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Choosing who maintains your aircraft is one of the most important decisions you make as an owner. A good maintenance shop or avionics installer keeps you safe, legal and flying. A poor one costs money, time and sometimes much more. This guide explains how to check that a shop holds the right approvals in the US and the UK, what to ask before you hand over the keys, how to compare quotes fairly, and the warning signs to watch for.

It starts with a simple principle: the owner carries the responsibility. In the US, the regulations make the owner or operator primarily responsible for maintaining the aircraft in an airworthy condition. In the UK, the CAA reminds new owners that they are responsible for ensuring the aircraft is maintained and operated in line with the requirements of its Certificate of Airworthiness or Permit to Fly. A shop does the work, but you need to choose it wisely and keep an eye on what is done.

Step 1: Understand who is allowed to do what

The first filter is legal: does the person or organisation actually hold the approval needed for the work you want? The rules differ between the US and the UK.

In the US: A&P mechanics, IAs and Part 145 repair stations

• A&P mechanic. An FAA-certificated mechanic with airframe and powerplant ratings. Under 14 CFR 65.85 and 65.87, a mechanic can approve an airframe, powerplant or propeller for return to service after maintenance or alteration, excluding major repairs and major alterations. They can also perform the 100-hour inspection.
• Inspection Authorization (IA). An additional authorization held by some mechanics. Under 14 CFR 65.95, an IA holder can perform an annual inspection and can approve an aircraft for return to service after a major repair or major alteration done in accordance with FAA-approved technical data.
• Part 145 repair station. An FAA-certificated maintenance organisation. Under 14 CFR 145.201, a repair station can carry out maintenance and alterations on articles for which it is rated, within the limitations in its operations specifications. In practice, a repair station is approved for specific kinds of work, not everything.

How to check: You can look up an individual mechanic’s certificate on the FAA’s Airmen Inquiry search. For a repair station, ask to see its certificate and the ratings and operations specifications that cover your aircraft or the equipment you want installed. The FAA’s Part 145 certification page explains how repair stations are approved.

In the UK: Part-145, Part-CAMO, Part-CAO and Part-ML

• UK Part-145. An approval for organisations maintaining Part 21 aircraft, their engines and components.
• Part-CAMO. A Continuing Airworthiness Management Organisation. It manages an aircraft’s continuing airworthiness (maintenance planning, records and airworthiness reviews) rather than necessarily turning the spanners.
• Part-CAO. A combined maintenance and continuing airworthiness management approval for general aviation. The CAA says a Part-CAO organisation may perform both on aircraft within the scope of UK Part-ML and on non-complex Part-M aircraft up to 5,700 kg MTOW, excluding aircraft used by a licensed air carrier.
• Part-ML. The proportionate continuing airworthiness rules for “light aircraft”. The CAA defines these as including aeroplanes of 2,730 kg maximum take-off mass or less, rotorcraft of 1,200 kg MTOM or less certified for up to four occupants, and other ELA2 aircraft. Under Part-ML, the Airworthiness Review Certificate (ARC) can be issued by the CAA, by an approved organisation, or by an independent Part-66 licensed engineer holding the appropriate authorisation.

How to check: The CAA publishes downloadable lists of approved airworthiness organisations, including UK Part-145, Part-CAMO and Part-CAO, plus separate lists of suspended and revoked approvals. Ask an organisation for its approval reference and confirm it appears on the current list, with a scope that covers your aircraft. For an independent engineer, ask which licence and authorisation they hold and whether it covers your aircraft type.

Aircraft operating on a Permit to Fly rather than a Certificate of Airworthiness follow different maintenance arrangements. Check the requirements that apply to your aircraft before choosing who maintains it.

Step 2: Annuals and recurring inspections

Most owners first meet a new shop at inspection time, so it helps to know what is legally required.

• US annual inspection: under 14 CFR 91.409, an aircraft generally may not be operated unless it has had an annual inspection within the preceding 12 calendar months. Aircraft carrying people for hire, or provided by the instructor for flight instruction for hire, also need a 100-hour or annual inspection within the preceding 100 hours of time in service.
• US IFR checks: to fly under IFR in controlled airspace, the altimeter and static system tests in 91.411 must have been done within the preceding 24 calendar months. Transponders covered by 91.413 also need testing within the preceding 24 calendar months, and after installation or maintenance where data correspondence errors could be introduced.
• UK Part-ML aircraft: maintenance follows a Part-ML-compliant aircraft maintenance programme, and the aircraft needs a valid ARC. The CAA notes that independent certifying staff with the appropriate authorisation can carry out the airworthiness review and issue the ARC in conjunction with the annual inspection. See the CAA’s Part-ML page.

When you get quotes for an annual, ask exactly what is included. Is it a fixed-price inspection with defects quoted separately, or an open-ended job? And how will the shop handle the airworthiness review, ARC or IFR checks that fall due at the same time?

