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Stalls, Deep Stalls, and What We Can Learn From The Air France Accident

Blue skies with an occasional puffy white cloud and a slight breeze made this the perfect day to practice landings with my student training for his commercial pilot certificate. As my student taxied to the runway I opened my window and stuck my hand out scooping up the cool morning air. I kept thinking what a perfect day to fly. That would soon change.

After doing his preflight checks my student taxied into position to practice a short field takeoff which requires a constant airspeed until we clear an obstacle. Most airports don’t have many obstacles so we normally pretend there is a one hundred foot tall obstacle at the end of the runway.

Since the airport was surrounded by many tall trees I told my student to climb at best angle of climb until we clear the trees. Then we would accelerate to best rate of climb. Best angle of climb, or Vx, is the airspeed which will give us the greatest increase in altitude over a given distance. Best rate of climb, or Vy, is the airspeed which will produce the greatest increase in altitude over a period of time.

We taxied into position and he applied full power and accelerated quickly to rotation speed. Then he did the unexpected, he aggressively pulled back on the yoke pointing the nose skyward at a much higher pitch attitude than normal. It was fairly obvious we would stall if he kept the nose high but he seemed to do nothing.

This is when natural instinct conflicts with the pilots aeronautical knowledge. To go up the pilot wants to pull up but if he pulls up to far the aircraft will stall and then come down quickly. Therefore, I spend much of my time with my students teaching them that to gain airspeed and to reduce the angle of attack we must push the nose forward. A very difficult thing to do at low altitude but necessary.

My student succumbed to his natural instinct instead of his airmanship and pulled further back because we where not climbing. As I reached for the yoke the stall horn began blaring and the wing began to buffet. As I recovered from the incipient stall I told my student to look out in front of us at the trees. And then I calmly said we sometimes must point our nose at the trees to prevent ourselves from crashing into them. After gaining airspeed I then climbed out at best angle of climb speed and gave him back the controls.

After debriefing my student I realized he did not have much practice in recovering from stalls and incipient stalls. I explained to him that as a commercial pilot you must be better at understanding when you are approaching a stall and how to recover immediately. I only pushed the yoke forward slightly and we where flying again.

This is why with all my students I do many stalls and even more incipient stalls to get them in the habit of reducing the pitch attitude immediately when they feel a buffet or hear the stall horn. This is especially important for my commercial students who are moving onto the airlines and swept wing jets. Swept wing aircraft stall abruptly with few indications.

I have noticed a lack of understanding concerning stalls in the past few years. This is especially true with pilots who only want to get their ratings quickly and move up to the airlines as opposed to actually learning how to fly the plane properly. This is a training issue and can be solved through practice. Let’s review some aspects of stalls and what you can do to prevent yourself from getting into this same situation no matter what type of aircraft you fly.

Stalls:

When we hear the word stall we normally think about our car engine quitting. This is what some of us thought the first time we heard our instructor tell us we where going to practice stalls. Who in there right mind would want to fly and stall the engine in the aircraft?

After some introductory ground school we learned that a stall has nothing to do with the engine but more to do with the aircraft. An aircraft stalls when it exceeds the critical angle of attack resulting in separation of airflow over the wing and a rapid decrease in lift. The point that the airflow no longer can produce enough lift to sustain level flight is the point at which the airplane stalls.

It is important to remember the aircraft is stalled but portions of the wing are still producing lift. The coefficient of lift increases as we increase our angle of attack and at the maximum coefficient of lift we are at the critical angle of attack. Any increase in the angle of attack will see a dramatic decrease in the coefficient of lift but we still have some lift. This can be seen in the diagram below.

Most light aircraft will tend to pitch down when a stall has occurred due to the forward center of gravity. Since the tail is still effective and is not stalled we only need to push the control column forward slightly to recover from the stall.

Deep Stalls:

Some aircraft will enable us to continue to increase the angle of attack even after the wing has stalled. This is the case with most t-tail aircraft. As we pull back the stall becomes greater, or deeper, and eventually we may have trouble recovering. It might be impossible to recover if we pull the aircraft tail into the area behind the wing where the airflow is disrupted by the wing during the stall. This is a condition that has led to accidents in some light aircraft.

Much of my instruction was in a Piper Tomahawk which has a t-tail and has been known to get into unrecoverable stalls and spins. Due these accidents, stall strips were attached to the front of the wing to cause early airflow separation and a stall at a lower angle of attack. Larger t-tail aircraft such as airliners have a device called a stall shaker and pusher which prevents an actual stall and will reduce the angle of attack automatically without pilot input. The most important thing to remember when flying a large jet or any aircraft is to always be aware of your airspeed and react properly if you find yourself getting slow. This can be accomplished through training and practice.

Training:

I feel stall training and recovery is important in every aircraft you fly. To be able to recover from an inadvertent stall is very important. Stalls seem sudden when we are not paying attention but they normally develop gradually as we pull back on the flight controls. When an inadvertent stall occurs we rely on our past experience and training to recover properly.

I also feel it is just as important to learn how to prevent a stall because you cannot have a deep stall if you never stall the aircraft in the first place. That is why I practice many imminent stalls with all my students. I want them to have an immediate reaction of reducing the angle of attack any time they hear the stall horn or feel a buffet.

