In his note in RISKS-26.15, Peter Wayner refers to the article Simulator training flaws tied to airline crashes in USA Today, 31 August 2010 (WWW version), which claims to have shown that «Flaws in flight simulator training helped trigger some of the worst airline accidents in the past decade» and that «More than half of the 522 fatalities in U.S. airline accidents since 2000 have been linked to problems with simulators».

I like to think I keep well up to date with commercial aircraft accidents, their analyses and causes, and am aware of simulator strengths and weaknesses. This suggestion struck me as somewhat thin. But if one reads the sentences literally, with their main verbs “helped trigger” and “have been linked to“, they do not speak of causes or causal factors. I can “help trigger” an accident if some USA Today journalist is so enraged by reading this note on hisher Blackberry that heshe runs a red light. And I can link USA Today with whom I wish simply by mentioning them in the same sentence in a Risks note. I am sure the newspaper intends stronger links than this, but it would be good to know what and how, and the article gives no clue. The NTSB uses the words “probable cause” and “contributing factors” in their conclusions and these terms have more precise meanings.

The article mentions three accidents: the November 12, 2001 American Airlines Airbus A300-600 loss of control on climb-out from New York; the December 20, 2008 Continental Airlines Boeing 737-500 takeoff loss of directional control at Denver; andthe February 12, 2009 Colgan Air Bombardier Q400 loss of control on approach to landing at Buffalo. The abstracts and links to the full reports are to be found on the NTSB WWW site as, respectively, DCA02MA001, NTSB Abstract AAR-10/04 and NTSB Abstract AAR-10/01. I invite readers to take a quick look at these very short synopses. These three accidents total 315 deaths and the USA Today article does not say which other accidents it counts.

Only the Denver accident causes and factors specifically mention simulators. The pilot flying lost directional control of the aircraft on the runway during takeoff, because of very high gusting crosswinds. The gust “exceeded the captain’s training and experience”, and according to the NTSB he failed effectively to use rudder to control the aircraft in the gust. The first contributing factor allows us to conclude that the crew did not receive timely and accurate information on the actual wind strength and direction. The second contributing factor is “inadequate crosswind training in the airline industry due to deficient simulator wind gust modeling“.

It is widely accepted in the industry that the most recurrent feature of most large-airplane commercial air accidents worldwide in the last few years has been loss of control. It used to be controlled flight into terrain, but it is now widely accepted that the Ground Proximity Warning System (GPWS) and its version Enhanced by terrain mapping using GPS and terrain maps (EGPWS) have reduced the incidence of such accidents considerably (although they still occur, as to an Airblue Airbus A321 on approach to Islamabad on 28 July, 2010 – see the Aviation Safety Net brief report).

The 2001 American Airlines accident was loss of control because of structural failure: the vertical fin separated from the aircraft. The NTSB found that the pilot flying had caused that separation by overstressing it through “rudder reversal” control inputs; contributing were the rudder control system design of Airbus, and American Airlines Advanced Aircraft Maneuvering [sic] Program AAMP. The NTSB heard both that AAMP discussed use of rudder to help recover from upsets, and that the FAA, Airbus and Boeing had expressed concern about this in a letter to American Airlines four years before. The pilot flying had been observed on a previous flight using rudder to control unwanted aircraft movement from environmental disturbance, and the captain on that flight, who gave evidence to the inquiry, had discussed it with him then. I refer Risks readers interested in more to the report, as well as to my paper The Crash of AA587: A Guide. The AAMP does involve simulator work, but a simulator cannot be known accurately to represent what would happen during unusual piloting rudder-reversal behavior because, well, until the accident nobody knew at what point airframe structure would fail (it turned out to be some one-third stronger than required by certification regulations)!

The pilot flying the Colgan Air accident aircraft reacted inappropriately to a stall warning, by pulling on the stick, and holding it back against the attempts of the automatic “stick pusher” system to push it forward. This resulted in the aircraft stalling at low altitude. Pushing the stick forward is the appropriate response. There was considerable discussion of the pilot’s aptitude, his level of awareness (relating to possible fatigue), and his overall Q400 training at Colgan Air. The NTSB remarked on features of that airline’s training program, which of course involves simulator work. But I don’t think it would be appropriate to conclude that there is anything much wrong with the simulators themselves.

Simulators do not necessarily accurately represent the behavior of aircraft close to the “edge” of their “flight envelope”, and they cannot be taken to do so for flight outside the envelope. Aerodynamicists study these “out of envelope” characteristics by use of wind tunnel models, but actual aircraft are not flown in flight test “out of envelope” except for certain restricted manoeuvres prescribed in the certification regulations (such as flying at “maximum operating airspeed” and initiating a 7.5° nose-down dive for 20 seconds, to mimic an overspeed excursion from cruise). For most “out of envelope” flight, aerodynamicists can make very well-educated guesses (from their wind-tunnel modelling) as to what might happen, but they are the first people to say that they are not at all certain. Nobody goes out to flight-test Boeing 747 aircraft in partially-inverted almost-vertical semi-spins, such as what happened to a China Air Lines Boeing 747 over the Pacific near San Francisco in 1985 (see the digitised version of the NTSB accident report in the entry in our Compendium. Incidentally, the human factors chair on this investigation tells me this was a watershed event for the investigation of human biorhythms and possible fatigue as potential contributors to accidents).

So there are limits to what simulators can achieve, and it is a matter for research how much “out of envelope” behavior can be usefully and veridically simulated. Since loss of control is now prominent amongst probable causal factors of accidents, it seems to me obviously worthwhile to perform this research. Where it will lead is anybody’s guess, as with most research. However, the NTSB’s concern in the Denver report is with situations that could be veridically modelled in flight simulators but currently are not. That could be, and probably should be, fixed.