Damage on one aircraft should be repaired/eliminated [by replacement] when no other similar-to examples exist.
EXCEPTIONs that I developed, working as a aerospace field service engineer in the 1990s, thus...
I worked with various fighters [squadrons, multiple acft] and was faced with this dilemma on a regular basis.
These particular acft had a mandatory redundancy in primary structures and systems elements and a slow/stable crack-growth requirement. These 'components' [almost] always had very clear guidance in the tech data for cracking, corrosion, wear, etc damage.
However secondary structure and systems components often had poorly defined cracking, corrosion, wear, etc damage limits: Inspect when it is required by tech data; IF you see IT then you must repair or replace IT.
Yeah Right.
A small fleet wide sampling was used to determine if a one-off problem was really a one-off problem... or if the sampling revealed more acft with similar issues... IF SO... then a fleet-wide inspection was accomplished.
During these inspections, the size of each detectable defect was determined/recorded for each location on each acft. See below for applicable examples. This data was tabulated and the damage [by acft/location on the acft] was charted on a spreadsheet... then arranged in worst-to-best case order [worst-defect to barely detectable defect order].
Here’s where the ‘logic’ became specific to multiple highly stressed fighters: most flights [training, actual combat, etc] were very similar [for the fleet], and all/most aircraft had similar flying hours and operational exposure. I ass-u-med that the worst case damage detected occurred on the last flight before it would fail [next flight]... which lead to the obvious plan to minimize effect on the fleet, thus...
Based on this analysis [and back-up simplified stress analysis] I set a max damage limit [below the worst case] for 'unserviceable' = too-close to failure and must be grounded and maintenance action taken.
Damage exceeding my cutoff limit required immediate repair or replacement to attain a fully serviceable condition. This 'usually' affected only a small population of acft, immediately.
Damage less than my cutoff limit was allowed to remain in service with recurring inspections that I set based on relative ratio of damage size to my-damage-limit size... and usually resulted in 0-5, 5-10, 10-20, 20-40, or up-to-100 etc, flying hour limits before mandatory re-inspections for damage growth.
Damage that grew to the cutoff limit required immediate repair or replacement [noted] to attain a fully serviceable condition.
Meanwhile repairs or parts replacements began immediately on a worst-to-best case recorded damage sequence. These repair/replacement actions were accelerated to ensure that 'the problem went-away quickly'... and allowed breathing room to still fly Full Mission Capable [FMC].
NOTE. I rarely worked in a vacuum [very few exceptions]. I always contacted my depot counterparts and laid-out my plan... and sent a few damage specimens [removed from service for repair/replacement] to them for failure analysis... and strongly suggested that the remaining fighter fleet [worldwide] be sample inspected for the same defect. Curiously, we often discovered that wildly unpredicted damaged was limited to an acft production-run [block] or to a particular operating base/environment [etc].
NOTE. Commanders were reluctant to allow unscheduled fleet sampling or fleet-wide inspections, since it ‘wasn’t in the book’ and manpower was scarce and FMC jet-count could drop for scheduled missions... temporarily or critically ‘depending on findings’.
NOTE. For military commanders, I learned that a clearly presented path/goals was vital... and reminded these ‘command-pilots’ [mostly] that these inspections to mechanics are similar in nature to combat reconnaissance for warfighters... had to be done for mission success/safety.
Examples where these judgments came in handy.
CRES bleed-air band-clamps that cracked thru spot welds.
Belly and bay Panels that chattered/wore-out-edges/holes by chaffing/fretting [especially during deployment in a desert environment due to trapped fine-sand].
Plain bearings/extruded hinges that wore-out prematurely.
Roller bearings that wore-out prematurely.
Wiring that was chaffing/abrading against a titanium bleed air duct.
Circuit breakers that tended to corrode the wipe-contacts ‘closed’ in a seacoast environment.
Titanium hydraulic tubing that abraded the wall [to 3/4 thickness] due to Teflon lined clamps in a desert deployment environment due to trapped fine-sand.
Titanium skin panel cracking damage located at the base of the leading edges of both vertical stabilizers.
Pulled/broken-off fastener heads in certain lightly loaded skin fairing panels.
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]
