I believe that there is a basic flaw in the mishap investigation report by the UK AAIB. The flaw is a lack of inclusion of important take off aerodynamics procedures in the investigation, due to referencing solely the events surrounding EFB procedure errors. [Did the board seek the Safety Purpose or the Legal Purpose? See the end of the article for more on that argument.]
Here is the basic flaw in the investigation. The tail scrape occurred because the crew raised the deck angle to take off climb attitude instead of raising the deck angle to lift off angle. This is a mishap due to a misunderstanding of aerodynamics, rather than the entry error in the weight and balance procedure. If pilots reading this article remember this very important point, they will be able to prevent tail scrapes or tail strikes even in the circumstances where there has been an underweight error made in the weight and balance procedure. Understanding the aerodynamics going on during the take off procedure will allow a flight crew member to easily overcome these errors and not damage the aircraft. Remember any damage on take off could lead to a compromise of the pressure vessel, which could lead to pressurization failures and even structural failure. So in my opinion it is very important that the UK AAIB considers reopening the investigation and delve into take off aerodynamics. In lieu of that happening, I have written this article to cover the important aerodynamics involved.
Take off really requires several steps. The first is the lift off rotation and the second is the rotation to take off climb. With our very powerful engines today, these two steps most often blend into one smooth step since the aircraft accelerates so quickly, even at heavier weights.
But in fact there are two separate steps and if flight crew member understands this, they will be able to both recognize that the lift off attitude, that is the deck angle read on the EADI, will be the same at any weight. By setting this deck angle and waiting for the wing to develop lift equal to weight at the correct speed for the weight, the crew will never drag the tail on the concrete. It is critical to recall that the wing lift is what raises the aircraft off of the runway. The thrust from the jet engines seems like the exhaust from a rocket, but if crew tries to raise the nose to “launch the aircraft into the air” with jet engine exhaust, much like a rocket takes off and if the wing needs more lift to raise the weight into the air, the rotation will not produce lift, but rather only drag the tail. Remember that raising the nose or deck angle up to the climb angle should never be done until the aircraft is airborne, that is wing borne, is lifted off the runway, hence my terminology “the lift off angle.” So, let us look again at a wing trying to produce lift with an aircraft heavier that it was thought to be.
The wing of the aircraft, any aircraft, flies when it attains the correct lift, a product of coefficient of lift and indicated airspeed creating a pressure differential spread over the wing’s surface area. The coefficient of lift itself is based on the shape of the wing and the angle of attack of the wing in the airflow. When the coefficient of lift combined with the indicated airspeed spread over the wing surface is sufficient, the wing will produce a lifting force equal to the aircraft weight, the aircraft will rise up off the wheels and onto the wing and flight will be achieved. This is the lift off rotation from the wheels and onto the wing.
Just pulling the nose up at any speed, thereby creating angle of attack above optimal, is therefore never the correct procedure for lifting off of the runway into the first flight on the wing. The angle of attack is created by the deck angle setting the wing angle of attack when the indicated airspeed is achieved. You can not yank the nose up at a slower indicated airspeed to get lift because the angle of attack will be excessive and lift will be less than aircraft weight. Therefore just pulling the nose up high past rotate angle and into climb angle is never the correct take off procedure. The weight will not yet be lifted onto the wing and therefore further rotation will just drop the tail to the runway and scrape it along at a very fast speed and with a very strong force. This is not good and may do a great deal of damage. This may be hard for crew members to remember especially if their previous training has been on smaller longitudinal axis aircraft such as a trainer. Since transport aircraft tend to be long bodied, the initial lift off rotation and take off climb procedures based on separate deck angles is so important to learn and remember.
The error that this crew made was pulling the plane into the air early at a lower than required indicated airspeed, to a deck angle well above the initial take off angle and up to the higher take off climb angle.
The lift off angle is often low, around 5-8 degrees. Then after the wing is producing lift and lifting the aircraft off the wheels and initially into the air, raising the nose slowly up, further to 15-20 degrees sets the climb angle and the take off climb. There rotation brings the entire aircraft into the air, not just the nose off the ground and the tail into the concrete.
