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High Altitude Stalls \u2013 how well do you understand them?<\/strong><\/p>\n By Captain Shem Malmquist<\/p>\n Acknowledgements<\/strong><\/p>\n Credit for the impetus of this article must be given to my friend, aerodynamicist Clive Leyman, who initiated a discussion on these issues. \u00a0He provided the technical foundations, including correcting and clarifying portions of this article.\u00a0 Portions of this article are based on a paper written by Clive Leyman, which have been revised as necessary for a more general pilot audience.<\/p>\n Stalls and modern wing designs<\/strong><\/p>\n There has been much written in the aftermath of the Air France 447 accident regarding aircraft stalls, pilot training and similar aspects.\u00a0 In a previous article<\/a> I outlined some of the cognitive aspects that were likely involved with the accident.\u00a0 Many pilots have wondered why the Air France crew did not recognize the stall itself.\u00a0 In this article I will explore some of the aerodynamic effects involved in high altitude stalls which can make the problem much more complex than many pilots might realize.<\/p>\n Modern airliner wings have been designed to minimize drag at the design cruising Mach number.\u00a0 The aerodynamic design of the wing is based on the necessity to reduce wave drag and lift induced drag, and may be modified to reduce wing bending moments and weight.\u00a0 These compromises mean that the way the air starts to separate as the angle of attack approaches the stall, and what forces are generated as it does so, may be significantly different than what most pilots are expecting.<\/p>\n As outlined in the figure below, on a typical airliner wing, the air will start to separate about 2\/3rds of the way from the root to the tip.\u00a0 It starts on the aft portion, so the forward section of the wing is still developing normal lift.\u00a0 This results in a gentle pitch up.<\/p>\n As the angle of attack is increased, the separation will move forward and across the wingspan, but these separations are still aft of the CG, so the aircraft will continue to have a gentle pitch up.\u00a0 It is only after the inboard part of the wing stalls that there will be any pitch down at all, but in reality, this will likely just appear as a cessation of the pitch up.<\/p>\n Modern wings are designed to be \u201csupercritical\u201d, meaning that they are designed such that during normal cruise over a large part of the wing upper surface the airflow is supersonic, decelerating through a shock wave lying about two thirds to three quarters wing chord.\u00a0 It is across this shock wave that the initial airflow separation will most likely begin, and that is near the trailing edge of the wing. \u00a0This will be perceptible as buffet. \u00a0Additionally, some of the lift is produced by positive pressure on the lower surface near the trailing edge.\u00a0 This has the effect of increasing the lift on the wing with increased angle of attack for quite some time after the air flow on the upper surface as begun to deteriorate.<\/p>\n The experience most pilots had in primary training is a bit different. \u00a0In most trainer aircraft during the stall the airflow separation results in a loss of lift early in the process. As the airflow continues to deteriorate there comes a point where there is a fairly significant pitch downwards known as the \u201cstall break\u201d which is coupled with a simultaneous significant loss of lift. \u00a0The buffet is significant, and very obvious.<\/p>\n By contrast, the modern airliner wing will still see the lift increasing somewhat\u00a0after <\/em>the point that the pre-stall buffet has occurred.\u00a0 This may, or may not coincide the the AoA at which the stall warning is triggered, with choice of that point left to the designer. Beyond that pre-stall buffet the lift goes on increasing very slowly (from the bottom surface flow), but the buffeting gets steadily worse.\u00a0 At some point there is a change in the character of the buffet (magnitude and frequency) accompanied by a loss of lift. Taken together these may define \u201cstall\u201d, but it is difficult to identify the exact point without recourse to instrumentation. This is quite unlike anything met in training.<\/p>\n A word about high speed buffet<\/strong><\/p>\n In addition to low speed buffet associated with the stall, many pilots also have read about, or been taught that the aircraft will experience a \u201chigh speed\u201d buffet if they fly above the maximum mach speeds.\u00a0 It is possible that some pilots might be concerned with lowering the nose as they might believe they are entering \u201ccoffin corner\u201d, and lose control of the aircraft.