switch tank, boost pump ON ?

[Ulysse]

My POH recommends switching boost pump On before changing fuel tank. I did that for the initial flights after I acquired the plane. However, on a downwind leg, I switched the boost pump to select the fullest tank, but before I had time to change, the engine stumbled. I immediately stopped the pump which restored normal operation after a few seconds. 

Since then, I do not switch ON the boost pump anymore and just change tanks watching closely any variation of FF.

What is the correct procedure ?

 

[ArtVandelay]

I think the boost pump is needed if you emptied the other tank and have air in the fuel lines.

I never do that and never use the boost pump when switching.

[smwash02]

I don't use low boost when switching tanks and have never had a stumble. I have turned low boost on to see its effects during a climb and while I see the GPH momentarily rise, it's never produced a stumble. I suspect because of your lower power setting it got overrich for a moment.

Unless I have to, I only switch tanks with altitude and hospitable terrain in case the fuel selector fails or it will require a restart because something was wrong with the other tank and I had to switch back.

[GeeBee]

Does the POH recommend, or does the POH say to do it?

 

[Ulysse]

17 minutes ago, GeeBee said:

Does the POH recommend, or does the POH say to do it?

it says so

 

[A64Pilot]

The answer is the correct procedure is boost on

‘I never do, but then I don’t touch that selector below cruise altitude either, I cringe when I see people moving that selector on downwind, but then I have had the handle come off in my hand too, between tanks of course, got it back on and tank selected before it stumbled, but what if I had dropped the thing and it went under my seat?

 

Edited July 2, 2021 by A64Pilot

[steingar]

I have to unhook my seatbelt and bend over to the point of autofellatio to switch tanks.  One of the few things I genuinely dislike about the Mooney.  On approach I switch at the top of the descent. I don't want to be head down in the vicinity of an airport, and I definitely do not want my seatbelt off that low and slow.

[Vance Harral]

The OP is flying an M20K, presumably with the Continental TSIO-360, and both high and low boost pump switches.  It's important to distinguish the engine and fuel system in answering his question.

I have nearly 1000 hours behind Lycoming 4-bangers in low wing airplanes, and regularly use the boost pump for takeoff, landing, and when switching tanks.  If a Mooney pilot told me their IO-360 variant (M20E/M20F/M20J) stumbled when turning on the boost pump, I'd say something is wrong with the fuel system and needs to be fixed.  There is no good reason why turning on the boost pump in these airplanes should cause an issue.

In contrast, I have limited experience with the TSIO-360, but SOP is to *not* use either high or low boost pump, except in an emergency.  The POH not withstanding, most pilots I know flying behind this engine never select either boost pump switch, except to prime.  My understanding is that turning on low boost should not cause issues, and the fact the POH prescribes it when switching tanks matches that understanding.  But my understanding is that selecting high boost can actually flood the engine to the point of quitting under normal circumstances, and is intended only for a situation in which some sort of fuel system malfunction is actively restricting fuel flow.  I don't claim to understand the details, and confess a bias that the Continental fuel flow system seems poorly designed.  If others chime in and say that low boost sometimes causing the engine to stumble is a "normal" characteristic of this engine, it will further reinforce my bias.  Happy to be educated otherwise, though.

Edited July 2, 2021 by Vance Harral

[jlunseth]

I am puzzled. My POH does not have that, although we have different engines. Mine is the TSIO360-LB, yours, I believe, would be the TSIO360SB. What I can offer you, is that the POH for my engine does not recommend boost pump use for any non-emergency flight regime and there are warnings against doing so because the engine may operate over-rich. I also have experience with the engine in an over-rich condition, in fact most or all 231 pilots do, and I believe the non-Encore 252 pilots as well. That happens if the pilot moves the fuel control to full rich for landing (just in case of a go-around) and then idles the engine back or at least sets a low MP (i.e. 15"). The engine becomes over-rich and does what I call "burbling," it is missing or ignition is greatly suppressed because of the surplus of fuel. The engine will not quit in this flight regime, because the descending aircraft drives the prop, but it does not sound very happy and people on the ground sometimes comment that the engine was missing. 

I think, reading that instruction, that they mean for you to switch the boost on, and I would use the Low not the High boost, but only long enough to switch tanks, and then immediately switch the boost off. They are probably concerned about air bubbles in the fuel line. You might try the opposite, that is, don't switch the Boost pump on, but do switch tanks, and if the engine then stumbles, immediately switch the Low boost on.

 

[PT20J]

The correct answer of course is whatever it says in the AFM for that airplane. The manufacturer sets the procedures. They don't tell us why. So we don't have enough information to know if something is necessary or not.

I also make it a practice not to switch tanks at low altitudes. I select my landing tank before or during descent. Also, I like to start on one tank and switch to the other during taxi to make sure that I can draw fuel from both tanks.

Skip

[A64Pilot]

I know on an IO-520 on both a C-210 and a Bonanza high boost will make one very rich, won’t kill it at high power but will make it real rich.

Apparently some Bonanza’s don’t have good baffling and some will use high boost on a hot day climb to keep cylinder head temps in check. I believe it’s likely that their fuel system isn’t set up correctly as many times on a Continental it’s not, it’s not a simple procedure and there is I believe an SB for that.

