MTBUR Calculation

[huron2016]

In ATA CSDD (Common Support Date Dictionary)

MTBUR Definition:
"A performance figure calculated by DIVIDING the total unit flying hours (airborne) accrued in a period by the number of unscheduled unit removals that occurred during the same period."

Q: What is MTBUR when number of unscheduled removals is ZERO (0) and the reasoning ? Thanks for any insights.

[Pantelis]

The short answer is to assume an exponential distribution (constant failure rate). In this case the MTTF (mean time to failure or what ever other event you are modeling, i.e. UR) is:

MTTF={2*(Total Time Without Failure)}/Chi^2(CL;2)

where CL is the desired confidence level, and Chi^2 is the value of the Chi^2 distribution at the desired CL and with 2 degrees of freedom.

To compute the Chi^2 value you can use the QSR Tool (http://www.reliasoft.org/_rsapps/itools/qsr.aspx) at http://www.weibull.com/itools/index.htm. In this tool use d=1-CL (i.e. 1-0.90) and v=2. As an example and for a 90% CL this is 4.61.

Now as to the statistics behind this, you can find that in many reliability and statistics textbooks. I am assuming you have some textbooks available. Otherwise if you need a specific reference see [13] as listed at http://www.reliasoft.com/books/index.html.