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: John Schweighardt
Date posted: Thu Jul 26 15:54:46 US/Eastern 2001
Subject: Flight Hours vs Operating Hours
Is there a stated relationship commonly in use between actual aircraft flight hours and operating hours (which includes ground time). I've used anywhere from a 1.0 to 1.5 relationship based on the type of aircraft.
Subject: Relation between F/H and operating hours
Reply Posted by: Larry George
Organization: Problem Solving Tools
Date Posted: Sat Jul 28 17:19:38 US/Eastern 2001
People used the same diversion tactic on me when I worked for the Air Force Logistics Command. The relation between flying hours and operating hours is irrelevant and you'll probably never get the data to quantify it and use it properly! Sure, engines operate while aircraft tootle around runways and await clearance, but that operation is not a serious cause of failure. Tell the people that harrassed you with this red herring that you want people to record the actual operating hours along with flying hours. The harrassment will end or take on other forms.
Field reliability requires data. You can expect to get flying hours at failures or survivors' flying hours for fracture critical parts; for all other parts, probably all you can get is installations and repair or spares usage data grouped by accounting period. That data is sufficient to estimate age-specific field reliability and failure rate functions, in terms of flying hours or calendar time. Please refer to http://web.utk.edu/~asaqp/newsletters/1299newsletter.pdf page 13 for more info.
What do you need reliability in terms of all operating hours for? Engineering is already accustomed to using reliability in terms of flying hours; they have already factored in total operating hours. Service and spare parts decisions take place in terms of calendar time, so you need to convert reliability in terms of flying hours to reliability in terms of calendar time. Don't rescale using flying hours per quarter. Don't make actuarial forecasts using the same operating hours per quarter for each aircraft. The ratio isn't the same for all aircraft. Please contact me for the statistical method to convert reliability units, make actuarial forecasts, and estimate the distribution of parts and service requirements.
Next, the people that harrass you will point out that life limits are in terms of the minimum of flying hours or cycles, so be prepared for bivariate reliability functions. Airlines track both flying hours and cycles for parts with both life limits, so get the data and send it to me. I'll send back nonparametric estimates of bivariate, field reliability and failure rate functions plus diagnostics and applications, free.