minutes after the crew declared a pan-pan,
36:01the fire disabled all electronics in the cockpit.
36:12In the aftermath, Swissair removed the flammable insulate from its entire MD-11 fleet.
36:18The rest of the industry was required to follow suit.
36:24In February, the fire spread to the first class galleys.
36:28In Phoenix, Arizona, flight engineers continue their sea check on the 737.
36:34It includes testing the plane's rudder.
36:37The rudder is one of the jet's most vital control surfaces.
36:41It allows a plane to turn left and right.
36:48A problem here could have terrifying consequences.
36:58In fact, despite years of proper maintenance,
37:01a problem with a tiny component with the 737 rudder killed more than a hundred people.
37:07Not even the most diligent maintenance workers could have spotted it.
37:14March the 3rd, 1991.
37:17United Flight 585 begins its final approach into Colorado Springs.
37:23Another 10-hour gain.
37:25Three flaps.
37:28Two.
37:33Oh, God, six.
37:35Fifteen flaps.
37:36Fifteen.
37:38Going up.
37:39Oh, my God.
37:41Oh, my God.
37:43Oh, my God.
37:45Oh, my God.
37:55All 20 passengers and five crew are killed.
38:04Investigators from the National Transportation Safety Board descend on Colorado Springs.
38:16My first sense that it was going to take some time to investigate the accident
38:21was the damage that we saw in the parts.
38:24An aerospace engineer by training,
38:27Greg Phillips is in charge of investigating United 585's flight control systems.
38:33We focused in after eliminating other flight control surfaces
38:38that we thought could contribute to the role.
38:41We started looking at the rudder.
38:42But investigators face a critical obstacle.
38:45Most of the plane's parts are too crushed or burned for testing.
38:50Luckily, one vital component is still reasonably intact.
38:55This is the power control unit, or PCU.
39:00Used constantly during flight, especially during landings,
39:04the PCU performs like a car's power steering.
39:08When the pilot pushes on a rudder pedal,
39:10the PCU uses hydraulic fluid to convert the gentle movements of a pilot's foot
39:15into the pressure needed to move the 737's enormous rudder.
39:21The heart of the PCU is the dual servo valve.
39:25This valve is roughly the size of a soda can.
39:29It contains two extremely thin slides that glide past one another.
39:35These slides direct the flow of hydraulic fluid
39:37which moves the rudder.
39:41When a technician opens up the power control unit,
39:45it seems to be in working order.
39:49We didn't have any absolute indication or information that we could point to
39:54that said the rudder, power control unit, the servo valve,
39:57or any part of that flight control system caused that accident.
40:03It's a burst.
40:05It's a burst.
40:10For only the fourth time in its history,
40:13the NTSB releases a report that does not reach a conclusion.
40:17We had put a lot of time and effort into the investigation
40:21and we just weren't sure what had happened.
40:26Less than two years later, Greg Phillips and the NTSB
40:30would be brought back to the mysterious disaster
40:32through the crash of another 737.
40:35Hold on, hold on.
40:37Hold on.
40:39Shoot!
40:41What the hell is this?
40:45Oh God, no!
40:52September the 8th, 1994.
40:55US Air Flight 427 has crashed near Pittsburgh,
40:59killing everyone on board.
41:03When we first arrived at the crash site,
41:05first of all, there was no aircraft there.
41:07There were only bits and pieces of the airplane.
41:09It wasn't really recognizable as an airplane.
41:14Investigators begin to see that this crash
41:17is a mirror image of United Flight 585.
41:22On final approach, United 585 rolled right,
41:26while US Air 427 rolled left.
41:28Both crews are caught by surprise.
41:32After a terrifying struggle, both crash with no survivors.
41:41Once again, investigators test the dual servo valve,
41:45but they come up empty-handed.
41:48That unit passed all its operational tests.
41:51There wasn't any indication that it had failed.
41:55We were going up again.
41:57We were going up against an aircraft
41:59that had an incredible safety history.
42:01It was really...
42:03Everything you could see for 30 years,
42:05this had been a great airplane.
42:07We were trying to prove that there was something wrong
42:10with the straight-A student.
42:14The team reaches another dead end.
42:18But almost two years later,
42:20they get a third chance to solve this deadly mystery.
42:27Here.
42:35In Phoenix, Arizona,
42:37a long night of maintenance is winding down.
42:40Over 30 mechanics and some half a dozen inspectors
42:44have combed through a Southwest Airlines 737.
42:47The team has made more than 400 separate checks.
42:51Parts have been replaced.
42:52Southwest 737 N427WN is almost ready to fly again.
42:58Hopefully, it'll push back out of the hangar.
43:01We'll do some leak checks, make sure everything's working,
43:03nothing's leaking,
43:05and then it'll go to the gate for departure time.
43:07But it's a truth in maintenance
43:09that engineers can only fix what they know is broken.
43:13For several years, every 737 that flew
43:16had a hidden danger
43:18that not even the most careful technician could have spotted.