\u00a0 In reality, while this was a factor in early jet transports, it is generally no longer the case in modern designs due to aerodynamic improvements. \u00a0From the accident report for Air France 447, page 42-43:<\/p>\n Pilots consider that in-flight overspeeds constitute a serious risk. This perception of the risk has a number of origins:<\/i><\/p>\n \u0088 Flying theory training (notably during ATPL):<\/i><\/p>\n \u2013 the danger of a \u201cshock stall\u201d is considered on a par with the more classical \u201clow-speed\u201d stall<\/i>;<\/p>\n -the dangers associated with high speed (e.g. the onset of flutter, or the tuck-under effect(2)) are presented even though modern aircraft generally no longer suffer from these characteristics, which could indeed be hazardous on some older-design aircraft;<\/i><\/p>\n \u0088- VMO\/MMO corresponds to an important limit in the performance and limitation curves, for airline pilots; although the \u201cclassic\u201d stall is perceived as being fairly well known, and is experienced by pilots (at least during their initial training), excursions well above the VMO\/MMO are not demonstrated in training<\/i>.<\/p>\n It would take a very large excursion above Mmo before adverse effects, and most such effects would be issues with flutter or hinge moments with large control inputs. \u00a0Any buffet experienced is almost certainly going to be due to pre-stall or stall buffet. \u00a0Also, high speed buffet will feel different (see discussion below), likely higher frequency and less likely to incite the aircraft\u2019s natural frequency.<\/p>\n Stall identification<\/strong><\/p>\n The JAR (see Annex) and FAR rules specify that; acceptable indications of a stall are \u2013<\/p>\n As previously described on many modern wing designs the airflow separation will slowly spread outwards and forwards from the initial point.\u00a0 This means that any changes in pitch or roll from the approaching stall can take place over a relatively long period of time (depending on the rate the AoA is increased), and there are no sudden indications.\u00a0 This can somewhat mask the approaching stall, which can then only be identified through the severe buffeting criteria.\u00a0 Further complicating this is that the initial buffeting could be mistaken for turbulence, as appeared to be the case in the other events described in my article on cognitive bias<\/a>. \u00a0It is possible that mountain wave action or turbulence can induce a stall warning and possibly even pre-stall buffet at high altitude, although it is unlikely to cause an actual stall.<\/p>\n To further elaborate on what the buffeting might be like for the pilots in a large transport, consider the following charts derived for the Air France 447 accident, notice the amount of buffet in both vertical and lateral, with almost plus or minus 2 g\u2019s vertically and half a g laterally, and at a fast cycle of around 2-3 times per second! The airplane fuselage\u2019s natural frequency will have a great influence here.\u00a0 Boeing test pilots have described it as akin to driving sideways across railroad tracks \u2013 and I don\u2019t mean tracks embedded in pavement, think driving sideways across a railway yard at 20 mph with no shock absorbers!\u00a0 If you were experiencing that what would YOU think was happening (absent prior knowledge of it)?\u00a0 What does an airplane feel like as an engine is falling off or some other sort of structural damage?\u00a0 Put yourself in the situation with zero prior knowledge (which you now have).<\/p>\n The first chart is lateral (Nyf) and the second vertical (Nzf), and these represent the values that were experienced at the location of the flight deck:<\/p>\n \u00a0<\/p>\n \u00a0<\/p>\n \u00a0<\/p>\n \u00a0<\/p>\n \u00a0<\/p>\n <\/p>\n <\/div>\n <\/div>\n One more point.\u00a0 If you do find yourself in buffet, high or low speed, and you really<\/em> do not know which it is, it is probably best to \u201cpush\u201d either way.\u00a0 That is obvious if it is low speed, but why high speed?\u00a0 Because you will descend away from the buffet boundary.<\/p>\n \u00a0<\/p>\n Air France 447<\/strong><\/p>\n AF 447 was in cruise flight at FL 350.\u00a0 The Captain had chosen to take the \u201cmiddle\u201d nap, which is typical.\u00a0 Unless unusually tired, most Captains will take a turn in the middle of the flight so they can be present for the more complex procedures during the first and last portions of the flight.\u00a0 This can be altered, of course, depending on when they are tired, the Captain might choose the first or last period also.\u00a0 The area of weather was still about 80 miles ahead of them when the Captain went back to take his nap.\u00a0 Thunderstorms are typically somewhat numerous crossing the tropics, and it is probable that the aircraft radar was not depicting anything of significance that far out.\u00a0 Furthermore, as I have outlined in other articles, thunderstorms in the ITCZ region do not depict in the same way as storms at higher latitudes, particularly at night over the water.\u00a0 The reflective portion tends to be much lower.<\/p>\n While some have questioned the decision of the Captain to take his rest at that time, it is not so surprising given the information present.\u00a0 The radar was not depicting anything much (as I have described in previous articles here<\/a> and here<\/a>) all all looked typical.\u00a0 Further, it was likely that there would be more storms over the next several hours as they continued across the tropics.\u00a0 Waiting was unlikely to improve things.