SAS pilots pretty good too.

toy264 · Jan 22, 2011

I came across this doing some unrelated research. David, you probably have studied this at some point. As said before, sometimes you stand in awe.

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Charlie Farley · Jan 22, 2011

Woops !

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David Morton · Jan 23, 2011

Contributory causes were: The pilots were not trained to identify and eliminate engine surging; ATR-which was unknown within SAS - was activated and increased the engine power without the pilot's knowledge."
IMHO both contributory causes shoud really have been factors and not causes. The contributory cause was the ice which was actually made clear in the report.
I flew out of Arlanda ARN Stockholm) for 6 months in Maersk Air on the B737 and I know the simulator details did deal with surges and all aspects of autothrottle but this ATR seems to be a peculiar thing and I've never heard it mentioned before. It certainly was not a feature with the Boeings. I flew through the winter in ARN and landed on packed and sanded snow up in Lulea and Umea in temps down to -40c but those temps are never a problem with wing and fuselage icing. We never had a problem with wing and fuselage de-icing in ARN either and it was a Maersk thing for either one of us Nigels (aces, studs) to physically check - especially in the upper wing root area where the performance would be so degraded it can be difficult to manage a 737.
BA found that out in Helsinki in a five minute 737 flight that barely got above tree top level using max thrust. The 737 does have a subtly different engine configuration though, so I have a friend how flew the DC9 in BMI and later on this morning I'll ask him if he has ever heard of this ATR thrust thing. It just goes to show that technology can really bite hard if you ignore it. Looks like the SAS guys including the extra dead head Captain did a really amazing job but I guess their Flight Crew Orders now insist on a Nigel (Sven?) check as well as the Engineers report.
In my subsequent time in BA we always checked the upper wing especially on the 747.

wolodymyr · Jan 23, 2011

John,
yes I see there were no fatalities, which is amazing considering the circumstances and damage to the plane.
Surprising that engine surge was off the SAS radar as it is a fundamental jet engine thing, known to be very destructive and not unheard of where engine damage or fault conditions occur, so anyone operating air or ground based should be fully aware.
Basically, too much back pressure causes blade aerodynamic stall, as a result, the compressor loses pressure, and the high pressure gas begins to flow back into the compressor, this causes the stall to recover, and the compressor begins to work again, but there is too much back pressure, so the blading stalls again. This cycle is self sustaining and can happen several times per second, putting severe oscillating axial loads on the engine and supporting structure. Imagine going from a few thousand pounds of thrust to zero and back several times a second!
I experienced it myself in ground based gas turbine generating plant, both aero engine and industrial where it almost feels like an earthquake, so definitely brown trouser time in the air.
When I worked in the Gas Turbine industry, and we developed a new design we would do practical testing to plot the surge line, which is essentially a graph of limiting compressor pressure ratio v engine speed, important to confirm for design knowledge. To do this, we would control pressure ratio using the blow off valves, increasing compressor back pressure slowly until surge occurred. I personally did this on a 100MW industrial engine, the procedure was to keep the engine speed constant, while increasing the back pressure slowly. I would rest my hand over the emergency stop button, and as soon as surge occurred, hit it quickly to avoid any chance of engine damage. If you tried to come out of surge by reducing back pressure, it would take too long because of the surge hysteresis. This was repeated for successive engine speeds to plot the graph. Normally I could hit the button quickly enough to kill it on the second or third cycle, but the chuffing noise it made was scary and you could feel the whole control room shake. Like an overspeed test, you only wanted to do this the one time.

sorry to bang on, but always find engine dynamics interesting

DaveB

SAS pilots pretty good too.

John Fitzpatrick

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