Understanding High-altitude Aerodynamics Is Critical

 - May 14, 2012, 4:25 PM
Airbus A330 attitude director indicator (ADI)

Reports about the 2009 Air France Flight 447 accident released last summer by the French safety board (BEA) said the three experienced Airbus A330 pilots were unable to recognize they were operating at a too high angle of attack to sustain flight. The reports also said the pilots were unable to see a remedy early enough to recover. But why were these three international pilots confounded by the events of that night?

AIN decided to take a look at some high-altitude basics, with the thought of stimulating a discussion on this topic.

We begin at the “Coffin Corner,” also called the “Q corner,” where AF 447 was operating at the time of the accident. (“Q” is the designation for dynamic pressure). The corner is best described as high-altitude operations where low indicated airspeeds, yield high true airspeeds and Mach numbers at relatively low angles of attack. Surprisingly, high-altitude stalls occur at a significantly lower angle of attack than many once believed, thereby providing a much narrower maneuvering margin. The stall occurs at a lower angle of attack because of the altered dynamics of airflow at higher Mach numbers and compressibility effects.

The recommended maximum altitude on the flight management system provides only 1.3-g stall protection (the g-load is already 1.2 in a level 30-degree bank), which translates to razor-thin margins. The airplane’s climb capability here is a minimum of 300 feet per minute in stable air, although in practical terms it is often less. All of this normally occurs at the upper portion of the maneuvering envelope. Turning maneuvers at high altitudes can increase the angle of attack and result in a significant reduction in stability, as well as a decrease in control effectiveness.

The relationship of stall speed to critical Mach narrows at high altitudes, to a point where any sudden increases in angle of attack or roll rate and disturbances, such as clear-air turbulence, can lead to a stall.

Training in this region in the actual aircraft is certainly dangerous, as well as impractical. Simulator training, however, is sometimes not realistic enough. Although a sophisticated simulator can replicate a high-altitude stall, the data used to run the simulator reflects only the flight-test data made available for approval of the simulator. Beyond that point, it’s guesswork at best, and the software may not actually offer characteristics that accurately duplicate those to be found in the airplane. 

So, here’s our discussion question: “How should a flight crew train to best understand high-altitude operations?”

Please reply or comment to this question in the “Add your comment” section below.

Comments

Joe Illing's picture

Part of the training equation needs to include an understanding and the use of angle of attack indicators. These underutilized pieces of equipment need to be elevated to a higher status in the training environment.

The second part of the training equation is education. Understanding that compressibility is different in the upper flight levels, stalls may occur differently than trained in the simulator and the reasons behind why these truisms exist.

Those would be my modest suggestions.

Joe Duval's picture

To me it is all about the education, if the pilots understood more about aerodynamics and the differences between a high altitude stall vs. low altitude, they would probably be alive today. There is probably more information about aerodynamics in your article than a number of pilots truely understand. We work in flight test and the engineers always want us to be in that upper-left hand corner of the envelope. This is in air transport category airplanes. We train for it by immersing ourselves, studying the different aspects, and then training in a simulator before we do it in an airplane.

Tom Greene's picture

Know your aircraft is one place to start.
Do you know the minimum Mach number for the last 15,000 feet of your climb for the weight and temperature? Can you apply that number to a deck angle and power setting? I bet you might know the Hi-Speed numbers ???
Should the manufactures provide more data???
Training? Ask for the unknown from your simulator provider? Ask the flight test team to speak at you departmental meetings?
Train in Up-Set recovery in the simulators and approved aircraft.

don Bateman's picture

An excellent summary of the lack of fundamentals but your findings need to be expanded into a paper for a broad leadership of business and comercial jet pilots.

