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A photo of an OC-135 with (presumably) P&W TF33-P-5 engines made me wonder how many of the components had been replaced, and what was left of the original airframe, engines, and other parts. There are many aircraft operated today that are at least 50 years old (no real significance to that particular number).

Over the lifetime of say, a C-135/707 airframe without engine upgrades, is any of the original engine left? Are body panels replaced? Landing gear? Control surfaces?

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    $\begingroup$ The B-52 is also interesting given it's extremely long life. $\endgroup$
    – zymhan
    Mar 14, 2019 at 14:39
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    $\begingroup$ My 1973 Cessna is 46 years old this year. All the structural aluminum and steel parts are all original. Brakes, inner tubes, tires, those are normal wear items. Engine cylinders when they developed cracks, opted to replace with less crack cylinders. Propeller blades, also a wear item. Misc. engine accessory bits too. Avionics were upgraded twice by me since 1996. Last year I replaced all the external ABS plastic fairings with new fiberglass parts. And it's finishing its 2nd ever paint job and a new front window. Many small planes get similar treatments, and undergo annual inspections. $\endgroup$
    – CrossRoads
    Mar 14, 2019 at 17:08
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    $\begingroup$ Control surfaces can be replaced if they get beat up badly from hail, ground vehicle, etc. Wings can be patched - damaged part is unriveted, new one riveted on (plane at my airport hit a deer on landing late at night and needed part of wing reskinned). Ultimately, almost anything is repairable if one wants to spend the money. Another plane had corrosion found in the wing - so the owner went out and got another wing. A wing for a 4-seater is less than a military plane, but there are plenty to choose from out in the 'boneyard' if one is needed. $\endgroup$
    – CrossRoads
    Mar 14, 2019 at 17:15
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    $\begingroup$ A nice nod at the Ship of Theseus paradox: en.wikipedia.org/wiki/Ship_of_Theseus $\endgroup$
    – DeepSpace
    Mar 14, 2019 at 18:46
  • $\begingroup$ @CrossRoads and many see much more. Was in Canada last year where someone was reskinning a Gypsy Moth, replaced the main wing spar, tension wires, brakes, tyres, propellor, control cables. $\endgroup$
    – jwenting
    Mar 15, 2019 at 4:37

2 Answers 2

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In the US an aircraft may be allowed to fly indefinitely so long as it continues to meet the requirements of the definition for "Airworthy" in Court Order 8130 (.2e par 9).

Note: Aircraft operated by government agencies or the military do not have to comply with any rules, laws, or regulations - they may operate in any manner the agency desires (as long as they are not used for paying passengers). These aircraft are described in FAR 1.1 as "Public Aircraft". Examples: military fighters and transport, firefighting, research, etc.

There are two possible scenarios for replacing a part.

1) Life Limited: The manufacture or FAA through an AD (airworthiness directive) requires a part to be replaced at a certain limit i.e TIS (time in service) - calendar or operating hours, number of operating cycles (i.e. number of landings or pressurization cycles), etc. The manufacture is required to provide a chapter/section of the POH (Pilots Operating Manual) that list all life limited parts.

2) On Condition: Maintenance personal (often during an inspection) find the part 1) inoperative, 2) near or beyond "Service Limits" published by the manufacture, 3) in the discretion of maintenance personal worn and no-longer safe for operation, deemed not airworthy, etc. These parts are repaired or replaced as needed.

In practice many older aircraft have been completely replaced (technically a "repair"). There is no limit on the amount an aircraft can be "replaced".

War birds are one such example. It is legal to salvage an airplane from the ocean floor, use the corroded metal as a template to make new parts and completely assemble an aircraft from new material. The restored aircraft must be identical to the original aircraft and is legally considered the same aircraft ("all" the parts where repaired!).

There are many examples, such as the P-38 "Glacier Girl" being one of the more recent well document examples. It was salvaged after being crushed under 265ft of glacier ice for over 50 years.

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  • $\begingroup$ Part 121/129 transport category aircraft have a hard life limit, thanks to the widespread fatigue damage rule in 2011. $\endgroup$
    – user71659
    Mar 14, 2019 at 19:58
  • $\begingroup$ I said, "may be allowed to fly indefinitely so long as it .. is airworthy". Part 135/121/129 opps are allowed to make their own rules subject to the approval of the FAA. Often companies like engine, prop, and Boeing set low time life limits, then get repeated amendments and extensions as fleet data becomes available. Even then a trashed 121 aircraft might be sold and used in another category such as research, firefighting, etc. $\endgroup$
    – jwzumwalt
    Mar 14, 2019 at 20:45
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    $\begingroup$ There is one part that must not be replaced: The type plate. $\endgroup$
    – Peter Kämpf
    Mar 14, 2019 at 20:57
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    $\begingroup$ Oldest aircraft in the world still flying is a 1909 Bleriot belonging to the Shuttleworth Collection in the UK. Might be interesting to research if any part installed by Bleriot in 1909 remains in that one :) $\endgroup$
    – jwenting
    Mar 15, 2019 at 4:38
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    $\begingroup$ @jwenting the type plate. :) $\endgroup$
    – FreeMan
    Mar 15, 2019 at 14:14
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That depends on how the aircraft is used, and how much its flown. Most aircraft parts that are life-limited are cycled out either after a certain number of hours, or after a certain number of cycles (takeoff to landing is one cycle). Years is a somewhat irrelevant term in aviation since it does not practically measure much more than the time that has elapsed since it rolled off the assembly line.

If a plane is privately owned, and/or lightly used and there are no AD's against any of the parts causing them to be replaced there may very well be old planes flying around with old parts. With that in mind, heavy use, commercial planes that get banged around, hit by birds etc may have some new body panels, fresh landing gear, updated avionics etc.

In your specific example, body panels will be replaced if they are damaged but provided there is no corrosion or damage and the airframe is within its cycle/hour lifetime nothing will be replaced. The same goes for the landing gear (although tires may be cycled faster than other parts).

According to this article, the JT4, which some 707s had, has a 2400 hour TBO. While TBO is not a legal limit you can bet that most commercial operations are going to pull them around there. Its possible an engine gets overhauled and replaced on the same airframe thus there may be planes flying around with their original engines. However most engines can only be overhauled so many times and in 50 years, considering regular usage, its likely the original engines are long gone.

There are even some rare 100-year-old early era airplanes still flying that likely have original parts due to the lack of availability of any replacements.

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  • $\begingroup$ TBO is a legal limit for all commercial op i.e. part 121/135 etc, it is not binding for privately operated aircraft. However, some commercial operators are able to show special circumstances and receive a waiver. The flight school I got my PPL in 1975 had a Cessna Cardinal with a 2200hr TBO that was being flown with more than 2400hrs as a complex checkout. The school was allowed to operate it so long as either it was only flown in the pattern or there was a flight instructor on-board. $\endgroup$
    – jwzumwalt
    Mar 14, 2019 at 20:36
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    $\begingroup$ The number of years since the plane rolled off the assembly line is by no means irrelevant - corrosion doesn't care whether the plane's flying or not. $\endgroup$
    – Vikki
    Mar 15, 2019 at 4:03

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