"BUT if you have an engine failure on a light twin ,757,767,A330 etc and also a loss of cabin pressure therefore decend to 10k feet to keep pax alive and you are at 179 mins flying from nearest diversion(this 179 minutes is calculated at criusing alt) there is a high possibility that the aircraft will not enough fuel on board to fly this distance.Depends what flight phase you are in when these disasters strike But we are talking about 2 or 3 major failures happing at once...A very bad day at the office."
Yes bolty, but that was my point. The Etops fuel requirements assume that in the worst case scenario you do have a pressurisation failure descend to 10,000ft and divert to your ETOPs alternate at 10,000ft whilst still arriving there with your normal holding reserves intact. Ironicaly perhaps the engine failure and pressurisation case would actually leave you better off since only one engine is using fuel at this altitude. However that notwithstanding and using your 180 minute assumption, you would even in the worst possible scenario still arive at your alternate with 30 minutes holding reserves. If you see what I mean. So it wouldn't be as critical as you suggest. Having said that I don't want to put it to the test, but I am sure you take the point. The fuel is Not calculated at cruising altitude but at 10,000ft. ( see items 1 and 3 )