Sorry folks, I will try again tomorow.
I can see the image now. Do you mean the switch (you called it sensor?)
It switches the electrical fuel pump at full throttle.
Jesse, yes that is exactly what I mean. The fuel pump is switched OFF via a toggle switch in the cabin and has high, low and off, so does this switch override the the toggle switch and why is this required? Surely the mechanical fuel pump is sufficient.
Not sure, I have replaced and adjusted on of those once. I think, it will switch to the electrical fuel pump to high mode if you give a certain amount of throttle and you have the electrical fuel pump on. Low mode might not deliver sufficient fuel if you have to accelerate quickly.
I will try to check today just to be sure.
I have fixed up the image.
It’s interesting if they switch on the electric fuel pump when the throttle is at maximum. That would do two things
The mechanical (engine driven) pump should always be sufficient but it can fail; also I think the electric pump can usefully pressurise the fuel pipework upstream of the engine and remove slight blockages (icing, etc).
Peter, thank you for fixing the image, still no idea where I went wrong with this.
The electric fuel pump in the OFF position is not activated at any throttle setting. When the pump is in the “low” setting then yes it operates with the throttle. So what is the purpose anyone please? I can only surmise it is an aid when starting the engine BUT as I was discussing with someone, if inadvertantly left on it will drain the tanks to dry in an hour of normal flight (so I was informed). Seems a little odd to me.
Just checked, it operates as indicated previously:
“When the switch is in the high or emergency setting, the auxillary fuel pump can operate at two flow rates depending on the setting of the throttle. With the thrtottle at a cruise setting, the auxiliary pump is operating at maximum capacity, supplying sufficient fuel flow to maintain flight with the engine-driven pump inoperative. When throttle is moved toward the closed position, as during let-down, landing and taxiing, a mechnically actuated switch electrically reduces the auxiliary fuel pump flow rate by means of a resistor in the pump power circuit. This action automatically prevents an excessively richt mixture during these periods of reduced engine speed.”
Jesse may I ask what airframe you have? In my 172 the high setting for the electric pump has a spring bias to the off position so it cannot be left on as your quote says. In normal flight all fuel is delivered by the engine driven mechanical pump, at any throttle setting.
I don’t have an airframe, I am just a B1/B2 engineer.
The quote is from the Cessna 100 series maintenance manual, can’t recall which year. It might work the other way around on your aircraft. I believe it was the other way around on that aircraft I mentioned as well.
So normal operation is with engine driven mechnical pump, emergency operation is with the electric fuel pump on. The electric fuel pump has two speeds / fuel flow rates. Yours seem to be low by default if switched on manually, and automatically goes high when you apply throttle.
In the section in the manual it’s the other way around, it is high by default, and reducing by retarding throttle.
Results would be the same, to make sure the correct fuel flow is available when operating the electrical fuel pump.