43:21In the early 1990s,
43:23two 737s crashed in mysterious accidents.
43:28In both cases, the jets spiraled out of control.
43:37In 1996, the same malfunction strikes again.
43:43It's June the 9th.
43:45Captain Brian Bishop prepares to land in Richmond, Virginia.
43:48Then, just like United 585 and US Air 427,
43:53his plane rolls out of control.
44:00I turned the yoke the opposite direction
44:03and stood on the opposite rudder pedal.
44:05The pedal didn't move for me.
44:08We didn't know to what extent,
44:10but we knew we had a problem with the rudder.
44:12For over 30 seconds,
44:14Bishop struggles to control his plane.
44:16And then, just as suddenly,
44:18the 737 calms down and goes back to horizontal.
44:27We had started the checklist
44:29almost before I could finish the sentence.
44:32All of a sudden, there was just a wham.
44:36The 737 is once again out of control.
44:39The 737 is once again out of control.
44:50Then, out of the blue, Eastwind 517 is back on track.
44:56Wasting no time, Captain Bishop gets it onto the tarmac.
45:03Taxing in is when I realized my legs were shaking.
45:09We launched to the scene.
45:11The airplane literally didn't move.
45:13It stayed in its location in the airport
45:15until we got down there.
45:17Suddenly, they had a 737
45:19that had had a rudder incident that was intact,
45:22and they had a pilot who was alive
45:24and who could talk about it.
45:26I think they were much happier to have the airplane than me.
45:30Investigators zero in on the 737's rudder controls.
45:34The power control unit is tested again and again
45:38but it performs perfectly.
45:41Refusing to give up,
45:43investigator Tom Howter decides to try a different test.
45:47One fellow mentioned a test they had done in the military
45:50of a thermal shock.
45:53The power control unit is soaked in dry ice
45:56and blasted with nitrogen gas at minus 40 degrees Celsius.
46:03Then it's injected with superheated hydraulic fluid.
46:06It's then given a command to start working.
46:11As we were standing there,
46:13listening to the actuator move left and right,
46:16left and right,
46:18it stopped.
46:20And it was not commanded to stop.
46:22It just jammed.
46:24Stopped working completely.
46:27The team has discovered that a small hydraulic valve
46:30that controls the rudder
46:32of the world's most popular jetliner
46:33can jam in the right circumstances.
46:38And the valve can jam without leaving behind any traces.
46:46When investigators double-check their results,
46:49they discover another major flaw.
46:54Careful analysis of the data,
46:56a couple of the engineers recognized
46:58that it not only stopped working,
47:00but actually left became right and right became left.
47:03There was actually a movement of fluid
47:06into places that it shouldn't have gone.
47:11And the reversal is like driving your car.
47:14You turn to the right, it goes left.
47:16You're not going to figure out this failure mode
47:19until you go off the road.
47:21And in these cases,
47:23that's the pilots were faced with something so unusual
47:26that they didn't understand what was happening.
47:27What the hell is this?
47:29That would explain why the first officer, Chuck Emmett,
47:33would keep his foot on the rudder pedal,
47:36because he's thinking,
47:38why isn't the plane going right?
47:40And he's feeling the plane go to the left.
47:46In the aftermath of these disasters,
47:49pilots received better training
47:51on how to deal with sudden rudder problems.
47:54Boeing spent hundreds of hours
47:55redesigning and replacing the rudder's dual servo valve
47:59on thousands of 737s around the world.
48:02One thing we don't like in the safety board
48:05is to have an undetermined accident,
48:08because then we can't make a change to improve safety.
48:11So out of US Air 427, United 585,
48:14we have a much safer 737 fleet.
48:19It's 7 a.m.
48:21After an eight-hour shift,
48:22the maintenance is finished on this Southwest Airlines 737.
48:28According to their maintenance reports,
48:30the team has conducted 78 unscheduled procedures
48:34and 339 scheduled inspections.
48:37I think each one of us out here
48:40has a sense of pride in themselves
48:42that we do the best job we possibly can,
48:44day in, day out.
48:46Obviously, the stakes are high.
48:48Every life is important.
48:49Despite the horror of airplane disasters,
48:52they're still extremely rare,
48:55especially given how often passenger planes take off and land.
49:00Sometimes we obscure the fact
49:02that we fly millions and millions and millions of people
49:06day in and day out
49:08without putting a scratch on even the airplane,
49:10let alone the people.
49:12This is the most amazing system.
49:14This system depends on the dedicated team of professionals
49:17committed to taking care of these 21st century masterpieces.
49:22Planes so well built
49:24that they could fly almost as long
49:26as we're willing to take care of them.
49:28We've learned now how to inspect and maintain these things
49:32and even rebuild them
49:34to where they should have an indefinite life.
49:36They're built tough,
49:38and they should be able to last forever
49:40if they're maintained properly.
49:47NASA Jet Propulsion Laboratory
49:49California Institute of Technology