Value of a flight test after maintenance

Air Safety Week, July 14, 2003

Recall that the Air Midwest accident aircraft was put back into revenue service immediately after the tension was adjusted on its elevator cable (see ASW, June 16). The notion of a flight test beforehand was raised before a return to line service. That suggestion elicited the following response from Kevin Horton, an engineering test pilot at Transport Canada:

"The discussion on the elevator cable rigging problem that was present on the Jan. 8 crash of an Air Midwest Beech 1900D ends with the statement, 'There is a very simple means of guarding against catastrophe. Any adjustment to flight controls should involve a dedicated flight test to prove the controls are set up correctly.' However, based on 15 years experience as an engineering test pilot, I can assure you that the vast majority of flights will only require a small portion of the total available travel on each axis. So the successful completion of a test flight will not guarantee that full travel is available on all control surfaces. The only way to ensure that full travel is available is for the maintenance personnel to measure the actual travel at the control surfaces as the cockpit control is moved from stop to stop."

ASW Contributing Editor John Sampson responded with these thoughts:

"There are possibly more than two schools of thought on this but the one that used to keep me airborne more often than not as a unit maintenance test pilot with Hawker de Havilland was the requirement for a functional check flight on control adjustments as minimal as re-jigging the electric trim [system]. A trims air-test would be a quick out and back again with CB's [circuit breakers] pulled to freeze the elevator, aileron and rudder trims at the appropriate position for nulling at the specified speed (normally 300kts). Sometimes a stall-check would be required due to flap or flight-control re-work. I can recall canopy removal/replacements where shims were inadvertently omitted and the canopy sprang its locking and remained precariously held by a weak secondary latch. I've experienced tip-tanks that came adrift and rotated around the central spigot due to improper retensioning. I've seen similar post-maintenance nasties in four engined aircraft that covered the whole spectrum from engines and flight-controls to fuel-dump and handling qualities.

"By definition a post-maintenance functional check-flight (FCF) is designed to avoid rude surprises for those persons not being paid to take those risks. An FCF pilot is looking for (and unsurprised by) glitches, whereas linepilots tend to trust that the airplane is serviceable. The accident aircraft in question was flown by a number of pilots in a variety of weight and balance conditions. If that aircraft had been loaded to a standard max load and C of G [center of gravity] and flown to a profile and the DFDR [digital flight data recorder] then pulled and examined, the fault may have been disclosed. But as I have said before this accident, air-tests nowadays, whether for engines or airframe, are carried out with a full load of fuel and passengers.

"One sees such hazards as both engines in a twin-engine aircraft being run out of oil because sump plugs were left out after a bore-scoping. Quite sanguinely, it is a commercially dictated risk that is being taken.

"I agree with Horton that the prime responsibility is that the maintenance task be done right in the first instance, but the conundrum is that quite often it isn't - and thus there is no double-check for shift-change misunderstandings, pencil-whipping or innocent error. That Emery Worldwide Airlines DC-8F that crashed Feb. 16, 2000, due to a faultily connected elevator could quite possibly have been full of passengers were it not for the fact that it was a freighter (see ASW, May 27, 2002, and July 1, 2002). Research could turn up a number of other instances. Indeed, a fatal 1978 accident involving a Beech 99 seems a remarkably similar precursor to the Air Midwest crash of the Beech 1900D earlier this year at Charlotte, N.C.

"Possibly it's the wording used in the ASW article that is the crux of the matter. The article said 'a dedicated flight test' rather than 'a dedicated test flight.' What's the difference? Well, simply this. If one were guarding against the worst-case possible outcome of getting airborne with a full passenger and freight load, i.e., the precise setting for that fateful Charlotte flight, one would be testing for that exact condition by flying it with a critical aft-biased loading.

"One of the cited advancements of aviation has been that BITE (built-in test equipment) has allegedly taken much of the uncertainty out of black-box swap-outs and allowed aircraft to be released with confidence about their systems. But every now and again this arrangement proves itself to be eminently fallible - and flight controls represent an area where there's little margin for recovery from error. Recall the near-accident March 20, 2001, of a Lufthansa A320. On its first flight following maintenance the airplane's wingtip came within two feet of contacting the ground during takeoff at Munich. It turned out the sidesticks had been cross-wired, which meant control inputs were reversed. The near-crash was saved by an alert first officer (see ASW, June 4, 2001).