Engine surge?

[flapsforty]

After 25 seconds (at 1124 feet height) bangs, vibrations and jerks were perceived in the aircraft. This was caused by a no. 2 engine surge. The engine was throttled down a little, but throttle control simultaneously changed to an automatic mode which increased throttle setting with altitude (Automatic Thrust Restoration - ATR). This in turn increased the intensity of the surging. The no. 1 engine surged 39 seconds later, but this was not noticed by the flight crew.

Using short words ;) is there anybody here who would explain 'engine surge' to me please?
As usual, I have an idea what it might be, but it's rather vague. Especially on why it is such an undesirable occurence.

[Notso Fantastic]

Flaps, a jet engine works by progressively building up air pressure as you go backwards in the engine. Located along the engine are a few valves which can release pressure when it builds up too much- these are called bleed valves. If they get a bit sticky, or the engine design is not quite right, air pressure can build up in certain areas and be too great to be restrained- sometimes this pressure will spill out forwards. So the engine does a 'cough', sometimes quite violent and throws out air forwards. It is accompanied by a loud 'boom' or sound of canon firing, great vibration, and sometimes startling indications of exhaust gas temperature rise- the steady state burning has been interrupted and the engine may not be working at the right speed for the air coming through. This can manifest itself as either EGT staying fairly stable after a rise, or rapidly running towards limits requiring a quick shutdown.

The VC10 of old ws prone to surging at high altitude- the junior cabin crew working in the rear galley would be a few feet away from a canon going off! You could count to 10 after one and a wide eyed junior crew would breathlessly burst onto the flight deck believing a bomb had gone off it was that bad! The 747 would sometimes not recover itself and need to be shut down. I have sat next to a 707 engine at night continually surging and it was spectacular, bangs and a continual firework display with sparks flying forward in the dark (complete hydraulics failure on the ground requiring power to stop rolling off the runway).
The amazing thing is, despite such a savage event, I don't think engines fall apart because of them- they can recover and run on quite happily in the majority of occasions.

[Snigs]

Flaps,

We all know that in the natural world air wants to move from a high pressure to a low pressure environment. In a gas turbine engine we are making the air flow from a low pressure environment (ambient) to a high pressure at the end of the aptly called "compressor".

This is all well and good if the engine is working properly (it's running line). If there is a disruption to the above described equlibrium (for example a disruption of flow into the engine due to a high angle of attack) then the pressure balance/running line is affected.

In the surge scenario the natural world wins and the air starts to flow from high to low pressure, i.e. towards the front of the engine. Naturally this has rather serious consequences on the combustion process (and hence thrust etc).

Normally the engine (especially the newer ones) will recover after a cough and splutter, but some will not, and will be shut down. Depending on the intensity of the surge the compressor blades and vanes may be damaged, but I've known them to carry on functioning well even after a massive surge.

Hope that helps. :D

[Brenoch]

And the reason for the surge in the particular case your quote is from was clear ice separating from the wings on rotation and subsequently was ingested by the tailmounted engines. (MD80)

These particular engines P&W JT8D-217 never recovered and the airplane was glided on to a nearby field and everyone amazingly walked away from it with only minor injuries. (I'm sure you know all this flaps but anyway) ;)

Regs

[redsnail]

Flaps,
Forgive me for I have partaken in incaholic substances. The engine "burped".
There, that's the simplest I can get it. :)

[flapsforty]

Another mystery cleared up. :ok:
Every FA in the world should have access to you kind people.

Knowing a bit more about what happens apart from the stuck trolleywheel and the U/S coffeemaker is fun, interesting and useful in explaining to the pax why the man on the little ladder is stroking the wing and shining his torch over it.

Reds, very ...errm........... descriptive! ;)