What if I lean the mix with fuel pump on, then turn off the pump?

[NT5224]

Hi team.

 

on a 50km local flight yesterday I was watching my fuel flow and realised it was rather high. As an impoverished aviator I adjusted mix to decrease my fuel usage.

 

Then I realised that, as I sometimes do, I had left my fuel pump on after takeoff and climb. There are no hard and fast rules about turning fuel pumps off in flight so I sometimes just let it keep running on local flights.

 

So, forgetting I had already leaned the mix, I switched off the pump. And my fuel flow plummeted…

 

I don’t think the reduced flow reached the engine because RPM didn’t fall, but the flow guage  read close to what it reads on idle. I immediately switched the pump back on, and we went back to normal.

 

Is there a lesson to learn here? Should I always switch off fuel pump before adjusting mix to run lean in the cruise? Nobody has ever told me that before.

 

I don’t want to fall out of the sky. 🤣

 

Alan 

[facthunter]

What engine and carburettor do you have and do you normally lean on fuel flow or WHAT?  Nev

[NT5224]

What engine and carburettor do you have and do you normally lean on fuel flow or WHAT?  Nev

 

 

Good morning Nev.

 

I fly behind a Lycoming  0320, if that matters.

 

Im looking for guidance from experienced pilots on a very simple issue which I hoped would be informative  to others.

 

i have been flying over 20 years myself, and in that time I can’t recall anybody ever telling me to turn off the fuel pump before leaning the engine. Not in my pilot training or in any subsequent flight review. Maybe it’s just so obvious they don’t bother mentioning it!

 

Now I think about it, it is bloody obvious, but I found out through making the error  myself in flight. No harm was done, so I’m just sharing the lesson learned and observation I made.

 

I thought that  advice is probably better coming from aviators with more experience than I. Somebody like you Nev.

 

So what would you advise? No capitals please! 😱

 

Alan 

 

 

 

 

Edited June 16 by NT5224

[Thruster88]

I have o-320 in the musketeer and o-360 in the RV. 

 

The rate of fuel consumption is only changed by mixture and power setting, there is no change with electric boost pump on or off, if the are no problems with the fuel system such as partially blocked filter, engine pump faulty etc.

 

How ever in my RV switching on the boost ALWAYS shows an instant increase in fuel flow. I believe this is due to fuel pulsing in the flow meter.

 

So if you lean to a known fuel flow with the pump on and then turn it off the mixture would be to lean in my RV by about 2 gph.

[NT5224]

41 minutes ago, Thruster88 said:

I have o-320 in the musketeer and o-360 in the RV. 

 

The rate of fuel consumption is only changed by mixture and power setting, there is no change with electric boost pump on or off, if the are no problems with the fuel system such as partially blocked filter, engine pump faulty etc.

 

How ever in my RV switching on the boost ALWAYS shows an instant increase in fuel flow. I believe this is due to fuel pulsing in the flow meter.

 

So if you lean to a known fuel flow with the pump on and then turn it off the mixture would be to lean in my RV by about 2 gph.

G’day Thruster88

 

thanks for this informed explanation.  I don’t doubt what you say, but my fuel flow guage reading definitely increases with boost pump on. But as you say, RPM does not correspondingly increase.
 

As mentioned in my original post, when I saw the fuel flow drop suddenly I immediately switched the pump back on again, so maybe that was just a temporary ‘blip’ in the reading, and it would have stabilised after a while.

 

So you are saying that if I lean the mixture with the fuel pump on, there should be no problem then switching it off? 
 

Or would it be advisable to first switch off the pump, then lean the engine?

 

I’ve never thought much about this before, but was staring at the fuel guage as I switched off the pump yesterday and the reduced readout was concerning.

 

Alan 

 

 

[Thruster88]

Yes any time the boost pump is on in my RV the indicated fuel flow is about 2 gallons per hour higher, this is obviously a false reading since there is never any change in power, engine sound or air speed.

 

Are you A, leaning to a fuel flow from the lycoming performance chart? or B, simply pulling the mixture out until the engine starts to run rough and then go  slightly richer? If the answer is B then fuel pump on or off will not matter. If answer is A and your flow meter is like mine then there will be a difference. 

