Without looking at prints, it's pretty straight forward. Either engine oil pressure switch showing pressure (engine running causes switch to provide a ground) activates the pitot/static heat control relays..... turning heat on.
Some aircraft use air/ground signals as a backup to the engine oil pressure switches.
Does 767 WDM include the circuit that conduct engine speed signal to the relay that when energizes the switch is closed and power is supplied to the pitot heater? Does the 767 automatic heating system only supply pitot or otherwise other air data probes?
Again, without looking at prints..... the 767 does not use oil pressure. It has two engine speed cards. When they sense that an engine(s) are running, all heaters are turned on. The pitot probes only receive half power while on the ground. When the aircraft goes airborne, pitot heat goes full.
I sent just a moment ago 4 files:
1-WDM pre modification
2-WDM post modification (incorporation of modification to make automated air data probe heating system)
3-SSM pre modification
4-Electrical Shelf E3-2 equipments(pre modification)
After modification 3 engine relays are added to the junction box J18 that I can not see how these relays get signal from engine oil pressure switch from the diagrams!??
I sent an email attached with pages of WDM that indicates engine accessory unit EAU that controls the reverse gets signal from oil pressure switch, and you can see that EAU is connected to JB18 that contain engine relays, so I believe that is how the automatic pitot heat system sense the oil pressure, correct me if I am wrong.
By the way, do you have any idea on C/B 1946&1947 in page 8/18 of file WDM-SB.pdf that I sent yesterday?
You're getting messed up with all your effectivity aircraft. It looks like some have a manual switch and some are automatic.
Each aircraft's system must be looked at individually. Of the ones which are automatic, air/ground relays control heat.
I'm not digging through all your prints, but I would still suspect that oil pressure plays a part in heating the Capt's and F/O's pitot probes "before" breaking ground. If icing conditions are present, those probes need to be heated for takeoff roll to guarantee proper airspeed indication.
Mark the modification should have been done in compliance with FAA AD2019-09-01 for all tail numbers, and the pre modification schematics/diagrams I sent for making the differences that were brought about by incorporation of relevant service bulletins clear. 737-300 airplanes are equipped with the engine accessory unit (EAU) that monitors thrust reverse sleeve position during all phases of operation. Thrust reverse proximity sensor faults show on the front of the EAU by NOT STOW lights. An automatic reset of the NOT STOW light on the front of EAU occurs when this sequence occurs:
-Both engines have high oil pressure
-At least one engine is shut down (low oil pressure)
-Both engines again have high oil pressure
So I am sure the oil pressure switch is wired to the EAU and the engine relays in the JB18 are fed by the oil pressure signal from EAU instead of being wired to the oil pressure switch directly. However I can't see what is the functions of two C/B's 1946&1947 (page 8/18 of file WDM-SB.pdf) which are in 28DCV supply lines from DC BUS 1/2 that are connected to the engine relays R1076,R1077,R1078 in JB18(file WDM-SB.pdf)?