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In aircraft reliability monitoring analysis when we calculate the alert level, why is the calculation of defect rate per 1000 FH?

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When calculating a rate, you have to decide what time unit you use as a basis.

For reliability monitoring, using wall clock time would not make sense, since most defects are caused by use of the aircraft, not degradation of materials over time.

Therefore flight time is more appropriate than wall clock time. Using a basis of 1 flight hour would result in very low numbers for the defect rate. Multiplying those numbers by 1000 gives a bigger number, easier to work with (fewer 0s after the decimal separator).

It must be noted that systems do also degrade over time when they are not flown. Temperature cycles during day and night causing expansion and retraction of materials, and humidity causing corrosion and evaporation of oil components in lubricants are some of the mechanisms that will cause defects over time but they are not driven by the number of flight hours.

When the defect rate is expressed in flight hours, some assumptions need to be made on the utilisation rate of the aircraft, in order to take these calendar time effects into account. This will result in higher MTBUR/MTBF values for aircraft that fly a lot (e.g. airline use) vs lower values for infrequently used aircraft (e.g. business aircraft).

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    $\begingroup$ I agree with everything you said. Despite the fact MTBUR/MTBF are measure in flying hours, you can't ignore non-flying hours when it comes to avionics. Just sitting on the ramp with daily temperature swings and humidity/condensation effects has a detriment on avionics. When I worked for one avionics supplier, we had separate reliability rates for the same components in aircraft in scheduled service (200-300 FH/month) and another for business aircraft (200-300 FH/year). Reliability (MTBUR/MTBF) in business aircraft was typically half of that obtained in scheduled service. $\endgroup$
    – Gerry
    Sep 21, 2022 at 17:22
  • $\begingroup$ Very interesing comment @Gerry, thank you! I might try and incorporate some of that into the answer if you don't mind. $\endgroup$
    – DeltaLima ♦
    Sep 21, 2022 at 21:20
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    $\begingroup$ Believe me big dollar corporate airplanes don't spend any time outside when inactive. They're pampered playthings. Low utilization IS a problem however, even kept in a heated hangar, because things both mechanical and electronic don't like to sit. In RJ land, you have corporate versions of the same airliner, and there were two separate maintenance programs each with its own analysis foundation. The bizjet program was the special Low Utilization program, where you would have calendar limits to take effect if the hours level was insufficient over some period, to account for the sitting around. $\endgroup$
    – John K
    Sep 21, 2022 at 22:45
  • $\begingroup$ Of course @JohnK and Gerry service intervals on cars work in a similar way - maintenance that happens every year or 20000 miles, for example. And part of this is the effect of prolonged storage on components, especially soft parts like seals sticking or deforming subtly. $\endgroup$
    – Chris H
    Sep 22, 2022 at 9:15
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    $\begingroup$ Another common measures is cycles (flights), i.e. from takeoff to landing. This is especially the case for pressure-relate fatigue defects. This is especially an issue for carriers that operate short routes, e.g. inter-island flights in Hawaii or Indonesia. $\endgroup$
    – MSalters
    Sep 22, 2022 at 15:31

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