 

If you are leaning to a fuel flow then I would switch the boost pump off. Normally the boost pump would be off by the time leaning is done. Leaning in a lycoming should be done any time the power is 75% or less, this has nothing to do with altitude. 

[NT5224]

Thanks Thruster88

 

That’s really valuable information and exactly what I wanted to hear from a fellow pilot!

 

To be honest my apparent drop in fuel  flow switching off the pump yesterday was even more than 2 gallons… it recorded a greater reduction in flow than when I had leaned the engine… 

 

I lean my engine by pulling the mix until I get a wobble on the RPM  needle then going back a bit. I then keep an eye on fuel flow and temps…

 

Not very scientific but I get a smooth running engine, better fuel efficiency and hopefully less muck in the cylinders

 

Alan
 

 

[facthunter]

On a float equipped Carburettor I can't see why extra fuel pressure would change anything, unless the flow is below what is should be without the Tank Pump. As a  rule I don't lean below about 5000 ft where you're then only getting about  75% with WOT. When a light plane goes into service you should  check and get a Fuel flow  of  about 150% of the Maximum it will ever need on take off. Maybe you have some impediment to flow?  Without a fuel flow indication to lean I just lean till it starts to  lose Power then push in till it runs smooth and then a "wee" bit more. to be sure it's not lean. That has always come out to what the Book figures would give you. With Bigger stuff you go by the Book. Manifold Pressure RPM and fuel flow for the HP you  aimed for dependant on the weight.  Nev

[dlegg]

Do you have a high wing aircraft or low? If high with tanks in them, you probably don't even need a pump. But you must have an engine driven pump?

[NT5224]

1 hour ago, dlegg said:

Do you have a high wing aircraft or low? If high with tanks in them, you probably don't even need a pump. But you must have an engine driven pump?

High wing, gravity fed motor. No I don’t need the pump in cruise so often turn it off.

 

It’s  an electric pump, not mechanical. 

 

Alan

[Headwind]

Personally I have always turned the fuel pump off at around 500ft on climb out, with some possible small drop in flow. But given aircraft age and the drop not affecting the noise up front I’ve never been too bothered; that said I also don’t look at it for long since I’m at a critical phase of flight and trying to look out. I also take it as a demonstration that the pump is working, and if the engine keeps going then gravity and engine driven pumps are also going ok too. Then when I reach cruise I lean out; I tend to above 3000 if it’s a decent cruise or maybe not bother if I’m stooging about. 
Not sure if that’s any help, but it’s what I was taught and have tended to follow since. 
cheers

[skippydiesel]

Sonex Legacy/Rotax 912 ULS - Boost pump on for Take-Off and all low level opps. Turn off Boost when at/above Circuit height - immediate drop in fuel pressure. Pressure return/recovers slowly over about 1- 2 minutes. Never been any change in engine note, so am of opinion that gauge is likly in error.😈

[Thruster88]

8 hours ago, skippydiesel said:

Sonex Legacy/Rotax 912 ULS - Boost pump on for Take-Off and all low level opps. Turn off Boost when at/above Circuit height - immediate drop in fuel pressure. Pressure return/recovers slowly over about 1- 2 minutes. Never been any change in engine note, so am of opinion that gauge is likly in error.😈

How would the fuel pressure gauge know that you had just turned the boost pump off and then decide to indicate a lower pressure for 1 to 2 minutes? It must be a very clever or mischievous one, or it might just be showing what is actually happening. 

[pmccarthy]

Same thing used to happen in my PA28. I checked and found it was common and not a problem.

[skippydiesel]

2 hours ago, Thruster88 said:

How would the fuel pressure gauge know that you had just turned the boost pump off and then decide to indicate a lower pressure for 1 to 2 minutes? It must be a very clever or mischievous one, or it might just be showing what is actually happening. 

Nope - no effect on engine performance ie engine still receiving a sufficient fuel - remember pressure , in this context, is a secondary indicator of supply  - the engine runs on volume not pressure..

