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