Idiot-proofing components to make maintenance mistakes impossible is a fine idea, but it can never be the complete answer
It looks as if reversed controls caused the Spectrum 33 crash. It may only be the National Transportation Safety Board preliminary statement on this tragic accident, but the incomplete evidence cited so far looks pretty conclusive and the accident followed the classic pattern for such a mishap - occurring as it did just after the wheels left the ground.
Meanwhile, two test pilots have died and the future of a promising aircraft type has been endangered.
It is easy to say the industry should have stamped out this kind of accident by now, but the task is more difficult to do than to say. "Murphy-proof the system" is usually the cry. It would be fine if it were that simple - if only one or two connections were involved. If that were so, all the designers would have to do is make it impossible to connect left to right because the male-female connections would not fit. The trouble is the systems involved are multiplex, with multiple connections and many interfaces in a single system, whether mechanical, digital, electrical or hydraulic or, more usually, a combination of several of them. Making the maintenance mistakes that can cause control reversal has been proved to be just as easy in a digital fly-by-wire (FBW) system as it is in a mechanical system of cables, pulleys, torque rods and bell cranks. And even digital electronic systems eventually become mechanical where they interface with control surfaces through electrical or hydraulic actuators.
Mis-rigging, cross-wiring, or even failure to reconnect flight controls still happens uncomfortably often for something so serious. Failure to reconnect cannot be Murphy-proofed, and it is difficult to make it impossible to pass a cable the wrong way around a pitch trim drum.
The two common factors in all recent events were that the mistakes originated in maintenance, and then the multiple opportunities for detecting them in the hangar and on the line were all missed.
It makes sense to Murphy-proof simple connections for line replaceable units or other items that are frequently disconnected during maintenance, but after that has been done the prevention strategies are all based on disciplines: ensuring the clarity of maintenance manuals and part numbers rigid respect for maintenance and checking routines. But finally it is up to the pilots - the people whose lives are on the line if they don't take seriously the entry in the maintenance log that makes it clear this is the first flight after a specific piece of maintenance - however routine it was. Often, for access to particular equipment, flight controls have to be disconnected. That entry in the log turns the crew into test pilots for this flight. They should be as assiduous as test pilots in every routine check they do. Particularly the check for "full and free" flight control movement - but that is not enough on its own: they must not omit to check that the control surfaces all move in the correct sense - for each sidestick individually in FBW aircraft.
Source: Flight International