It can only be an anomaly of my fuel distribution system or some inherent fault in the pressure sensing.

IF the pressure is actually dropping , as indicated, the engine is somehow still receiving sufficient volume of fuel to keep functioning normally.😈

[Moneybox]

The carburettor needle and seat controls the fuel level and therefore mixture regardless of fuel pump pressure (unless excessive). Your fuel flow measurement will be fuel consumed plus by-pass fuel that is returning to the tank. The electric fuel pump should not have any effect on engine performance or fuel mixture unless the mechanical pump is faulty.

[Thruster88]

18 hours ago, skippydiesel said:

Sonex Legacy/Rotax 912 ULS - Boost pump on for Take-Off and all low level opps. Turn off Boost when at/above Circuit height - immediate drop in fuel pressure. Pressure return/recovers slowly over about 1- 2 minutes. Never been any change in engine note, so am of opinion that gauge is likly in error.😈

Regarding your posts that the carburetor engine still receives adequate flow even at low fuel pressure, I TOTALLY AGREE, however to say the fuel pressure gauge is in error sometimes makes no sense. How can it be in error sometimes?

 

Is your fuel pressure on the ground, Boost pump only the same as while flying, engine pump only?

 

In a tecnam /912 I have been flying lately they are the same. Turning off the boost pump in flight results in zero change in fuel pressure on the mechanical fuel pressure gauge. Below is a 912 fuel pump. A blockage in the vent line might explain your pressure rising over a minute or two. The little spring controls fuel pressure and should immediately take over from the boost pump unless perhaps the diaphragm is being held down and not able to stroke.

 

 

 

[turboplanner]

Electric fuel pumps on Basic (training) GA airctraft were introduced as an extra safety feature in the case of a mechanical fuel pump failure near the ground.

The Check list was:

Pre Start:                                                  Fuel pump on, check making pressure.

After Start:                                                Fuel pump off

Pre Take Off:                                            Fuel pump on, check pressure

1000 feet:                                                 Fuel pump off

Pre-Landing checks (downwind leg):    Fuel Pump On

After landing:                                           Fuel Pump Off 

 

The reason for turning off after engine start and in flight was to avoid the situation where a fuel line might be cracked, or blow off at a connection and cause a runaway situation spraying fuel in the engine compartment until there's a fire or the smell makes you turn it off and hope.

 

The pumps were not designed as "boost" pumps. They were simply a redundancy item like twin spark plugs and twin magnetos.

The carburettors determined how much fuel they wanted to draw.

 

On these airctaft if you get correct fuel burn, such as 49 litres/hr climb, 32 litres/hr cruise at 75% power (flight where no leaning is involved) they are consistent, and if you do your 10 minute (Not 10 Nm) checks the fuel tanks will usually be confirming your calculations, as will the refill at the end.

 

If you record the details of every flight, and you've calculated the correct amount of taxy fuel, climb fuel, cruise fuel, descent fuel and taxy fuel, based on minutes, you should be seeing actual vs results (for equal flight times (not distances).

(Using this data, if there's a strong head wind you can calculate and decide to return to base after 10 minutes cruise rather than an hour and a half later hoping you can make it home).

 

Remember, fuel burn is always time based, not distance based; (the wind direction and strength decides distance.)

 

Notice so far that I have stuck to basic training GA aircraft all built on production lines, all with a lot of testing, all with production components and all producing consistent results without tinkering.

 

RA Airctraft, including self built are likely to have anything in their system, and in some cases producing less efficiency than the kit designer specified.

 

RA "Boost" Pumps

This implies a pump that boosts the pressure.

If it really is a boost pump, and not just an incorrect name, this implies that a mechanical pump is fitted, but doesn't have enough capacity to supply fuel to the engine at full power. In this case if you don't have your boost pump working you could run the engine lean and  get combustion chamber damage. (I suspect that people here are just mis-calling the name of the pump, but you never know.)

