The French in Denial

It could happen to anyone. This time it happened to be a French airplane with French pilots flying for a French airline.

For two years the “black boxes” (the voice recorder and the DFDR) lay in peace on the floor of the Atlantic Ocean, 13,000 feet below the waves. For two years there was conjecture, speculation, and (some quite fine) attempts at reconstruction. Then the black boxes surfaced, along with other hard evidence, including the jackscrew from the Trimmable Horizontal Stabilizer.

For months as the Bureau d’Enquêtes et d’Analyses slowly released information, we (and the BEA) put together the tragic and terrifying story. But the story stopped abruptly, the last chapter removed or never written.

Now a French aviation writer, Jean-Pierre Otelli, has published that last chapter independently of Air France, Airbus Industrie, and The BEA. The story ends as we knew it would – badly and sadly – but now we have more grisly detail and less room for denial.

The Bureau d’Enquêtes et d’Analyses is incensed. In a press release on October 13, 2011, (look under News in the sidebar) they claim that the transcription released by Otelli “mentions personal conversations between the crew members that have no bearing on the event, which shows a lack of respect for the memory of the late crew members” (my emphasis). The same day the London Telegraph published an account of the final minutes. The account seems to have been shortened since October 13, and I have been unable to find the original. Those who are interested may add the following after “According to an official report released earlier this year, the last words were from Captain Dubois who said: ‘Ten degrees pitch.‘”:

But in his new book Mr Otelli asks who will be held responsible ‘for this mess’. ‘Is it a training problem, fatigue, lack of sleep, or is it due to the fact the pilots are confident that an Airbus can make up for all errors?,’ he writes. France’s air accident investigation unit, the BEA, reacted angrily to the publication of the book, with a spokesman saying printing the conversation showed a ‘lack of respect to the memory of the crew who died’. Air France has denied that its pilots were incompetent, but has since improved training, concentrating on how to fly a plane manually when there is a stall. Both Air France and Airbus are facing manslaughter charges, with a judicial investigation led by Paris judges already under way. A judge has already ordered Air France to pay some £120,000 in compensation to the families of each victim, but this is just a provisional figure which is likely to multiply many times over. THE FINAL MOMENTS Marc Dubois (captain): ‘Get your wings horizontal.’ David Robert (pilot): ‘Level your wings. ‘Pierre-Cedric Bonin (pilot): ‘That’s what I’m trying to do… What the… how is it we are going down like this?’Robert: ‘See what you can do with the commands up there, the primaries and so on…Climb climb, climb, climb. ‘Bonin: ‘But I have been pulling back on the stick all the way for a while. ‘Dubois: ‘No,no, no, don’t climb. ‘Robert: ‘Ok give me control, give me control.’Dubois: ‘Watch out you are pulling up. ‘Robert: ‘Am I?’Bonin: ‘Well you should, we are at 4,000.’As they approach the water, the on-board computer is heard to announce: ‘Sink rate. Pull up, pull up, pull up. ‘To which Captain Dubois reacts with the words: ‘Go on: pull.’Bonin: ‘We’re pulling, pulling, pulling, pulling.’The crew never discuss the possibility that they are about to crash, instead concentrating on trying to right the plane throughout the final four minutes. Dubois: ‘Ten degrees pitch. ‘Robert: ‘Go back up!…Go back up!…Go back up!… Go back up! ‘Bonin: ‘But I’ve been going down at maximum level for a while.’Dubois: ‘No, No, No!… Don’t go up !… No, No! ‘Bonin: ‘Go down, then!’Robert: ‘Damn it! We’re going to crash. It can’t be true!’Bonin: ‘But what’s happening?!’The recording stops.

What we know, briefly, is this: Air France 447 ventured into a line of thunderstorms along the InterTropical Convergence Zone. Four other flights diverted around the storms. In the zone the flight encountered unusually warm temperatures and supercooled water droplets – enough to briefly overwhelm the heaters in all three pitot tubes, denying airspeed information to the Flight Control Computers for long enough to cause them to kick off the autopilot and to degrade the flight controls from Normal Law to Roll Direct/Pitch Alternate Law. Despite the fact that by the book they were too heavy to climb, the pilot flying (First Officer David Robert) zoomed up from 35,000 feet to almost 38,000 feet, dissipating the aircraft’s energy and exposing it to coffin corner, where Mach buffet meets stalling speed. With brief lapses he held back pressure on the sidestick for the remainder of the flight.

