ATA 30-31, Pitot Heat

I'm somewhat perplexed on the electrical theory behind the 767's pitot heat operation in the air mode.

To start with, there is no crew interaction with the system (there is no control On/Off switch on the overhead panel). The heaters are off while the aircraft is on the ground with the engines not running. 115 VAC (A phase) power is always present on one side of the heater coil. With either (or both) engine(s) up to speed, a ground path is provided via two air/ground relays (K516 and K528) and a heat control relay (K644). K644 remains energized at all times with engines running, but it is not used once the aircraft goes airborne.

The confusing aspect of this is how it functions in the air mode. Once the aircraft is off the ground, both air/ground relays relax. At that time the previous supplied ground is replaced with 115 VAC (B phase) power. I "do not" understand how this is possible. You have two phases of power acting across a heater coil. Exactly how does current flow??? It must, because the current sensing module has to see current flow to keep the "Pitot Heat Off" light extinguished.




767-300 AMM 30-31-00 SSM 30-31-01

I'd bet the Window Heat Controllers have logic that figures this out and provides a path.

I just don't understand how the "trons" flow. There's no hard ground in the system when they're in the air..