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Fellow aviators, a question, Why do we turn off the fuel boost pump after take off ?

My regular instructor, good bloke, makes me turn the pump off once airborne, I asked him the other day why, his answer was it’s normal procedure and as per flight manual, still no explanation.

Recently I did some mustering flying with a bloke in FNQ in a Jabby 160 and after take off noticed he left the pump on, I pointed to the pump and said u left it on, his answer was why turn it off, you aren’t one of those DH’s that turns it off to save the fuel pump and my answer was yes I do, so he turns off a magneto and I said WTF, his answer was let’s save a magneto.

Later that nite I got talkin to him regarding the fuel pump issue and his answer was that turning the pump off is old GA kerfuffle to save the expensive, certified electric pump, the modern Jabiru has a cheep very reliable electric pump and he doesn’t want to have a pump failure down low when mustering, so he runs both pumps and both mags on. I did take note that when he starts the engine the electric pump is off and he checks the pressure at run up then turns the electric pump on, checks for a slight pressure increase and u can just hear the pump ticking in the back ground noise.

OK who is doing the right thing ???

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There is no one answer for all aircraft, so follow the aircraft's POH. Fuel pumps are used for different reasons One is to transfer fuel, another is to prime the engine when starting (fuel injection old system) a common reason is to back up the EDP when it's failure will make the engine unreliable. then you use it on take off till a safe height and on approach . Some tanks won't feed by suction alone. (tip tanks on a Piper Twin Commanche) . so without electrical power it's not available. (Use it before PNR).. If you are at low level it's covered by the safe height consideration and you leave it on.. Nev

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I can think of two possible considerations for the Jab.

1. If the boost pump is in series with the EDFP then you may provide too much pressure to the carb, overriding the needle and seat because the way the EDFP usually regulates is a differential pressure, ie: maintains 4 psi difference between input and output.

 

2. If you have a leak in your fuel system, with the boost pump off, the EDFP will suck air, and possibly cause the engine to starve, alternatively, with the boost pump on, your engine probably wont starve but a leak under pressure risks a fire.

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Standard Jabiru procedure is to turn on master switch, electric fuel pump on for 5 seconds then off. Start engine without electric pump. This shows the mechanical pump is working. If it wasn't the engine might start on the fuel in the carburettor bowl but would quickly stop. If the electric pump was left on you would not know whether the mechanical pump is operating or not. Standard procedure is then to warm up & run up with electric pump off. Final pre takeoff check includes pump on. This allows for mechanical pump failure on takeoff so you won't need a forced landing.

 

Following this is arbitrary but mostly it is advised to turn the pump off at the top of the climb. This means your mechanical pump is continuing to work. If the engine falters turn the pump back on and land.

 

Downwind checks should include pump on for reasons as above.

 

Low wing aircraft can be different as fuel has to be pumped up to the engine. Once the fuel lines are fully primed to the carb the mechanical pump will suck the fuel through except of course if it fails or you have a vaporisation issue in the lines.

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IF it's a mechanical diaphragm (the usual) Engine driven pump the spring determines the absolute max pressure. An auxiliary pump will just prevent it stroking and the second pump's pressure doesn't add to the other. Nev

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Electric pump used at critical phases of flight, ie where a engine stoppage may result in unrecoverable accident.

Like take off and a possible go around

Its also where fuel flow is maximum and gravity flow may not be able to keep up.

Not many Jabirus can achieve 1.5x max fuel without the electric pump on so it is required at all critical phases

An interesting question is how do you know the electric pump is delivering working properly before you need it?

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An interesting question is how do you know the electric pump is delivering working properly before you need it?

 

Those with a fuel flow indicator will notice a short term temporary increase in flow when pump energised..... Bob

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Turn the electric pump on before starting engine. Read pressure?

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Turn the electric pump on before starting engine. Read pressure?

 

Only if you have a fuel pressure gauge. Most recreational and GA single engine aircraft do not have fuel flow or fuel pressure gauges.

 

Going back to the original post I would be suspicious of an instructor who just says the electric pump is used because it is in the POH. Either he did not feel like explaining it or didn't know. If it was the latter I'd get a new instructor.

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Only if you have a fuel pressure gauge. Most recreational and GA single engine aircraft do not have fuel flow or fuel pressure gauges

The electric and the mechanical should then be checked on the annual.... it's not hard to fit a temporary gauge.

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IF it's a mechanical diaphragm (the usual) Engine driven pump the spring determines the absolute max pressure. An auxiliary pump will just prevent it stroking and the second pump's pressure doesn't add to the other. Nev

 

Hi Nev - I have a slightly different observation (naturally) - with engine running at fast idle (start/warm up conditions) when I turn on/off my auxiliary pump I note a small but consistent, change in pressure. Could be the auxiliary is delivering a small increase in pressure over the main diaphragm pump.

 

It was a long time ago, however in my GA days I recall being able to dicier a small change in engine note when auxiliary pump selected on/off during run up checks (age & imagination?)

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Turn the electric pump on before starting engine. Read pressure?

 

Agreed ! Don't know if this applies ac cross other aircraft but checking auxiliary fuel pressure is one of my pre- engine start checks.

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Most recreational and GA single engine aircraft do not have fuel flow or fuel pressure gauges.
Most? I think you'll find that all certified aeroplanes requiring a fuel pump will have a fuel pressure gauge. eg Airtourer, PA-28s, Cessna 172 with fuel injected engines, Decathlons ...... - I think that about covers "most GA single engine aircraft" here?
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Most? I think you'll find that all certified aeroplanes requiring a fuel pump will have a fuel pressure gauge. eg Airtourer, PA-28s, Cessna 172 with fuel injected engines, Decathlons ...... - I think that about covers "most GA single engine aircraft" here?

