Function of my Boost Pumps

[aviatoreb]

Hi All,

Don't berate me for not knowing - at least I am asking.

What is the function of my two boost pump switches on my M20K. I know what they operate, so what I mean is what scenarios call for use of each.

1) Low Boost.

Possibilities

- when I switch tanks? This was the function per POH in my previous airplane, the DA40, but I don't know here. Is this standard operating procedure to run low boost when switching tanks in case at that very instant the mechanical pump fails?

- when cooling is an issue like an extended climb?

- with a hot start, and with cutoff on full lean the low boost works nicely to pressurize the fuel lines and chase vapor lock. This function i am sure works well.

2) High Boost? It has a switch lock over it so you cannot touch it by accident. Is this for failure of the mechanical pump? But then how would turning this high boost on do anything but flood if activated when trying to restart after the mechanical might fail in flight. So what scenario calls for this?

Please feel to correct as I am asking.

BTW I have a TSIO520 with a rocket version of M20k but I don't think don't think this effects my question.

[aviatoreb]

Hi All,

Don't berate me for not knowing - at least I am asking.

What is the function of my two boost pump switches on my M20K. I know what they operate, so what I mean is what scenarios call for use of each.

1) Low Boost.

Possibilities

- when I switch tanks? This was the function per POH in my previous airplane, the DA40, but I don't know here. Is this standard operating procedure to run low boost when switching tanks in case at that very instant the mechanical pump fails?

- when cooling is an issue like an extended climb?

- with a hot start, and with cutoff on full lean the low boost works nicely to pressurize the fuel lines and chase vapor lock. This function i am sure works well.

2) High Boost? It has a switch lock over it so you cannot touch it by accident. Is this for failure of the mechanical pump? But then how would turning this high boost on do anything but flood if activated when trying to restart after the mechanical might fail in flight. So what scenario calls for this?

Please feel to correct as I am asking.

BTW I have a TSIO520 with a rocket version of M20k but I don't think don't think this effects my question.

Darn I multiposted! This is the edited version. How do I remove the rest?

[fluffysheap]

All this information is in the POH, but I'm not sure if the Rocket conversion changes it. I can only tell you what the basic M20K POH says.

Most of the time , the boost pumps in the M20K are not used during flight.

The low boost pump's primary function is to mitigate fuel vaporization. You've discovered the use of it for hot starting, but a secondary function is for high altitude operation (above 18K) where low ambient pressure and the suction of the engine-driven pump might cause the fuel to vaporize. The Diamond has a similar recommendation in the POH. You should not use it while taking off, landing, or switching tanks. In the Mooney, the low boost pump alters the fuel flow and so operating it only part of the time will result in an inaccurate mixture setting. At idle, it may even cause the engine to stop running due to an over-rich mixture. This is unusual as most aircraft specify the use of the boost pump during landing, takeoff, tank switching, and various other times.

The high boost pump is there for emergencies in case the engine driven pump fails. You can operate it briefly without removing the guard. The high boost pump does not have as much flow capacity as the engine driven pump and my POH specifies a maximum power and other limitations while using the high boost pump. In general, you can expect to never turn this pump on although I occasionally run it momentarily on the ground just to make sure it works.

[aviatoreb]

All this information is in the POH, but I'm not sure if the Rocket conversion changes it. I can only tell you what the basic M20K POH says.

Most of the time , the boost pumps in the M20K are not used during flight.

The low boost pump's primary function is to mitigate fuel vaporization. You've discovered the use of it for hot starting, but a secondary function is for high altitude operation (above 18K) where low ambient pressure and the suction of the engine-driven pump might cause the fuel to vaporize. The Diamond has a similar recommendation in the POH. You should not use it while taking off, landing, or switching tanks. In the Mooney, the low boost pump alters the fuel flow and so operating it only part of the time will result in an inaccurate mixture setting. At idle, it may even cause the engine to stop running due to an over-rich mixture. This is unusual as most aircraft specify the use of the boost pump during landing, takeoff, tank switching, and various other times.

