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 yesterday at 04:00 AM 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