The Archer 2 & 172 M,N & S models I used to fly didn't have them. Maybe they do now but there are plenty of old GA singles around.

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Those 172s don't have a fuel pump. I think you'll find that the Archer 2 does. Even an old Cherokee 180 that I used to fly.

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Those 172s don't have a fuel pump. I think you'll find that the Archer 2 does. Even an old Cherokee 180 that I used to fly.

Correct, I'd forgotten that there was a fuel pressure gauge on the Archer. That was always a check when changing tanks very 1/2 hour or so.

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Some high wing planes don't have any fuel pump. It still has t meet the stipulated fuel flow rate for the engine with the added % above full power..( Nearly double. ) I recall priming some injected engines with pump on and move throttle forward slightly to indicate flow and then pump off. or you would start a grass fire under the plane...Nev

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Fellow aviators, a question, Why do we turn off the fuel boost pump after take off ?

My regular instructor, good bloke, makes me turn the pump off once airborne, I asked him the other day why, his answer was it’s normal procedure and as per flight manual, still no explanation.

Recently I did some mustering flying with a bloke in FNQ in a Jabby 160 and after take off noticed he left the pump on, I pointed to the pump and said u left it on, his answer was why turn it off, you aren’t one of those DH’s that turns it off to save the fuel pump and my answer was yes I do, so he turns off a magneto and I said WTF, his answer was let’s save a magneto.

Later that nite I got talkin to him regarding the fuel pump issue and his answer was that turning the pump off is old GA kerfuffle to save the expensive, certified electric pump, the modern Jabiru has a cheep very reliable electric pump and he doesn’t want to have a pump failure down low when mustering, so he runs both pumps and both mags on. I did take note that when he starts the engine the electric pump is off and he checks the pressure at run up then turns the electric pump on, checks for a slight pressure increase and u can just hear the pump ticking in the back ground noise.

OK who is doing the right thing ???

I would consider takeoff, landing, climbing to altitude and mustering flying all situations where I too would use the boost pump continuously.

However, in normal flying where I am at cruising altitude, I turn the boost pump off. I have a fuel pressure gauge and I monitor it regularly during flight. If there is sufficient fuel pressure, the boost pump is superfluous and just adds a little extra electrical load on the engine. Also, the more you run anything, the sooner it will wear out. That is a simple law of physics.

So, I run the boost pump when an engine pump failure would cause danger and leave it off otherwise.

But, if the POH states when the boost pump should be used, you should use it at those times regardless of any advice from me or anyone else.

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I don't have an electric pump so I'm unsure if they normally run in series or parallel with the mechanical pump?

If in series, that is fine as you have flow through the electric pump to the mechanical keeping it cool.

If in parallel, could it be that the reason it is turned off is that lack of flow through it makes it hot?

The mechanical pump is effectively pumping the fuel and therefore the electric is dead heading. Ok for short periods but it will get hot after a period of time.

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I have a fuel pressure gauge and I monitor it regularly during flight.
me too

Plus, if at any time the engine stops I will then know the fuel pressure is zero and ...

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I don't have an electric pump so I'm unsure if they normally run in series or parallel with the mechanical pump?

If in series, that is fine as you have flow through the electric pump to the mechanical keeping it cool.

If in parallel, could it be that the reason it is turned off is that lack of flow through it makes it hot?

The mechanical pump is effectively pumping the fuel and therefore the electric is dead heading. Ok for short periods but it will get hot after a period of time.

It depends on the type of pump I imagine. I use a Facet cube solid state electronic pump which delivers 1.5-3 PSI in series with the mechanical pump. Fuel flows through it unimpeded when it is turned off. There is very little wear as the mechanism is a free moving magnet that is pulsed multiple times a second, hence the distinctive brrrrr sound. I also have a similar but larger 4-6 psi one to transfer fuel from the wing tanks into the main tank.

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I have read the previous posts and don't think the risk of engine fire has been mentioned.

Pre-start - run the electric pump to pressurise the line and check pressure on the gauge. You now know the pump works.

Then turn it off before starting. If you get an engine fire and stop the engine, you dont want the electric pump to continue delivering fuel to the fire.

Pre-takeoff - tun the pump on again.

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Posted (edited)

me too

Plus, if at any time the engine stops I will then know the fuel pressure is zero and ...

I have GoPro footage of my panel as my engine runs out of fuel- the fuel pressure gauge starts to quiver and read lower only a couple of seconds before the audio clearly indicates fuel starvation.

During most flights, the gauge doesn't get much attention. Your hearing is more likely warn of fuel shortage.

Edited by Guest
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Most? I think you'll find that all certified aeroplanes requiring a fuel pump will have a fuel pressure gauge. eg Airtourer, PA-28s, Cessna 172 with fuel injected engines, Decathlons ...... - I think that about covers "most GA single engine aircraft" here?

 

I'm fairly sure there isn't a fuel pressure gauge in the injected C172 I have flown. Fuel flow definitely, but not fuel pressure.

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Posted (edited)

Been a while since I've sat in a 185, 206, 210 or Agwagon but from distant memory they don't have fuel pressure only flow. Same with Fletcher, Brave, they only have fuel flow.

Edited by Guest

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