Mayday (2013) s05e08 Episode Script

Mixed Signals

(THEME MUSIC) VOICEOVER: Gregorio Luperon International Airport in Puerto Plata, the Dominican Republic.
February 6, 1996.
The small Caribbean island is a popular winter getaway.
A group of German tourists has been delayed.
There are mechanical problems with the jet that was supposed to take them back to Frankfurt.
The airline has arranged to lease another plane for the flight.
The replacement jet is owned by Turkish charter company Birgenair.
They have a 757 that's been sitting on the tarmac for almost three weeks.
But it takes several hours to get the plane ready to go and to get the Turkish crew aboard.
By 10:15pm, the plane and most of its crew are at the gate.
Alpha lima whisky, 301.
Shortly after 11:30, Birgenair Flight 301 is given permission to leave the gate.
Moments later, it taxis to the threshold of the runway.
Cabin crew, take your seats ready for takeoff.
The cabin announcement is completed.
First Officer Aykut Gergin is new to the 757, with fewer than 75 hours in the plane, all in the last three months.
Thank you.
Ready for takeoff.
In contrast, Captain Ahmet Erdem is one of Birgenair's most senior pilots.
He's logged thousands of hours in this type of plane.
Alpha lima whisky, 301.
Ready for takeoff.
MAN: 301 cleared for takeoff, runway 08.
Clear for takeoff, runway 08, 301.
Thank you.
- A good flight.
- Good flight.
Good flight.
Mulis Evrenesoglu is on this flight as a relief pilot.
Like First Officer Gergin, he's been flying the 757 for less than three months.
As the plane accelerates to take-off speed, a light rain begins to fall.
- Power set.
- OK.
First Officer Aykut Gergin watches his airspeed indicator for a routine instrument check.
As the airplane's accelerating to take-off speed, the first officer calls 80 knots.
- 80 knots.
- Check.
The captain, in theory, should verify that his air speed indicator also reads 80 knots.
My airspeed indicator is not working.
The captain's air speed should read the same as his first officer's.
120 But the readings do not match.
- Is yours working? - Yes, sir.
You tell me.
Erdem wants his first officer to tell him when the plane hits take-off speed.
At 150 knots, the plane hits V1, the point of no return.
The captain must pull back on his column and get the plane in the air.
Positive climb.
Gear up.
Positive climb.
Gear is up.
At 11:42 the plane takes off.
Seconds later, Captain Erdem's airspeed indicator comes to life.
Is it possible to turn off the wipers? OK.
Wipers off.
Climb thrust.
Climb thrust.
First Officer Gergin reduces power to the engines for a gradual climb to cruising altitude.
MAN: 301 airborne 45.
Switch over to Santo Domingo 1243.
Bye-bye, sir.
Climbing through 2,500 feet, First Officer Gergin establishes contact with the island's main tower in Santo Domingo.
Alpha lima whisky 301, climb and maintain 280.
Centre autopilot on, please.
Centre autopilot has command.
Thank you.
One minute and 30 seconds into the flight, the autopilot takes over.
Onboard computers now make all the calculations and adjustments necessary to keep the plane climbing safely.
Almost immediately, the computer reports a problem.
Rudder ratio? Mach airspeed trim? Two different alerts warn the pilots that the plane is travelling far too fast.
But the first officer's airspeed indicator shows the plane climbing at a normal speed - 220 knots.
Something's wrong here.
Unaware that there are any problems, the controller directs Birgenair Flight 301 to continue climbing.
Alpha lima whisky 301, report POKEG.
OK, 280, I'll call you over POKEG.
OK, there's something crazy.
Do you see it? In the cockpit, the situation gets more confusing.
The first officer's airspeed indicator shows that the plane is flying much too slowly.
Mine shows only 200 now and decreasing, sir.
But the captain's gauge shows the plane flying far too fast - 325 knots.
Both of them are wrong.
