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Is it possible for an air force to operate a fighter aircraft indefinitely?

Suppose a country has some Mig-21s or Mirage-IIIs. Is it possible for them to operate those aircraft indefinitely by continuously overhauling and upgrading them?

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    $\begingroup$ Well, if you manufacture and replace enough of the aircraft including the airframe you could keep anything flying for ever - wouldn't really be the same plane though...? Infinity is a very long time, too... Bear in mind that air forces don't generally design and build their own aircraft though, so for most modern militaries the answer would be no without the aircraft builders cooperation. $\endgroup$ – Dan Nov 29 '15 at 21:12
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    $\begingroup$ youtube.com/watch?v=s1VNNbSYdt0 $\endgroup$ – Simon Nov 29 '15 at 21:53
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    $\begingroup$ In 1983 I saw a military flight line of beautiful P-51's in Central America. That same year I saw a F-100 flying in Turkey. The B-52 is over 60 yrs old and may still be flying 100 years after first flight. Israel and Germany had essentially brand new F-4's when they were all but gone from the US arsenal. Many DC-3's still flying today doing real work. Sooner or later a cost-benefit analysis sends them all to the scrap heap. $\endgroup$ – radarbob Nov 30 '15 at 1:43
  • $\begingroup$ I don't see how the question can be too broad when there's an accepted answer five paragraphs long. "Too broad" generally means the answer is a list or the answer is a book. $\endgroup$ – Dan Hulme Nov 30 '15 at 8:47
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    $\begingroup$ Cf. en.wikipedia.org/wiki/Ship_of_Theseus $\endgroup$ – Relaxed Nov 30 '15 at 16:01
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Yes and no. In Cambridgeshire we still have some Spitfires and other Second World War aircraft which are still operated more than 60 years after being built. The Shuttleworth Collection even flies some First World War and pre-war aircraft. It's demonstrably possible to keep operating a military aircraft for at least a century after it's built.

But that's not useful if the aircraft has no military value. None of the examples I've mentioned are still operated as military aircraft: they're in civilian hands, performing for displays, mostly for their historic value (though many ex-military aircraft in civil use are performing other functions, such as air transport or crop spraying).

Aircraft get retired for two reasons. The most obvious is that they're superseded by newer types, newer designs with better technologies and materials. The first air force that tried to field one of these Spitfires in combat against modern jet fighters would quickly realise it had no military value whatsoever.

The second reason is that they get increasingly expensive to maintain. Military aircraft especially can be very fussy about fuels and parts. Maintenance might require special fabrication of small numbers of parts or tools, which gets more expensive when the rest of the industry has moved on. (For example, the Tiger Moths I fly have one particular part that's impossible to obtain, because the alloy it's made of isn't produced any more.)

For this reason, although there are still surviving, operating aircraft from the earliest days of military aviation, it wouldn't be feasible or useful to continue operating them as combat aircraft.

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  • $\begingroup$ So what do you do on the Tiger Moth when that part needs replaced? $\endgroup$ – TomMcW Nov 30 '15 at 0:32
  • $\begingroup$ If it breaks you ground it, unless you can manufacture a substitute or cannibalise another aircraft. $\endgroup$ – Jon Story Nov 30 '15 at 0:40
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    $\begingroup$ It's an optional part, so many flying Tiger Moths have a workaround instead. If that weren't the case, I'm sure an equivalent made of a newer alloy would have been certified for it by now. $\endgroup$ – Dan Hulme Nov 30 '15 at 8:44
  • $\begingroup$ @DanHulme Or, possibly, used under "experimental" category regulations. $\endgroup$ – a CVn Dec 14 '17 at 14:13
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In addition to the other answer: The magic number is the fatigue life of the aircraft. This is the number of hours an aircraft can be flown before metal fatigue possibly sets in and the manufacturer can no longer guarantee the safety of the aircraft. There have been cases (Vulcan and Victor in RAF service come to mind) where this limit was reached fairly quickly, and to keep the aircraft in service they would have had to replace the main wing spars, i.e. rebuild half the aircraft. For complex aircraft like those, that's generally only possible with support from the manufacturer.

However, if you do have support, a lot is possible. You can get avionics upgrades for the MiG-21 that allow it to fire modern weapons, for example.

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An aircraft can be flown for as long as there is an operational demand for it. Keep in mind that the aircraft will likely have been through multiple engines and subsystems thereby "refreshing" the aircraft over time, but technically there is no reason it can't continue to fly as long as it continues to serve a purpose.

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