Its my understanding that high boost is to keep the engine running in the event of a filed mechanical fuel pump. but I’ve seen a a Cirrus make a forced landing from a failed mechanical fuel pump and the pilot said high boost didn’t keep it running.

‘So not sure.

210 didn’t call for boost pump under normal conditions that I remember, neither did the Maule I don’t think and it was a Lycoming, but perhaps I didn’t read the POH well  enough. 

Maule and the 210 were high wings. and that may make a difference.

However he included a section of his POH for a K model, and it seems to plainly say boost on, so I think it’s tough to make a case to not follow the POH, as it’s required by at least the FAA to do so.

 

I’ve not considered switching tanks during taxi, probably a good idea as if nothing else exercising the valve isn’t a bad idea, many of my flights I don’t switch tanks 

Edited July 2, 2021 by A64Pilot

[sleeper-319]

3 hours ago, steingar said:

I have to unhook my seatbelt and bend over to the point of autofellatio to switch tanks.  One of the few things I genuinely dislike about the Mooney. 

I don’t want to dissuade you from self care, but if you’re after a better solution, our club has a homemade switcher made out of a small length of PVC pipe with a notch cut in once end that fits nicely over the the fuel selector. It’s a game changer for Mooney’s with the tank selector where yours is.

I can take a pic and post it later today, but basically it’s this… a 12” length of 1” schedule 40 PVC. One end has two notches cut in it to fit over the selector, and the other end has a hole drilled through about a half inch from the end, with a 2.” long 1/8 in bolt through it to act as a sliding T handle. It’s simple and effective and sits in the right seat seat back pocket when you’re not using it. 

Edited July 2, 2021 by sleeper-319

[midlifeflyer]

Just now, sleeper-319 said:

I don’t want to dissuade you from self care, but if you’re after a better solution, our club has a homemade switcher made out of a small length of PVC pipe with a notch cut in once end that fits nicely over the the fuel selector. It’s a game changer for Mooney’s with the tank selector where yours is.

I can take a pic and post it later today, but basically it’s this… a 12” length of 1” schedule 40 PVC. One end has two notches cut in it to fit over the selector, and the other end has a hole drilled through about a half inch from the end, with a 2.” long 1/8 in bolt through it to act as a sliding T handle. It’s simple and effective and sits in the right seat seat back pocket when you’re not using it. 

A friend of mine has the same PVC pipe assist as you. Works great.

[Niko182]

2 hours ago, A64Pilot said:

Its my understanding that high boost is to keep the engine running in the event of a filed mechanical fuel pump. but I’ve seen a a Cirrus make a forced landing from a failed mechanical fuel pump and the pilot said high boost didn’t keep it running.

The high boost pump on the IO550s will push out right around 15-17gph. Plently enough to get you to an airport somewhere around at least.

as for switching tanks, I usually don't switch tanks with the boost pump. I really never touch the low boost pump unless I need to deal with vapor lock. I use the high boost pump to prime. Other than that I never touch them during any phase of flight.

[Vance Harral]

4 hours ago, jlunseth said:

the POH for my engine does not recommend boost pump use for any non-emergency flight regime and there are warnings against doing so because the engine may operate over-rich. I also have experience with the engine in an over-rich condition, in fact most or all 231 pilots do, and I believe the non-Encore 252 pilots as well. That happens if the pilot moves the fuel control to full rich for landing (just in case of a go-around) and then idles the engine back or at least sets a low MP (i.e. 15"). The engine becomes over-rich and does what I call "burbling," it is missing or ignition is greatly suppressed because of the surplus of fuel. The engine will not quit in this flight regime, because the descending aircraft drives the prop, but it does not sound very happy and people on the ground sometimes comment that the engine was missing.

 

3 hours ago, A64Pilot said:

I know on an IO-520 on both a C-210 and a Bonanza high boost will make one very rich, won’t kill it at high power but will make it real rich.

 

3 hours ago, A64Pilot said:

Its my understanding that high boost is to keep the engine running in the event of a filed mechanical fuel pump. but I’ve seen a a Cirrus make a forced landing from a failed mechanical fuel pump and the pilot said high boost didn’t keep it running.

Good grief, this all seems so complex and error prone.  Why is the fuel flow and mixture control systems in these turbo Continental engines so sensitive that it requires two different boost pump settings, one of which can actually cause more problems than it solves?  Why is choking and coughing and sputtering at idle/low power so common that it's considered normal?  Is everyone running around with mal-adjusted engines because it's impractical to get them tuned correctly, or is the design just that poor?

If these problems are just inherent to turbocharged engines, please educate me.

[GeeBee]

4 hours ago, PT20J said:

The correct answer of course is whatever it says in the AFM for that airplane. The manufacturer sets the procedures. They don't tell us why. So we don't have enough information to know if something is necessary or not.

I also make it a practice not to switch tanks at low altitudes. I select my landing tank before or during descent. Also, I like to start on one tank and switch to the other during taxi to make sure that I can draw fuel from both tanks.

