Aircraft Repair Lifetime

If an "older" aircraft type, and if the repair is done according to the structural repair manual, then the life of the repair is as good as the life of the aircraft - with certain conditions of inspection and protection needing to be applied which vary from aircraft to aircraft.
As you get to newer and newer types of aircraft, and larger types of aircraft, many other considerations apply. In those cases, the life of the repair is the life to which the repair is designed - and the conditions of inspection, protection become even more strict.

I can't be more specific unless you have a specific problem with a specific aircraft.

Note that I did not touch the word "minor". No way to determine that without an examination of the aircraft damage.

STF

The repair is not done according to the SRM but thru the approval of DOA. The aircraft types are more specifically on PART 23, 27 and 29 (e.g AS350, EC120, AW139 etc).

Examples of minor repair performed:
Aft Fuselage Upper Panel delaminated - repaired by removing the delaminated area and installed doublers on the affected area.
Baggage compartment floor panel cracked - repaired by patch.

The problem now is to determine the lifetime of the repair (permanent or temporary). Are there any guidelines for non OEM (DOA) to refer to?

Are there any guidelines for non OEM (DOA) to refer to?
Start with one of the 5 day DTA courses like this one maybe (although much of it is about pressurized fuselages)

Otherwise comparative analysis with a existing feature (rivet line, SRM repair, etc) is always a good place to start. Floor panels aren't typically cyclically loaded nor primary structure. Not sure there is any easy methods for fatigue critical structure in Rotorcraft structure. Although if you are supporting suitable sized fleet, the cost of a strain gauge is equal to about 20 hr's engineering.

If you are tying to justify the life impact of oop's to riveted SRM repairs this book is really good "Riveted Lap Joints in Aircraft Fuselage: Design, Analysis and Properties".

the DAO (or DOA) should have referred to xx.571 or other rules for continuing airworthiness as part of certification of the repair; possibly also Part 26.

Some areas may be considered non-structural, so I'd look to what the OEM does in these areas (in terms of inspections).

Otherwise if structural, then you need F/DTA to demonstrate that the repair is good to the remaining life of the aircraft.



another day in paradise, or is paradise one day closer ?

its.ME.lah

In addition to everyone else's comments... another perspective/implications based on hard-lessons-learned [military].

Primary structure is required for continued safe flight: in-flight failure can cause major structural damage and may/may not cause loss of a major component [engine, weapons pylon, etc], loss of individual human life or loss of the aircraft.

Secondary structure is not absolutely required for continued safe flight. In-flight failure may result in degraded function and/or actual 'dropped/lost/ component/assembly. NOTE: IF aircrew is unaware of damage then this may become a deadly serious problem. NOTE: Sometimes a 'dropped-object'... even though it is benign to the survival of the aircraft... can create a host of political/economic or public-safety issues when it hits the ground/roof/car/person, etc.

Class of repair = class of damage.

Minor damage
I tend to see 'minor repairs' and gouge/scratch/corrosion/small crack repairs... where hand-sanding to remove the Kt or pitting, and/or stop-drill-rivet-fill of small crack-tips, followed by restoration of corrosion protection finishes and moisture/pressure sealing.

Major damage
Flight safety and airworthiness not immediate concern; but will become a concern in relative near term. IF un-repaired, then may evolve to be (a) economically infeasible to repair; or may become (b) non-airworthy [critical] and cause grounding or possible in-flight failure.

Critical damage
Damage-repair cannot be deferred/postponed due to possibility of immanent failure an/or economic infeasible to salvage-repair the component/airframe.

Type of repair

Permanent = Full expectation that no further repair required during remaining lifetime of the aircraft.

Life-limited = repair life is expected to no greater than a specified service intervals W/WO inspections.

Temporary/restricted = repairs that allow less than 1/2 a typical service interval but allows aircraft to be flown full mission capable for a short period or to be flown partial mission capable for a slightly longer period. Flying limits are strictly observed, such as gross takeoff weight, G-load, speed, distances, etc.

Battle Damage [ferry] = minimal/partial required repairs that allow reasonably safe restricting the aircraft to extremely short duration flight, minimum crew and no payload, and which allows it to be ferried safely to a suitable location for major repair [W/WO imposed flight limits and total flying hour limits]. In some cases these type repairs may not be feasible; forcing the repairs to be accomplished on-site; or causing the airframe to be scrapped on-sight.

IF battle damage repair is really a critical temp repair then the entire world has to know about it and real action taken as early as practical to ensure specified flight/hour limits are not over-flown. NOTE: I have done 'battle damage' type repairs used simply to get aircraft safely back to USAF Depots for major repair [W/WO imposed flight limits and total flying hour limits]; or to get the airframe to Davis-Monthan AFB ['the desert'] for permanent grounding/storage/salvage/demolition.

