Boost Pump Question

[jemax]

Hi,

Question ref boost pump,

I'm asking in relation to an EC120, but I guess the question applies across a number of types.

1. It has been suggested to me by a pilot more experienced than myself that when conducting a hot start, the T4 temp control is a lot more easier to control if you do not select boost pump in contradiction to the POH, is this indeed the case or sensible practice? I find the T4 pretty easy to control anyway as long as the engine has been cranked down to 150 degrees or less.

2. Today I quickly repositioned the 120 and in my haste, from a v cold start I forgot to select boost pump, didn't realise until I went to deselect it once NG was stable. Other than opening the throttle open a little more positively initially it seemed to make very little difference to the way the engine started.

My conclusion is that the Boost pump makes very little difference to the start that you cannot control effectively with the throttle. Other than applying more fuel pressure if the flow becomes blocked what does it do? As it seems to make little difference when starting?

Thanks

[wg13_dummy]

Relating to the Gazelle, it was to be used for high alt but by default, we turn it on for start and leave it on (unless it looks like its going to be a hot T4) in all regimes. Engine starts quite happily with it deselected. The boost pump opens the micro pump which bypasses the main FCU to 'prime' the system. Not really required seeings how youre going to squirt a shed load of juice in when selecting 'Run'.

[NickLappos]

On most turbines, the boost pump cannot affect the start or start temperature other than assuring that fuel is available at the fuel control. The boost pump is a low pressure pump whose sole purpose is to lift the fuel into the plumbing, and prevent any bubbles from gathering at the input to the high pressure pump, which is usually next to or inside the fuel control.

The boost pump has 20 to 40 psi, generally, and is located inside the tank so there is direct pressure, vice the suction that the engine pump must make.

The high pressure pump in the engine does the real work on the fuel (all turbines are actually fuel injected) and the high pressure pump is usually about 400 to 600 psi.

Most engine installations can suck the fuel up under benign conditions using only the engine pump (low alt, cool fuel) but most have limits. Altitude counts because the higher the altitude, the less "suction" an engine pump can deliver, since the pump is actually using atmospheric pressure to lift the fuel to fill in the slight vacuum the pump creates at the top of the pipe. The lower the atmospheric pressure (the higher the altitude) the less oomph the atmosphere has to push the fuel up into the vacuum, so the worse the suction pump behaves.

Hot fuel is an problem for a suction pump, because the hotter the fuel, the more it wants to just become a vapor, so the easier it cavitates when the suction condition is present.

Leaving a boost pump on is no problem, except for pump wear. Leaving it off is a problem if you are outside the suction-capable conditions. The "fuel Pressure" light on your caution panel is designed to tell you when the pressure drops at the fuel control inlet (high pressure pump inlet) so that you know when the suction has given up the ghost.

[jemax]

Thanks Nick, that explains it nicely

[ShyTorque]

But N..N..Nick....I'm sooo upset! For years you have been preaching to us that booster pumps are BAD things and suction is so much better.....

NOW you are saying that fuel pressure is GOOD. Gosh - I'm so confused!

Aha! Do Gulfstreams have boost pumps? :E

[NickLappos]

Shy,
I stayed off my soapbox, but here goes:

The NEED for a boost pump is a given for older technology engines and airframes. Engine high pressure pumps exist that can lift the fuel at all altitudes and fuel temperatures, without the need for boost pumps. The S-76 has been to 26,000 feet, and has never had a boost pump.

Why eliminate boost pumps? Because the 20 to 40 psi fuel system is just waiting to turn some section of the helicopter into an oil burner, and especially after a crash or hard landing, is the biggest single cause of post crash fires.

The S-76 and S-92 operate without boost pumps, and the Black Hawk needs boost pumps only for takeoff with high temp fuel at high alt. This was done for safety, and measurably contributes to reduction in post-crash fires.

[ShyTorque]

I knew that, you already told me. :E

I have an old Ford Fiesta Mk3 (the first fuel injected model) which has a high pressure fuel pump in the boot. In the event of an impact the crash switch operates and shuts off the electrical supply to the pump. I recall that some older Sikorsky designs had a similar system. Maybe that is the best of both worlds?

[NickLappos]

The problem is that fuel leaks can occur without crashes as well, and why have a pump, and then a shut-off mechanism, when you need neither?

The pumps that GE, Rolls and turbomeca developed for Sikorsky are not owned by Sikorsky, everyone can use them. I think it is time.