I recommend (if you haven't already) that you print your technical-operational safety paper in your wonderful monthly publication. I am amazed how many pilots read this great monthly journal---it is one of the very best in our industry for all pilots, engineers and maintenance technicans.

don Bateman

Randall Brooks's picture

The differences in high and low altitude stalls can be significant, as outlined by the article. However, the appropriate recovery technique is substantially the same in each case (reduce the angle of attack by lowering the pitch attitude). Unfortunately, decades of training focused on minimizing loss of altitude in carefuly scripted approach to stall maneuvers have not adequately prepared pilots for the diversity of stalls that can be encountered in actual operations.

Mauricio Ferrari's picture

I believe the answer are AoA and energy management. I have to credit my best sim instructor, Mr. Phil Hass, (ex-Navy pilot from Vietnam era) that in l986 gave me an introduction on AoA.
All these years I have not encountered a pilot that I remember, ex-airline or GA pilot, that use (understand) AoA on a regular bases. Also, I wonder why and how there are any certified aircraft that operates at a high altitude and doesn't have a dedicated AoA indicator available in the cockpit. FMS can give you performance based on weight, but the AoA will tell where the CG is...

Wayne Gorsek's picture

I have three jet type ratings including two single pilot in the Embraer Phenom 100 and 300. One is from Flight Safety and the other two are from CAE Simuflite. What I learned at APS in three days about stalls, upsets and recovery made the total 20 years flight training and three jet type ratings seem grossly inadequate. I strongly recommend all pilots but in particular jet pilots take this training and also do two hours in the ERJ sim there. It can save the lives of you, your passengers and people on the ground.
http://www.apstraining.com/

Wayne Gorsek
PIC and owner Embraer Phenom 300 N67WG

Sergio's picture

I recommend reading 1) Airplane Upset Recovery Higt (Flight Safety Foundation), 2)Boeing article http://www.boeing.com/commercial/aeromagazine/aero_03/textonly/fo01txt.html and 3) High-Altitude Upset Recovery by Fred George (Aviation Week)

philip rushton's picture

I was fortunate enough to train/fly in the military..(single-seat ground attack fighters). Although we spent most of or flying lives at a couple of hundred feet or below, we were 'drilled' with high-alt. aerodynamics and a not-inconsiderable respect for the definition and use of......AoA !

Situational awareness is a much-hackneyed phrase in the flying lexicon....but vigilance at high-altitude will go a long way to keeping the blue side up.

Robert P. Mark's picture

Isn’t it interesting that the Air France 447 A330 didn’t have an AoA indicator that was easy to read? My secret Airbus check airman pals tell me that on some aircraft it’s an option in fact.

How much easier could we make the transition from steam to glass – and back – in an emergency than an instructor explaining how to fly AoA with cruise N1, when everything else falls apart?

Do you think they will now?

Larry D'Oench's picture

Through 43 years of professional flying the only aerodynamic principles training I received outside of what I did myself was while at Pensacola. Perhaps now there'll be a more formalized approach to that aspect of our craft

Aluizio V. Fleury da Fonseca's picture

Back to basics, back to basics, training, training. In the begining, in the middle and
during high time flying.
Flying the basics will never ends, being part of the whole process.
Nice flights Happy landings for all

Rolland Sicard's picture

My instrument instructor had me do several sessions of partial panel flying. This should be mandatory along with SYSTEMS knowledge training. If properly trained in Aircraft Systems, the pilot would have known that Airspeed, Altimeter and Rate of Climb Indicators can all MALFUNCTION as they are based on a centralized sensor which is the Pitot mast which also incorporates Static pressure sensing for Altimeter and Rate of Climb. Merely holding the same Pitch Attitude and Power Setting and ignoring the three associated Pitot Static Indicators would have kept them out of trouble. It boils down to cross comparing cockpit indicators of Pitch Attitude, Airspeed, Altitude and Rate of climb and knowing at a SYSTEMS level the cockpit instrument systems.

Felix Hernandez's picture

There a many things that cannot be accurately depicted in the simulator. Windshear recovery comes to mind, because of varying factors. In training, there should be demonstrations showing different factors eg. with some turbulence, in a turn, erroneous speed indications just to see what it may look like.

