I flew a Sequoia Falco fitted with a Lycoming IO-320 engine recently. It’s the same engine as fitted to Twin Commanches but instead of a fuel flow gauge this aircraft has one that measures fuel pressure at the fuel injection distributor and is marked with low and high limits of 1.5 and 8 psi. On priming the engine with the electric pump no fuel pressure is indicated, though the pump is clearly working as the engine starts well from cold after 5 seconds of priming.
But the fuel pressure gauge reads less than 1 psi on the ground until the tacho reads 1800rpm and even at nearly full power it won’t read more than 3 psi, even with the boost pump on. We have checked fuel flow from the electric pump which is 25 gallons per hour so no problem with the pump. I can’t find any reference to fuel pressure limits in the Falco pilots notes nor the Lycoming handbook.
Any ideas, maybe they are like that?
Well, firstly it would appear that you are confusing fuel delivery rate with fuel pressure.
Secondly, my Falco does not have a fuel pressure gauge and neither does any other Falco that I have seen. It seems a strange thing to have.
However, since it is there it does seem to indicate some rather low pressures. If you’ve looked in the Lycoming book and there’s no mention of fuel pressures, perhaps try the bending book? I will look over the weekend and see if I have any technical data on this.
I don’t know if this aircraft’s system is fitted with a return line (mine isn’t but I do recall reading somewhere that some installations do have a return line) but if so then perhaps there is a fuel pressure regulator in the return line? If the regulator has failed in the open position then low fuel pressure will result.
This is not my specialist field but I do have considerable experience with other forms of fuel injection….
Have you asked the question on the Falco Forum?
If you do a search here on
fuel pressure
you find several threads where puzzled people are wondering why these pressures are so low. I don’t know if a conclusion was reached but it’s worth a read.
On a fuel injected engine, the “fuel flow” gauge is just a pressure gauge which is fed from the fuel distributor. It’s a crude but fairly accurate system. It works because the pressure in the fuel distributor is a direct measure of how much fuel is coming out of it, into the 4 or 6 injectors.
I would probably replace the pressure gauge – pick up something on Ebay, just to test with on the ground.
The fuel distributor rarely gives trouble but it has been known. It is an overhaul item when the engine gets done. It’s also not expensive to just replace. But if the engine is running OK then maybe there is a blockage in the pipe to the gauge?
“Bending” should of course be “Bendix”!
Thank you Peter and stickandrudderman. I will look more closely and see whether there is a fuel return. I have joined the Falco Forum and am waiting to be accepted.
A google images search shows a couple of Sequoia Falcos with the same gauge, its a dual MP and fuel pressure one. Will find a way to check the gauge readings anyway.
I have heard there is an obligation to share photos when you own/fly the most beautiful aircraft in existence, isn’t that so ?
(I know, I am being inappropriate … I just could not help it)
