Current Sensing Relay Needed for my Private Plane.

I do not think you have a plane, but ..

On cars with towing provision there is current sensing in the lines to the indicators to flash a light on the dash to show that the trailer is flashing its indicators too. In older vehicles this is nothing more than a reed switch of the type that operates by magnet, with a slack handful of turns  of stiff wire around it, so that when you get a dozen amp turns or more the contacts close. Nowadays electronics senses the droop across a series resistance of a few tens of milli-ohms

You do not normally do it the reed switch way with AC, as it leads to reed chatter, but a current transformer and rectifier may be used, or nowadays on low-ish currents an  optopcoupler wired so that the LED part is in series with the supply (via a bridge rectifier so the LED polarity is unimportant ).

Cheap current transformers are made that are the same type used as the basis for the home energy monitors. These are sold to hobbyists as  when combined with a suitable load resistor and voltage divider, these create waveforms that can be played into the audio input of a modern PC to give waveform capture, for a 'scope' view and with some extra post-processing effort, spectral analysis for harmonic distortion and so on. The output can also be rectified and used to fire a low current triac.

Mike.

Sounds like the same principle as a load shedding relay - e.g. www.kempstoncontrols.co.uk/.../1810847 (just a random google found example) - although the output might be reversed in terms of on/off for load/noload.

(I've used one similar to that to knock together a box that switches the workshop vacuum on when a power tool starts up).

   - Andy.

Understand this is tongue-in-cheek, but .....

Please don't mess with anything on airplanes unless you have a supplemental type certificate for the kit and its installation. Unless the craft is airworthiness-certified as "experimental", and even then you'll need a qualified mechanic to sign off on it (and who stands to lose her qualification if it messes up).