Ask Captain Lim

Dear Captain Lim,

First I want to say how much I love your insightful answers. I have read through a lot on this site and the information you give has increased my confidence in flying.

My question has to do with faulty servo valves in a plane*s rudder, causing *rudder hard-over.* I have read about several planes that have virtually dropped out of the sky and crashed in a near vertical position and faulty servo valves were named as the cause.

Can you please explain what these are, what they do and what exactly a fault in them would cause? Also, how can pilots prevent this from happening? Is a dive the only outcome when this happens or is there varying degrees of crash landings that could be possible? What precautions have been taken since these accidents and how do I not feel scared that this will happen again?

Many many thanks,

Kathleen
London, UK

Hi Kathleen,

It is believed that at least 2 crashes on the Boeing 737s were attributed to the faulty servo valves in the rudder system. It caused the loss of control of the airplanes resulting from the opposite movement of the rudder or "uncommanded rudder reversal". With this problem, the rudder surface will deflect in a direction opposite to that commanded by the pilots.

For instance, if the right rudder was applied, the rudder would move to its full-blown left position causing the plane to roll further left, stall and go into a dive as had happened on USAir Flight 427  from Chicago to Pittsburgh. I cannot say for certain as to the consequences of such a misfortune. Anything can happen when a plane loses control and fall out of the sky.

Some speculated that if the pilot had pushed the control forward to gain some speed rather than pulled it back, it was possible to recover from the upset. Well, there was no mention of such a recovery by the manufacturer nor was there any training for this kind of occurrence before. Following the investigation, pilots were then warned of and trained on how to deal with this if it were to arise again.

In the meantime, the manufacturer has redesigned the rudder system on the Boeing 737s and retrofitted the existing aircraft until the affected systems could be replaced. So by 12 November 2008, according to sources, all 737*s should have been retrofitted with a new rudder control system. Hopefully it would then solve the problem and put the whole sorry episode to rest.