Why was the mishap crew procedure an error? Was it because they may have been “trained” with incorrect practices? Moreover was there another error that this airline made either by teaching or allowing their flight crew to use incorrect and unwritten and unapproved “practices” (set by who knows who), instead of employing the correct, approved and published procedures? In my opinion written Procedures, not generally accepted Practice, is the key to safety and success.
Remember that there is a separate and lower lift off deck angle, the first-part-of-rotation angle, an angle meant purely for lift off of the aircraft from its landing gear and up onto its wing. Once this flight has been achieved, through the production of lift equal to weight by the wing, the aircraft can next accelerate and then climb upward on the wing. These two steps, initial lift off rotation from the wheels and onto the wing and take off climb on the wing, are all really separate aerodynamic events, even if we blend them into one smooth take off rotation procedure due to the rapid take off acceleration of powerful high bypass ration turbo fan engines.
In my opinion, it is a major flaw in aerodynamics to assume that raising the nose at any speed will be sufficient to achieve flight through lift. It appears that this flaw was not identified in the mishap investigation report. If it was in there, I did not find it. The flaw of the airline or the crew members is not uncommon. However, wouldn’t this information from the mishap board investigation identifying the procedural flaw, be of value to other pilots? In my opinion this a major mistake of omission and I believe a correct accounting of the correct take off procedures should be offered here.
Every flight crew member flying all transport aircraft would be better off if they knew the basics of aerodynamics related to take off, so that they could prevent scraping the tail on takeoff, even in the hazardous event the take off weights are given wrong, calculated wrong or entered in the flight management computers wrong.
Here is the correct procedure in my opinion and in my opinion it is the same or very similar for all transport aircraft:
1. After application of whatever takeoff power is chosen and as the aircraft approaches rotate speed, the crew should rotate to and then stop the rotation at the lift off deck attitude and do not go any higher. Stop here, hesitate here, be ready to remain here in the event that the aircraft is not yet at the correct lift off speed. If you do this, you will never ever drag the tail of the aircraft even if the weight and balance are wrong. The speed will continue to accelerate and momentarily will be traveling fast enough for lift off to occur on the wing. If the take off data, the rotate speeds and take off speeds are correct and if the wing camber (flap setting) is correct, the wing will achieve the take off coefficient of lift at take off speed, as the angle of attack lowers to the lift off angle of attack and the aircraft will lift off the wheels and onto the wing through the production of lift. Again, if the speeds are slow, it will be only by 15 to 20 knots at the most, and with large turbo fan engines, this 15-20 acceleration will take place within a few seconds, in my experience.
2. At that point the wing will lift the aircraft off the ground, because the lift is generated at the correct angle of attack and camber creating the take off coefficient of lift, at the correct indicated air speed. (This is not what is called ground effect, wherein the downwash angle is reduced by close proximity to the ground, thereby reducing induced drag and allowing acceleration and more lift from the aerodynamic force.)
Rather, this is actually the wing producing lift to get the weight of the aircraft up and off the ground. This is a very important step to recognize as a separate event, a separate and distinct part of the lift off take off procedure. It may very well occur at an indicated airspeed above what you may have expected, but not to worry. When the lift is being produced to lift the aircraft into the air, it will occur due to the wing flying at the proper speed and angle of attack. This is how you can overcome a weight entry error and not drag the tail. This is critical training information that every transport pilot should know and be trained on.
3. As the wing lifts the aircraft off the ground and out of ground effect, the feel of pitch control movement will demonstrate an increase in lift as pitch is increased. This is really important and critical to understand. If the aircraft is flying in ground effect and is not at the take off coefficient of lift, any increase in pitch will only result in aircraft longitudinal axis rotation but no increase in lift and will not result in any upward movement of the aircraft due to lift. This is the tell-tale indication that the indicated airspeed is too low for the actual total weight of the aircraft in my opinion and experience. This is trouble and the best thing to do is hold the stick steady, hold the rotation angle steady and wait for more speed. Many pilots refer to a feeling of a mushy control column in this situation. Be very careful here and be patient for the aircraft to accelerate to a higher speed. Patience and a light touch on the controls will serve you well at this point. Do not panic and do not yank the nose up, but rather wait, wait for the continuation of acceleration by the engines. Remember that the wing can fly at a range of speeds and weight, but only at the correct angle of attack needed to create sufficient coefficient of lift when the speed is at the minimum for flight and then decreasing angle of attack as the speed increases. So, wait for that correct angle of attack to lower onto the wing before moving the stick back any further.
