Flight testing after engine changes.
I would be grateful for some help for researching the answer to a "what-if?" from my boss.
I fly an old 4-jet for the military and often have to perform the post-maintenance airtest. Each airframe is given a 'major' service about every 6 years, which takes 4 months and is quite an invasive process. I believe the civil terminology is a D-check? All 4 engines are removed and often, but not always, any combination of engines may then be changed depending on other criteria. After a major there is a 4-hour airtest to be flown, a small part of which includes individual engine slam-acceleration tests and shutdown/relight tests. The engine tests take place regardless of whether any of engines have been changed for new units.
Between these majors there is also 'minor' service, which sometimes requires engines to be removed/re-fitted or changed. We have a regulation that requires the aircraft to be flight-checked after a double-engine change and therefore the slams, relight checks etc are performed on the new engines. Traditionally we have treated the removal/refit of an engine as a 'change', on the basis that the integrity of the unit has been disturbed. So the removal and refit of 2 engines would generate the requirement for an airtest. There is no requirement to airtest an aircraft after a single engine change, and it goes straight back into normal service.
For financial reasons we have been asked to consider dropping the requirement to airtest the aircraft after the removal/refit of engines and to regard only the fitting 2 or more new engines as triggering an airtest. If we went down this route, the worst case scenario could see an aircraft have 3 engines removed/refitted and the 4th changed, before being be released straight back into service with only ground checks.
I am curious to know how other civil or military aircraft are managed in equivalent circumstances. It is possible that we are being too cautious or maybe we are following normal practice, particularly for older types?
Do other organizations/companies require a flight test of aircraft after multiple engine changes?
Do other 4-jet operators (particularly older types) have similar, or perhaps much more lenient, requirements?
Is the removal and refit of an engine regarded as equivalent to a change by anyone else?
Many thanks in advance.