\u00a0 Personally, I would have chosen the first or third rest period just because I do not sleep well in turbulence and I have a bit more training with radar usage than many, but it is hard to second guess this Captain\u2019s decision.\u00a0 This left the First Officer in the right seat, as the flying pilot, and the relief First Officer in the left seat, as pilot monitoring.<\/p>\n When the aircraft encountered an area of high altitude ice crystals resulting in the loss of airspeed indications the autopilot and autothrust disconnected, unable to function without airspeed.\u00a0 With the loss of airspeed came a multitude of various warnings as each system that utilized airspeed in some way indicated a problem.\u00a0 Furthermore, as the system incorporates what is known as integrated modular avionics, these failures propagate across the other computers in very unpredictable ways, a cascading series of faults, with each box now receiving bad information sending its failure messages out.\u00a0 The flight controls reverted to a degraded state where the pitch mode was still working normally in terms of response, but no longer had any of the stall protection features.\u00a0 Meanwhile, the roll mode went to \u201cdirect law\u201d. In \u201cnormal\u201d operation the pilot would have become accustomed to the Airbus FBW mode where stick movement commands roll rate and centralizing the stick gives zero roll rate (holds the bank angle). However in direct law stick movement commands roll acceleration and centralizing the stick leaves the pilot with a residual roll rate. To cancel that one must apply opposite stick motion. \u00a0The A330 is also a surprisingly nimble airplane, and it is easy to over-control the roll when in direct law at altitude.\u00a0 It is a bit like going from flying a big jet to a Learjet, instantly.\u00a0 Few pilots would not \u201covercontrol\u201d at least initially.<\/p>\n The pilot found himself flying an aircraft at altitude by hand.\u00a0 Due to the lower dynamic pressure and higher true airspeeds for the same equivalent airspeed (EAS), there is less damping at that altitude, so not only is the aircraft flight control system in a degraded state that is not normally seen outside of a demonstration in the simulator during initial training, but it was in a flight regime most pilots today have never \u201chand-flown\u201d an aircraft at due to RVSM rules.<\/p>\n With such a sudden change in aircraft dynamics coupled with the low damping at altitude, it is not surprising, then, that the pilot was focused on trying to keep the wings level, which was occupying a good deal of his ability.\u00a0 It also would not be unusual for a pilot to be subconsciously pulling a bit with each lateral control input.\u00a0 Furthermore, the flight directors, which were biasing in and out of view, were commanding a pitch up as a consequence of the altimeters now showing that they were 400 feet low with the loss of pitot input.\u00a0 Exacerbating this, pilots today often see a lot of emphasis on staying on the flight director.\u00a0 Even assuming the indications are correct, attempting to stay right on the flight director bars at altitude becomes what the test community calls a \u201ctight tracking task\u201d.\u00a0 Tight tracking tasks are utilized in flight test to elicit PIO.\u00a0 Not ideal.<\/p>\n The pilot pulled the controls back enough to increase the g-force momentarily.\u00a0 This led to an increase in angle of attack to the stall warning threshold.\u00a0 The stall warning responded with a momentary \u201cStall, Stall\u201d, but discontinued before the \u201ccricket\u201d tone was generated.\u00a0 The monitoring pilot in the left seat asked \u201cWhat was that?\u201d, but beyond that, the Air France crew did not discuss this momentary indication.\u00a0 Did they just attribute it to turbulence?\u00a0 Based on the lack of any secondary indications, it is very possible that they assumed that the momentary warning was connected with the lack of airspeed indication.\u00a0 This is a training problem, as modern stall warning systems on transport aircraft utilize angle of attack.\u00a0 However, the aircraft continued to fly normally.<\/p>\n Although not discussed by the crew or mentioned in the BEA report, flight tests reproducing AF 447 clearly showed buffet.\u00a0 It is hypothesized that the pilot perception of buffet may be strongly linked to fuselage flexibility, so the g-forces generated by the buffet might not represent what the pilots are actually experiencing.\u00a0 These might lead to a pilot mistaking buffet for turbulence during the initial stages.\u00a0 It is also possible that, while in the turbulence, the buffet is somewhat masked, or perhaps not as salient as the turbulence itself. \u00a0As the buffet became severe it is possible that it seemed like some sort of unknown structural failure as discussed above.\u00a0 It would absolutely be outside of anything most pilots had experienced at that time.<\/p>\n Many have wondered why the crew might have ignored the stall warning in the first place. It is likely that they viewed it as a false or spurious warning.