Mixture Rich, Boost Pump On

[Jaybird180]

When I did my transition to the DA-40 (par example) the pre-takeoff and the pre-landing checklists both include the titled procedure. I've always wondered why.

Reading Deakins' article on Mixture it brought the question back to mind on why the procedure exists.

He says:

It's All in the Mixture


How does this translate into the gasoline-powered, spark-fired, internal combustion engines found in our airplanes (and also in tractors, lawn mowers, and cars)? For purposes of this article, I'll limit my discussion to normally-aspirated engines, leaving turbocharged ones for a future column.


In the engine that powers my Bonanza (a TCM IO-550), if I flip the boost pump to high with the mixture rich, I can literally "flood" the engine and kill it, even at takeoff or climb power settings. (Update Oct. 2005: This column was written in 1999, when I believed that to be true. Alas, it is not completely so. At low power settings, it is true, and illustrates the principle nicely. But most of these engines -- including mine -- tolerate the boost pump very well at high power settings. Indeed, the boost pump can be your friend if taking off with hot fuel in the tanks, or if you climb rapidly, as with the Turbo'd aircraft.)
Any malfunction that pumps "too much" fuel into the combustion chamber (relative to the air) will do the same thing. The fuel is flowing, the spark is sparking, and the airspeed is still turning the prop, but there's just too much fuel and not enough air. Flip the boost to "Low" or "Off" or lean it out with the mixture control and the engine will run again. Given the correct fuel flow, air flow and spark, it must start and run.

Can someone help me understand why? Why not just leave the engine-driven pump to do the job? The procedure also is there on other low-wing planes.

 

[MAKG1]

The engine-driven pump is a siphon on low-wing aircraft, and is subject to vapor lock. The boost pump is not. It's mostly an issue when the fuel lines are heated externally, such as with a warm engine on a warm day, on the ground.

On a high wing aircraft, fuel is fed by gravity, and it is not an issue.

 

[wsuffa]

Can someone help me understand why? Why not just leave the engine-driven pump to do the job? The procedure also is there on other low-wing planes.

I don't know about his engine, but the engine on my plane is barely adequate at full fuel-flow, especially at higher altitudes.

 

[dell30rb]

It has something to do with vapor lock and hot fuel lines. The extra fuel pressure with the boost pump forces extra fuel through the lines and will overcome vapor lock. If its at a cruiser or higher power setting it can handle this extra fuel without extinguishing the flame.

 

[Jaybird180]

So why is vapor lock only a concern at takeoff and landings. Seems like a design problem. What am I missing here?

 

[Clark1961]

So why is vapor lock only a concern at takeoff and landings. Seems like a design problem. What am I missing here?

I believe it's more of a consequence thing. If the mechanical fuel pump quits then you've got a real problem.

On the Frankenkota, the POH specifies that the electric fuel pump has no standby function. Low side of the electric fuel pump is to be used to suppress vapor lock if variation is observed in fuel pressure. High side of the electric fuel pump is to be used if the mechanical fuel pump fails (fuel pressure/flow goes to zero). Basically if the mechanical fuel pump fails on departure then chances are the aircraft is landing.

 

[Geico266]

So why is vapor lock only a concern at takeoff and landings. Seems like a design problem. What am I missing here?

Mostly on take off, and yes, it is a design "issue". On a hot day, fast climb out, the heat builds up very fast and heat soaks the engine compartment. The mechanical fuel pump is sucking fuel. The vapor point is lowered under these conditions and a vapor "lock" can occur. It can be eliminated or prevented by pressurizing the lines as close to the fuel tanks as possible. This is why today's cars have the pump in the tanks.

 

[Sac Arrow]

Electric fuel pumps have different functionalities in different planes. On Lycoming engines (DA 40's, Lycoming equipped Cherokees) the fuel pump is used on takeoff and landing. Turbo Arrows have a two position boost pump that is not used in normal operation, just if vapor lock occurs, which is probably the case with the quoted TCM IO 550 in the article. Author is just cautioning people to be careful in its use. Not really an issue with the DA 40 or normally aspirated Pipers.

The engine driven pump normally works fine in these planes on startup and takeoff. The use of the electrical pump is just a precaution to prevent vapor lock at a critical time. Although, the DA 40 I flew wouldn't idle correctly on the ground at high density altitudes without the boost pump running, probably due to internal heat under the cowling.

 

[Anthony]

I was taught the aux. fuel pump was put on during takeoff, and landing in case the engine driven, mechanical fuel pump failed.

 

[Sac Arrow]

I was taught the aux. fuel pump was put on during takeoff, and landing in case the engine driven, mechanical fuel pump failed.

That too, but vapor lock is a more likely occurrence. In some low wing planes, the aux pump isn't used at all in normal operation.

 

[Geico266]

That too, but vapor lock is a more likely occurrence. In some low wing planes, the aux pump isn't used at all in normal operation.

Unless you build an RV-12.

This has been a major problem of mine with Van's and a Rotax engine set up. We had to build our E-LSA -12's to the plans which included having both pumps on all the time. The first thing I did after certification was to add a cut off switch for the electric pump. Many builders have reported mechanical pump failures. Mine has been fine.

 

[MAKG1]

So why is vapor lock only a concern at takeoff and landings. Seems like a design problem. What am I missing here?

Not takeoff and landing. Takeoff and go-around.

That's when you need the most fuel and put the most stress on the fuel system.

 

[TMetzinger]

Also, at altitude in cruise, sometimes the fuel pressure in the DA40 can get low enough to trigger the G1000 alert, in at least two of the planes I've flown. Turning on the electric fuel pump eliminates this.

So far every low-wing airplane I've flown (haven't flown the commanche) requires the electric fuel pump on for takeoff and landing (really for takeoff and go-around as stated above). It's just safer to have positive pressure in the fuel line all the way to the engine.

 

[flyingcheesehead]

Also, at altitude in cruise, sometimes the fuel pressure in the DA40 can get low enough to trigger the G1000 alert, in at least two of the planes I've flown. Turning on the electric fuel pump eliminates this.

The DA40 POH specifically states to use the electric pump at high DA, at least in climbs... (Time to go read it again.)

So far every low-wing airplane I've flown (haven't flown the commanche) requires the electric fuel pump on for takeoff and landing (really for takeoff and go-around as stated above). It's just safer to have positive pressure in the fuel line all the way to the engine.

The Mooney Ovation (TCM IO-550) uses the pump only for priming at engine start, absent an emergency. If you don't turn it off after engine start, it'll run so rich that it'll barely stay on at idle, and it has a significant and noticeable effect on mixture. And that's just the low boost switch! The high boost switch has a guard over it, and I'm guessing that's because it'd probably kill the engine entirely if you accidentally flipped it in many phases of flight. Kind of the opposite of fuel starvation - Fuel gorging!

FWIW, DA40's used to have an issue with the fuel pumps not lasting very long. Duke's finally made a new rev just a couple of years ago that actually seems to last. So, I make it a habit to turn the electric pump off after takeoff as soon as I'm high enough to make it back to the runway, whereas in most low-wings I'll leave it on until I level off at cruise. I figure it's better to be gentle to it in normal operations, lest it fail when I need it most!