Sir - With reference to the continuing investigation into the Boeing 737 accidents in March 1991 at Colorado, and in September 1994 at Pittsburgh, I wonder whether old lessons can be re-learned.

In the 1950s, the single pole, electrically operated tailplane on the Canberra often ran away to full deflection. Double pole switches were then introduced on aircraft such as the Valiant. This cured the runaways but, after some time and two or three fatal accidents, another fault was diagnosed. This involved what was often called reversal but was, in fact, unidirectional movement of the control surface. Whatever direction you selected, the surface would only move in one direction.

For simplicity, let me describe double-pole switches as one for power (P) and one for direction (D). It was the shorting of D which caused unidirectional movement and confusion, leading to fatal accidents. It can also be seen that, if a foreign body makes a permanent contact at P, this would have no noticeable effect and would not be discovered unless each switch was tested to check no movement. This was done before each flight.

I believe that the Boeing rudder trim is a knob rotating in azimuth left and right around centre. The two contacts are made during travel to the limit of control knob movement; the first contact is close to the centre and the second near the limit of movement. This makes it difficult to check each contact but, in any case, I gather this is not part of the pilot's checks. I wonder how often it is checked.

I appreciate that for a runaway two shorts have to occur, but if one at P has been present for some time then this makes runaway much more likely. I also appreciate that, unlike the Canberra/Valiant, we are not talking about runaway of a control surface, only the trim. Under normal conditions this could be held, but perhaps not if you are unaware of what is happening.

Have the investigators looked closely at the possibility of unidirectional movement or runaway of the rudder trim?


Worthing, West Sussex, UK

Source: Flight International