Flight operations, crew duty and documents
Which Part applies
Part 91 is the default for all aircraft. Commercial air transport needs an air operator certificate under Part 119 and then Part 121 (large aeroplanes), Part 125 (medium aeroplanes) or Part 135 (helicopters and small aeroplanes). The higher Parts add performance, crew, training, dispatch and documentation requirements.
Documents and fuel
The flight must carry the C of A, certificate of registration, maintenance release, flight manual, pilot licences and medical certificates, and journey or technical logs; air operations also carry the operations specifications, MEL and weight-and-balance data. Fuel planning under Part 91 is trip fuel plus alternate where required plus a final reserve of 30 minutes by day VFR and 45 minutes for night VFR and IFR, plus taxi and contingency fuel. Nothing may be planned that relies on the reserve.
Performance
Take-off and landing must be within the runway available with the required obstacle clearance and climb performance; Part 121 and 125 aircraft apply the certified performance class data, including net take-off flight path and stopway considerations.
Flight and duty time
Parts 121, 125 and 135 require the operator to have an approved flight and duty time scheme, with maximum duty, minimum rest and cumulative limits, and a fatigue risk management approach.
Worked example
Trip 2:00, destination alternate 0:30, final reserve 0:45, taxi 0:10 - required fuel 3:25. If endurance is 3:40 the flight is legal, but a 20-minute hold at destination would eat the margin; the pilot must divert or hold only while the reserve remains intact. At night or IFR the reserve is 45 minutes, not 30.
Traps
Using day-VFR reserves at night; forgetting alternate fuel; treating the MEL as permission to fly anything; assuming Part 91 covers commercial passenger operations.