 

Flow gauges

Flow gauges are notoriously inaccurate; they can be getting different electrical inputs or they can be uncalibrated and showing inaccurate readings or they can be showing accurate flow at that second, but reading a situation which is temporary, such as reading high when lower pressure fuel is rushing to get up to pressure. Better to build fuel records on your engine based on Taxy, Climb, Cuise, Descent on your RA engine as a whole.

 

If you want to save money, fly on calm days or where you can avoid headwinds.

 

 

 

 

 

 

 

 

 

 

 

[facthunter]

Depends on the pressure of the electric pump setting (Built into it) Your gauge will read the higher figure, never the combined figure. If there's a partial blockage anywhere it will limit the flow at higher power settings and that  would lean it out. You need to go back to the FLOW rate checks to know what's happening.. IF the pressure gauge reading changes when you change something there's a near 100% chance the pressure is changing. Nev

[skippydiesel]

5 hours ago, Thruster88 said:

Regarding your posts that the carburetor engine still receives adequate flow even at low fuel pressure, I TOTALLY AGREE, however to say the fuel pressure gauge is in error sometimes makes no sense. How can it be in error sometimes?  I don't recall using the word "sometimes" in this context. Have checked back on posts  - nope!

 

Is your fuel pressure on the ground, Boost pump only the same as while flying, engine pump only? Boost pump only, on the ground, about 5 psi - in Air, mechanical pump only, 4.5 psi. Mechanical + Boost delivers about 5.5 psi

 

In a tecnam /912 I have been flying lately they are the same. Turning off the boost pump in flight results in zero change in fuel pressure on the mechanical fuel pressure gauge. Below is a 912 fuel pump. A blockage in the vent line might explain your pressure rising over a minute or two. The little spring controls fuel pressure and should immediately take over from the boost pump unless perhaps the diaphragm is being held down and not able to stroke.

There is no blockage - I have tried to "fix" this anomaly over the last 18 months to no avail. As there has never been even a hint of engine hesitation, I will just live with it. 

In about 3 years (5 year scheduled replacement) I will replace the mechanical pump - it will be interesting to see if the new pump delivers the same/diffrent performance/characteristics.😈

 

 

 

 

[skippydiesel]

3 hours ago, turboplanner said:

Electric fuel pumps on Basic (training) GA airctraft were introduced as an extra safety feature in the case of a mechanical fuel pump failure near the ground.

The Check list was:

Pre Start:                                                  Fuel pump on, check making pressure.

After Start:                                                Fuel pump off

Pre Take Off:                                            Fuel pump on, check pressure

1000 feet:                                                 Fuel pump off

Pre-Landing checks (downwind leg):    Fuel Pump On

After landing:                                           Fuel Pump Off 

 

The reason for turning off after engine start and in flight was to avoid the situation where a fuel line might be cracked, or blow off at a connection and cause a runaway situation spraying fuel in the engine compartment until there's a fire or the smell makes you turn it off and hope.

 

The pumps were not designed as "boost" pumps. They were simply a redundancy item like twin spark plugs and twin magnetos.

The carburettors determined how much fuel they wanted to draw.

 

RA Airctraft, including self built are likely to have anything in their system, and in some cases producing less efficiency than the kit designer specified. Speculative negativity, without foundation

 

RA "Boost" Pumps

This implies a pump that boosts the pressure. Some do - mine by about .5psi

If it really is a boost pump, and not just an incorrect name, this implies that a mechanical pump is fitted, but doesn't have enough capacity to supply fuel to the engine at full power. In this case if you don't have your boost pump working you could run the engine lean and  get combustion chamber damage. (I suspect that people here are just mis-calling the name of the pump, but you never know.) Agreed - see below.

 

 

Some comments on your statements above;

NOTE: My comments are with reference to Rotax naturally aspirated (carburetted) engines. Fuel injection is a whole diffrent ball game.

3 hours ago, turboplanner said:

 

The term "Boost Pump" is as you say a misnomer - Back Up/ or Auxiliary Pump  would be more a acurate description of the function.

