To: Blacksheep
Dear Blacksheep or, should I say Baa Baa (joke).
Skytrucker87 was addressing what he felt was a high failure rate of the starter motors on the RR engines on a B757 and that is not the same as MTBUR. If your figures defined the MTBUR as 20,000 hours then the mean time between failure for that component is 23,529 hours. Another point to consider is that the calculated or predicted MTBF is a statistical mean for the fleet of B757s and not the failure rate for one starter motor. That means from a statistical point of view every 23,529 operating hours for starter motors in the fleet you would expect one starter motor to fail. However, since the starter motors are just used to start then you can use cycles instead of operating hours as for a generator which operates from block to block. If it were determined that one cycle was .1 of the block to block time then the MTBF would be one failure every 117,645 hours of aircraft operation since there are two starter motors per aircraft.
A calculated MTBUR is derived by multiplying the calculated or predicted MTBF by a constant which for this type of equipment is .85 or, 23,529x.85= 20,000 hours.
These figures are developed by the manufacturer of the unit under analysis.
The MTBUR figure you quoted was derived from the operational history of the starter motor. What you have to do is to calculate the differences between the predicted MTBF and the demonstrated MTBF. If the demonstrated MTBF is less than the predicted MTBF then you have a claim on the warranty
for the starter motor. I am not disputing what was mentioned in the posts above that referencec magnetic plugs and oil or, operator error. All of these things come into play as modes of failure and their resultant effects. That's why the manufacturer is required to perform a Failure Mode Effects Analysis (FMEA)
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The Cat