First the airplane stalled (quit flying because the Angle of Attack was too great). Then, because of the steady back pressure on the sidestick, the autotrim wound the Trimmable Horizontal Stabilizer (more powerful than the elevators) to full nose up. (The THS jackscrew was found in this full nose up condition). By now the aircraft was in a deep stall, falling almost straight down in a near-level attitude.

There is plenty of room for argument about why it happened this way. Many (including David Learmount at Flight Global and myself) have started that discussion. It must continue, because we must know not only why F/O Robert stalled the aircraft, but much more importantly why he didn’t know he had stalled it, why he had a totally inaccurate picture of what was happening, and why there was a complete absence of situational awareness on that Flight Deck.

It may look as if I am placing blame solely on F/O Robert. Absolutely not. That would be much too easy. I and others have already written many pages (see AF447 on my blog) trying to piece together all the factors at work in this accident. We will write many more.

As in all accidents, there is a chain of events and decisions which gradually (at first!) reduce maneuvering room. The first of these was Captain Dubois’ decision to take crew rest approaching the ITCZ.

But before that came Air France’s decision to carry less fuel than the spirit of the regulations requires, by filing the Flight Plan as Rio to Bordeaux, alternate Paris. Even earlier, the brilliant (I am not being ironic or facetious, I admire the man) Bernard Ziegler designed the Airbus to be “pilot-proof” and impossible to stall. However he (or his designers) also left the autotrim functional in Pitch Alternate Law, an oversight I believe should be corrected ASAP. Finally, (and earliest of all) you and I and everyone else who has traveled since Airline Deregulation in 1978 believes or wants to believe in cheap seats.

It could happen to anyone.

Sadly, it has all been foreseen. Recently I read an article which bluntly calls out the forces that led to this accident. It is called The Training Paradox, and was written by pilot, engineer, and lawyer Mark. H. Goodrich. Some of the accidents and incidents he describes stood my hair on end. Unfortunately I cannot provide a link to it. I read it in Position Report, November 2011, Volume VIII Number 3. (This is the magazine of the Retired Airline Pilots of Canada).

The very experienced and knowledgeable Mr. Goodrich shows how the forces of deregulation have derailed traditional career paths and interrupted the passing along of knowledge. As a result, the craft, the trade, the profession if you will of flying is dying a slow death. Neither are regulatory bodies or airline management immune from this decay.

This time it was the French. It is not surprising they are in denial. But it could happen to anyone, and it will.

 

AF 447: Let’s Talk About Why – 2: Virtual Reality

How many times have you heard Airbus pilots say, “It’s not an airplane, it’s a video game.”?

In this blog I will explore the fact and fiction in this statement. My objective is not to praise a great airplane or run down a flawed one, but rather to find how to live with a perfectly good one.

For almost a decade I flew the A320 (and the A319 and A321) as Captain and Training Captain. I came to see her not as a video game but as a person with whom I had to deal. I came to appreciate her many sterling qualities and also her weaknesses. Both informed our work together.

Fundamentals

First, she is an airplane like any other. If you accelerate her to Vr and raise the nose, does she not fly? If you provoke her into an Angle of Attack above 16°, does she not stall? Do not the laws of aerodynamics still hold?

These fundamental things will always apply as we work through her many wonders: Fly-by-Wire, Envelope Protection, and Flight Guidance systems. These wonders are what software people call a “front end” to her conventional aircraft qualities. But the wonders can be a powerful distraction as well as a boon.

Another pilot comment heard (more frequently in the first decade of operation, roughly the 1990’s) is “What the #*%# is it doing now?”. The question was being asked because the pilots didn’t know where to look for the answer, and also because an airplane maneuvering on her own was a novelty. When things happen that we don’t understand, we human beings tend to see them as acts of God. We substitute reverence for understanding. A320 software – partly because it is so good, most of the time – has been an object of such reverence.

But just as we are not perfect, neither is this marvellous software. As the history of the airplane in service demonstrates, it is only as good as its interface with the pilots.

The Crashes

The first three crashes – Mulhouse, Bangalore, and Strasbourg – are illustrative. In the first two the engines were at idle and the crew were unaware that the power was being commanded to idle by the Auto-thrust. This information is clearly presented at the left end of the Flight Mode Annunciator or FMA, which appears in a band across the top of the Primary Flight Display, or PFD. The Auto-thrust Mode is what the Auto-thrust thinks it is doing. The only acceptable modes for approach are Speed and Off. At Mulhouse and Bangalore the Auto-thrust Mode was reading Idle.

