Does an overhaul reset the 'life limit' of a turbine engine?

Question

I've come across a very unclear maintenance document. It's often referred to as a "Hard Card" for an Engine Axial Compressor Turbine. This specific part happens to come from a French manufacturer, so there may be a language barrier issue involved, but I'm trying to determine if anyone has clarified documentation that discusses this issue.

(Source: helisupportnz.com)

On this card, beneath a section translated as "Available Life", "Module Life Limited Parts", and "Usage Limited Parts", there is a section translated as "Time for the Calendar Limit". Next to this is another section translated as "Date for First Entry into Service" and a logical third calculated section translated as "Limited Date for Usage".

These connect with each other very clearly and easily. For example, one card marked with a 15 Year Calendar Limit is showing a Limited Date for Usage of 2020 when the Entry into Service was 2005.

I have a maintenance technician who is under the impression that this Calendar Limit is reset after every Overhaul of this Engine Module. Unfortunately, no where in this card does it say that this is the case, and if this module gets reset when it is actually a Life Limit, then we will run into an overfly.

Has anyone come across this and received a clear answer from someone in authority over this?

Answer 1

All nations operate with very similar rules. For the US, the short answer is nearly all turbine powered aircraft operate under part 119/121 (air carriers) rules. Each company has maintenance and operations specifically tailored and tied to their operating certificate and so it depends on the approved procedures of that companies operating certificate.

In your example, no mention is made of the part being "rebuilt", therefor the use of the part will continue where the previous use left off. The card says it is a "life limited" part so it's use will run out according to operating hours, calendar time from first installation, or number of operating cycles, etc. The parts life span is set by the mfg or operating certificate rules.

For example if a part has a 7yr calendar "life limit" (like some seat belts) and the part was installed January 1, 2015 then removed 10 days later, the part would become worthless Jan 1, 2022 even though it was used less than 1 month - the clock starts ticking from it's first use.

This card says the part has a 15yr calendar limit which will usually be in addition to other limits. It will either be limited from it's first date of use, or it's date of manufacture (whether the part is used or not). Once again, exactly how it should be interpreted will be based on the mfg or operating certificate rules.

Most life limited part life spans are TIS (time in service) and therefore expire after a certain number of hours of operation. If TIS for the turbine blade is 5000hrs and airplane "A" used it for 4000hrs, it could remain on the shelf indefinitely - be re-installed on airplane "B" and used for an additional 1000hrs. The card is designed to keep track of the dates and how long the part has been used - TIS, calendar, cycles, or whatever.

[I'm guessing for this particular part, but for many critical parts on helicopters and turbines a combination of TIS and calendar are used. So it would be something like 5000hrs TIS or 15yrs calendar from date of mfg - whichever comes first. Even if it sat on a shelf and was never used, it would have to be destroyed when the 15yr limit was up.]

NOTE: Due to a problem of expensive time limited parts being counterfeited, re-sold, or re-used, the FAA requires companies using life limited parts (once the service life is up) to make the part unusable by returning to a mfg, defacing, or destroying it.

Parts

On any US registered aircraft, "overhaul" and "rebuild" have very specific legal and maintenance meaning. "overhaul" per FAR 43.2(a) means "Disassemble, cleaned, inspected, repaired, reassembled, and tested to service limits", whereas Rebuild FAR 43(2)(b), 43(3)(j) means "disassemble, clean, inspect, repair, reassemble, and test to new part tolerances".

"Overhaul" means repair or fix the old part and does not change the wear or life of a part.

"Rebuild" means to make a "new" part out of the old one and the part's life is started over - "zero time". The part is treated as if it is a brand new part and many mfg even stamp a new serial number on the part.

Entire Engines

The above FAR's relates to "parts". For an entire engine, the FAA's AC-43-11 (Reciprocating Engine Overhaul Terminology and Standards) goes into great detail on what the terms mean and who may perform the work.

Why would AC-43-11 (entire engine) be limited to reciprocating engines? Because nearly all turbine engines are operated under Part 119/121 (not 91 or 135) operators. Part 119/121 receive FAA approval on a company by company basis and each company has maintenance and operations specifically tailored and tied to their operating certificate.

When a part 119/121 company has their operations approved, the FAA's standard practice is to hold them to all existing FAR's unless they show a "as good" or "better" way of maintaining the same level of safety. So, the spirit of AC-43-11 would be applicable unless some exception was given.

Answer 2

In my experience, engine parts with a life limit are thrown away when they reach that limit, and are definitely not reset by an overhaul (or repair). But... I have never ever seen a life limit in "calendar years". Normally the life is in engine hours, or throttle cycles. (See here for a description of cycles, and turbine blade failure modes.)

So, I would guess this life is for a static part, as I am unaware of a failure mode for a rotating part that is so loosely based on just being in the engine for x years.

However, without knowing what the part is, and what the failure mode is, I would be making a total guess if performing an overhaul, or repair, reset this life for this (unknown) part.

Answer 3

There are limits for "Calendar Life" based upon the degredation of a material's physical properties due to exposure to air, oil, ultraviolet life, fuel etc. Typically batteries in emergency devices, aircraft fabric, the fabrics used in life vests and rafts as well as the rubber used in fluid carrying hoses, orings used in propellers etc all have calendar lives

Flight hour life limits are typically there to set "inspection" times based upon flying hours. Some parts may be subject to material property strength limits based on the number of flying hours accumulated and so an inspection is required to ensure no degredation has occurred. Typically the "flight hour inspection period limits are reset, however some parts may have a cumulative requirement for additional inspections to be accomplished as the part's flight hour life grows.

Cycle life limits are directly related to Material Fatigue and repetative stress cycles that the part's material is subjected to. These Material fatigue limits may be related to flexing where the material tensile strength is effected ans the material is work hardened thru repeated flex cycles and becomes brittle - resulting in cracks developing etc.

Cycle life limits may also be related to "thermal Stress cycles" such as the thermal stresses experienced by turbine engine rotating parts during engine start, shift from a low to a high idle, or from idle to take off power.

Thermal stresses also effect material strength and if the part's thermal stress cycles are not properly tracked and accounted for the part may suffer a catastrophic failure.

So, if a part that has any, all or a combination of these limits has it's "Life reset at overhaul" the end user may face an "unexpected event" that can cost them a lot of $$$$$$ to fix or defend against.