From: firstname.lastname@example.org (Peter J. Coe) Organization: NETCOM On-line Communication Services (408 261-4700 guest) Date: 05 May 95 03:27:15 References: 1 2 3 Followups: 1
View raw article or MIME structure
kls@ohare.Chicago.COM (Karl Swartz) writes: >>I would think that the statistics used to say ETOPS are safe, are >>flawed. They are based on the reliability of an engine in a twin >>engine set up. Not a single engine at greater power. >Do you have any facts to substantiate those last two sentences? I >seriously doubt you do. OK, I'm ranting. Maybe I should say the statistics are misleading. The reliability statistics are, I am fairly sure gathered from actual data, and then projections are made based on that data. Boeing is flying around 777's for all they are worth in order to gather the necessary data for the 777. As I understand it, before an individual aircraft is certified as ETOP's capable, certain criteria have to be met, namely: The aircraft type has to have accumulated enough (reliable) hours. The operator has to have accumulated enough experience with the type. The individual aircraft and it's engines must have proven their reliability. Virtually all of those criteria will be met using aircraft flying under normal (twin engine) operation. So the number of hours that a twin engine aircraft type is operated under single engine power will be minimal. It's all very well to say that 200% of normal cruise power is still well within the limits of the engine, but how long is it designed to run at that power setting? I realise that normal cruise rating is probably only 10% of maximum power, but for a long range aircraft, maximum power is probably only generated for, at the very most 30 minutes of a typical flight. It's actually probably no more than 5 minutes on and around take-off. The life and reliability of the engine is based on the statistics of normal operation, and as we all hope, ETOPS is an abnormal situation. As an example of how engine reliability statistics can be flawed, who remembers the Kegworth crash of a British Midland 737-400? The original problem with the plane was an engine failure of one of it's CFM-56's. I can't remember the exact cause of the failure, but it was basically because the engine had not been tested at altitude. It was tested at sea-level, and it's performance and reliability statistics were derived from that data. Come real life, the engine failed, and as I recall, the BM plane was not the only plane to have problems, but was the only one to crash. The point of the story, is that we don't have enough real life experience of operating these aircraft under these circumstances, and the result is we _will_ get surprised now and then, and people will eventually die.