What was not clearly understood at the time of these crashes was how to change the Auto-thrust mode from Idle to Speed, which is to turn off both Flight Directors. Further, the annunciation of the Flight Director modes on the FMA was not as communicative as it is today. At Bangalore one pilot turned off his Flight Director and the other did not. As a result the Auto-thrust mode remained in Idle. Today in that situation the FMA would show 1FD- , meaning that FD 1 is operating on the left side and that FD2, on the right side, is off. (With both FD’s on the FMA would show 1FD2). Both pilots can see what is going on. This improvement was implemented after analysis of these crashes.

The Strasbourg crash resulted in another improvement in the airplane-pilot interface. The flight was performing a non-precision approach which specified a Flight Path Angle. The crew selected -3.3 into the Flight Control Unit but failed to switch it to Track/Flight Path Angle mode. The FCU remained in Heading/Vertical Speed mode and interpreted the command as -3300 feet per minute. (There is a big difference between the two. At normal approach speeds a Flight Path Angle of -3.3 would be 800-900 fpm.) The presentation has since been changed in two important ways: first, to change the HDG/VS mode on the FCU to show 3300 while leaving the TRK/FPA mode as 3.3. Second, the commanded rates are now repeated on the Flight Mode Annunciator.

In these accidents the crew were not aware of what the software was doing. In the following example, the loss of an A330 in flight test at Toulouse in 1994, the crew were not aware of a crucial software limitation.

In most autopilots there is an altitude capture mode. In Airbus aircraft this is known as ALT*, or “Alt Star.” The computer uses the selected altitude and the vertical speed to calculate how far ahead to begin the capture maneuver, which is an asymptotic curve. Higher vertical speeds require that the maneuver be begun earlier if “G” forces are to remain within limits. Crucially, because the software calculates the curve based vertical speed, it de facto assumes that the thrust available at the start of the capture maneuver will remain available. Thus the loss of an engine while in ALT* is a first-rate emergency requiring flight crew intervention within a few seconds.

Man/Machine Communication

I present these examples not as an exhaustive course on Airbus software, but as an illustration of how extra intelligence brings with it extra complication. First, the communication between man and machine is of paramount importance. The interface cannot be too well-designed and the pilot cannot take too much care in maintaining effective two-way communication. This is why at my airline any change in the FMA was verbalized by the Pilot Flying, in effect giving voice to the machine and keeping the three pilots (two human, one cybernetic) on the same page.

Second, each time a task is assigned to automation the process must remain transparent to the pilot. He must understand in general terms what the computers are doing, and even more importantly what they are not doing. Should the automation for any reason drop the task it must be immediately obvious to the pilot and he must have steps rehearsed which let him take control and do the task himself.

Engine failure in ALT* is a good example. With today’s improved FCU interface the pilot can push the Vertical Speed knob, which simultaneously selects V/S as the vertical mode and sets the target V/S to zero. In less than a second he has intervened, taken control, and given himself time.

If altitude cannot be maintained on the remaining engine(s) he can twist the knob to set a modest descent. Then the drill calls for getting a clearance to a lower altitude, turning off the Auto-thrust and setting Maximum Continuous Thrust on the good engine(s), selecting the cleared altitude and Pulling the Altitude knob to select Open Descent. Speed and thrust can then be adjusted to suit the situation.

The above procedure is not difficult, is easily performed in the time available before losing control, and requires no particular skill. What it does require of the pilot is that he view the airplane (and her wonders of automation) as an equal: a skilled pilot who nevertheless can have a bad day, make a mistake, or be simply unavailable.

Anthropomorphism

I know I am not alone in assigning a personality to the Airbus. I have said elsewhere that I came to regard her as a friend, or more than a friend. I (ahem) even loved her. Perhaps I still do and that why I am writing this.

Wait, though. I know full well she is aluminum, carbon fibre, and Intel and Motorola Assembler. I also know she is a damn good pilot and that she can be trusted like a close friend. But – and this is the important part – she is my equal. I can fly too, but I sometimes make mistakes, have a bad day, or fail to communicate effectively. Ditto my software friend. I can be blinded by pride. Ditto my software friend. She is French and she has pride in her DNA.

In the Simulator we practice Pilot Incapacitation, recognizing that to err is human. What we have a harder time with is Automation Incapacitation. This is perhaps a symptom of our reverence for something that is beyond our understanding, for our unrecognized assumption that technology is perfect, or at least better than we are. This unrecognized and unwarranted assumption can be fatal.

It is much better to appreciate her as an equal and deal with her as a whole person, warts and all.

