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Bill Wade, you did an excellent job of describing the
engine driven fuel pump operation. "Bill's question is:
could the output of the two pumps combine and overwhelm the return line causing
pressure and thus fuel flow to go out of limits?" Yes but not just because it overwhelms the return line.
The pump sends fuel to the metering valve based on more functions than the
just the relief valve pressure setting. If you increase the engine driven
fuel pump inlet side fuel pressure, you will increase the fuel going to the
metering valve. For all folks other than the ultimate "gear heads", I feel
this is what needs to be remembered. While I may be considered a gear
head, I am still a little fuzzy as to the exact interactions of all the pump
features. The same fuzziness emulsions tubes created when trying to figure
out what the part throttle fuel curve would look like in a
carburetor.
For those who feel this is an unacceptable risk
posed by the fuel injection design, let me pose this comparison. If you
are flying very close to stall and you increase the angle of attack, the plane
will stall and may create a life threatening event. Did the aircraft
engineer design an unacceptable aircraft? I believe there are rules that
must be followed to help insure safe flight. Don't turn the freaking high
boost on during short final. Don't do a mag check on short final.
Don't stall the airplane on short final. The list is quite long.
Personally, I never change fuel tanks while I'm in the pattern.
This irritates a lot of instructors.
Craig Berland
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