Some impressions from listening to some of the 2nd and 3rd days of the AA587 NTSB hearing, and later looking at some of the
on-line material including the “Reconstruction video” of the flight.
Reconstruction video: This shows “pictures” of the plane and the attitude indicator, together with graphic displays of positions of the rudder pedals and rudder, as well as the vertical and lateral accelerations of the a/c, all derived from FDR data. One of the most interesting features (to me) is that you can see the operation of the yaw damper/auto-coordination system on the ground. As the a/c exits the taxiway it does a 180º right turn onto the runway, during which the rudder deflects left without the rudder pedals moving, i.e. the system thinks the a/c is yawing right (nose right) and tries to correct. When the a/c holds (stops) on the runway the rudder makes a large deflection right which then slowly decays to zero, again without the rudder pedals moving.
During the first wake encounter the rudder makes only tiny movements. Evidently feet were on the pedals but they were used with great restraint.
The second wake encounter is extremely short, about ten seconds from start to fin separation. Its so quick that its hard to see exactly what happens, and I have not found any way to slow the video down. In contrast to the first encounter, extreme coordinated yoke and rudder movements are used almost from the start. The delay between a rudder pedal movement and the consequent rudder movement can be seen. The delay is apparently due to digital processing. It was stated in the hearing that pilot induced oscillations are always more of a concern in fly-by-wire a/c because of this delay. The delay usually has to be reduced to less than 1/10 second. It was also stated that with PIO it is possible to reach limit loads in two cycles and ultimate loads in four cycles.
Vertical Motion Simulator: FDR data are used to back drive a simulator specially suited to reproducing large-amplitude vertical motion. The observers noted that on take-off from JFK 31L the runway roughness was much less than they knew it to be from experience, probably due to the filtering of the FDR data on the a/c and/or limitations in the simulator. They thought the encounter with the first wake felt like a wake encounter. In contrast, they did not feel anything in the second encounter that would justify the extreme yoke and rudder pedal movements recorded. They described these movements as being more typical of an avoidance maneuver.
Wakes from JAL47: The atmospheric conditions were favorable for stable wakes, and it was estimated that 63% of the original energy was still in the wake in both encounters. However, the maximum velocity was estimated to be only 10 m/s (about 20 kts), and too low to be of any concern to the a/c structure. The a/c crossed the wakes at very narrow angles, so that it was almost flying along the wake. One structures witness stated he was not aware of any study of the effects of wakes on structure while in a yaw.
FAR25: It seemed that everybody worked from the regulations only. The FAA decided in the 1950’s that there had to be some standards as to what a/c have to be able to do in order to carry people about, and started to list certain maneuvers, which become the “envelope”. Because manufacturing a/c is a competitive business, manufactures design to this “envelope” and little more, or they will not be competitive. The aircrew are then assumed to stay within the envelope. This whole approach was best illustrated by the first sentence of Mr. Winkler’s prepared presentation, “The most important thing for an a/c structure is that it should be strong enough to resist all load conditions as required by FAR25”. He did not say “strong enough to withstand possible loads“. The same with maintenance. Anything which happens outside the envelope is “non-scheduled maintenance”, e.g. being hit by a truck on the ground. I am not commenting on this, just stating everybody’s attitude.
One question posed to the hearing but not directed to any particular individual was, “how can we continue to certify an a/c which can be broken by use of its certified flight controls?”. The answer given was along the lines that it has always been the case that a/c can be broken by use of the pitch controls. The pilot has always been expected to stay within the pitch envelope. It struck that this is like the case of a bus - you rely on the driver not to drive over a cliff. But perhaps a better analogy is a ship, where a storm may put you on the rocks whatever you do. But at least a ship Captain does not expect the rudder to fall off if he goes “hard a port”. (Continued in the next post.)
One the most interesting (to me) on-line documents is an (undated)
talk by Captain William Wainwright, Chief Pilot of Airbus Industrie. I have no idea if this is old or recent. Here are some extracts. The omission of text in these extracts is indicated by “………”.
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AIRPLANE UPSET RECOVERY TRAINING AID, By Captain William Wainwright, Airbus Industrie.
INTRODUCTION
The idea for a joint industry working group to produce an Airplane Upset Recovery Training Aid was first proposed by ATA in June 1996. ,,,,,,,,,.The end result of 2 years work is a training package including a video and a CD-ROM, giving an airplane upset recovery training aid. This package is on free issue to all of you, to use or not to use as you wish. ………..The content of the package is not my subject today, but there are a few issues of general interest which I gained from my experience as a member of the working group which I would like to talk about.
2. THE BEGINNING
…… Right from the beginning there was a conflict between the technical advice given by the manufacturers' training pilots and that expressed by those of the principal airlines already practicing upset training. They naturally considered themselves to be the experts on this subject, based on the many .hours of training that they had already conducted on a large number of pilots in their simulators. At the beginning of 1997, the Flight Test Departments were asked to come in to support their training pilots. From then on, the chief test pilots of the 3 major manufacturers became members of the working group; XXX of Boeing, YYY of McDonnell-Douglas (now Boeing - Douglas Products), and myself. But the conflict over the different opinions on aircraft handling and recovery techniques continued for a long time until we finally achieved agreement at the last meeting in January 1998. The reasons for these differences of opinion are the subject of my talk today.
