Fuel pressure gauge

[G-BLEW]

From time to time I fly aeroplanes that have a fuel pressure gauge, and while it is interesting to see the needle move I'm not entirely sure what use it is when compared to say fuel flow. The last aircraft I flew with one also had an autopilot, so I had a good look through the POH, but wasn't any the wiser afterwards.

As far as I can tell it is of little use, what am I missing?

Thanks

Ian

[Keef]

Little Concorde has one (one of the few gauges she does have). It tells you the fuel pump is working.

[Skyhawk-N]

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[G-BLEW]

Seems to me that it takes up a lot of space for not very much, fuel flow is significantly more useful.

Ian

[Paul_Sengupta]

Most low wing carburetted aircraft have a small one don't they? You check it on the pre-flight. Fuel pump on, check pressure, pump off. My Bulldog has a fuel pressure gauge, a large proper one, and it shows the fuel flow equivalent. As I understand it, on other VP prop things which are fuel injected, the gauge might be labelled "Fuel flow" but it actually measures fuel pressure. Is this right?

(note, I don't think I've ever flown a carburetted aeroplane with a VP prop)

From here:

http://www.aerotech.net/On_Line_Schedule/stuffforcfiam/8162S_Fuel_System.pdf

The fuel flow gauge translates fuel pressure in the fuel flow divider into an indication of fuel consumption in gallons per hour

There's a diagram showing it linked to the spider/divider on the top of the engine.

Here's my panel:



You can see to the left of the RPM gauge is a combined instrument which shows manifold pressure and fuel pressure. The fuel pressure gauge can, and in some cases is, marked up in gallons per hour. I think you can get stickers to put on the front of it!

Here's a flight sim photo of the same instrument in a Piper Arrow...you can see the pressure gauge is marked up in GPH.



Above it though is the fuel pressure gauge, the same as it is in a Warrior. I'm guessing this is taken from a different part of the fuel system? I don't have a separate fuel pressure gauge in mine.

[leiafee]

It's certainly of naff all use in well over 50% of tommyhawks I've flown -being almost invariably unservicable! Stuck on one end or the other regardless of the actually state of affairs. We replaced every component part of TOMS' one and never made it work...

[G-BLEW]

Most low wing carburetted aircraft have a small one don't they?

I guess, but the last aircraft I flew with one was a C182 in the US

Ian

[Paul_Sengupta]

I guess, but the last aircraft I flew with one was a C182 in the US

Was it in lieu of a fuel flow gauge or as well as? Do you have a photo?

[G-BLEW]

Instead of fuel flow - that was what surprised me. I'm not sure I have a pic, I will take a look.

Ian

[G-BLEW]

Here you go



Ian

[Paul_Sengupta]

Ah right, so that *is* the fuel pressure/flow gauge, same as it is in mine or in the Arrow. It's just how it's marked up that's the issue I guess.

[Keef]

Not really.

G-UTSY had a fuel flow gauge (went from 0 to 20 GPH as I recall) which settled to 10 GPH in the cruise, all leaned. She also had a fuel pressure gauge which sat resolutely at 4½ whatevers.

At each fuel tank change, the sequence was "fuel pump on ... change tank ... fuel pressure remains ... pump off ... fuel pressure remains."

[Paul_Sengupta]

Not really.

G-UTSY had a fuel flow gauge (went from 0 to 20 GPH as I recall) which settled to 10 GPH in the cruise, all leaned. She also had a fuel pressure gauge which sat resolutely at 4½ whatevers.

At each fuel tank change, the sequence was "fuel pump on ... change tank ... fuel pressure remains ... pump off ... fuel pressure remains."

Yes, but this goes back to what the gauge measures and where. The "fuel flow gauge" actually measures fuel pressure at the point where the fuel injection system supplies the fuel to the injectors. The pressure here is equivalent to the fuel flow, so the gauge can be marked up as fuel flow rather than fuel pressure. It's in the diagram in the link I put in my first post, I found it for an Arrow. On some aeroplanes it's marked as fuel pressure (as on mine and the 182 that G-BLEW posted), on others (such as the Arrow) it's marked with fuel flow.

I don't know where the small "fuel pressure gauge" is taken from, you'd have to ask one of the forum engineery people. I suspect it's taken from after the fuel pump but before the throttle body? Edit: Ah, looking at the Cherokee 6 link below, yes, it does indeed connect to the fuel line after the fuel pump, just shy of the throttle body. So my reasoning is sound!

There's another one here for the Cherokee 6.

http://www.pcflyers.org/fleet/N4072W/files/PA-32-300POH.pdf

A combination fuel flow indicator and manifold pressure gauge is installed in the left side of the instrument panel. The fuel flow indicator is connected to the fuel flow divider and monitors fuel pressure. The instrument converts fuel pressure to an accurate indication of fuel flow in gallons per hour and percentage of cruise power.

Anyway, 2 psi on mine equates to about 29-30 litres an hour.

[Skyhawk-N]

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[Ridders]

Interesting discussion, as I currently have a fuel pressure gauge on the Pup carbed engine.

The Bulldog POH actually has a graph that gives a direct Fuel flow in UK gal/h vs fuel pressure (thats shown on the gauge as PSI).
In your picture paul, its showing 2.4, which equates to 7.6 gal/hr = 34.5 ltr/hr.

In the Pup, its a pressure indicator, and shows pressure and change when the electricical pump is switched on/off. The manual is pretty basic, stating if the engine driven pump fails with electric pump off, the engine will stop, which I can understand. It does however state that if the mechanical pump fails with the electric pump on, it could be detected, but doesnt really state how. I would expect to see reduced pressure to that which I would normally see with pump on. Is that reasonable chris?