SRC Forum - Message Replies

Forum: Reliability & Maintainability Questions and Answers

Topic: Reliability & Maintainability Questions and Answers

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

Original Message:

Posted by: GRIB
Date posted: Mon Mar 5 16:02:07 US/Eastern 2007
Subject: Conversion factor between OH/FH
Message:
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?

Reply:

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
Message:
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: www.fieldreliability.com/NWSLT3.DOC www.fieldreliability.com/NWSLT4.DOC “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.