SRC Forum - Message Replies

Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

Topic Posted by: Reliability & Maintainability Forum ( )
Organization: System Reliability Center
Date Posted: Mon Aug 31 12:47:36 US/Eastern 1998

Back to message list Topics List About this forum
Original Message:

Posted by: GRIB
Date posted: Mon Mar 5 16:02:07 US/Eastern 2007
Subject: Conversion factor between OH/FH
The average flight time for an aircraft is 1.75 hrs. the conversion factor between operating hrs and flight hrs OH/FH is 1.24. According to the above information, shall I multiply each failure rate issued from the NPRD per 1.24 in order to obtain the total failure or the MTBF per Flight hours?


Subject: Convert flying hour failure rate to operating hour failure rate
Reply Posted by: Larry George
Date Posted: Fri Mar 9 12:58:24 US/Eastern 2007
I first tried to persuade the Air Force AMC (formerly known as AFLC) to do this right in ~1975. OH/FH is a random variable, so converting FH to OH scaling the failure rate by 1.24 biases the failure rate, even if the failure rate is constant. I simulated OH MTBF using a normal distribution with a mean of 1.24 and a standard deviation of 0.2. The MTBF was ~820 instead of 805.3 hours you might expect. The MTBF has a rather large standard deviation, over 100 hours. Convert FH to OH using the observed distribution(s) of OH/FH instead of a fixed average, to make an unbiased estimate of MTBF and to represent the variation in OH. All it takes is a sample of OH/FH. References: “Convert From Calendar to Operating Time Reliability,” ASQ Reliability Review, Vol. 18, No. 3, Sept. 1998 (contains more references) If your want the spreadsheet simulation, ask.

Back to message list Topics List About this forum
Reply to this message