Feedback and Feel

Let’s dig a step further. I believe what pilots are talking about, when they say Airbus aircraft are video games, is the lack of feedback and feel in the controls. The throttles, for example, do not move when the Auto-thrust is active. The pilot sees only Speed on the FMA and the engine indications on the ECAM. To take control smoothly (for example to do a manual approach) he must pull the thrust levers back until the little green donuts match the current thrust, and click the off button on the lever. The FMA says Off and he’s on his own. But the approach is still a bit of a parlor trick because there is no feel in the sidestick. When a conventional aircraft gets slow increasing back pressure is necessary to keep the nose from dropping. Not so in an Airbus. Instead, the Autotrim will move the stabilizer nose-up to maintain 1G flight. The pilot’s eye has to dart to the airspeed indicator to get what he might have sensed in the stick or control column. All of this contributes to the “video game” feel.

Perhaps a direct Angle of Attack readout in a Heads Up Display would compensate for the lack of feel. But this is ignoring an essential fact: the Airbus is a conventional airframe, with positive aerodynamic longitudinal stability. It is not like some fighter aircraft with neutral or negative longitudinal stability, where the aircraft is uncontrollable without fly-by-wire. The stability is there, but it is shielded from the pilot.

It must be pointed out that the Airbus is a beautiful airplane and a joy to fly and that it has hundreds of wonderful design features I would not like to see disappear. Just one example is “the hook” (the display on the airspeed tape of Vls (lowest selectable speed)) and its relationship to “the bug” (Vapp, or final approach speed). The bug speed is calculated by the FMCG (Flight Management and Guidance Computer) based on the Gross Weight (or Zero Fuel Weight) entered by the pilots. The hook is calculated from first principles by comparing Angle of Attack with dynamic pressure (airspeed). In a normal approach these are 0.5 cm (1/4 inch) apart. This is one of those comfort crosschecks for pilots. If the bug and the hook are too close together, the weight entered in the FMCD is likely wrong, and the calculated Vapp is too slow.

But even here an intelligence has been interposed between the pilot and his aircraft. Why not also display the Angle of Attack directly, and always fly the approach at the same angle of attack regardless of weight? (See my blog AF 447 – Let’s Talk about Why – 1: Angle of Attack). It is this interposition of intelligence that contributes to what I see as the problem: the illusion of Virtual Reality.

Virtual Reality

Flying an airplane, any airplane, is a very real job. The airplane can be a bear or sweet to fly, it can be automated or not, it can “land itself.” But the bottom line of the captain’s job does not change, and that is to be the arbiter of last resort: the man or woman who imagines, constructs, and sees the picture that determines the outcome. It is his or her job to maintain that picture. In the trade we call it situational awareness. If something goes wrong and that picture is wrong people die. And if the captain believes the glass display before him is superior to his own mental image, then he will be more likely to abdicate his responsibility to maintain situational awareness.

Today’s glass cockpit is seductive. A wealth of information sits before the pilot: some of it is raw data; often it has been extensively processed into a colourful and sometimes beautiful picture. Like a video game, this is virtual reality. Software is doing the imagining for the pilot.

It can be argued that the picture in the pilot’s head is also virtual reality, merely a representation of the external world. But this argument does not acknowledge the survival instinct that guides the pilot’s doubt and questioning, his constant checking for consistency, his testing of the obvious.

Airbus aircraft are beautiful and a joy to fly. But they are not perfect. Like all of us, they have a fatal flaw. The Ancient Greeks knew this hamartia as an essential component of human character. Bernard Ziegler, the brilliant designer of the Airbus software, has been quoted as saying he wanted to make the airplane pilot-proof. Consequentially, as I have shown, there are areas where the pilots have been shielded from useful, even essential, information. The Airbus pilot must work hard to ensure he is not entirely removed from the loop.

Reality for an airline passenger is not virtual. This game cannot be started over. The next time you hear someone say, this airplane lands itself, will you be comforted? Or will you be hoping that the pilots are not just along for the ride?

 

AF 447: Let's Talk About Why – 1

Thanks to the work of David Learmount at Flight Global, and that of the Wood’s Hole Oceanographic Institution and the Bureau d’Enquêtes et d’Analyses, enough is now known about this accident to start looking for useful lessons and to analyze the data along with the BEA. Flight safety and the future of the piloting profession depend on this becoming a wide and serious conversation.

Pilots obsess about accidents for good reason. There is always so much to learn. The AF447 tragedy is an epochal example.