Step 3: Questions to ask before you book

• Which approvals or certificates do you hold, and do they cover my aircraft type, engine and the avionics I have, or want installed?
• How often do you work on my type? Can I speak to another owner of the same type you maintain?
• Who will actually work on the aircraft, and who signs the release to service?
• How do you handle defects found during an inspection? Will you stop and ask for my approval before doing extra work?
• Do you provide written estimates? How do you charge: fixed price, labour rate plus parts, or a mix?
• Can I supply my own parts? If so, what documentation must they have?
• What is your typical turnaround, and where will the aircraft be stored while it is with you?
• Can I be present for part of the inspection, or see the aircraft opened up?
• What warranty do you give on workmanship and on parts you supply?
• Will I receive detailed logbook or maintenance record entries, plus copies of any modification paperwork?

Step 4: Comparing quotes fairly

Two quotes for “an annual” or “a new radio” can describe very different jobs. To compare like with like:

• Get the scope in writing. List the inspections, tasks and parts included, and those that are excluded.
• Separate the inspection from the repairs. A low inspection price means little if every defect is then charged at a high rate. Ask how defects will be quoted and approved.
• Agree an approval threshold. Tell the shop in writing that any additional work over an amount you choose must be approved by you first.
• Check parts pricing and sourcing. Ask whether parts are new, overhauled or used, and what documentation comes with them.
• Account for downtime and logistics. A cheaper shop that is a long positioning flight away, or that will keep the aircraft for weeks, may not be cheaper overall.
• Look beyond price. Type experience, communication and record-keeping quality often matter more than the headline number.

Step 5: Avionics installation considerations

Avionics upgrades are where shop choice matters most, because the work affects the aircraft’s certification paperwork as well as the panel.

US installs

• Approval route. Ask how the installation will be approved: for example, under a Supplemental Type Certificate (STC) or other FAA-approved data. Major alterations must be recorded on FAA Form 337. Under Part 43 Appendix B, the person doing the work gives a signed copy to the owner and forwards a copy to the FAA Aircraft Registration Branch within 48 hours.
• ADS-B Out. Under 14 CFR 91.225, since 1 January 2020 ADS-B Out equipment meeting the specified standards has been required to operate in specified airspace, unless otherwise authorised by ATC. The FAA’s Equip ADS-B pages explain the options.
• Post-install checks. After transponder installation or maintenance, the 91.413 test requirement applies. Confirm the shop will complete it and record it.

UK installs

• CS-STAN standard changes. For eligible light aircraft, some installations can be done as a Standard Change under CS-STAN without separate CAA approval. The CAA notes that a CAA Form 123 must be completed for the aircraft records. EASA’s CS-STAN text says it cannot be used to install or exchange integrated avionics or communication, navigation or surveillance systems unless explicitly allowed. See the CAA’s modifications and repairs page.
• Other modifications. Changes outside CS-STAN need approval as a minor or major change. Ask the installer which route applies and who will handle the paperwork.

Questions for any avionics shop

• Will you update the weight and balance, equipment list and any flight manual supplements?
• What happens to the old equipment, and will the panel need cutting or re-labelling?
• Which databases or subscriptions will the new equipment need, and who sets them up?
• How will the new equipment integrate with what I already have (autopilot, intercom, existing GPS)?
• What training or demonstration will you give me before I fly it?

Red flags to watch for

• Reluctance to show certificates, approval references or the scope of approval.
• An approval that does not appear on the regulator’s current list, or appears on a suspended or revoked list.
• No written estimate, or vague answers about how extra work is approved.
• Pressure to approve large amounts of extra work immediately, without photos or an explanation.
• Sparse or illegible record entries, or missing paperwork for modifications.
• Offers to “sign off” work they did not do or inspect, or to skip required inspections or checks.
• Poor communication: unanswered calls, missed deadlines with no explanation.

Remember that maintenance matters for safety beyond the obvious. Exhaust and cabin-heat defects can introduce carbon monoxide into the cabin, which is one reason we also recommend an active CO detector. See our guide to carbon monoxide detectors for UK pilots.

Do your own homework too

You will get more from any shop if your records are in order. In the US, 14 CFR 91.417 lists the records an owner must keep, including total time in service, the status of life-limited parts, time since overhaul, current inspection status and the status of applicable airworthiness directives. Under 43.9 and 43.11, each maintenance or inspection entry must include specific details, such as a description of the work, the date and the certifying person’s signature and certificate number. Keep your records organised and bring them along, so a new shop can quickly understand your aircraft’s history. If you are still shopping for an aircraft, our guide to buying your first aircraft explains how a pre-buy inspection and a records review fit together.

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Official sources

• 14 CFR 91.403: owner/operator responsibility
• 14 CFR 65.85 (airframe rating privileges) and 65.87 (powerplant)
• 14 CFR 65.95: Inspection Authorization privileges
• 14 CFR 43.7: who may approve return to service
• 14 CFR 145.201: repair station privileges and limitations
• FAA: Part 145 repair station certification
• FAA Airmen Inquiry
• UK CAA: Part 145
• UK CAA: Part CAMO
• UK CAA: Part-CAO
• UK CAA: Part-ML
• UK CAA: approved airworthiness organisations
• UK CAA: modifications and repairs (CS-STAN)

This guide is general information, not legal or engineering advice. Regulations change: always check the current FAA or CAA requirements for your aircraft and operation.