It depends on your context. Are you alluding to Mode C / ADS-B waivers for aircraft without engine-driven electrical systems? If so, here it is in a nutshell:
If an aircraft was originally certificated without an engine-driven electrical system, and has never had one installed in it's history, then it is eligible for the Mode C / ADS-B waiver. The key gotcha here is that it cannot have ever had a generator/alternator driven by the engine ever in it's history. Example: If you have a J3 cub that came from the factory with an A65-8 engine without a generator, and then subsequently in it's history had a had a C85-12 fitted to it with the generator pad on the accessory case blocked off and it never gets used for a generator or alternator it is still eligible for the waiver. However, if a previous owner installed a generator and operated the aircraft, even for a day, then the airframe is never eligible for the waiver. Even if a subsequent owner removed the generator or installed a A65-8 in its place. It cannot revert to being eligible. Only aircraft that have never had an engine-driven electrical system in their history can operate under the waiver. This means that most Rotax-powered experimentals, even with a rope pull starter, are not eligible as the Rotax has a lighting coil on the flywheel. Hence it has engine-driven electrical syatem.
Note that a wind-driven generator or alternator does not affect the eligibility as it is not engine driven.
Jim
