What constitutes an "engine-driven electrical system" per FAA?

3 Replies

What constitutes an "engine-driven electrical system" per FAA?

Posted by John Doyle on Feb 20, 2019 4:29 pm

I have been told by a former FAA employee (who shall remain nameless) that per the FAA (regarding piston engines) the term "engine-driven electrical system" means only a generator or alternator mounted on the exterior of of the engine and driven by engine power.  If true, the Rotax 912 on my aircraft, which produces 15 amps from the stators, does not have an "engine-driven electrical system," and could legally fly inside the Mode C veil, and below the floor of Class B & C airspace, without a Mode C transponder and (next year) without ADS-B out.  I am looking for a definitive citation for this question.  In a related matter, after 2020 how do I announce/explain this to ATC as I do have a Mode C transponder?
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Re: What constitutes an "engine-driven electrical system" per FAA?

Posted by Ronald Levy on Feb 21, 2019 8:21 am

I gather your question involved 14 CFR 91.215(b) and 91.225 regarding operation inside the Mode C veil around Class B airspace or in/over Class C airspace without a transponder/ADS-B-out.  I know of no FAA guidance which says what your source says.  I think you should ask this question of a current FAA employee, preferably an Inspector at your local FSDO.  They can provide you with a definitive and reliable answer either from existing guidance or by raising the question to the cognizant office at HQ.  You should also ask your source to show you the written FAA guidance which s/he thinks supports this position.

However, my personal guess is that the FAA's official reading on this will not be as limited as your source's, and that they will consider your system as being an "engine-driven electrical system" because the engine is driving the electrical system, even if it is doing so directly rather than via an external generator/alternator.  I believe what the FAA does not consider an "engine-driven electrical system" are the purely battery-powered systems installed in many gliders and some older simple planes like Cubs with no electrical power generation system at all (i.e., relying only on stored electrical energy, not energy developed in flight), which would be rapidly depleted by the 200-250 watt pulses of a transponder.  I say this based on the following legal interpretations, which makes differentiation only between engine-driven systems versus battery-powered systems, and reference primarily gliders/sailplanes/balloons which have no engines and rely solely on batteries for their electrical power.



Re: What constitutes an "engine-driven electrical system" per FAA?

Posted by John Doyle on Feb 21, 2019 12:10 pm


Thank you for the citations.  What the FAA seems to be saying in those citations is something like this:
     Sorry folks, our bad.  When we wrote "electrical system" in 91.225 (pertaining to ADS-B Out) we meant
     for it to have the same meaning as "engine-driven electrical system" in 91.215(b) (pertaining to Mode C
     Transponders).  Therefore, for the purposes of ADS-B Out and Mode C Transponders, please consider
     the phrase "electrical system" to have the same meaning as "engine-driven electrical system."  Maybe
     one of these days we will get around to fixing it.

As I tracked down the citations I came across the phrase "engine driven electrical system" which I assume
has the same meaning as "engine-driven electrical system."  I also encountered the phrase
"self contained electrical power system" which may include magnetos and stators. 
If so, the question becomes -
      In the eyes of the FAA, is a "self contained electrical power system" merely one type of an "engine-driven
     electrical system,"
or is it a different type of system?
Obviously a typical magneto could not power a Transponder or ADS-B Out.


Re: What constitutes an "engine-driven electrical system" per FAA?

Posted by Ronald Levy on Feb 21, 2019 12:29 pm

I think you think right.