What can we do? Let's check the circuit-breakers.
Yes, sir.
- As the first warning is eliminated - (ALARM SOUNDS) a more persistent warning replaces it.
The overspeed warning tells the crew they're approaching 350 knots, the maximum speed at which the plane was designed to fly at this altitude.
OK, it's no matter.
Let's pull the air speed.
Let's see.
Resetting the circuit-breakers turns off the alarm but it doesn't fix the problem.
Captain Erdem's airspeed indicator still shows he's flying much too fast.
Now it's 350, yes? Confused by the conflicting information, Captain Erdem decides to do what the plane wants.
He slows down (LOUD RATTLING) .
with terrifying results.
The 757 is sending out warnings that are confusing the crew.
Bewildered, the crew struggles to solve the problem.
The lives of 189 people depend on them getting the answer right.
Shortly after taking off from the Dominican Republic, Birgenair Flight 301 is in trouble.
The cockpit is filled with an ominous sound - the stick-shaker alert.
It means that the airplane is about to attain a speed so low that it cannot sustain flight.
The alert is so serious that it actually shakes the pilots' control columns.
Combined with a loud rattling, it's impossible to ignore.
To add to the crew's confusion, the plane begins to vibrate and dip wildly.
(SCREAMING) ADI! The Attitude Deviation Indicator is basically a round gauge that on this airplane is a screen, but it's blue on the top and brown on the bottom, and it shows not only the pitch of the airplane but also roll.
On the gauge, blue represents sky.
The more blue, the steeper the climb.
Right now it's showing that the plane's nose is pointing dangerously high.
Then, more than 7,000 feet above the ocean, the plane rolls hard to the right and begins to plummet from the sky.
Captain Erdem struggles desperately to get his plane to climb.
Nose down.
He has just a few thousand feet to pull a 100-ton airliner out of a deadly stall.
The less experienced pilots utter prayers and offer suggestions.
Thrust? ADI? 301, squawk 377.
Stand by.
Air traffic control is still unaware that Flight 301 is in grave danger.
It's not climbing! What am I to do?! If you can level off, our altitude's OK.
The plane is falling fast.
OK, 5,000 feet.
Captain Erdem is trying to fly a plane that's become virtually uncontrollable.
(SCREAMING) Thrust levers! Thrust! Gergin pushes the throttles to full power but it doesn't help.
The plane spirals towards the sea.
Pull up! Oh! What's happening?! Ahhhhh! Alpha lima whisky 301, Santo Domingo.
Come in, please.
Alpha lima whisky 301, Santo Domingo.
Come in, please.
Less than five minutes after take-off, Birgenair Flight 301 vanishes from radar.
Come in, please! The Dominican navy begins searching for the missing plane and its passengers.
(MAN SPEAKS SPANISH) TRANSLATION: En route, I was always thinking of what I would find.
I thought I would find people screaming, people yelling, asking for help.
They discover something very different.
The strong smell of jet fuel hangs over the water.
Pieces of wreckage float on the waves.
Within hours, more than just wreckage begins floating to the surface.
The search continues by the light of the next morning.
American and Dominican rescue ships scour a 1,300 square kilometre area for survivors.
None are found.
Now it's up to the Dominican Republic's Aircraft Accident Investigation Bureau to find out what caused this accident.
The USA's National Transportation Safety Board agrees to assist the Dominicans with their investigation.
Welcome to the Dominican Republic.
I am Enmanuel Souffront.
They send an investigator to Puerto Plata.
We could see that this was gonna take some international cooperation to get to the bottom of the event.
Robert Macintosh would also provide support from the NTSB's Washington offices.
Any signal from the recorders? This particular accident was the first major loss of a Boeing 757 in the water.
There was added urgency because the aircraft was going into the delivery of many American-operated airlines and on the market for the world-wide passenger service.
Like all commercial airliners, the Boeing 757 carries a cockpit voice recorder that records all the sounds in the cockpit, and a flight data recorder that records a wealth of information about the plane's operation during flight.