There is a mind-boggling number of lessons to be learned here, in a host of areas and disciplines: Pilot Training, Standard Operating Procedures, Instrument Flight, and Aircraft Design are but a few of them.

I will commit today to joining the conversation. I begin with a consideration of Angle of Attack.

Angle of Attack

Wolfgang Langewiesche (father of William) emphasized Angle of Attack in his excellent Stick and Rudder, published in 1944. Advocacy of AoA was an uphill battle then and it still is today. Instead of talking about AoA, we prefer to use airspeed and explain why certain speeds we use change with aircraft weight and G loading. Many or even most aircraft flying today have no Angle of Attack indication. The accident aircraft had two AoA sensors. The flight recorders had access to the signals from these sensors but the pilots did not, at least not when they needed it most.

Lift is produced when the air flowing over the top of a wing has a longer distance to travel than the air flowing underneath. The air “stretching out” over the top produces a lower pressure, allowing the higher pressure underneath to push the wing up. There is a caveat, however. The airflow must remain attached to the upper surface of the wing.

Imagine a cross-section of wing, with a line drawn from the middle of the rounded leading edge to the pointed trailing edge. This is the chord line. Now imagine an arrow pointing at the leading edge. This is the airflow.

If the arrow meets a (symmetrical) wing head-on there will be no lift. But let the wing meet the air at a slight angle and the airflow around the wing will no longer be symmetrical: it will meet the rounded leading edge at an angle and it will divide lower on the curve of the leading edge. The air flowing over the wing will have a longer distance to travel. Lift will be produced.

The angle at which the airflow meets the chord line is called the Angle of Attack. Up to a point, increasing the Angle of Attack will increase lift. But beyond a certain point – usually about 16° – lift will instead decrease because the airflow is beginning to separate from the upper surface of the wing. This is called the aerodynamic stall, and it always happens at the same Angle of Attack.

Angle of Attack is controlled by the elevators, the control surfaces on the trailing edge of the horizontal tail. When the pilot pulls back on the stick, the elevators lift, causing a down-force on the tail and forcing the wing to meet the air at a higher Angle of Attack. Trim tabs (small surfaces at the trailing edge of the elevators) can be moved to change the neutral position of the stick. (Another way to think of it is the trim tabs change the Angle of Attack at which there is zero stick force.)

In a modern jet transport the entire horizontal tail is usually moveable. This is because of the very wide speed range of the jet and because flaps and leading edge slats also change the “trim.” The other side of the coin is that this horizontal tail, or stabilizer, is very powerful in modern jet transports. A runaway stabilizer is a true emergency. Traditionally there has been a STAB IN MOTION aural warning, and an emergency cutout switch close to hand. Most cases where the stabilizer ran all the way up or down in flight have resulted in the loss of all on board.

In most aircraft the pilot is used to trimming as he flies. A change of speed or configuration, be it in a Beech Bonanza or a DC-9, will require a trim change. With some experience on type the pilot knows (for example on a DC-9) that extending the leading edge slats will require two beeps (of the STAB IN MOTION aural warning) of nose-up trim. He can use the thumb switches on the yoke to move the stabilizer as the slats are extending and thus remain stick-neutral during the configuration change. This is part of anticipation, or staying ahead of the aircraft.

Airbus aircraft, from the A320 onwards, are different. They are fly by wire, where computers are interposed between the pilots’ sidestick inputs and the control surfaces. This arrangement allows some elegant additions to aircraft design, such as envelope protection (which among other things makes it impossible for the pilot to stall the aircraft) and, relevant to our discussion today, stick force per G and autotrim.

In Normal Law, which is where the Airbus is most (and the pilot hopes, all) of the time, configuration changes can be made hands off, even flying by hand. Of course the pilot has the tips of his fingers on the sidestick, but he can make a configuration change with no pitch input because the control system, in Normal Law, will maintain 1G flight. When he calls for FLAP 1 and the leading edge slats extend, the nose-down pitch is sensed and countered by the system, maintaining 1G flight. (1G is what you experience sitting in a chair at home or in an aircraft at cruise in smooth air). In effect, the airplane is doing the anticipation for the pilot.

Like the transition in the late 1950’s from props to jets, fly-by-wire has been a major change for pilots. In general we welcome it for the many advantages it offers.

Experience has shown that to do his job, which is to ensure the safe arrival of his aircraft, the pilot must fully understand a much more complex airplane. Chesley Sullenberger reached up and started the APU (the Auxiliary Power Unit, a small turbine in the tail which can supply electrical and hydraulic power on the ground or in flight) as soon as his engines lost power. Why? Because he knew his airplane and he knew he wanted to keep it in Normal Law until touchdown.