Robert P. Mark's picture

I want to focus on Larry’s comment for a moment … “Through 43 years of professional flying the only aerodynamic principles training I received outside of what I did myself was while at Pensacola.”

I thought it was just me who’d been figuring out much of the aerodynamics I do understand on my own.

What about the rest of you here?

How much education have you had in high-altitude aerodynamics - or any aerodynamics for that matter - but at least before you began flying jets?

Come to think of it, I can’t recall an interview where anyone ever asked me anything about how an airplane flew … regs, ATC, weather … that sort of thing, but not aerodynamics.

Imagine if just before my first type-rating in the Citation, the guy had asked me to explain “coffin corner.” I’d have flunked.

Rob

Harold's picture

In a word; use gliders. They are inexpensive high aspect ratio airfoils and force pilots to use ONLY aerodynamics instead of engine performance to resolve angle
of attack issues.

I'll probably get lambasted for saying this but so what. 15,000 hours later my extensive glider training has served me well.

Harold's picture

As a glider pilot, we learned all about coffin corner, adverse yaw, etc., Glider pilots survive by aerodynamics and in America we have pushed this skill aside for V8 horsepower and the result is what we see such as the Colgan air accident.

R. Randall Padfield's picture

Harold, you are right on about gliders!

This is an excerpt from an AIN Blog I wrote in August 2011, following reports about the Air France 447 accident.

“Years ago, pilot/author Richard Bach wrote a short story about a mythical flight school based on the historical development of flight. As Bach related it, the students in this hypothetical school build and fly balloons and gliders, not unlike those built and flown by the early pioneers of flight. Then the students move to building engines and powered airplanes and flying them, too, continuing this step-by-step approach to more complex aircraft.

“Along the way, these budding pilots learn hands-on all the basics of flight, as well as the mechanical aspects of their aircraft. They learn to recognize when their aircraft is reaching the limits of its capabilities and can tell when the engine is running poorly by its sound. Their experiences serve them well, regardless of how heavy, fast or complicated the aircraft they eventually fly are. It was the ultimate flight training for real pilots.”

Bach’s idea is cool, but the concept is even more improbable now than it was the 40 or 50 years ago when he wrote the story. Nevertheless, the point is sound. As many others have said in so many words, pilots need good initial training and much experience in the basics and good recurrent training in both common and rare emergencies.

P Nelson's picture

Admitting that we don't 'know it all'. Re-learning basics. Quality training.
All of us come from different aviation backgrounds and have had different aviation experiences that we tap into throughout our careers. All of us have 'holes' in our knowledge and understanding, either by never learning a topic, never learning properly or having forgotten. The hardest thing for an experienced pilot to admit about any aviation-related topic is "I don't understand this". Most of us make it through a career with 'holes', never falling into them, or flying with someone that doesn't have the same holes. But for those valiant AirFrance and Colgane crews, they shared the same holes at the wrong time.

Richard C-Jones. England's picture

What a wealth of informed comment! After 50 years from Tiger Moth to Gulfstream via the RAF I am amazed that there now have to be special schools for upset training and situational awareness. Isn't that what pilots do to become proficient? I was fortunate to have had high altitude training in a single jet but not before a comprehensive aerodynamics course. 25 degrees of bank + hiccup = buffet. Ask any navy pilot about AoA, the unsung hero of aerodynamics. Not just for high speed jet jocks, it also shows the most efficient configuguration in cruise. It has kept me out of trouble when in the Q corner in turbulence and unable to raise ATC for a lower level. Rolland's comment on the number of systems needing pitot input is particularly relevent. Wasn't Sully a glider pilorichardc-j

Nalliah Thayabharan's picture

Once the airplane is stalled, it will lose altitude about 150 feet per second. The pilots have to unstall to stop severe altitude loss by lowering the nose by manually reposition the All Flying Horizontal Stabilizer (Trimmable Horizontal Stabilizer - THS) nose down. If close to the ground, reducing altitude loss would be of up most importance during the recovery. A stall at high altitude would allow a generous degree of nose down pitch and altitude loss during the recovery. Air France and other airlines need a serious review of basic aerodynamic facts and amend their stall recovery procedure.

