Expert: Paul Soderman - 9/23/2006

Question
Hi Paul, I have a couple of questions regarding aeronautics. I was wondering if u could answer them. Is it possible for a commercial aicraft  such as the 747 to break up in the air if it keeps on porpoising? Would an uncommanded slats deployment be dangerous for a commerical aircraft at cruising speeds?  

Answer
Hi Kevin

The type of porpoising that is dangerous for aircraft is pilot-induced-oscillation.  That has been the demise of certain fighter aircraft because of their quick response.  Once the pitch oscillations begin, the pilot's response lags the motion and causes the oscillations to worsen and diverge.  It can happen quickly.  But a 747... I don't know if you could get it in that situation.  It would be very difficult because of its slow response.  It might be possible, but I doubt it.  On the other hand, porpoising could be caused by other things - a commercial aircraft on landing in Iowa lost hydraulic power and could only be controlled by thrust.  This led to porpoising and loss of aircraft, but it required a serious system failure.  

A series of stalls on landing could cause porpoising and loss of aircraft.  But I don't know how a 747 could get into that situation.

An uncommanded slat deployment can be very dangerous because it can lead to a very abrupt and strong nose-down pitching moment.  Aircraft have been lost by inadvertant slat deployment.

Paul