Tinmug. Where'd you get that novel idea? "the engine is pretty much electrically isolated from the airframe" In a metal aircraft the engine is very deliberately electrically connected to the airframe by substantial flexible metal straps or cables to create a path for current to flow between the battery (also thus connected to the airframe) and the engine starter motor.
Furthermore the alternator or generator needs a similar connection for the rectifier/regulator wherever that is located and some other electrical components like the pressure and temperature sensors also need the engine to be well bonded to the fuselage.
Even wooden and composite aircraft need their engines electrically connected, usually to the battery negative and perhaps to provide connection to some bus-bar for the negative sides of some electrical bits.
Think again!
Last edited by Colibri49; 19th Aug 2016 at 23:42.