We've got underwater locator beacons on these recorders.
They work for 30 days upon immersion.
But the ocean where the plane crashed is over 7,200 feet deep.
The NTSB enlists the help of the US navy, who hire a submersible called the 'CURV'.
The CURV is essentially a remote-controlled submarine which can work at depths no manned submarine could handle.
While the CURV travels from the United States, investigators study the radar records, along with the conversation between the ground controllers and the crew.
I have the recording from the control tower.
Santo Domingo, good evening.
Alpha lima whisky 301, climbing.
Alpha lima whisky 301, climb and maintain.
OK, 280.
The exchange between the controller and Flight 301 is normal.
The investigators detect no signs of trouble.
- 301, squawk 377.
- Stand by.
As you heard, there is no reason why this plane went down.
What had caused something to go wrong to interrupt the flight path of the aircraft so rapidly? Certainly it was a good question for us.
Any debris that's found is taken to a Dominican military base to be examined.
Investigators comb through the wreckage they've recovered - pieces of the cabin, life vests, even part of the landing gear have all been found.
Every piece of debris is studied for signs of an explosion or fire.
Investigators also check if any of the life vests are inflated.
That would suggest passengers had some warning before the plane ploughed into the sea.
Cords on some of the life vests are hanging loose but investigators conclude that could be the result of the plane's violent impact.
The crash was so powerful it has compressed coffee cans into flat pieces of tin.
The wreckage tells investigators that the crash of Birgenair Flight 301 was sudden and violent.
But they find no evidence that there was an explosion on board.
Major Souffront looks into the possibility that the plane, which was called into service at the last minute, may not have been ready for the flight.
- Enmanuel Souffront.
- Hi.
I'd like to see some records of the Birgenair plane.
They're right here.
This is it.
It had been on the ground for some time in Puerto Plata, and that became interesting to us.
Why would an operating company have an aircraft sitting there? Investigators discover that the plane wasn't on the ground for repairs.
Based on maintenance records, the plane appears to have been mechanically sound.
Birgenair simply didn't have enough passengers to justify the flight financially.
So they kept the plane and the crew in the Dominican Republic for almost three weeks.
The plane's maintenance records turn out to be one more in a series of empty leads.
The CURV arrives in Puerto Plata Harbour.
On February 28, more than three weeks after the crash, it slips below the waves, looking for the remains of the Birgenair jet.
It takes the robotic submarine two hours just to descend the 7,200 feet to the ocean floor.
From there, it sends back images of the wreck of Flight 301.
MAN: The cockpit pretty much was sitting upright and it was pretty much the nose of the plane and you could see the front part of it.
It was obviously banged up and cracked and fragmented.
The clues investigators need to solve the mystery of this plane crash lie somewhere amongst the plane's twisted wreckage.
The flight's black boxes are the top priority.
The CURV quickly picks up the signal from a pinger on one of the units.
Operators must now manoeuvre the sub towards the sound.
They have to be able to see the boxes to pick them up with the robotic arm.
It takes just 90 minutes to find the first black box.
UNZICKER: The first one was sitting out in the open.
OK? It was where they could see it and they picked the first box up.
They grabbed a hold of it and tucked the arm up in place so that they wouldn't lose it.
The second black box is also heard but after almost two hours, the CURV's cameras still can't see it.
They knew it was right there and they were searching around a debris pile and they could not physically see it, you know, with the cameras of the vehicle.
So, as they went around a couple of times, they started lifting metal up and moving pieces out of the way and they did find the second box under some debris.
Flight 301's flight data recorder and cockpit voice recorder are brought to the surface and loaded onto a waiting jet.
Within hours, the black boxes are at the NTSB labs in Washington, DC.
Technicians prepare to extract the precious data from the boxes.
Investigators hope it will tell them what happened aboard Birgenair Flight 301.