 

My usage (slightly diffrent to yours) & reasoning;

• Pre Engine Start - Boost Pump ON - observe pressure rise  - OFF. Reason for off, is that there is no further benefit to the starting regime (unless vapour lock suspected). Fuel system has been primed and there is no good reason to add the small additional electrical load when demanding a high starting load.
• Pre Take -Off - Boost Pump ON - observe any changes to fuel pressure.
• At or about Circuit Hight  - Boost Pump OFF (when convenient to do so. There is no reason for hast)
• Pre Landing (including any low level opps) - Boost Pump ON
• Post Landing - Boost Pump OFF (when convenient to do so)

While it is possible to run the Boost Pump continually (some do) there is no benefit in this practise. There are two potential disadvantaged in continual use - shorten the pumps service life and may hide a deterioration in the performance of the Primary mechanical pump. 

The Boost Pumps primary purpose is to ensure fuel supply to the carburettors at times of potential or actual high engine power demand, when a loss of power may result in crash. Its secondary function is to prime the fuel supply system to ensure a "good" engine start. The latter may include hot starts where vapour lock is a potential or suspected 😈

3 hours ago, turboplanner said:

 

 

[BrendAn]

this is exactly what we are taught in raaus

 

turboplanner

Posted 13 hours ago

Electric fuel pumps on Basic (training) GA airctraft were introduced as an extra safety feature in the case of a mechanical fuel pump failure near the ground.

The Check list was:

Pre Start:                                                  Fuel pump on, check making pressure.

After Start:                                                Fuel pump off

Pre Take Off:                                            Fuel pump on, check pressure

1000 feet:                                                 Fuel pump off

Pre-Landing checks (downwind leg):    Fuel Pump On

After landing:                                           Fuel Pump Off 

[BrendAn]

RA Airctraft, including self built are likely to have anything in their system, and in some cases producing less efficiency than the kit designer specified.

 

RA "Boost" Pumps

This implies a pump that boosts the pressure.

If it really is a boost pump, and not just an incorrect name, this implies that a mechanical pump is fitted, but doesn't have enough capacity to supply fuel to the engine at full power. In this case if you don't have your boost pump working you could run the engine lean and  get combustion chamber damage. (I suspect that people here are just mis-calling the name of the pump, but you never know.

its actually the backup or secondary pump.  if you need it to get full power then there is a problem with the mechanical pump.

boost pump is not a term i have heard any instructors use.

 

 

[Thruster88]

5 hours ago, BrendAn said:

RA Airctraft, including self built are likely to have anything in their system, and in some cases producing less efficiency than the kit designer specified.

 

RA "Boost" Pumps

This implies a pump that boosts the pressure.

If it really is a boost pump, and not just an incorrect name, this implies that a mechanical pump is fitted, but doesn't have enough capacity to supply fuel to the engine at full power. In this case if you don't have your boost pump working you could run the engine lean and  get combustion chamber damage. (I suspect that people here are just mis-calling the name of the pump, but you never know.

its actually the backup or secondary pump.  if you need it to get full power then there is a problem with the mechanical pump.

boost pump is not a term i have heard any instructors use.

 

 

Brendan, the POH for my 1963 Beechcraft 23 always calls the electric fuel pump the boost pump. It does this in the normal and the emergency sections of the manual. In the event of engine failure BOOST PUMP ON and change tank is first in the list.

 

The engine pump can supply enough fuel pressure at all times unless there is a mechanical pump failure, vapour lock, partial blockage or air in the system due to a dry tank selected, the BOOST pump can over come these issues. 

 

I will continue to call the secondary, electric, backup, other pump the BOOST pump in any aircraft I fly. Unless it is an iS rotax, then it is pump A and pump B.

[BrendAn]

Fair enough. I only know raaus terms. And  the training for use is exactly as turbo described with ga.

 

Skippy is implying that you need the pump on to get full power at critical phases of flight. 

In training you are told it's only on as a backup in case the mechanical pump fails.

It's just words I suppose.

 

My latest xair has a 618 and if I leave the electric pump on too long at idle it overcomes the the needles and starts flooding.  Will have to swap for a lower pressure pump or fit a regulator.

Standard xair backup is a squeeze bulb, not much fun operating that if mechanical pump fails.