High / low boost pump useage

[Ragsf15e]

Ok, @kortopates brought this up in a different thread, and he had me questioning my own knowledge, so here goes.  Has anyone had to use these? Especially the high boost pump?

Looking through the checklist, low boost pump would most likely be taking off somewhere real hot and getting vapor in the fuel lines which might show up as a surging engine or possibly fuel flow.  I haven’t seen that yet.

High boost is only for loss of the engine driven pump and can flood the engine if the engine pump is still running.  If the engine driven pump dies, the engine dies, so I see this as probably being something to consider if your engine dies and you have time to try to restart it or if you happen to realize your engine driven pump is going out - maybe you noticed deteriorating fuel pressure and low boost didn’t help? Anyone seen this?

I don’t know what I don’t know, so feel free to add your thoughts!

[Z W]

I have never used the high boost. Per the POH, its only use is to try to re-start the engine after the engine-driven fuel pump has failed. You test its operation by turning it on on the ground and making sure it floods the engine and stops it. Wouldn't want to do that in the air.

I did this year find a new use for low boost beyond engine start. I have intermittently experienced manifold pressure oscillations in cruise at higher altitudes and power settings for years. At normal cruise of 27" MP, for example, it would start fluctuating between 25-29". Fuel flow would fluctuate with it and you could detect a difference in how the engine was running. Overhauling the turbo controller, wastegate, and turbo made no difference. I thought mousemilk on the wastegate fixed it, but it would randomly re-occur. It only occurs on random flights so it was very hard to chase down. Reducing power in flight would make it stop. Descending always made it stop. Once it stopped it usually would not re-occur.

Turns out if you turn on the low boost pump, the fluctuations go away. If you run the low boost for 1-2 minutes, then turn it back off, the fluctuations do not return. I believe the cause is vapor in the fuel system at higher altitudes, temperatures, and pressures (most often saw this on warm days up high). This has worked maybe two times now, those being the only times I've seen it since I figured this out. Might be a good tip for other K owners to know and try since it's not in the POH. I read about it being the reason some later model POH's leave the boost pump on for high altitude climbs, which led me to try it out.

[kortopates]

I have never used the high boost. Per the POH, its only use is to try to re-start the engine after the engine-driven fuel pump has failed. You test its operation by turning it on on the ground and making sure it floods the engine and stops it. Wouldn't want to do that in the air.
I did this year find a new use for low boost beyond engine start. I have intermittently experienced manifold pressure oscillations in cruise at higher altitudes and power settings for years. At normal cruise of 27" MP, for example, it would start fluctuating between 25-29". Fuel flow would fluctuate with it and you could detect a difference in how the engine was running. Overhauling the turbo controller, wastegate, and turbo made no difference. I thought mousemilk on the wastegate fixed it, but it would randomly re-occur. It only occurs on random flights so it was very hard to chase down. Reducing power in flight would make it stop. Descending always made it stop. Once it stopped it usually would not re-occur.
Turns out if you turn on the low boost pump, the fluctuations go away. If you run the low boost for 1-2 minutes, then turn it back off, the fluctuations do not return. I believe the cause is vapor in the fuel system at higher altitudes, temperatures, and pressures (most often saw this on warm days up high). This has worked maybe two times now, those being the only times I've seen it since I figured this out. Might be a good tip for other K owners to know and try since it's not in the POH. I read about it being the reason some later model POH's leave the boost pump on for high altitude climbs, which led me to try it out.

Vapor Suppression is the main purpose of the low boost pump. True the prime button uses the same circuit but either pump circuit can be used in starting. It is just one electric boost pump with a dropped voltage for the low boost pump.

The K POH’s do a very poor job explaining there use which i think stems from back in the day vapor lock didn’t get as much attention. But this changed drastically in modern POH’s. In fact, given the huge market share of Cirrus and the fact that their planes are much more likely to be flown by relatively inexperienced new pilots they decided they needed to be much more proactive to reduce the accident rate. Where all the other manufacturers (Cessna and Mooney) now recommend turning on the boost pump climbing past 9-12k DA, whereas Cirrus went a step further because pilots would forget to do so. Cirrus have the engine set up at idle with the boost pump on so that the pilot can takeoff and climb with the boost pump on without needing to remember to flip it on in climb later.
But the main point is that we should all be using the pump in climb, climbing above 9-12K DA to avoid vapor lock symptoms. Once leveling off below 18K we can usually turn it off but on some days we’ll need to turn it back on if we see fluctuating FF or FP.
This is where having fuel pressures so helpful as you will see fluctuating FP well before it’s so severe to cause fluctuating FF which is accompanied by a rough engine that you will no doubt feel.
Flying in our flight levels at 18K and above the pump should just stay on.

FP, fuel pressure instrumentation, is also vital in recognizing if you need to use the high boost pump from loss of fuel pressure or the low boost pump for vapor suppression.
Choosing the wrong pump it going to greatly intensify the magnitude and duration of the helmet fire the pilot will be going through till they get the engine mixture back to a usable state.

Another less common use of the high boost is at WOT takeoff and climb and you’re not getting enough FF seeing your TIT heading north of 1450. This is a bonafide emergency that Rocket and Acclaim pilots should recognize since their POH’s spell it out but not the older K model POH’s. Yet I’ve seen many K pilots experience a partial power failure when this happens. Flipping on the high boost pump may raise the FF enough to bring down the TIT to safe level but shouldn’t be relied on. I try to empathize aborting takeoff if we see the TIT hit 1450 before getting airborne.

Lastly, the turbo fuel pressure sensor is not the same as the NA engine equivalent. For the turbo we’re looking for the fuel pressure above upper deck pressure which represents the positive gradient of fuel pressure over the manifold pressure in the cylinder that must be overcome to successfully inject fuel into the cylinder. They’re about double the cost of NA fuel pressure since they are taking the difference in pressure between two ports.
For some reason i can’t fathom Mooney dropped the fuel pressure sensor requirement in the 252 & Encore after having it on the 231. But in my opinion it’s a vital instrument for any Turbo pilot.


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[Fly Boomer]

26 minutes ago, kortopates said:

For some reason i can’t fathom Mooney dropped the fuel pressure sensor requirement in the 252 & Encore after having it on the 231. But in my opinion it’s a vital instrument for any Turbo pilot.

How did you get fuel pressure instrumentation?

[kortopates]

i have it on my EDM-900. You should be able to add it to most if not all engine monitors.


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[Ragsf15e]

1 hour ago, kortopates said:

i have it on my EDM-900. You should be able to add it to most if not all engine monitors.


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Do you have to use two probes to measure the difference between them as you indicate above?

[kortopates]

No it’s a differential pressure sensor that has two ports, one plumbed to UDP like your pressurized mags and the other the fuel pressure off a fuel divider port. They are just expensive relative to a normal single port pressure transducer.


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