JOHN WILLIAMS. One of the, the things that we’re interested in is what were the conditions at thirty-five thousand feet, what kind of ice or liquid was there at that altitude?
31:30
Williams can work out the precise conditions at Flight 447’s altitude.
JOHN WILLIAMS. One of the first things we want to do is try to figure out what the temperatures were at that level.
31:42
He creates a cross section, showing the temperature at different altitudes.
JOHN WILLIAMS. What it shows, starting at the surface and going up to the top of the atmosphere, they were flying at this level here and you can see that the temperatures were about forty below Celsius at the time and location of the accident.
32:04
Minus 40 Celsius may seem extremely cold - but in fact it’s much warmer than is usual at 35 000 feet.
And, Williams thinks, just right for a highly unusual phenomenon.
JOHN WILLIAMS. What we’ve found out from this analysis is that it’s possible that there was super-cooled liquid water at the altitude of the aircraft.
32:28
Super-cooled water is a strange quirk of physics.
32:32
In 32 years’ experience, Tony Cable has never seen it up close.
32:40
The water in these bottles is well below zero – the normal freezing point.
32:50
But it’s still liquid.
32:55
Ice crystals can only grow around tiny particles - and this water is extremely pure.
33:04
But if Cable inserts a metal tube…
TONY CABLE. Hey!
33:11
Instant, solid ice.
TONY CABLE. That is incredible. JOHN WILLIAMS. The fact that air is really clean over the oceans suggests that if there is super-cooled liquid water in the atmosphere and an aircraft flies through that, those little droplets are ready to freeze as soon as they hit a surface.
33:38
When hit by super-cooled water…
…a pitot tube could freeze in seconds.
MARTIN ALDER. It is possible that the aircraft encountered conditions which were more severe than those to which it had been designed. The pitot heads may not have been sufficient to cope with these severe conditions.
34:02
The official reports agree that more research is required into super-cooled water at high altitude.
34:12
And Tony Cable discovers that Flight 447 wasn’t an isolated pitot tube incident.
34:18
A catalogue of failure has since come to light.
34:24
From 2003, thirty-six frozen pitot tube incidents, involving A330s or the similar A340s.
34:34
And in 2009, they were coming thick and fast.
TONY CABLE. It was in the order of one a week for getting on for two months in the period leading up to the 1st of June.
34:43
All the failed pitot tubes met existing safety standards. But in late 2007, Airbus recommended a refit of all A330s, with uprated pitots.
Air France was in the process of upgrading its entire fleet - when Flight 447 ran into the Atlantic storm…
35:03
…still with old-model pitots.
34:43
ALT
All the failed pitot tubes met existing safety standards. But in late 2007, Airbus recommended a refit of all A330s, with upgraded pitots.
34:54
Air France was in the process of refitting its entire fleet - when Flight 447 ran into the Atlantic storm…
35:03
…still with old-model pitots.
35:11
Our independent team believes they may have encountered super-cooled water..
…causing the pitot tubes to freeze.
35:28
With no airspeed data, Flight 447’s automatic systems collapse one…
by one…
by one.
35:40
In total darkness, and heavy turbulence, the crew are forced to retake manual control.
JOHN COX. Pilots are the last line of defence, so when things go very wrong the last line of defence is the aviator.
36:09
After more than 3 hours on auto-pilot, the pilots are suddenly faced by information overload.
JOHN COX. That crew faced an almost unheard of series of failures, one right behind the other, and for them to sort through it would have been very difficult that night. JOHN COX. Why is the airplane doing what it’s doing, what are all these failures, why are they all coming at one time?
36:22
Bombarded by faults – the pilot must cope with the most serious problem of all.
He must maintain speed - or they will go out of control.
MARTIN ALDER. The acceleration forces caused by the turbulence means that we might stall the aircraft.
36:40
If Flight 447 slows down by just a few knots, it could go into the catastrophic condition known as a stall.
36:56
Back in the wind tunnel - Tony Cable demonstrates how an aircraft’s wing can cease to function.
TONY CABLE. OK Cliff, you ready to go?
37:06
An aircraft is kept aloft by smooth, streamlined airflow over the wings.
But too slow, and there’s a critical angle, where the airflow breaks up.
JOHN COX. If you disrupt that flow the wing cannot produce lift. And that’s really all a stall is, is it’s a disruption of the air flow across the top of the wing. TONY CABLE. If it slows down there’s a tendency for the nose to come up and at a fairly shallow angle the wings will stall, the air flow breaks down, there’s a loss of lift and the aircraft begins to descend.
37:53
Stalled wings would mean a dramatic, uncontrolled descent.
At 35 000 feet, in heavy turbulence, even a very small reduction in speed would increase the risk.
MARTIN ALDER. Our speed range is quite limited. Typically it could be ten knots either side of the cruise speed.
38:18
The pilots must somehow avoid slowing down - by even ten knots.
The only trouble is…
…with their pitot probes frozen..
– they have no way of knowing how fast they’re going.
JOHN COX. The ability for the crew to recognise if they were at the proper speed is going to be a much more complex problem now.
38:46
A complex problem – but can it be solved?
39:00
In the simulator, Martin Alder plans to recreate the known technical failures on Flight 447.
39:10
Two experienced pilot instructors will attempt to maintain speed and avoid the stall - using standard operating procedure.
39:24
Like Flight 447, they are cruising normally at 35 000 feet, at night, over the ocean.
39:33
Alder triggers the storm.
MARTIN ALDER. Storm control. I like the look of that one, so now, activating…
39:42
Thunderclouds loom ahead on the pilots’ radar.
