Aircraft Operations and Air Operator Certification (Parts 91, 121, 125, 135)
The operating parts
Part 91 is the general operating and flight rules that apply to all aircraft. Parts 121, 125 and 135 build certification and additional operating requirements on top, scaled by aircraft size and operation: Part 121 for large aeroplanes and high-capacity operations, Part 125 for medium aeroplanes, Part 135 for small aeroplanes and helicopters. The larger the aircraft and the more passengers carried, the deeper the management, training and equipment requirements.
What an AOC actually proves
The certificate shows the Director is satisfied the applicant has an organisation, facilities, competent and checked personnel, an exposition and operations manual, maintenance arrangements, training and checking programmes, a safety management system, dangerous goods acceptance arrangements, security measures, record keeping and adequate financial resources. Authorisations then appear in the operations specifications: types, routes, instrument approaches, RVSM, extended diversion times, special approvals.
Worked example
An operator holding a Part 135 AOC buys a larger turboprop that falls under Part 125. This is not covered by the existing certificate. It needs a variation: amended exposition, revised maintenance programme, crew training and checking, a proving flight where required, then amended operations specifications. Buying the aircraft does not buy the authorisation.
Traps
- Treating Part 91 as a "small aircraft only" part - it applies across the board.
- Assuming operations specifications are advisory documents; they are binding conditions.
- Forgetting that the PIC retains final authority for the flight even inside a tightly written exposition.
- Overlooking fuel policy, MEL, load control and flight and duty time limits as certification items.