The transition from props to jets was all about speed range, speed brakes and spoilers, high Mach number, coffin corner, Dutch Roll and super-stall, but in everyday life it was more about high drag on approach, no propwash, slow spool-up times, and operating on the back side of the power curve. This change took some adjustment on the part of pilots: the more experienced pilots had more adjustments to make. The same is true with the transition to fly-by-wire.

In a traditional airplane the pilot controls Angle of Attack with the elevator and the trim tabs or stabilizer. (More often he will be thinking of Airspeed, which is the constant-weight, 1G manifestation of AofA). He is used to feel, which is essentially the change in elevator neutral point with AofA. Should the aircraft slow on approach, the nose will get “heavy”, prompting him to pull back or trim nose-up.

That feel is totally absent in Airbus aircraft. (Boeing, in the B777, have added artificial feel to their fly-by-wire system). The Airbus pilot points and shoots, so to speak. Flying by hand he can take the bird, turning on a symbol (like a bird or an aircraft seen from behind) on his Primary Flight Display. The bird shows where his velocity vector is pointed; in other words, where is airplane will be so many seconds from now if he makes no further adjustments. On approach he can pin the bird on his flare point on the runway and either let the autothrust take care of the speed or adjust the thrust levers manually. If he does the latter, he must remember that there is no feel or feedback in the sidestick.

Obviously there are quite different assumptions operating during an approach in a Bonanza, one one hand, and an Airbus, on the other. This is not necessarily a bad thing. Take for example driving a car versus riding a motorcycle. In a car you steer with the steering wheel. In a motorcycle you counter-steer, putting pressure on the inside foot-peg and forward pressure on the inside bar, in effect trying to steer the front wheel the opposite way.

But you know you’re on a motorcycle and not in a car. You have learned how to ride a motorcycle.

Consider, however, flying an Airbus if something goes wrong with a sensor or a computer and you wind up in Alternate Law or Direct Law. You are in the same vehicle but suddenly the rules have changed; the assumptions have changed. It is, in effect, no longer the same machine. This is a recipe which messes with a pilot’s head.

Unfortunately, experience has shown that Direct Law, where control displacement is proportional to stick force and the airplane handles like a wet fish, is actually the more benign of the two degraded modes. There is a big message in red on the ECAM saying USE MAN PITCH TRIM. The pilot moves the THS (Trimmable Horizontal Stabilizer) by moving a wheel almost a foot in diameter. This is old-style, normal airplane flying, commanding AofA with stick force and trim. There is still no feel in the sidestick, but the procedure is familiar.

Alas, in Alternate Law there is no such familiarity. It is still point-and-shoot, sort of, but autotrim is still working. As long as there is back pressure on the stick the THS trims nose-up, and vice-versa. There is NO Stabilizer in Motion warning except the movement of the trim wheels. That would seem to be an easy thing to detect, but I can testify from personal experience that it is not. On every landing (in Normal Law) the flight control computers memorize the attitude at 50 feet Radio Altitude and at 30 feet start rolling in nose-down trim, in effect trying to mimic the feel of a normal aircraft slowing in the flare. In almost a decade of flying as Captain and Training Captain, whether as Pilot Flying or Pilot Not Flying, I cannot remember ever seeing the trim wheels move.

In two recent accidents an Airbus has hit the ocean with the THS wound to full nose up. In both cases the aircraft was in Alternate Law.

I am not an engineer. There are likely many ramifications that have not crossed my mind. But sitting here this afternoon my personal recommendation would be as follows:

Disable Autotrim in Alternate Law

AF 447: Let’s Talk About Why – 1

Thanks to the work of David Learmount at Flight Global, and that of the Wood’s Hole Oceanographic Institution and the Bureau d’Enquêtes et d’Analyses, enough is now known about this accident to start looking for useful lessons and to analyze the data along with the BEA. Flight safety and the future of the piloting profession depend on this becoming a wide and serious conversation.

Pilots obsess about accidents for good reason. There is always so much to learn. The AF447 tragedy is an epochal example.

There is a mind-boggling number of lessons to be learned here, in a host of areas and disciplines: Pilot Training, Standard Operating Procedures, Instrument Flight, and Aircraft Design are but a few of them.

I will commit today to joining the conversation. I begin with a consideration of Angle of Attack.