They'll soon uncover a stunning miscommunication between a seasoned pilot and his plane.
To unravel the mystery of Birgenair Flight 301, investigators are counting on the plane's black boxes.
That flight data recorder was our key so the technicians got busy and gave us plots, visual plots, of what was going on with engines and air speeds and so on to allow us to try and understand why that aircraft slowed down .
and then simply departed controlled flight and entered the ocean.
It's on a time line.
Top is the pitch.
We've got the air speed and the altitude.
But check out the time line - 44.
Investigators immediately notice something unusual about the flight.
15 degrees pitch.
Nose up.
Seems high.
- It's almost the maximum.
- And then it stays that way.
The plane began climbing normally.
Centre autopilot on, please.
But investigators notice that shortly after the autopilot was switched on, the plane's nose pitched upward.
They also see that the plane's airspeed seems much higher than it should be.
350 knots.
No, it it can't be right.
Oh, there's definitely something wonky about the airspeed numbers.
Can you cue that up and play it? Investigators soon start filling in the missing pieces of the puzzle.
80 knots.
My airspeed indicator's not working.
They learn that Captain Erdem noticed that his airspeed indicator wasn't working.
- Is yours working? - Yes, sir.
- You tell me.
- V1.
But Erdem didn't think the problem was serious enough to abort his take-off.
The tape reveals that once they were airborne, the crew quickly became overwhelmed by a series of warnings.
Rudder ratio? Mach airspeed trim? To investigators, the captain seems to become increasingly bewildered by the messages he was getting from his plane.
GERGIN: Mine shows only 200 now and decreasing, sir.
Both of them are wrong.
What can we do? Investigators don't know why the captain's airspeed indicator wasn't working.
They do notice that the captain's gauge came back to life as the plane started to climb.
OK, it's no matter.
Let's pull the airspeed.
Let's see.
It's a telling discovery, which leads investigators to focus their attention on the device that feeds the gauges airspeed information .
the Pitot tube.
A Pitot tube is an airspeed sensor - a pipe, open at one end, that responds to air pressure.
When the plane travels forward, an increase in air pressure inside the Pitot tube causes the airspeed indicator's needle to move.
My airspeed indicator's not working.
But if a Pitot tube becomes blocked, it can send faulty readings to the plane's gauges.
Blocked Pitot tubes have been a factor in previous plane crashes.
In 1982, a Boeing 737 crashed in heavy snow in Washington, DC shortly after take-off.
The Pitot tubes were blocked with ice.
Investigators suspect that the Pitot tube which fed the captain's airspeed indicator on the Birgenair jet was somehow blocked.
But they know ice can't have blocked the tubes of a plane taking off from a Caribbean island.
We started carefully looking at what could cause that kind of thing to happen.
MAN: Senor! Major Souffront has a theory.
He returns to question the airplane's mechanics.
Senor Souffront.
I'd like to ask you some more questions about the Birgenair plane.
Souffront suspects that mechanics may have taped over the Pitot tubes during maintenance.
It's a common procedure, but if the tape wasn't removed, it could have caused the deadly accident.
Maybe a piece of tape was left on accidentally? No, sir.
It didn't have to be taped.
We never did anything with the Pitots.
Did you put the Pitot covers back on them when the maintenance was finished? It didn't have any covers with it.
We didn't take any off and we didn't put any on.
TRANSLATION: And that's when we discovered that the Pitots had not been covered for the 25 days that the aircraft remained parked at the international airport in Puerto Plata.
A Pitot cover slips over the end of the tube.
Regulations state that these covers must be installed any time a plane will be on the ground for an extended period of time.
A prominent flag is meant to remind pilots and technicians to remove them again before take-off.
Recovering the tube from the ocean floor is the only way for investigators to answer a pressing question - what blocked the Pitot tubes? But even if the Pitot tubes were blocked, how could it have caused the crash of a modern jet .
and the death of 189 people? It's not like a car where you have only one speedometer.
