Regarding the engine follow the manufacturer advice. For the most common types it is air borne time but for some engines its approval is the actual start up to shutdown hours. The airframe standard is most commonly airborne time. The most common Hobbs (Datcon) meter is electric and is activated by the engine pressure so it is very close to start up to shutdown (there are some simple types that are activated immediately by the master switch). The law states that flight commences from when the aircraft moves with the intention to fly and ends when the aircraft comes a halt after landing. For airborne time it is common to keep it simple and to deduct 0 .2 from start up to shut down times allowing for non flight taxying time, this simpler method means that there is no need to note airborne and landing times although some do. It is the tacho that varies with RPM (not the Hobbs meter) so from start and during taxying at low rpm it records very little change over time. It is common therefore for non profit ops such as group owned aeroplanes to use the tacho for logging as it is very close for the average airborne flights of a short duration. The tacho is optimised to a constant 2300rpm over 1 hour (known as a 'tacho hour'). On longer flights of over two hours say (not precise) the tacho can be close to the clock hour or even exceed it if cruise is at a high rpm. The pilot should always log the flight time (see above) in their logbook distinct from the engine and airframe that is defined in accordance with the manufacturer .
Many clubs charge start up to shut down but retain the log books for its own use and record the airborne time (or as required by the manufacturer) for maintenance cycles. All the above method are acceptable to the CAA.
Last edited by Fl1ingfrog; 31st Oct 2023 at 21:13.