The high boost pump is there for emergencies in case the engine driven pump fails. You can operate it briefly without removing the guard. The high boost pump does not have as much flow capacity as the engine driven pump and my POH specifies a maximum power and other limitations while using the high boost pump. In general, you can expect to never turn this pump on although I occasionally run it momentarily on the ground just to make sure it works.

Thanks fluffy sheep.

I can't find tell of their function in my POH which is why the question. Could pages be missing?! What else might be missing?

So running low boost alters mixture setting - makes sense and makes sense that running it at idle might even over rich and cause engine to shut off in an over rich condition.

What about running it on a prolonged climb to improve cooling? Is that a good idea? Not that I am having a cooling problem but just asking what is sop.

I have never operated my high boost. Should I to test it - I presume it is like the low boost pump on steroids. So more fuel flow is even more likely to cause cut-off if not at a higher setting and it would be like running the engine in an over rich setting even if leaned by the red nob. Is it a toggle switch that stays on or is it a press and hold switch?

Most of all - if I really need the high boost - how do I use it? I mean I guess I need it if the engine shuts off because the mechanical pump has failed (knock on wood everyone). So the engine has shut off. Is there any other scenario? But then to try to restart on high boost wouldn't I just have a stopped and flooded engine if I try running the high bust in flight (dead stick) while trying to restart while windmilling. Lets all knock on wood again.

[Seth]

Hi Erik-

You may want to give Rocket Engineering a call. They have been helpful with my Missile questions when I get someone on the phone. I'm sure they'll be able to assist you. They'll know exactly what the boost pump will do to your engine setup.

Sorry I missed your Rocket the last time you were in town - I'm looking forward to seeing the updates since I last saw it.

Take care,

-seth

[co2bruce]

I fly an Eagle, and the POH specifies using the Low boost pump during tank change. I have changed tanks with and without it, there doesn't seem to be any difference. High boost is only ised to prime a cold engine in the Eagle. Not sure about Missile.

[fluffysheap]

I have climbed at Vy all the way up to 18,000 feet without any cooling problems (hottest CHT at top of climb was about 410, which is hot, but not excessively hot for such a prolonged climb). A higher airspeed climb will lower these temperatures and be better than yet more cooling fuel, since fuel used for cooling is wasted but a higher airspeed climb helps get you where you are going. I think full rich is too rich already and I suspect it would be totally fine to take off and climb with mixture leaned to 1350 TIT or so. However, everyone I've talked to has recoiled in horror at this idea.

In general, my experience has been that oil temperatures are much more of a limitation than CHTs, and cooling fuel has little to no impact on oil temperature. Your experience may differ.

As for the high boost, if you suspect fuel pump failure, just move the guard out of the way and turn it on. You only have to hold it in if you didn't move the guard. It is just about impossible to flood a windmilling engine because the pistons will still be moving and will pump any unburned fuel out of the cylinders. If the mechanical fuel pump wasn't the issue, then turning on the boost pump isn't going to make much difference one way or another although it will make your mixture richer. There are not a lot of engine failures that you can fix in flight anyway, so if the boost pump doesn't fix it, there's a decent chance it's just not getting fixed.

Remember the most common cause of engine stoppage in flight, by far, is running out of fuel. Always switch tanks before trying anything else.

The only time I can imagine I would use the high boost pump in the "hold on/release off" mode, other than for testing, is if I ran a tank dry and needed to use it to prime the engine driven pump to draw fuel from the other tank. I've never run a tank dry in the Mooney so I have never done this. It might not be necessary.

I don't have my POH here but next time I go to the plane I will take a look and tell you what page(s) the boost pump is discussed on. Keep in mind I have a 231, so if your Rocket was made out of a 252, or if the Rocket conversion changes things, then it might be different.

For me, operation of the high boost pump is strictly a hot start affair. Sometimes I push it in for a couple seconds instead of the low boost just to hear it make noise which I take to mean it probably works. I expect you could use it on cold starts too with no ill effects.