In this kind of an aircraft, you have a total of three airspeed indicators and there's a flight data computer which is computing the velocity in relation to the ground.
(LOUD RATTLING) Oh, what's happening? (SCREAMS) I'd like to welcome you all this afternoon.
As you know, I'm Bob MacIntosh Using information from the black boxes, the NTSB pieces together a real-time animation from lift-off to the final moments of Flight 301.
This is the indicator.
But the captain actually realised his speed indicator wasn't working.
- Is yours working? - Yes, sir.
The big question for investigators is how could one faulty airspeed source result in a crash? He realised at a time when he could have aborted.
He could have turned back but he chose to continue.
Right now Investigators analyse the captain's every move and find that he allowed a small error to escalate and ultimately overwhelm him.
Positive climb.
Gear up.
Moments after lift-off, Captain Erdem's airspeed indicator appears to be working.
And then that means it's at odds.
But investigators suspect the gauge is responding to changes in altitude.
GERGIN: Climb thrust.
As the plane climbs through the thinning atmosphere, the air trapped inside the tube expands, causing a build-up of pressure.
Inside the cockpit, this causes the airspeed indicator needle to deflect.
Even though altitude is causing the increase in pressure, the sensors mistakenly read it as an increase in airspeed.
So at this point the take-off is standard Captain Erdem may have had five separate sources of airspeed to rely on, but investigators notice that when the trouble started, he wasn't flying the plane Centre autopilot on, please.
- Centre autopilot has command.
- Thank you.
the autopilot was.
Remember, the autopilot gets its data only from the captain's Pitot tube - the one that was blocked.
The data shows that the trouble on Birgenair Flight 301 began when the autopilot took over.
Right after the autopilot is engaged, the plane's nose begins to rise.
Investigators suspect that the crew didn't realise that the blocked Pitot tube was feeding the autopilot faulty information.
The computer registered that the plane was travelling too fast and raised the nose to slow it down.
It soon rises to 15 degrees but it stays there.
Raising the nose works like an air brake - it slows the plane by creating drag.
The autopilot is programmed to never bring the nose higher than about 15 degrees.
Any higher and the plane would slow down too much and stall.
The autopilot's a pretty smart guy - he already knows he's got all the power that he's gonna get for the climb.
The only thing for the autopilot to do is raise the nose, and it raised the nose of the aeroplane to its limits of authority.
But the autopilot was reacting to faulty information.
Moments later it sent out two different warnings Rudder ratio? Mach airspeed trim? .
that the plane was travelling too fast to be controlled safely.
OK, there's something crazy here.
Do you see it? Yes, there is something crazy.
Mine shows only 200 now and decreasing, sir.
Both of them are wrong.
Investigators realise that Captain Erdem wrongly concluded that both airspeed indicators were malfunctioning.
In fact, his first officer's gauge was always correct - the plane was travelling much too slowly.
What can we do? Captain Erdem no longer knows which instruments to trust.
Let's check the circuit-breakers.
The actions of trying to reset circuit-breakers is something that pilots of older generations aircraft have learned via experience of sometimes being able to get an errant system back functional.
Resetting the circuit-breakers turns off the alarm.
MACINTOSH: But that needle continued to climb around the clock face until it activated the overspeed warning.
(ALARM BEEPS) The autopilot system sent yet another warning that the plane was travelling too fast.
But the reality was just the opposite - the plane was slowing down.
At the point, the overspeed indicator is on.
They're going slow - they think they're going too fast and confusion has set in.
- (ALARM BEEPS) - Let's pull the airspeed.
That's when Captain Erdem made the gravest error of all.
You can see that he now pulls back on his throttles.
Investigators realise that at the plane's already slow speed (ALL SCREAM) .
pulling back on the throttles was disastrous.
The crew got the most severe warning that a plane can send out.
And that's when he gets the stick-shaker.
