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
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.
“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.