Angle of Attack

Wolfgang Langewiesche (father of William) emphasized Angle of Attack in his excellent Stick and Rudder, published in 1944. Advocacy of AoA was an uphill battle then and it still is today. Instead of talking about AoA, we prefer to use airspeed and explain why certain speeds we use change with aircraft weight and G loading. Many or even most aircraft flying today have no Angle of Attack indication. The accident aircraft had two AoA sensors. The flight recorders had access to the signals from these sensors but the pilots did not, at least not when they needed it most.

Lift is produced when the air flowing over the top of a wing has a longer distance to travel than the air flowing underneath. The air “stretching out” over the top produces a lower pressure, allowing the higher pressure underneath to push the wing up. There is a caveat, however. The airflow must remain attached to the upper surface of the wing.

Imagine a cross-section of wing, with a line drawn from the middle of the rounded leading edge to the pointed trailing edge. This is the chord line. Now imagine an arrow pointing at the leading edge. This is the airflow.

If the arrow meets a (symmetrical) wing head-on there will be no lift. But let the wing meet the air at a slight angle and the airflow around the wing will no longer be symmetrical: it will meet the rounded leading edge at an angle and it will divide lower on the curve of the leading edge. The air flowing over the wing will have a longer distance to travel. Lift will be produced.

The angle at which the airflow meets the chord line is called the Angle of Attack. Up to a point, increasing the Angle of Attack will increase lift. But beyond a certain point – usually about 16° – lift will instead decrease because the airflow is beginning to separate from the upper surface of the wing. This is called the aerodynamic stall, and it always happens at the same Angle of Attack.

Angle of Attack is controlled by the elevators, the control surfaces on the trailing edge of the horizontal tail. When the pilot pulls back on the stick, the elevators lift, causing a down-force on the tail and forcing the wing to meet the air at a higher Angle of Attack. Trim tabs (small surfaces at the trailing edge of the elevators) can be moved to change the neutral position of the stick. (Another way to think of it is the trim tabs change the Angle of Attack at which there is zero stick force.)

In a modern jet transport the entire horizontal tail is usually moveable. This is because of the very wide speed range of the jet and because flaps and leading edge slats also change the “trim.” The other side of the coin is that this horizontal tail, or stabilizer, is very powerful in modern jet transports. A runaway stabilizer is a true emergency. Traditionally there has been a STAB IN MOTION aural warning, and an emergency cutout switch close to hand. Most cases where the stabilizer ran all the way up or down in flight have resulted in the loss of all on board.

In most aircraft the pilot is used to trimming as he flies. A change of speed or configuration, be it in a Beech Bonanza or a DC-9, will require a trim change. With some experience on type the pilot knows (for example on a DC-9) that extending the leading edge slats will require two beeps (of the STAB IN MOTION aural warning) of nose-up trim. He can use the thumb switches on the yoke to move the stabilizer as the slats are extending and thus remain stick-neutral during the configuration change. This is part of anticipation, or staying ahead of the aircraft.

Airbus aircraft, from the A320 onwards, are different. They are fly by wire, where computers are interposed between the pilots’ sidestick inputs and the control surfaces. This arrangement allows some elegant additions to aircraft design, such as envelope protection (which among other things makes it impossible for the pilot to stall the aircraft) and, relevant to our discussion today, stick force per G and autotrim.

In Normal Law, which is where the Airbus is most (and the pilot hopes, all) of the time, configuration changes can be made hands off, even flying by hand. Of course the pilot has the tips of his fingers on the sidestick, but he can make a configuration change with no pitch input because the control system, in Normal Law, will maintain 1G flight. When he calls for FLAP 1 and the leading edge slats extend, the nose-down pitch is sensed and countered by the system, maintaining 1G flight. (1G is what you experience sitting in a chair at home or in an aircraft at cruise in smooth air). In effect, the airplane is doing the anticipation for the pilot.

Like the transition in the late 1950’s from props to jets, fly-by-wire has been a major change for pilots. In general we welcome it for the many advantages it offers.

Experience has shown that to do his job, which is to ensure the safe arrival of his aircraft, the pilot must fully understand a much more complex airplane. Chesley Sullenberger reached up and started the APU (the Auxiliary Power Unit, a small turbine in the tail which can supply electrical and hydraulic power on the ground or in flight) as soon as his engines lost power. Why? Because he knew his airplane and he knew he wanted to keep it in Normal Law until touchdown.