God! God! He's got a tactile sense coming through his flight controls that literally shakes the stick and says, "You've got to lower this nose.
" In a matter of seconds, Captain Erdem was first warned that his plane was travelling too fast and then that it was travelling dangerously slowly.
The autopilot is programmed to always disconnect when the stick-shaker activates.
It's up to the pilot to get the plane out of a stall.
Once the autopilot reached its limits of authority, it said, "I've done all I can do.
I'm out of here.
" When the autopilot disengaged, Captain Erdem suddenly found himself in control of the plane at the moment of his greatest confusion.
If you watch the plane at the top of the screen The cockpit recordings lead investigators to a stunning conclusion.
ADI! Captain Erdem may not have recognised that his plane is about to stall.
But the relief pilot behind him did.
ADI! This additional pilot intervened to say, "ADI! ADI!" In other words, "Look at that ADI and put yourself "where you would normally see the nose of the aircraft.
" 5 degrees nose high.
10 degrees.
The relief pilot wanted Captain Erdem to recognise that the plane's nose was pitched dangerously skyward.
Investigators can hear First Officer Gergin trying to convey the same message.
Nose down! At this point, the first officer's actually trying to lead the captain to some solutions but not actually take control of the airplane.
What the 757 desperately needed was airflow over the wings to generate lift.
The only way to get that was to point the nose down and dive.
What puzzles investigators is that the first officer had a control column identical to the captain's.
He could have pushed it and brought the nose down himself.
He may have been able to save the plane (ALL SCREAM) .
but he didn't.
Instead, he and the other Turkish crew member continued offering suggestions to their more experienced but overwhelmed captain.
We're climbing! What am I to do? You can level off.
Our altitude's OK.
The recording shows that Captain Erdem ignored valuable advice that could have saved the plane.
JOHN COX: And in the Birgenair case, there's the case of a relatively junior first officer looking at one of the most senior captains on the airline.
It is not culturally appropriate for him to say, "I'm gonna take the aeroplane away from you.
" I think the social atmosphere in the cockpit will prevail to revere age and experience to the point where it can kill somebody.
And in this case, it looks like it did.
At this point, the crew goes to full power.
Thrust levers.
Thrust! At the angle the plane was falling, the engines couldn't get enough air.
Applying full power was more than they could handle.
The left engine quit first.
With the right side at full throttle, the airliner swings around as though its left wing were caught on a branch.
The airplane goes into a classic full stall where the nose drops, it falls on a wing, which is now a very life-threatening condition.
The 757 itself makes the situation worse.
Like many modern jets, it uses a so-called 'sweptwing' design.
The wings angle slightly backwards to reduce drag and increase fuel efficiency.
But the design has a downside.
One of the characteristics of sweptwing jets is they get less and less stable.
They're much harder to fly as they approach stall.
- Sir, pull up! - ERDEM: What's happening? Oh, what's happening? (PASSENGERS SCREAM) Investigators now know why Birgenair Flight 301 crashed.
What they can't understand is why the flight ever left the ground.
80 knots.
My airspeed indicator's not working.
At 80 knots, if the pilot and copilot's instruments disagree, take-off should be aborted.
Investigators are troubled that the captain took off knowing he had malfunctioning instruments.
This decision on the part of the captain has been criticised.
But there is a very short time window for this decision to be made.
Members of the investigative team in Puerto Plata try to find any clues that might explain why Captain Erdem didn't abort his take-off.
And we certainly looked at the parameters of the runway to ensure that there was adequate runway in that particular situation.
Careful measurements are taken.
Investigators conclude that at 80 knots, when he first noticed the problem, Captain Erdem had enough runway left to bring his plane to a stop.
He could have aborted his take-off.
There's also the question of the hastily assembled crew.
Birgenair started with a crew that probably, er .
didn't expect to fly that night.
They didn't have adequate rest.
They got out to the aircraft and perhaps were rushed in some of their planning.