3. THE DIFFERENCES
The differences of opinion were mainly concentrated in the following areas:
(a) Procedures versus general advice.
(b) Ease of training versus failure cases.
(c) Stalling.
(d) Use of rudder.
(e) Use of simulators.
It is worth saying that there was never any difference of opinion between the 3 test pilots on the group. Although we come from different backgrounds and have worked in different organisations with different work cultures, we always agreed on our technical advice. ………………………..
6. STALLING
Another aspect that was being ignored in the existing training was the stall. ……… However, the technique being taught only works if the aircraft is not stalled. ………….This is something that we are well aware of in testing, but it was either being totally ignored, or misunderstood. I consider the inclusion of this note to be one of our most important contributions. ………
7. USE OF RUDDER
We also spent a lot of time discussing the use of rudder. The existing training courses all emphasised using rudder for roll control at low speeds. It is true that the rudder remains effective down to very low speeds, and fighter pilots are accustomed to using it for "scissor" evasive manoeuvres when flying not far from the stall. But large airliners, with all the inertias that they possess, are not like fighter aircraft. Based on our experience as test pilots we are very wary of using rudder close to the stall. It is the best way to provoke' a loss of control if not used very carefully, particularly with flaps out. We finally got the training managers to agree to play down the use of rudder in their existing courses. But we do not say never use the rudder at low speed. We say that, if necessary, the aileron inputs can be assisted by coordinated rudder in the direction of the desired roll, We also caution that "excessive rudder can cause excessive sideslip, which could lead to departure from controlled flight".
But why did we have so much difficulty in convincing the training pilots that it is not a good idea to go kicking the rudder around at low speed? Their reply was always the same; “but it works in the simulator“. This leads me on to my last point.
8. USE OF SIMULATORS
We manufacturers were very concerned over the types of manoeuvres being flown in simulators and the conclusions that were being drawn from them. Simulators, like any computer system, are only as good as the data that goes into them. That means the data package that is given to the simulator manufacturer. And we test pilots do not deliberately lose control of our aircraft just to get data for the simulator. …………..
The complete data package includes a part that is drawn from actual flight tests, a part that uses wind tunnel data, and the rest which is pure extrapolation. If should be obvious that conclusions about aircraft behaviour can only be drawn from the parts of the flight envelope that are based on hard data. This in fact means being not far from the centre of the flight envelope; the part that is used in normal service. It does not cover the edges of the envelope. I should also add that most of the data actually collected in flight is from quasi-static manoeuvres. Thus, dynamic manoeuvring is not very well represented. …………
In other words, you have reasonable cover up to quite high sideslips and quite high AOAs, but not at the same time. Furthermore, the matching between aircraft stalling tests and the simulator concentrates mainly on the longitudinal axis. This means that the simulator model is able to correctly reproduce the stalling speeds and the pitching behaviour, but fidelity is not ensured for rolling efficiency (based on a sirrpfified model of wind tunnel data) or for possible asymmetric stalling of Ihe wings. Also, the engine out range is much less than !he all engines operating one, and linear interpolation is assumed between low and high Mach numbers. Wind tunnel data goes further. ,,,,,,,,,,,,,,,,,,,,
In fact, this is a perfectly adequate coverage to conduct all normal training needs. But it is insufficient to evaluate recovery techniques from loss of control incidents. Whereas, the training managers were all in the habit of demonstrating the handling characteristics beyond the stall; often telling their trainees that the rudder is far more effective than aileron and induces less drag and has no vices In short, they were developing handling techniques from simulators that were outside their guaranteed domain. Simulators can be used for upset training, but the training should be confined to the normal flight envelope; For example, training should stop at the stall warning. They are "virtual" aircraft and they should not be used to develop techniques at the edges of the flight envelope. This is work for test pilots and flight test engineers using their knowledge gained from flight testing the "real" aircraft.
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Several test pilots (some USA) made the point (if I understood them correctly) that pilots do not make control inputs of set amounts. They make a control input and wait to see what happens. On the basis of the result they then make another input. And so on. They do not just stamp on the pedals. So to some extent it may be argued that it should not matter what the rudder travel gearing is. Certainly the Airbus Industrie engineering test pilot witness said he was very happy with the present system. On the other hand it seemed to me that events in the reconstruction were happening so fast that a “try-it-and-see" approach wasn’t really possible.
If you got this far you may be amused by a comment by one of the simulator instructors being questioned about the psychology of instruction who said, “I know that pilots have a pretty short attention span, and if you have a point to make you had better get it over early”. This same interviewe said that simulator instructors have no way of telling which behaviors of the simulator are based on data and which on extrapolations.
These are just some personal impressions of the hearing. I was not there.