\u00a0 Perhaps they just assumed it was another system failure associated with the loss of airspeed, as many pilots incorrectly believe that the stall warning might be impacted by loss of airspeed indications. Several crews reported that in previous probe icing events they had a single stall warning sound but ignored it as being \u201ca blip\u201d\u00a0 Regardless, this likely had an effect on the subsequent stall warnings, as research has shown that when a system warning is perceived to be false once (accurately or not), people will ignore subsequent warnings. \u00a0As they continued to slow, the aircraft entered a stall again.\u00a0 Again, it appears that the warning was still not accompanied by salient secondary indications, or, at the very least, as described previously, not the secondary indications most pilots have been trained to expect.<\/p>\n In any event, it is clear that warning was essentially ignored from that point onwards.\u00a0 No other discussion or mention of it occurred, even when it was repeatedly calling \u201cStall, Stall..cricket\u201d\u00a0 The warning was just noise at that point.\u00a0 If there was buffet at this point it would be easily masked by the turbulence as they flew through the tops of thunderstorms.\u00a0 The aircraft would be experiencing a gentle pitch up due to aerodynamic factors discussed earlier, but the A330 FBW system would have just kept the pitch constant. \u00a0Further, the system trimmed the stabilizer to the full up position.<\/p>\n The stall warning continued for the next two and a half minutes.\u00a0 Clearly they would have heard it so why did they not react? \u00a0Again, the most probable explanation is that they considered it a false warning.\u00a0 The aircraft AoA continued to increase, and that is when the shaking would have gotten to be severe.\u00a0 That coupled with the sound of the ice crystals may be why they expressed concern about overspeed.\u00a0 The airplane entered a \u201cdeep stall\u201d.\u00a0 The airspeed become so low and angle of attack so high that the stall warning system stopped based on the assumption that the combination would be a false indication.\u00a0 At that point, a relatively dramatic nose down pitch would have been required to recover, and with full nose up trim, even full forward control stick would result in just a very slow pitch over.\u00a0 Transport aircraft rarely see nose-down pitches over a few degrees in normal operations, but after reaching this point, the aircraft would have required something in the neighborhood of 15 degrees nose down to start a serious recovery.\u00a0 Coupled with this, though, was that as the aircraft finally fully stalled it started to descend.\u00a0 Fast.<\/p>\n The descent rate resulted in the measured g-force falling to around 0.6 g, vacillating between that and .75 g.\u00a0 Pushing forward on the controls under normal circumstances to get to 15 degrees nose down would be outside the experience of most pilots.\u00a0 How many pilots would still recognize the need when they were subject to g-forces where they felt they were already falling?\u00a0 Pilots are taught to \u201cunload\u201d to break the stall, but what if they are already <\/em>\u201cunloaded\u201d? Under normal circumstances a stall at this altitude could require more than 5,000 feet to recover. \u00a0In this case, would have taken much more.\u00a0 This means pushing forward to around zero-g for perhaps half a minute, at night, in a storm, when you\u2019re not sure what is going on, while the aircraft is shaking like driving sideways across railroad tracks!<\/p>\n From the time the airplane actually stalled to when it hit the water was just around 3 minutes.\u00a0 Not a lot of time to sort out what was happening.<\/p>\n In addition to the confusion the crew was experiencing regarding what was happening, there was also a good amount of oscillation in roll.\u00a0 This is likely due to the aerodynamics of very high angles of attack, where the flow can have some very difficult to predict effects.\u00a0 The pilot flying had all he could do to try to keep the wings level.\u00a0 Sadly, allowing the aircraft to \u201croll off\u201d might have pushed the aircraft out of the stall, but they did not know that.<\/p>\n Again, if you do encounter buffet, one method that should work for about any airplane is to lower the nose and maintain a constant Mach number by reducing thrust. \u00a0Whether you are in high or low speed buffet this will move you away from the buffet boundaries.<\/p>\n In conclusion, it should be clear that the aspects surrounding high altitude stalls are complex.\u00a0 As outlined in previous articles<\/a>, expectation bias and confirmation bias play their parts as well.\u00a0 In truth, there was really not a lot of time to sort it all out, and simulators are not able to replicate the situation adequately.\u00a0 It is hoped that this article will provide some insight and \u201cfood for thought\u201d for pilots confronted with such a situation. \u00a0For those interested in more on the topic, may I recommend my book written with Roger Rapoport, \u201cAngle of Attack<\/a>\u201c. \u00a0We explore this and many other aspects in much greater detail there.<\/p>\n =====================================================<\/p>\n![\"contrail\"](\"https:\/\/airlinesafety.blog\/wp-content\/uploads\/2015\/04\/contrail.jpg\")
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