I will take my students up and increase the angle of attack until the stall horn or light comes on. Then we will reduce the back pressure slightly and the horn will go off. Then I will have the student apply enough back pressure on the yoke until it comes on. Next I will have them turn it off by reducing the back pressure. We will do this over and over again in many different configurations until the student automatically responds to the stall horn in the correct manner. It never correct to keep increasing the back pressure and the angle off attack.

Conclusion:

It is important to practice stalls and stall prevention at all levels of flying. Over the years it concerns me when I see pilots, especially commercial pilots, react incorrectly to an imminent stall. It is important that instructors teach all skill levels, from student to airline transport pilots, and validate the pilot has the skill and instinctive reaction when recovering from an imminent stall.

As you can see using the incorrect input of pulling back on the flight controls with great force can cause the nose of the aircraft, especially a t-tail aircraft, to reach a high angle of attack that it may cause a deep stall. If you feel uncomfortable with your ability to recover from a stall, you need to tell your instructor and review the procedures until the recovery becomes second nature. Do not be embarrassed to ask your instructor or check airman if you work for an airline to review the procedures with you. I”m sure they would be happy to help. Remember, a good pilot is always learning.

Safe Flying!

For More Information:

Stuck Mic AvCast Episode #2 : Air France 447, Deep Stalls, & LiveATC.net

USA Today: Air France crash calls for better pilot training, experts say.


{ 4 comments… add one }
  • Nalliah Thayabharan July 7, 2012, 7:19 pm

    This Airbus A330-203 did not have multiple independent systems for detecting speed of the aircraft such as a GPS based system that would at least cross check the readings being given by the pitot tubes and then provide a cockpit warning that the airspeed could be wrong, or another safety mechanism whereby the pitot tubes are heated as long as this would not impact the reading so that ice could not occlude them.

    The accident was caused by the co-pilot induced Deep Stall condition and remained in that condition until impact.

    To recover from deep 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 Deep Stall

    (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 Deep Stall

    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 A330-203 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 and turbulence and amid a tumult of conflicting warnings of mysterious system failures, 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 stabilizer, the airplane would have recovered from the stall and flown normally.

    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.

  • CarlValeri July 8, 2012, 11:35 am

    Certification of Transport Category Aircraft such as the 330 require separate pitot static systems to indicate airspeed. If they fail the GPS in the airplane can be used to estimate your airspeed based on the winds aloft and the ground speed.

    Most pilots are trained to use the pitch/power combination to achieve an airspeed for a specific aircraft configuration. I agree more recurrent training in loss of all airspeed indications is a good idea.

  • Nalliah Thayabharan August 1, 2012, 12:05 pm

    Air France complain that the pilots did not have enough time to analyze the situation. Gravity does not allow timeouts, so a round table could be called together to thoroughly discuss the situation to find out what went wrong? The pilots 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.
    Since then Air France changed the Thales pitots (which were already slated for change), its training, including cockpit resource management, as a tacit acknowledgement of its own fault. Further, the aircraft performed exactly as it was designed and described when the stall warning cut out at the end of valid values (extreme stall), except the 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 is clearly the airline’s fault. Of course, afterwards it is easy to make various arguments of how the situation could have been saved by others, but in case pilots do not or cannot fly by the book, the blame is solely theirs. Air France also argues that the stall warning system in the A330 is too “confusing”. Well, it must be realized, that an 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 miracles inside, which can look very confusing indeed, but it is the pilot’s duty to reign on them, or not to be pilot. You simply cannot be a pilot if you are only familiar with the fun part.
    With respect to the big confusion, the question is, was this stall warning device the straw that broke the back of the camel? In other words, if the pilots would not have had to remember just this one thing that the stall warning stops in extreme situations, then confusion would not have set in, and they could have perfectly saved the situation? Well, the A330 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 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 447 started, such as UA flight 232 at Sioux City, or Air Canada 143, 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 pilots of 447 to be confused in a generally much simpler and easier-to-fly aircraft.
    Some say the A330 is a “video-game” aircraft because of its side-stick control, which does not match up in real hard situations. But who can say that after the brilliant ditching of US Airways 1549 on the Hudson River? It was an A320 with the same side-stick control, and it matched up with the hardest situation very well, of course, with a seasoned pilot at the controls. The A330 is not a video-game aircraft, 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 pilots of flight 447, who seemed to be incapable to react even on a basic level to the phenomenon of the aerodynamic stall. Evidently, it might not be what Airbus had on its mind designing the aircraft. They might have meant the best of the two, 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 one 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.
    However, beyond the reasoning and explanations there is still some eeriness about the crash, taking in consideration that Air France is certainly no third-world airline, and still three of their 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, the arrival of the captain in the cockpit not making much a difference either. The question might arise whether weren’t they in a mentally incapacitating state of shock and disbelief? Whether do (or can) airlines test their pilots of how well they can keep their mental stability under the duress of a catastrophic situation? Wasn’t it a twist of fate unknown to anyone that three pilots prone to loose their cool and judgement in life-threatening situations got together in one cockpit and got into this situation, as stipulated by Murphy’ law, a true scourge of aviation?
    None of it seems to be the fault of the airplane, which seems to need only matchingly good, trained pilots to give superior performance for the good of the flying public.

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