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While reading the FAA presentation on Part 135 Airworthiness Requirements, on slide 7 where it discusses maintenance, the list includes

inspection, overhaul, repair, preservation, and the replacement of parts, but excluding preventive maintenance

I am sure that the other regulatory bodies have their own, similar lists.

I know that an aircraft overhaul is one of the major processes involved in (especially commercial) aircraft maintenance. What exactly is meant by overhaul? What is done to an aircraft when it is overhauled? And finally, why is it so important for an aircraft?

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    $\begingroup$ Could you perhaps elaborate your question? $\endgroup$ – Zizouz212 Jun 2 '15 at 16:41
  • $\begingroup$ Done! Is it okay now? Anyways, the answer will make the question absolutely clear to you. $\endgroup$ – anshabhi Jun 3 '15 at 14:27
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    $\begingroup$ But if I don't even understand the question, why would I care for the answers? $\endgroup$ – Zizouz212 Jun 3 '15 at 16:24
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    $\begingroup$ Does this edit help the down-voters? $\endgroup$ – CGCampbell Jun 3 '15 at 19:03
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    $\begingroup$ @MikeFoxtrot Why does this question need to be limited to commercial jets? Plenty of piston-engined and turboprop aircraft operate under part 135. $\endgroup$ – Pondlife Jun 3 '15 at 20:24
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Briefly, "overhaul" isn't a strictly defined term but it's generally used to refer to the intensive inspection, maintenance and repair of an aircraft component, frequently - but not exclusively - the engines.

The word overhaul by itself is a general term that's used in all sorts of places but it's not defined in 14 CFR 1.1, for example. The most specific definition I could find is in this agreement between the FAA and EASA:

"Overhaul" means a process that ensures the aeronautical article is in complete conformity with the applicable service tolerances specified in the type certificate holder's, or equipment manufacturer's instructions for continued airworthiness, or in the data which is approved or accepted by the Authority.

No person may describe an article as being overhauled unless it has been at least disassembled, cleaned, inspected, repaired as necessary, reassembled and tested in accordance with the above-specified data.

Note that that definition only mentions "aeronautical articles", which is very broad indeed and not limited to just engines, airframes or anything else.

In the general aviation world, a very common usage of "overhaul" is in the context of piston engines, which have a Time Between Overhaul (TBO or TBOH) set by the manufacturer. Once an engine reaches that number of hours, it needs to be overhauled. But even there, the term isn't precise and the FAA has a circular called Reciprocating Engine Overhaul Terminology and Standards that explains what it usually means:

c. Overhaul. In the general aviation industry, the term engine overhaul has two identifications that make a distinction between the degrees of work on an engine:

(1) Major Overhaul. A major overhaul consists of the complete disassembly of an engine. The overhaul facility inspects the engine, repairs it as necessary, reassembles, tests, and approves it for return to service within the fits and limits specified by the manufacturer’s overhaul data. This could be to new fits and limits or serviceable limits. The engine owner should clearly understand what fits and limits should be used when the engine is presented for overhaul. The owner should also be aware of any replaced parts, regardless of condition, as a result of a manufacturer’s overhaul data, SB, or an Airworthiness Directive (AD).

(2) Top Overhaul. Top overhaul consists of repair to parts outside of the crankcase, and can be accomplished without completely disassembling the entire engine. It can include the removal of cylinders, inspection and repair to cylinders, inspection and repair to cylinder walls, pistons, valve-operation mechanisms, valve guides, valve seats, and the replacement of piston and piston rings. All manufacturers do not recommend a top overhaul. Some manufacturers indicate that a powerplant requiring work to this extent should receive a complete overhaul.

Apart from that usage, the term MRO (Maintenance, Repair and Overhaul) seems to be used in the airline industry to describe maintenance facilities, but there isn't any specific definition that I could find.

So there's no single definition of "overhaul". It just means that you've very thoroughly inspected and tested something on an aircraft, whether it's an engine or a wheel, and you've verified that it's fit to be used in flight, i.e. it's airworthy. The reasons why it's important are probably obvious; they're the same reasons that your car needs regular servicing to run safely, efficiently and reliably.

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Overhauled. Describes an airframe, aircraft engine , propeller, appliance, or component part using methods, techniques, and practices acceptable to the Administrator, which has undergone the following:
(1) Has been disassembled, cleaned, inspected, repaired when necessary, and reassembled to the extent possible.

(2) Has been tested in accordance with approved standards and technical data, or current standards and technical data acceptable to the Administrator (i.e., manufacturer’s data), which have been developed and documented by the holder of one of the following:

  • TC,
  • Supplemental Type Certificate (STC),or article approval under § 21.8, or
  • PMA.

Rebuilt. Describes an aircraft, airframe, aircraft engine, propeller, or appliance, using new or used parts that conform to new part tolerances and limits or to approved oversized or undersized dimensions that has undergone the following:

(1) Has been disassembled, cleaned, inspected, repaired as necessary, and to the extent possible.

(2) Has been tested to the same tolerances and limits as a new item.

http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC%2020-62E.pdf

Also peruse 14CFR43 PART 43—MAINTENANCE, PREVENTIVE MAINTENANCE, REBUILDING, AND ALTERATION in particular 43.2

Records of overhaul and rebuilding.

(a) No person may describe in any required maintenance entry or form an aircraft, airframe, aircraft engine, propeller, appliance, or component part as being overhauled unless—

(1) Using methods, techniques, and practices acceptable to the Administrator, it has been disassembled, cleaned, inspected, repaired as necessary, and reassembled; and

(2) It has been tested in accordance with approved standards and technical data, or in accordance with current standards and technical data acceptable to the Administrator, which have been developed and documented by the holder of the type certificate, supplemental type certificate, or a material, part, process, or appliance approval under part 21 of this chapter.

(b) No person may describe in any required maintenance entry or form an aircraft, airframe, aircraft engine, propeller, appliance, or component part as being rebuilt unless it has been disassembled, cleaned, inspected, repaired as necessary, reassembled, and tested to the same tolerances and limits as a new item, using either new parts or used parts that either conform to new part tolerances and limits or to approved oversized or undersized dimensions.

When dealing with regulations, you may not find an exact, all encompassing definition.

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