[jhbehrens]

co2bruce

My Eagle POH says to use low boost for priming a cold engine during start. I've never used the high boost for start. High boost is actually also useful for pumping fuel out of the tank when you're opening it up to patch a leak.

afaik on the Eagle low and high boost are just two different speeds for the same electrical fuel pump, not two different pumps.

Jorgen

[kortopates]

I believe Fluffysheap(?) and Jorgen have it about right, and as Jorgen points out the low boost and high boost switches actually operate the same pump in different circuits and in different places or modes:

For K models:

• The low boost pump is to mitigate fuel vaporization in the way that Fluffysheap very accurately describes above. Another indication of its need is a fluctuating fuel flow.
  
• The prime switch, or low boost pump's prime circuit, actually pumps fuel directly into the main induction tube inlet by the controller and the preferred priming method when so equipped.
  
• The high boost pump is for the emergency situation should your engine driven pump fail - its use is documented in the emergency procedures section of your POH where it also describes it limitations.
  

Exceptions for other Models:

• Although the K does not use low boost for tank switching some of the long bodies do, such as the TN and S mentioned by c02Bruce earlier.
  
• Not all the Continental equipped models have a Prime Circuit on the low boost. For example the TN does not and relies on the High Boost for 5 seconds or Low for 25 sec; as compared to 5 to 6 seconds for the K models with the low boost prime circuit.
  

If your Mooney has a prime circuit, it’s the preferred method from a safety standpoint. If the circuit has failed, then High Boost is the easy way to get it started. But be careful, this does work well but is risking a much greater chance of an engine fire because the high boost pump is pumping large quantities of fuel directly into the cylinders and over doing it could lead to a fire. Its also helpful to wait 30 seconds for the fuel to vaporize in the induction tube (or cylinder) before engaging the starter (after following the recommended number of seconds for priming based on ambient temperature from the graph in TCM Engine Operations manual if you have it).

There is an additional use of the High Boost pump documented in your TCM Engines Operation guide and some of the later model POHs. This is specifically only for Hot Starts, and the procedure has been discussed elsewhere but is basically, pull the mixture and throttle all the way back (idle cut off), then run the high boost for approx 15 seconds. Running the high boost with mixture at Idle cut off prevents fuel from getting past the fuel divider (only on Continentals) and circulates the hot vaporizing fuel back to the tank replacing it with cool fuel to aid starting. After running the high boost at idle cut off, put the mixture forward and start up without priming.

Lastly, running the low boost needlessly for very long isn't a good practice for the pocket book. I know of number of owners that burned out their electric pumps using it to drain their tanks. A neighbor of mine did that to a brand new pump - ouch! Also, having it on at idle shouldn't cause a properly leaned engine to stop, but you'll likely note the engine won't stop when you pull the mixture to idle cut-off if the boost is running. When it’s on, some fuel is going to dribble past the fuel divider into the cylinders enough to keep it running.

[aviatoreb]

Hi Erik-

You may want to give Rocket Engineering a call. They have been helpful with my Missile questions when I get someone on the phone. I'm sure they'll be able to assist you. They'll know exactly what the boost pump will do to your engine setup.

Sorry I missed your Rocket the last time you were in town - I'm looking forward to seeing the updates since I last saw it.

Take care,

-seth

Hi Seth - yes that is my next stop to call Rocket Engineering. Its a good suggestion thanks.

I was down south in late August - We did miss. Lots of tinkering and facelifting to my Mooney since you last saw it. Oh I had a prop balance yesterday - you can't see it but wow that really made her purr!

I tried out Frederick this time - much more civilized now with the new tower. Not to mention fuel was >$1/gal less than KGAI. I MAY be down again in November sometime to see mom and gma again. I will drop a line for sure. Now over to pm for further correspondence....

[aviatoreb]

I believe Fluffysheap(?) and Jorgen

Thank you fluffysheap, Jorgen and kortopates - just the kind of information I was hoping for.

I had not noticed the high boost discussed in emergency procedures but you can bet I will dig out my poh from the airplane this weekend to look for it. Thanks.

Erik