Investigators consider the possibility that the crew, who had been away from home for more than two weeks, were simply too eager to get home.
08, 301.
Thank you.
- A good flight.
- MAN: Good flight.
Good flight.
(SPEAKS SPANISH) TRANSLATION: This is the homesick factor, where minor problems are ignored in order to get back home.
Investigators will never know what was going through Captain Erdem's mind when he opted to continue his take-off.
In this case, the airplane is accelerating rapidly enough.
The first officer responds V1.
which is the commit-to-fly speed and by training, now the decision window has closed, they need to fly.
And immediately they're after it and they're airborne.
Once the plane was in the air, its blocked Pitot tubes caused Captain Erdem to make a series of critical mistakes.
But how were the tubes blocked in the first place? Investigators will find that the deaths of 189 people was caused by something the size of a paper clip.
Investigators now know that a blocked Pitot tube led to a series of conflicting warnings that confounded Flight 301's captain.
Now they want to know how those same warnings would affect other pilots.
(SPEAKS SPANISH) TRANSLATION: We went to a flight simulator, and in the simulator we tried to recreate the conditions of what happened on the night of February 6, 1996.
Gentlemen, stand by.
The simulator showed investigators that an overspeed warning followed by a stick-shaker warning, caused even the most seasoned pilots to freeze.
The contradictory warnings were potentially dangerous.
(ALARM BEEPS) When the stick-shaker activated, it was very unnerving.
It's really overwhelming.
That would tell me that that mach or airspeed warning horn, combined with the stick-shaker, was a tremendously mind-boggling experience to a line pilot.
As a result, the FAA issues a directive that simulator training for all airline pilots must include a blocked Pitot tube scenario.
The flight crew within Birgenair was faced with a large number of warnings that kept coming and each warning added complexity to the environment.
There's a lot of warning lights going off.
This captain is in a condition that is deteriorating now very rapidly.
So there is a dramatically increased demand on the captain to fly the airplane.
The FAA asks Boeing to change some of those warnings.
Those changes include the addition of a new warning which tells both pilots that their instruments disagree, and the ability for pilots to more easily silence troublesome alarms.
Finally, Boeing modifies its planes so that pilots can easily choose which Pitot tube the autopilot is using for airspeed readings.
All told, more than 1,400 Boeing planes worldwide are affected by the new directives.
One final question remains - what had blocked the plane's Pitot tubes? (SPEAKS SPANISH) Investigators conduct an extensive search for Birgenair Flight 301's Pitot tubes.
They are never found.
But at Puerto Plata's airport, they don't have to look far to find the likeliest suspect.
It's not ice and it's not dirt.
We know that the area around Puerto Plata has a lot of bees and wasps and animals and birds and insects that like to build nests.
One of the insects is well-known to pilots flying out of the Dominican Republic.
It's called the mud dauber wasp.
Bug experts tell investigators about an extraordinary connection between the wasps and a Pitot tube.
(SPEAKS SPANISH) TRANSLATION: When a mud dauber is looking for an area to build its nest, it's looking for a site - a place that's more or less tubular.
When the mud daubers make their mud nest, the mud, when it dries, hardens and condenses.
It gets hard.
Mud dauber wasps are squatters that make their nests in available places, like crevices in homes or even the Pitot tubes of planes.
(SPEAKS SPANISH) TRANSLATION: That the plane was stopped for so long - 25 days - was enough time for any species of the mud dauber to build its nest in the Pitot tubes.
Investigators can only conclude that mud dauber wasps blocked the uncovered Pitot tubes that fed the captain's airspeed indicator, which caused it to malfunction.
MACINTOSH: They didn't put covers on the Pitot tubes.
So at some point in time - extended time - there was an opportunity to get something like a mud dauber in that Pitot tube.
Investigators have their answer.
On February 6, 1996, a tiny insect led to a series of mistakes that brought down an airplane and changed the design of the world's most successful series of airliners.