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For example let’s take the PT6A-27.

In every video related to engine start I see, the fuel boost pump is turned on, and then the fuel pressure goes up.

Example:

My question is: where is that fuel going if the condition lever is still at the idle cutoff position? Don’t you flood the engine?

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I can’t say for sure about the jet engine mentioned. If I think of it like a piston engine, the fuel pump pressurizes the fuel lines. That in no way means that the fuel is entering the combustion part of the engine (cylinders in my case). It just means that the fuel lines leading to the fuel distribution point (carburetor or EFI) are being fed fuel at a certain pressure. A primer is a different story. It is feeding fuel directly into the combustion chamber by using pressure.

As mentioned in another post, more pressure and fuel is supplied by the fuel pump than is needed by the engine. In the case of a Cessna 172SP, the excess is routed to a reserve fuel reservoir. Here is a schematic of the fuel system from the POH.

enter image description here

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  • $\begingroup$ … and turbines don't have primers. Fuel is only introduced once the starter spun it up to certain RPM and the igniters are on. $\endgroup$ – Jan Hudec Jun 7 '20 at 20:58
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On most (probably all) PT6 installations, the fuel pumps send much more fuel than the engine needs. That extra fuel is then returned to the fuel tanks. This is true both during operation and before engine start.

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  • $\begingroup$ So your position is that fuel just starts dumping into the combustion chamber as soon as the boost pump is turned on? What does the fuel condition lever do then? $\endgroup$ – Michael Hall Jun 7 '20 at 18:32
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    $\begingroup$ @MichaelHall, not the combustion chamber, just circulating more fuel through the pipes than the injectors will use when actually open. But they are controlled by the condition lever, so they are still closed at that point. $\endgroup$ – Jan Hudec Jun 7 '20 at 21:06
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The pump just creates the pressure. The fuel doesn’t have anywhere to go until you move the condition lever to idle. Think of it like your faucet: There is water pressure there, but it doesn’t flow anywhere until you open the valve.

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  • $\begingroup$ fuel pumps don't like to run without fuel moving through the lines. The excess fuel is returned to a collector tank. The condition that fuel has nowhere to go should never happen. $\endgroup$ – Peter Kämpf Jun 9 '20 at 13:25
  • $\begingroup$ @Peter Kampf, yes good point. Still, the pump simply creates pressure, and doesn't flood the engine until the valve is opened. $\endgroup$ – Michael Hall Jun 9 '20 at 15:24

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