Key Takeaways
- Overhaul has a precise meaning in aviation. Under FAA rules, an item can be called overhauled only if it has been disassembled, cleaned, inspected, repaired as necessary, reassembled and tested to approved standards.
- A repair fixes a defect, and an overhaul restores a whole item. “Rebuilt” is a stricter label again, and “maintenance” is the umbrella term for all of it.
- Release paperwork has its own vocabulary. In the UAE, a certificate of release to service must be issued before flight, and a component gets an AW Form 1.
The overhaul meaning in aviation is narrower than most people expect, and the terms around it get mixed up. This glossary defines the words you’ll meet in an MRO quote, with the regulation behind each. For the costs, timelines and UAE rules, read our full guide to aircraft repair and overhaul.
Core Terms
Maintenance
The umbrella term. The FAA defines it in 14 CFR Part 1 as inspection, overhaul, repair, preservation and the replacement of parts, excluding preventive maintenance. So overhaul is one kind of maintenance, and “MRO” simply lists maintenance, repair and overhaul side by side.
Overhaul
Under 14 CFR 43.2(a), an item may be described as overhauled only if it has been disassembled, cleaned, inspected, repaired as necessary and reassembled, and then tested to approved standards and technical data. The test step matters: a large repair isn’t an overhaul without it. The UAE applies its own GCAA rules, but this wording shows what the word means.
Rebuilt
A stricter label. Under 14 CFR 43.2(b), an item can be called rebuilt only if it has been taken apart, inspected, repaired as necessary, reassembled and tested to the same tolerances and limits as a new item, using new parts or used parts that meet new-part limits or approved oversize or undersize dimensions.
Repair
Work that restores a damaged or defective item to an airworthy condition. Under GCAA CAR 145.48(d), damage is assessed and repairs are carried out using the data specified in CAR M.304. A repair targets the affected area, while an overhaul covers the whole item. See the repair versus overhaul comparison.
Major repair
The FAA’s definition covers a repair that, if done improperly, might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics or other airworthiness qualities, or one that isn’t done by accepted practices or elementary operations.
Line and base maintenance
Line maintenance is fast, at-gate work. Base maintenance is in-depth, hangar-based work that can take weeks. Our MRO guide compares the two.
Maintenance Programme Terms
These describe when work is scheduled. The FAA’s Advisory Circular 120-17A (1978) sets them out.
Hard-time
A maximum interval for performing maintenance tasks. These intervals usually apply to overhaul, but can also apply to the total life of a part or unit.
On-condition
Assigned to components whose continued airworthiness can be judged by inspection, measurement or test without disassembly or overhaul, so they aren’t overhauled on a fixed clock.
Condition monitoring
No scheduled services or inspections to check integrity. Performance is monitored and analysed, but limits and mandatory actions aren’t prescribed.
TBO (time between overhaul)
The interval a manufacturer recommends before an engine is overhauled. Under Part 91, it isn’t automatically mandatory. An FAA Chief Counsel interpretation, answering a turbine-engine question, said a manufacturer’s recommended TBO isn’t mandatory unless it appears in an FAA-approved Airworthiness Limitations section or is required by an Airworthiness Directive or other regulation.
Release and Paperwork Terms
Sign-off
A statement by an authorised person that a task or group of tasks has been correctly performed. In GCAA CAR-145 guidance it relates to one step and is different from a certificate of release to service.
Certificate of release to service
Under CAR 145.50, appropriately authorised certifying staff issue it once all maintenance ordered has been properly carried out, and it must be issued before flight at the completion of any maintenance.
AW Form 1
The authorised release certificate for a component maintained off the aircraft. CAR 145.50(d) says it constitutes the component’s certificate of release to service.
Frequently Asked Questions
What does overhaul mean in aviation?
It means an item has been disassembled, cleaned, inspected, repaired as necessary, reassembled and tested to approved standards and technical data, as 14 CFR 43.2(a) describes it.
Is an overhaul the same as a repair?
No. A repair fixes a defect or damage, usually in one area. An overhaul opens up and restores the whole item, and ends with testing.
Is TBO mandatory?
Not automatically under Part 91. A manufacturer’s recommended TBO becomes mandatory only if it’s in an FAA-approved Airworthiness Limitations section or required by an Airworthiness Directive or other regulation.
Read the Full Guide
For what repair and overhaul cost, how long they take and what UAE compliance requires, see our full guide.