The crash of Air France flight 447 into the Atlantic Ocean, killing all 216 passengers was caused by the co-pilot induced stalled glide condition and the airplane - Airbus A330-203 remained in that condition until impact. To recover from stall is to set engine to idle to reduce nose up side effect and try full nose down input. If no success roll the aircraft to above 60° bank angle and rudder input to lower the nose in a steep engaged turn. Pilots lack of familiarity and training along with system malfunction contributed to this terrible accident. Also the following contributed to the accident
(1)the absence of proper immediate actions to correct the stalled glide
(2) Insufficient and inappropriate situation awareness disabling the co-pilots and the captain to become aware of what was happening regarding the performance and behaviour of the aircraft
(3)lack of effective communication between the co-pilots and the captain which limited the decision making processes, the ability to choose appropriate alternatives and establish priorities in the actions to counter the stalled glide
During most of its long descent into the Atlantic Ocean, Airbus A330-203 was in a stalled glide. Far from a deep stall, this seems to have been a conventional stall in which the Airbus A330 displayed exemplary behavior. The aircraft responded to roll inputs, maintained the commanded pitch attitude, and neither departed nor spun. The only thing the Airbus A330-203 failed to do well was to make clear to its cockpit crew what was going on.Its pitch attitude was about 15 degrees nose up and its flight path was around 25 degrees downward, giving an angle of attack of 35 degrees or more. Its vertical speed was about 100 knots, and its true airspeed was about 250 knots. It remained in this unusual attitude not because it could not recover, but because the co-pilots did not comprehend in darkness, the actual attitude of the aircraft. The co-pilots held the nose up. If the co-pilots had pushed the stick forward, held it there, and manually retrimmed the Trimmable Horizontal Stabilizer(THS), the airplane would have recovered from the stall and flown normally.

