Aircraft Overhaul, despite its frequent use in ‘aviation speak’, seems to lack a precise definition by the authorities charged with overseeing such actions. But the Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA) agree on a somewhat specific definition.
"'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,” (Source: FAA).
Put more generically, ‘Overhaul’ means returning an aviation article back to like-new condition by disassembling it, repairing or replacing out-of-specification parts with in-spec parts, reassembling the article, testing it and returning it to service.
Articles subject to manufacturer-specified overhaul cycles include engines, some engine components, propellers and other items subject to wear and tear.
When is Aircraft Overhaul Needed?
When an aircraft overhaul is needed will depend on the Time Between Overhaul (TBO) - a number that has been established by the component, prop or powerplant manufacturer as part of the process of earning regulatory approval for that part or engine.
For engines, TBO periods tend to be described in terms of hours of operation. Other components' life limits are expressed in “cycles” – one cycle being how many times the part has been started, used and stopped.
An example of TBO might be a popular jet engine with a 6,000-hour TBO and a Hot-Section Inspection (HSI) mandated at 3,000hrs. The HSI involves physically examining and/or testing components in the hot section of the turbine engine (i.e. the burner assembly, turbine wheels subject to the hot gases expelled from the burner, or combustion chamber as some are called, fuel and burner nozzles and related bearings).
For commercial operators the TBO is a regulatory limit. This is because FARs require the part to undergo an overhaul at the specified number of hours. For Part 91 operations – private-aircraft not flown for hire or revenue – TBO is an option, as long as the operator takes steps to assure the part remains within the manufacturer's specification. That's called “On-Condition.”
Nevertheless, many private Part 91 operations adhere to the TBO requirements as a matter of policy and practice.
What Does an Aircraft Overhaul Involve?
An overhaul involves several specific and detailed steps. As cited by the FAA and EASA: “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.”
All this takes time – and often requires the aircraft to be taken out of service for the duration of the process, which could take several weeks to complete.
There are some solutions available to keep operators flying during this time, however. One example available to operators enrolled on an hourly maintenance program is the provision of loaner engines that can be attached to the airframe in place of the usual powerplants that have been removed for overhaul.
Renting an exchange engine is an alternative option for those not enrolled in a maintenance-service program. Depending on the operator's location and situation, an engine or powerplant may need to be shipped to the overhauling facility, adding time and expense to the process.
Another way operators can hedge the time involved in an overhaul is to couple an overhaul job with other work on the aircraft. For example, when the airplane is in the hangar for a phase check, such as a C-Check or D-Check, an avionics upgrade or interior work could also be undertaken.
Who can Perform Aircraft Overhaul Work?
While, technically, any A&P technician can undertake an overhaul, given the complexity and sophistication of modern turbine engines, it’s wise to consider using a specialized service center.
These can provide faster service, specialized expertise and possess the tools and testing equipment required – not to mention the service warranties. As one engine OEM’s overhaul expert said, "you wouldn't want your heart-bypass surgery done by a general practitioner – you'd want a specialist".
If you think of your aircraft’s powerplants in the same context as your heart, you'll understand the value of turning to a specialist for an engine or propeller overhaul.
What Does an Aircraft Overhaul Cost?
Between an HSI and TBO, operators can expect to spend more than $1m a full overhaul. While overhauls are not cheap, they cost less than the problems an operator is exposed to if they don’t comply with a necessary overhaul.
In the case of an engine overhaul, the process requires removing the powerplant from the aircraft, transporting it to the overhaul shop, the work itself, any new or reconditioned parts required, reassembly and re-installation on the aircraft, testing and completing the paperwork.
And remember, none of these processes is complete without the appropriate paperwork completed, logged and filed for inspection. When it comes to aircraft overhaul, plan accordingly!