CO-PILOT. Probably a line of thunderstorms…
39:50
The Captain plans a detour, and prepares for turbulence.
CAPTAIN. So Turbulence Airspeed then, I’m selecting decimal seven six. CO-PILOT. Check.
40:02
As was likely on Flight 447, the pilot selects a slightly lower speed, to reduce stresses on the aircraft.
40:11
Auto-Thrust automatically reduces engine power.
CAPTAIN. Hi Ladies and Gentlemen, the Captain speaking, we are just approaching an area where there are a few rain showers around, we’ve got the Seatbelts sign on for approximately ten minutes or so.
40:28
As they edge around the storm, Alder triggers the critical moment of Flight 447…
40:36
He fails the airspeed indicators.
ALT
He fails all three airspeed indicators.
CO-PILOT. OK we have Nav ADR One disagree. We have unreliable airspeed.
40:51
The automatic systems shut down.
CO-PILOT. We’re flying with no auto-pilot, or auto-thrust. CAPTAIN. OK. Auto-pilot’s off, I have control. CO-PILOT. You have control.
41:03
If their airspeed drops by as little as 10 knots, they could stall - and fall out of the sky.
But the Captain relies on established standard procedure.
41:16
He pushes the throttle levers - to set thrust at exactly 85%
CAPTAIN. And I’m selecting - I’ve got 85% set.
41:30
Then, he raises the elevators…
…to pitch the nose up, at precisely 5 degrees.
With 85% power, and 5 degrees pitch up, they should always settle at the same safe speed.
MARTIN ALDER. It’s quite possible to fly the aircraft to actually quite high standards of accuracy, something in the region of about five knots or so of the, of the desired target speed would be quite achievable for most crews.
42:02
Pitch and Power is the pilots’ lifeline.
They ignore any fault messages until they’re safely in control.
CAPTAIN. I’ve got 5 degrees on the standby, 4 degrees on the other. MARTIN ALDER. Flying the aeroplane is the prime objective. No matter how attractive the messages might be to anybody on the flight deck, you both concentrate in ensuring that the person who should be flying the aeroplane is flying it, and flying it in the manner which is safe. CAPTAIN. OK, so we’ve got what I think is basic control of the attitude, we’re bumping a little bit with the weather, but generally that’s safe.
42:46
Maintaining Pitch and Power will get them to the nearest airport.
The storm has passed…
…the emergency is over.
MARTIN ALDER. That’s it, it’s finished.
43:03
Martin Alder’s simulation shows that speed can be maintained even if all 3 pitot probes fail.
43:03
ALT
Martin Alder’s simulation shows that speed can be maintained even if the pitot probes fail.
43:15
But was this standard pitch and power technique applied by the crew of Flight 447? And why didn’t they recover control?
43:24
The official reports have no direct evidence….
… but former Airbus pilot John Cox identifies a crucial detail in the standard procedures.
JOHN COX. If they were just entering an area of choppy air, they may very well have been slowing the airplane down.
43:40
CAPTION: RECONSTRUCTION
43:42
When Flight 447 first entered the storm, they would reduce speed….
43:50
… so auto-thrust decreases engine power automatically.
But it does not give the pilots an important visual signal.
JOHN COX. The thrust levers themselves, the throttles, don’t move. Unlike some other airplanes where you can feel the throttle in your hand moving, with Airbus aircraft that throttle doesn’t move with auto-thrust engaged, so you have to look at specific engine power indications.
44:18
The power indications are displayed here, on the central control panel.
But in John Cox’s scenario, the pilot doesn’t notice it.
JOHN COX. If you’re very task-saturated your concentration’s going to be directly in front of you. What’s the power output of the engines, you’re going to have to physically turn your attention and look to the centre console area. JOHN COX. This is not going to be done as frequently as looking at, at the things right in front of you, it, it’s certainly going to be in the scan, the question is how often?
44:58
Overloaded by fault warnings - the pilot overlooks the low power indication.
JOHN COX. Was this crew one that was very attentive and picked up this information very early, were they very slow to pick up the information? We don’t know. JOHN COX. Were the demands so high that they were unable to, to keep up with it? We don’t know.
45:23
Only Flight 447’s black boxes could provide conclusive evidence.
But now, Tony Cable discovers a worrying pattern, to support Cox’s theory.
45:39
In 10 previous incidents of airspeed failure, the crews failed to increase thrust immediately.
TONY CABLE. In quite a number of them, it’s clear that the crews were very slow to get onto manual throttle operation.
46:00
In 5 cases, crews did not take manual control of thrust for more than sixty seconds.
For Flight 447, that would spell rapid deceleration…
…and the risk of a sudden stall.
MARTIN ALDER. Ten to fifteen knots would not be unusual, if you decelerate at a knot a second it’s fifteen seconds. The aircraft would slow down to a much lower speed, and you could approach a stall quite quickly in that manner at altitude. JOHN COX. There is a good possibility that at some point in the last four minutes that there was a stall event.
46:39
A sudden and critical loss of lift from the wings.
But even in a stall, the aircraft can be saved.
Tony Cable explains the standard recovery technique.
TONY CABLE. The procedure is full thrust on both engines, reduce the pitch attitude and the aircraft will then resume normal flight.
47:06
By pitching the nose down, the pilot restores smooth airflow over the wings, and escapes the stall.
47:18
But Flight 447’s pilots may have found themselves - in totally unknown territory.
47:14
ALT
But a stall can be an extreme event. Flight 447’s pilots may have found themselves - in totally unknown territory.