NOTE: Any 'life limited repair' should actually be designed to last ~2X longer than intended or imagined. Reason: temporary repairs can 'over-flown' by circumstances beyond everyone's control... or may be 'forgotten' in the heat of flying missions.... or may prove tempting to 'get one-more-flight out of the acft. YES THIS HAS HAPPENED [TO ME] MANY TIMES!!!

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

If the component or part is not subjected to fatigue cycles or if the fatigue stresses are within endurance limit, then the repair could be safely categorized as permanent (having said that it should meet the static strength requirements). Otherwise a F/DT analysis must be carried out arrive at fatigue life. There could a life limitation or an additional inspection program as a result of F/DT analysis.

I'd expect the repair to be permanent, but it should (if it is on primary structure) have an inspection program. It probably affects the inspectability of the underlying structure. In the commercial world (at least AFAIK) there are limits for inspection intervals, to ensure all primary structure is inspected; military have "slow crack growth, non-inspectable".

another day in paradise, or is paradise one day closer ?

Has anyone here used the Detail Fatigue Rating [DFR] system for estimating fatigue life of repairs to individual parts?

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

someone stole my Boeing manual ... ok, I admit it wasn't a controlled copy ...

another day in paradise, or is paradise one day closer ?

Temporary repairs are also helpful when it is the first time that damage has turned up, also temporary repairs can be a helpful work smoothing practice, with the repairs paper work being reissued with maintenance instructions with 12 months when there was a gap in the traffic (mainly heavy maintenance support).

One aircraft turned up with popped rivet heads due to the steel emergency exit doubler corroding, doubler was partially unriveted to allow removal &treatment, and with a repair doubler installed released to service. The next aircraft in had, similar damage of a greater extent that was found to be non-inspectible (visual or NDT) without part removal (we found corrosion 50% of thickness on the best looking doubler that was only replaced only because it will be trouble & never cheaper to replace than now have done 2 others on the same aircraft).

Certification basis matters!

AS350: FAR 27-amendment 10, prior to the amendments that made 27.571 apply to many structures.

For these models, there is no fatigue requirement for structures such as baggage bay floors. For rotating components, there certainly is, but most of the airframe is not "flight critical" where failure could be "catastrophic".

Later amendments of these paragraphs expanded the requirement, therefore newer models of helicopters begin to have more structure that are fatigue life-limited or subject to damage tolerant inspections. However, if there is no life on the part to begin with, then it's still not fatigue critical.

AB139 & AW139: FAR 29-amendment 45, prior to the introduction of 29.573, but 29.571 does apply.
EC120 & EC130: FAR 27-amendment 32, so FAR 27.571 is applicable to flight-critical structure.
And there still isn't any "27.573" yet.

For these models, FAR 29.571/27.571 is applicable, but 29.573 is not. Fatigue life should have been established for the repair by the DOA and appropriate maintenance/inspection documents generated, but only if necessary. If the original structure that needed the repair didn't have a life or a mandatory inspection interval, then it's not fatigue critical and the repair won't be fatigue critical, either. In that case, life of the repair is unlimited. If there is an inspection interval on the original structure, then the repair must permit the inspection, and be effective for long enough for subsequent inspections to find subsequent damage.


STF

Thank you all for the reply.

But the examples that I states earlier are not Major repair and does not involves the primary structures therefore I don't think it should complies with para .571 (the aircraft is unpressurized) and .572.

Or should I make an assumption that any repairs that does not involves any primary structures and the loss of it will not cause a catastrophic failure can be considered as permanent repairs and shall be inspect on-condition or annually.

IMHO, you're mixing a whole lot of terms.

If the repair is not on primary structure, then yes that's good logic for 571 being not applicable ... but that depends on your definition of "primary structure". Fairings for example are often considered not primary structure, but a failure could impact aerodynamics of the plane, so something to be considered.

the plane is non-pressurised ... new info (I think) and suggests part 23 (or 27).

most repairs on commerical aircraft are considered (AFAIK) to be permanent. temporary repairs would be used only to fly the plane to a proper repair base. the military have their own guidelines. i'm assuming the plane is commercial.

an annual "on condition" visual inspection doesn't sound like a bad idea, though it wouldn't be mandatory (it'd be recommended), and probably not a hardship to the operator.

another day in paradise, or is paradise one day closer ?

Its.ME.lah,

Even a temporary repair can have inspections program though not for airworthiness reasons, it could be purely for economic reasons.
Some times it is recommended to inspect temporary repairs, so that it doesn't lead to a situation were permanent repair becomes too expensive.

So, just having an inspection program doesn't classify the repair as permanent or temporary.

All commercial aircrafts are pressurized (except for some sections), so it depends which section of the aircraft you are dealing with.
Even a secondary structures (like fairing, panels....) can have a temporary repair (permanent solution being a replacement of the part). So it is not necessary that all secondary structure repairs should be categorized as permanent.