Automatic Pitot Heat

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2 months 1 week ago - 2 months 1 week ago #1329 by dynamics
Would you mind explaining thoroughly how automatic pitot heat system senses the engine running using low oil pressure switch in B737-300?
Last edit: 2 months 1 week ago by dynamics.

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2 months 1 week ago - 2 months 1 week ago #1330 by Mark
Replied by Mark on topic Automatic Pitot Heat
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.

767's have a low/high mode for pitot heat.  rotate.aero/forum/767-ice30/242-ata-30-31-pitot-heat

 
Last edit: 2 months 1 week ago by Mark.
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2 months 1 week ago #1331 by dynamics
Replied by dynamics on topic Automatic Pitot Heat
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?

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2 months 1 week ago #1332 by Mark
Replied by Mark on topic Automatic Pitot Heat
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.

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2 months 1 week ago #1334 by dynamics
Replied by dynamics on topic Automatic Pitot Heat
I sent an email and pages of the diagram and the schematic attached with
Do you have any idea on how engine oil pressure signal is fed into the heater control system?

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2 months 1 week ago - 2 months 1 week ago #1335 by dynamics
Replied by dynamics on topic Automatic Pitot Heat
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!??
Last edit: 2 months 1 week ago by dynamics.

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2 months 1 week ago #1338 by dynamics
Replied by dynamics on topic Automatic Pitot Heat
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?

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2 months 1 week ago #1341 by Mark
Replied by Mark on topic Automatic Pitot Heat
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.

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2 months 1 week ago - 2 months 1 week ago #1342 by dynamics
Replied by dynamics on topic Automatic Pitot Heat
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)?
Last edit: 2 months 1 week ago by dynamics.

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2 months 1 week ago #1345 by Mark
Replied by Mark on topic Automatic Pitot Heat
Are you having a problem with an aircraft?

All these sheets are bits and pieces of the circuit. You need to lay them all out and follow each wire. This modification "should" be reflected in the schematic and wiring diagram manuals already.

It's a lot easier to follow everything on the schematics.

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