Intercom and 1/4" jack
 

Is a standard intercom earphone socket compatible with a normal 14" mono audio jack? Need to take a feed off the intercom.

Usually yes. Some are stereo, it's safest to use a stereo jack with tip contact for both ears, to ensure not shorting out one channel of stereo intercom systems.

Cheers Fitter. It's not actually going to a headset. Whatever output we get, stereo or mono is fine.

I think you lost Fitter's point, Lost man ....
Using a mono jack, you "short out" the stereo intercom of your 'provider', who may NOT appreciate that. It probably wouldn't do any damage, but the system would no longer be stereo...

CJ

Why not have a look at a compatible headset for the aircraft then you can provide a connector of the same type.