Dear Captain Lim,
Thanks for an amazing site. I*ve learned an incredible amount since I discovered it a few weeks back.
I've always enjoyed flying as a passenger without any problems, but a recent incident on a domestic flight between Johannesburg and Cape Town has frayed my nerves a little, and prompted a few questions.
I think the aircraft concerned was a Boeing 737-200, operated by Nationwide Airlines of South Africa. Soon after take-off, about 40 seconds into the ascent, a loud banging noise was heard coming from the left engine. It was considerably louder than the normal engine noise, and caused a few anxious looks among fellow passengers. The banging lasted about 20 seconds before abating. Shortly afterward the Captain came on the intercom and explained that the noise was related to a build-up of compressed air in the engine, resulting in the equivalent of an engine "backfire". He went on to stress that there was absolutely nothing wrong with the engine, and we continued on our journey to Cape Town without incident.
I certainly appreciated the Captain*s comments, as I*m sure the experience spooked many passengers on the flight, but it has really shaken my confidence. I recently felt compelled to spend a morning at Cape Town International just watching aircraft take off and land, to convince myself that thousands of these planes operate every day smoothly, happily and without incident. I know how ridiculous this is.
What I would like to know is how often such an engine "backfire" occurs, and what problems might it lead to if it did not automatically resolve itself?
Secondly, my (foolhardy?) Internet research on this matter led me to an FAA-issued report (related to separate incidents) which stated that many airline pilots and co-pilots actually took off and landed real planes maybe two times a month, and sometimes less, with the bulk of their regular experience occurring on simulators. I had previously taken great comfort in the idea that pilots take off and land these aircraft "day in and day out", so I found this revelation a little unsettling! Could you shed any light on this?
I'm flying from Cape Town to Los Angeles next week, and will be taking 4 or 6 domestic flights within the US during my trip. The recent flight with the engine backfire has made me a little anxious, and I would appreciate any reassuring words you might be able to offer!
Thanks for your time,
Department of Archaeology
University of Cape Town
Here are the answers to your two questions:
1. How often such an engine "backfire" occurs, and what problems might it lead to if it did not automatically resolve itself?
A jet engine doesn't "backfire". Only aircraft with piston engines is capable of "backfiring" like the old engine of a jalopy! I suppose this term was used by the captain to simplify what actually happened in your flight. In reality, the loud banging noise was due to the jet engine suffering a "compressor stall".
What is a compressor stall? Well, it occurs when the airflow through the engines is temporarily disrupted. Basically, the compressors of a jet engine consist of a series of rotating turbine blades. If air stops flowing smoothly around these airfoils, or if there are backflows between the various stages, the compressors are stalling. It is possible that this phenomenon can damage the engine, but chances are, it won't.
The loud bang may sound frightening - but the engine is neither exploding nor on fire. Well, it can happen; any time a jet is turning, fuel is being burned and any anomalies will unleash this abnormal combustion right out of the back of the engine with a bang. On the Boeing 777, we do have a drill for this and during the 8 years of flying on this airplane, I have yet to experience it except during tests in the simulator.
2. "Many airline pilots and co-pilots actually took off and landed real planes maybe two times a month, and sometimes less."
The statement is partly true when you talked about long-haul flights that have two sets of crew flying, say a 3-return flights of about 12-13 hours each. When auto landings are performed, then it even lessens the frequency of the real handling of the planes. Remember, pilots must undergo a mandatory number of take offs and landings each month to be legally current. Otherwise, he must undergo a check flight before he is put back on line flying again. However, on short domestic flights, pilots do much more take offs and landings per month.