Engine Problem Question.

[mallyR]

Hello all,

Long time lurker, hopefully this is the right place to put this.

Many years ago while travelling on a Northwest 747-200 around 15 mins after departure there was an almighty bang, and as you will imagine, the cabin went rather quiet. We seemed to turn, possibly circle for a while but there was no sign of a descent. Pretty soon there was an announcement where the crew explained what I vaguely remember as when reducing engine power the engine stopped, then due to the pressure of air entering it, started again, and I believe he equated it to a backfire ( I think thats about right). Anyway, all was apparantly well, we continued and the Captain said they would get it checked out on arrival (5 Hours later).

Would that be right, can jet engines backfire?

[rcav8r]

aka compressor stall.

http://www.airliners.net/open.file/0963368/M/ (http://www.airliners.net/open.file/0963368/M/)

[PAXboy]

A compressor stall sounds pretty bad from inside the a/c, especially to the pax who may travel for 20 years and never get one. Speaking as pax and not technical or air crew, I understand that it happens like this: [FC + techs pls correct!!]

Air is rushing into the front of the engine, where it is compressed by a series of whirling fan blades and then enters the combustion chamber. The first blade is what you can see when the a/c is on the ground.

Sometimes, the airflow into the running engine is disrupted and this can be for any number of reasons, such as a sudden change in air pressure and of course the engines are designed to accept this and are tested to see that they continue to operate smoothly. But, once in a while, the change in airflow is very great and the result is that the flow of air into the engine is broken.

One example that was given to me was this: Think of water flowing out of a dam, through a pipe. Sometimes the pipe cannot take all of the water at once and the flow is broken and the water is said to 'back up'. Shortly, the overwhelming pressure of water in the dam will overcome this and flow will be re-established.

For water into a pipe - think air into an engine! This means that the flow may actually stop for a moment and it is said that the air has 'stalled' (stopped moving) and it happens at the compressor, so it is a compressor stall.

Now the engine finds that it has no air but is still pumping fuel into the combustion chamber! So then there is a loud BANG! and you get a small flame out of the back. This is why the term 'back-fire' is sometimes used to explain the situation to pax.

Usually the engine will recover itself as the airflow returns to normal a few seconds later and sometimes the flight crew will need to make adjustments to it. If the engine does not return to normal function, it will be shut down. (The topic of what happens then is for another thread!!)

I experienced a compressor stall when departing HKG (the old Kai Tak) in a fully loaded 747-400 bound for the UK and the whole machine shuddered. It was just a couple of seconds after rotation but the other three engines were more than capable of taking us up.

So it sounds bad but it is nothing to worry about. :)

[Beeline]

Good explaination, one technical pointer or useless information, however you wish to interpret it!

A loud bang is heard on a compressor stall due to the formentioned 'stage' 'stalling'; airflow has disrupted and has ceased to move over the area due to a number of variables. The high pressure air compressed beyond the area of disruption moves back into the low pressure region causing the surge and a large bang.

The combustion area can become very hot at this stage as the airflow through it has stopped. If the temperature has exceeded engine limits the engine cannot be re-started, but more than often and in this case the aircrew were able to see it was within limitations, cool the engine down by allowing it to windmill and commence a re-start. :ok:

[strake]

..and it is indeed a bit "startling" to say the least. ..here's my "there I was....!"

Departing Helsinki with SAS in an MD something or other in 1994. It was a very cold morning, we'd been de-iced and set off in driving snow and no visibility to speak of. About 20 secs after rotation....big bang. However, before anyone in the cabin had much time to react, pilot was on the PA and explained exactly what the problem was and that was that.

In twenty one years, regularly flying at least once a month, that's the most drama I've ever had...apart, of course, from yesterdays flight back from Newark with VS when they ran out of ice...

[PAXboy]

Hi Beeline thanks for the elucidation. It's natural for pax to think it's the fuel that goes bang on account of wot that iz wot it normally doz ... :}

When I experienced the compressor stall ex-HKG I had the chance to ask the flight crew about it later in the night (ah, the delights of travelling in 1991!). They were 'shrug shoulders' about it as they had been able to recover it soon after and we toddled home without further ado.

(Interesting to see the name Beeline appear again!)

[Pax Vobiscum]

A compressor stall on an MD-8x? I really wouldn't want to be seated down the back (where the engines appear to be about a foot from the window) when that happened, strake!