Air France complained that the copilots did not have enough time to analyze the situation. Gravitational stalled glide does not allow timeouts, to thoroughly discuss the situation to find out what went wrong. The co-pilots – 37 year old David Robert and 32 year old Pierre-Cédric Bonin missed the cardinal rule that first they must fly the airplane, and after start analyzing the situation, since a falling airplane is not going to wait for them. If they did not understand the instruments, then instead of pondering on it they should have come to the quick conclusion that they did not understand those instruments, and apply the unreliable airspeed procedure clearly prescribed for that situation, which is a blind, given thrust and pitch setting for the given configuration, and let the airplane fly itself, and only after get to analyzing what went wrong, and by the time they finished, the root-cause (pitot icing) would have probably cured itself. It was the safe solution to the problem, but not applied.
The Airbus A330 performed exactly as it was designed and described when the stall warning cut out at the end of valid values, except the co-pilots did not know it. Unfortunately, it happens too often with catastrophic results that pilots are not familiar with the systems of their own airplane, such as in the case of American Airlines 587 over Queens, which was clearly the airline’s fault.
Air France also argued that the stall warning system in the A330 is too “confusing”. Every modern airplane is quite a confusing piece of machinery. It is full of buttons, levers, all kinds of red, yellow, green lights with buzzers, and a host of other indicators and controls inside, which can look very confusing indeed, but it is the pilot’s duty to reign on them, or not to be pilot.
Airbus A330-203 is a new generation, highly automated piece of equipment with drastically simplified controls, displays, and instrumentation compared to older models. Still, pilots with the same human capabilities as the ones on Air France flight 447 could very well stay in full control in those planes, and many times acted heroically saving situations much graver than where the plight of Air France flight 447 started, such as United Airlines flight UA232 at Sioux City, or Air Canada flight AC143, the Gimli Glider. If those pilots could perform well in those older, much more complicated aircraft in tougher situations, then there is no excuse for the co-pilots of AF flight 447 to be confused in a generally much simpler and easier to fly aircraft.
The Airbus A320 is a digital fly-by-wire aircraft as the flight control surfaces are moved by electrical and hydraulic actuators controlled by a digital computer. The computer interprets pilot commands via input from a side-stick, making adjustments on its own to keep the plane stable and on course, which is particularly useful after engine failure by allowing the pilots to concentrate on engine restart and landing planning. Some say the Airbus A330 is a “video-game” airplane due to its side-stick control, which does not match up in real hard situations. But who can say that after the successful ditching of US Airways flight 1549 on the Hudson River? It was an Airbus A320 with the same side-stick control, and it matched up with the hardest situation very well with an experienced 57 year old Captain Chesley Sullenberger at the command. The Airbus A330 is not a video-game airplane, it is the airlines that make it a video-game by cutting corners, taking advantage of its superior automated capabilities thinking that it flies by itself, and no training and no knowledge of even the basics of the principles of flying is required in them for their pilots, as was demonstrated by the co-pilots of flight 447, who seemed to be incapable to react even on a basic level to the phenomenon of the aerodynamic stall. The co-pilots had not applied the unreliable airspeed procedure. The co-pilots apparently did not notice that the plane had reached its maximum permissible altitude. The co-pilots did not read out the available data like vertical velocity, altitude, etc. The stall warning sounded continuously for 54 seconds. The absence of any training, at high altitude, in manual airplane handling and in the procedure for ”Vol avec IAS douteuse” (Flight with questionable Indicated Airspeed) caused this terrible accident. Evidently, it might not be what Airbus had on its mind designing the aircraft. They might have meant the best of the both, an airplane with superior controls, matched with seasoned pilots with superior education in the principles of flying and the handling of hard situations, best of the best, as airlines are prone to boast of their flying personnel, to represent quality improvement in flying safety by this pairing. Now, if this piece of equipment falls in the hands of the airlines who use it as a video game to save training costs, telling only their pilots that “if the red light on the right side blinks, just pull the stick back as hard as you can, and let the system do the rest”, they can get away with it as long as everything is normal, the airplane is good enough for that, but in unforeseeable situations, such as the flight 447 en-route to Paris on that night, without any independent knowledge of flying in general, the video-gaming with the aircraft may ultimately come to a fatal end.
Beyond the reasoning and explanations there is still some eeriness about the fatal crash, taking in consideration that Air France flight 447′s pilots just sat there in daze squeezing the control stick, barely being able to do more than commenting on how the airplane was falling out of the sky until crashing into the Atlantic Ocean, the arrival of the 58 year old flight captain Marc Dubois in the cockpit not making much a difference either. The question might arise whether weren’t the pilots in a mentally incapacitating state of shock and disbelief? Whether do or can Air France test pilots of how well they can keep their mental stability under the duress of a catastrophic situation? None of it seems to be the fault of the Airbus A330, which needs only good, trained pilots to give superior performance for the good of the flying public. Very similarly 3 decades ago Captain Madan Kukar’s mistaken perception of the Air India Flight 855 situation resulted in causing the Boeing 747-237 to rapidly lose altitude and the airplane hit the Arabian Sea at 35 degree nose-down angle.
Practicing recovery from “Loss of Control” situations and improve flight crew training for high altitude stalls (simulator training usually has low altitude stalls which are significantly different due to energy status of the aircraft) should become the mandatory part of recurrent training.

Phil Stradling's picture

I read that approaching areas of turbulence in an Airbus 330/200 fuel should be pumped back to restore full longitudinal stability - was this done on AF447 and if not did it add to the pilot's problems?

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