4. If the situation occurs that the aircraft is airborne solely in ground effect, no further rotation should be attempted until an additional increase of 15-20 knots of indicated airspeed has been achieved. Wait, wait for more speed. At that point the aircraft should be going fast enough at the correct angle of attack to begin to rise from the runway, to lift off out of ground effect. Another take off rotation attempt can now be made. This 15-20 knot increase should move the aircraft closer towards the correct take off speed, the correct angle of attack, the correct coefficient of lift and the correct lift from the wing for flight. Remember that at this point you are not sure what kind of weight error has been made, but you do not need to worry about that at that moment. Just wait for the angle of attack to lower to climb angle of attack, then you can gently raise the nose.
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In my professional opinion, the mishap crew just over rotated the nose of the aircraft up to the climb deck angle and into ground effect by not hesitating long enough at the lift off rotation deck angle. The coefficient of lift was too low for flight because the speed was too slow for flight and the angle of attack was too high. Therefore the lift being generated was too low to climb, even though it was sufficient to allow rotation of the nose up and the tail down, with the main gear as the fulcrum of the lever. When the crew rotated the nose up to a higher angle of attack to attempt raise the coefficient of lift and lift, it appears that the crew did not understand the basic aerodynamics related to take off flight in that lift off angle of attack was not yet achieved.
Did not knowing what to do next, when faced with incorrect rotate speeds and take off speeds at rotation result from the use of “common practices” in lieu of standard operating procedures? Is scraping the tail the result of rotation to lift off angle or to climb angle?
If the crew had rotated the deck angle to the correct lift off deck angle and then waited for whatever indicated airspeed was needed to produce lift at whatever weight the aircraft was actually at, and not tried to raise the aircraft into the air to climb, by rotating above that deck angle, this mishap could have been avoided, it would have never happened and the data error would have been the only problem the airline needed to deal with. In the same manner future take off over rotation mishaps can be avoided, even when the incorrect take off weight is entered in the EFB or other weight and balance calculation error occurs during preflight procedures.
My guess is that during a career in commercial aviation, many crew members are going to be faced with an incorrect, over-weights or other weight and balance issues due to human error, human factors. But by knowing how to deal with the aerodynamics of take off, and not by just doing a rote “this is how we do it here” practice, they will have the keys to success in any circumstance. In my opinion, during a 40 plus year flying career, you are likely going to see just about one of everything. But if you are prepared by training, you can overcome the hazard and continue to fly safely.
In my opinion this crew and perhaps crew members at other airlines need more takeoff training. This company and perhaps other companies needs more detailed take off procedures. In my opinion this mishap board needs to redo the investigation with a much more detailed investigation of the basic aerodynamics related to take off. I believe that an investigation of errors in the calculation or entry of take off data in the Electronic Flight Book was insufficient to both explain this mishap and explain how to avoid the occurrence of this mishap again, anywhere.
If you were to ask me, this mishap board sought to answer the legal question, who was at fault for the damage and who is to pay. In so doing they failed to answer the Safety Question, “How did this mishap happen and what can be done to keep this mishap from happening again?” In my opinion, Mishap Investigation Boards, Accident Investigation Boards should be called upon to serve the Safety Purpose of making commercial aviation safer, and not served the Legal Purpose of finding blame and determining who pays?
Feel free to question any of my arguments. I’d be happy to respond. It is critical that we all learn something from these mishaps through the mishap investigation reports and the Safety Purpose.
You can send me an email: paulmiller@safetyforecast.com. I look forward to your correspondence.
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