The transition from props to jets was all about speed range, speed brakes and spoilers, high Mach number, coffin corner, Dutch Roll and super-stall, but in everyday life it was more about high drag on approach, no propwash, slow spool-up times, and operating on the back side of the power curve. This change took some adjustment on the part of pilots: the more experienced pilots had more adjustments to make. The same is true with the transition to fly-by-wire.

In a traditional airplane the pilot controls Angle of Attack with the elevator and the trim tabs or stabilizer. (More often he will be thinking of Airspeed, which is the constant-weight, 1G manifestation of AofA). He is used to feel, which is essentially the change in elevator neutral point with AofA. Should the aircraft slow on approach, the nose will get “heavy”, prompting him to pull back or trim nose-up.

That feel is totally absent in Airbus aircraft. (Boeing, in the B777, have added artificial feel to their fly-by-wire system). The Airbus pilot points and shoots, so to speak. Flying by hand he can take the bird, turning on a symbol (like a bird or an aircraft seen from behind) on his Primary Flight Display. The bird shows where his velocity vector is pointed; in other words, where is airplane will be so many seconds from now if he makes no further adjustments. On approach he can pin the bird on his flare point on the runway and either let the autothrust take care of the speed or adjust the thrust levers manually. If he does the latter, he must remember that there is no feel or feedback in the sidestick.

Obviously there are quite different assumptions operating during an approach in a Bonanza, one one hand, and an Airbus, on the other. This is not necessarily a bad thing. Take for example driving a car versus riding a motorcycle. In a car you steer with the steering wheel. In a motorcycle you counter-steer, putting pressure on the inside foot-peg and forward pressure on the inside bar, in effect trying to steer the front wheel the opposite way.

But you know you’re on a motorcycle and not in a car. You have learned how to ride a motorcycle.

Consider, however, flying an Airbus if something goes wrong with a sensor or a computer and you wind up in Alternate Law or Direct Law. You are in the same vehicle but suddenly the rules have changed; the assumptions have changed. It is, in effect, no longer the same machine. This is a recipe which messes with a pilot’s head.

Unfortunately, experience has shown that Direct Law, where control displacement is proportional to stick force and the airplane handles like a wet fish, is actually the more benign of the two degraded modes. There is a big message in red on the ECAM saying USE MAN PITCH TRIM. The pilot moves the THS (Trimmable Horizontal Stabilizer) by moving a wheel almost a foot in diameter. This is old-style, normal airplane flying, commanding AofA with stick force and trim. There is still no feel in the sidestick, but the procedure is familiar.

Alas, in Alternate Law there is no such familiarity. It is still point-and-shoot, sort of, but autotrim is still working. As long as there is back pressure on the stick the THS trims nose-up, and vice-versa. There is NO Stabilizer in Motion warning except the movement of the trim wheels. That would seem to be an easy thing to detect, but I can testify from personal experience that it is not. On every landing (in Normal Law) the flight control computers memorize the attitude at 50 feet Radio Altitude and at 30 feet start rolling in nose-down trim, in effect trying to mimic the feel of a normal aircraft slowing in the flare. In almost a decade of flying as Captain and Training Captain, whether as Pilot Flying or Pilot Not Flying, I cannot remember ever seeing the trim wheels move.

In two recent accidents an Airbus has hit the ocean with the THS wound to full nose up. In both cases the aircraft was in Alternate Law.

I am not an engineer. There are likely many ramifications that have not crossed my mind. But sitting here this afternoon my personal recommendation would be as follows:

Disable Autotrim in Alternate Law

Treasure in Tragedy: AF 447

The treasure in the tragedy of AF 447 is what it can teach us.

Thank you, David Learmount at Flight Global, for calling for dialogue. Thank you Wood’s Hole Oceanographic Institution and Bureau d’Enquêtes et d’Analyses for recovering data we can all work from.

Thank you, David, for offering to share the blame. All of us share the blame, all the way down to the passenger who wants a cheaper flight. But in the end, blame is irrelevant. Blame only serves economic interests which demand simple answers. These answers will not be simple.

Let us all be brave and try to speak truth as we see it. Let us all bring our training and experience to bear on the recovered data and mine it for lessons and solutions.

AF447 Transcript

David Learmount’s article today reveals two key weaknesses: one in the crew and the other in the aircraft.

The crew had not clearly declared who was flying the aircraft – in airline lingo who was PF and who was PNF.

Airbus sidesticks add algebraically. When the aircraft hit the water the left sidestick had some nose-down input and the right sidestick was at full nose-up. The Trimmable Horizonal Stabilizer (the horizontal tail) was already at full nose-up, most likely because of continuous nose-up sidestick applied while in Alternate Law.