So I am a little unclear on what the regulations recommend.

Say you have a component that has a Restoration (Ignore the nomenclature) workscope but it is strictly an Inspection/Cleaning/Testing process. The FAA/EASA tags would say Inspected/Tested.

Does that mean the TSR/CSR (Time Since Repair / Cycles Since Repair) need to be reset to zero? Or would the TSR/CSR continue as-is because there was no Repair work carried out? Any assistance would be appreciated.

  • $\begingroup$ If there is a Restoration task in the Maintenance Program, the time will be tracked based on the hours/cycles the Restoration was performed. How does "repair" come into it? If it goes in for a repair "out of phase" it can be considered compliant with the restoration interval and zero timed from that standpoint, but may require language in the repair documentation that says the repair meets the restoration requirement. Restorations like that that don't involve parts replacement are usually due to a grease/lubricant replacement requirement. $\endgroup$
    – John K
    Jul 18, 2019 at 4:22
  • $\begingroup$ @JohnK Thanks for your reply. I have observed various EASA/FAA tags mentioning TSN/CSN, TSR/CSR and TSO/CSO. Generally the maintenance organistions will change the TSR/CSR to zero after a Repair work but for Test/Inspect as well some of them reset TSR/CSR to zero but some continue the TSR/CSR as per last known values. What do best practices recommend? $\endgroup$
    – Cameron
    Jul 18, 2019 at 5:21
  • $\begingroup$ Is this under an MSG-3 based maintenance program or something else? $\endgroup$
    – John K
    Jul 18, 2019 at 15:33
  • $\begingroup$ Yes, this is MSG-3 indeed $\endgroup$
    – Cameron
    Jul 26, 2019 at 9:47

2 Answers 2


In an MSG-3 based maintenance program there's no such thing as "overhauled" as in "put back in new condition and reset to zero". Line Replaceable Units will be qualified operationally and structurally under an endurance/fatigue test program and will normally have a structural life limit where a unit carries structural loads of some kind. If there is no applied life limit the part can be theoretically run forever (or at least the certified life of the airframe), because under MSG-3, components can be run indefinitely until some form of inspection or test in the field detects a degradation beyond a limit. Think functional test, leak test, NDT, visual inspection etc.

What happens with LRUs is they may fail or some internal parts may fail during endurance testing. The part was designed to run 80000 cycles, was tested to 120000 cycles (1.5 scatter factor margin for mechanical parts), and say a bearing and thrust washer failed during the testing at 60000 cycles of testing. Those parts will be replaced and the endurance test continued to the end.

But now when it's certified, there will be a need to send the unit in at 40000 cycles (1.5 scatter factor applied to 60k) to "restore" it to its pre-test failure condition. The unit has to go in, be disassembled and reassembled and any parts that are out of serviceable specs replaced, but the parts mentioned in the restoration task must be replaced, serviceable or not.

Sometimes, a part with a grease charged gearbox will have a restoration applied because the grease was found to be breaking down during endurance testing, and the grease is the only component that must be replaced. The restoration task in the maintenance program will say specifically what has to be done, as in "replacement of the grease" or "replacement of the Main Ballscrew Thrust Bearing and Washers, PNxxxx and xxxx" (Operators get mighty mad when they send some part in for replacement of grease or 40 dollars worth of needle bearings because the Maintenance Program requires it, expecting a 2000 dollar bill, and the unit comes back with half the guts replaced and a 15000 dollar repair bill because other parts were damaged or worn out prematurely).

When an LRU goes in for repair not related to the maintenance program (like a random breakdown), whatever parts are replaced will be those that are out of service limits or are considered needed for the unit to pass an outgoing Acceptance Test Procedure (functional test in a test rig). When it comes in for repair, it will get an incoming Acceptance Test done (the same rig test), and if it passes, it may go right back out No Fault Found unless the operator specifically demands it be torn down anyway. In any case, a repaired part does not come with any reset to a Life Limit, but it MAY reset the restoration interval if the restoration requirements were met during the repair.

The Life Limit applies to the root housing (the structural part with the serial number) and when a unit gets to the life limit in total hours/cycles it is scrap even if it was fully "rebuilt" 5000 cycles ago when it was sent in for a repair (so you generally try to avoid doing that).

Anyway, under this type of maintenance program, what needs to be tracked are total hours/cycles since manufacture to which the LRU's Life Limit applies, and hours/cycles since Restoration, (and hours/cycles since functional test, or some inspection obviously) where the maintenance program has a Restoration task applicable to an LRU (like a screw jack that has to go in every 30000 cycles to get a set of bearings changed).

So.. if you had a part with a tag that has "time since repair" on it, that information is not sufficient, and has no real effect on anything, because the only mandatory removal interval under MSG-3 is the Restoration task and the Life Limit. If a screwjack's tag says it was repaired at 20000 cycles, and there is a 30000 cycle Restoration in the maintenance program, you're going to have to send it back in in 10000 cycles (when it gets to 30k) unless there is language in the shop paperwork that states that parts mentioned in the Restoration were replaced, or there is a statement something to that effect. If that's the case, you can now run it to 50k, the next 30k interval.

Then when it gets to the life limit, it is trash even if it just went in and had a complete and total "overhaul" done where everything but the gearbox was replaced, following a random breakdown in the field. An LRU that breaks down a few thousand cycles short of its life limit will normally go in the trash.


Did the LRU require Repair or Overhaul? If not, then no reset. Your description of clean, inspect, and test would be applicable to "Inspected/Tested" status.

Use the status block on FormONE or 8130-3 to guide reset of TSO/CSO or TSR/CSR.

Typical regulatory guidance on this block is use the status which represented the majority of the work performed this shop visit: e.g. Modified, Inspected/Tested, Repaired, or Overhauled.

Note: TSN/CSN should never be reset.

Airlines track "soft" times on LRU's to optimize scheduling times between shop visits to minimize unplanned removals during Aircraft Operations which cause flight delays and interruptions to their schedule.

Only Life-Limited-Parts (LLP's) have "hard" limits. These are the fatigue parts previously mentioned by John K. LLP's are subject to strict regulatory rules for technical record keeping and mutilation at disposal.


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