Keygrip
What can I say
- would you believe it was a deliberate mistake? 'thought not!
Benix
That's not what I'm saying. DFC told us that the electrically induced deviation had to be less than 5 degrees. I simply made the point that there could therefore be an error of up to 5 degrees. Try looking at the compass when you turn things on and off; it moves noticeably in many aircraft although I'd admit the most I've seen is probably 5-7 degrees.
Personally I don't switch on pitot heat as a matter of course for VFR trips but it's part of my line-up checklist for IFR departures.
DFC
The "right hand grip rule" illustrates the direction of the field around a current. The magnetic field around a pair of long wires with equal and opposite currents varies directly with both the current and the separation of the wires and inversely with the square of the distance between the centre of the pair and the victim. The answer to disturbance from leakage fields is to keep the wires very close (coaxial would be good), the current low and the distance maximised.
"Why do the radios get special treatment? - Simple - many sources of alternating voltages and currents as various frequencies" You seem to think that alternating currents cause more compass deviation than direct currents; this is not correct. The compass cannot respond to alternating fields above a few Hz so effectively only static or half wave rectified fields are an issue. Therefore any significant load may have an effect.
I'm surprised that it's legit to swing with electrics off, do you have a link to where the rules are defined?
HFD