Skip

That is correct. Lot of speculation and conjecture here. However, the AFM is the Bible for operation the airplane. If it says to do it, you do it unless you have an emergency where using the normal procedure would be contra indicated. That all said, if your airplane cannot run smoothly using the normal procedure, an investigation into why it will not should be done. My guess is your injection system may need adjustment.

 

[A64Pilot]

I was speaking of non turbo motors.

‘it’s not complex. you had a split switch on the 210, left half low and right half high, or maybe it was both high, not sure. I used low for priming, awful sure it was actually the same motor with different windings or something for high and low, but other than to check it’s function I never used high boost, it was I believe meant in case of mechanical pump failure

its really not complicated, I found an article that gets pretty deep in explaining the IO and TIO-360 fuel system, which has some important differences to their big brothers on the 520 etc, but this article does bring up that the positive displacement mechanical pump does have a bypass so that the electric pump can keep it running if the mechanical one fails, cause you can’t flow though a positive displacement pump.

‘The TSIO in particular has a pretty high fuel pressure, so assumption is the high pressure is why high boost is needed, but that’s me making an assumption.

https://www.avweb.com/ownership/continental-io-360-fuel-injection/

[A64Pilot]

15 minutes ago, Vance Harral said:

 

 

  Is everyone running around with mal-adjusted engines because it's impractical to get them tuned correctly, or is the design just that poor?

Not everyone but some are, the Continental system requires a complex set up, that isn’t often done, and it’s not a set it once and forget it, it needs to be checked every so often, but rarely is.

‘I’ll look for the SB explaining it.

[A64Pilot]

I found it, if you read it,it’s not real simple. It’s not a turn a device and check for 50 RPM increase when leaning to idle cut off and your done. It also  requires instrumentation and fittings and hoses the average mechanic likely doesn’t have, so you should find someone that adjusts Conti fuel injection systems often.

I for example do not have the instrumentation and hoses etc.

Somewhere in here it says you MUST have max fuel flow at full throttle or engine damage can occur or something similar, it’s been a while so I’m going from memory.

https://documents.lancair.com/aircraft_documents/ES-ESP/SID97-3E.pdf

Its supposed to be done at every annual or 100 hour inspection

Edited July 3, 2021 by A64Pilot

[Guest]

2 hours ago, Vance Harral said:

 

 

Good grief, this all seems so complex and error prone.  Why is the fuel flow and mixture control systems in these turbo Continental engines so sensitive that it requires two different boost pump settings, one of which can actually cause more problems than it solves?  Why is choking and coughing and sputtering at idle/low power so common that it's considered normal?  Is everyone running around with mal-adjusted engines because it's impractical to get them tuned correctly, or is the design just that poor?

If these problems are just inherent to turbocharged engines, please educate me.

Continental engine fuel set up is not difficult, it requires a pair of calibrated pressure gauges, some hoses and a copy of the current engine maintenance manual which supersedes SID97-3G, the last version before the procedure was incorporated into the manual.  One part often overlooked is airframe boost pump adjustment.  When both systems are set correctly “Low” setting has no effect on engine idle performance.

Most Continental powered airframes has a two speed pump, Low for vapour purging and tank selector changes and High for emergency use should the engine pump fail.

Clarence

[Guest]

There is no real need or benefit from using the fuel pump while switching tanks.  It takes only a split second to change tanks, the engine fuel pump won’t suck the fuel lines empty in that time.  Both the engine pump and fuel boost pump would be drawing fuel against the same selector valve, neither will suck fuel past the closed valve while switching tanks.

Clarence

[jlunseth]

3 hours ago, Vance Harral said:

 

 

Good grief, this all seems so complex and error prone.  Why is the fuel flow and mixture control systems in these turbo Continental engines so sensitive that it requires two different boost pump settings, one of which can actually cause more problems than it solves?  Why is choking and coughing and sputtering at idle/low power so common that it's considered normal?  Is everyone running around with mal-adjusted engines because it's impractical to get them tuned correctly, or is the design just that poor?

If these problems are just inherent to turbocharged engines, please educate me.

The problem is inherent to turbocharged engines, not Continentals. Well, the one's in our Mooneys, I can't speak for all turbocharged engines. The engines need to be set up quite rich to keep the engine cool in a climb. For reasons perhaps you can explain to me, mechanics do not seem to be able to set the full power mixture screw for rich, and the idle screw not for rich. Its easy to deal with though. The pilot has a knob for that. New pilots just have to learn that operating as one did with an NA engine does not apply to a turbocharged engine. Some get nervous, think the engine might fall off, and do it the wrong way anyway. The boost switch is simple, leave it off, we don't use it except for true emergencies. The switch operates one pump, the one pump operates on low for an emergency and high for a really big emergency. 

My Conti has forgiven all my early mistakes, is currently about 300 hours over TBO, the compressions are actually improving from one annual to the next, and it is faster than it has ever been, all the result of ditching NA methods and fears. That, and settling on a good mechanic after sorting through some others.

[Shadrach]

13 hours ago, ArtVandelay said:

I think the boost pump is needed if you emptied the other tank and have air in the fuel lines.

I never do that and never use the boost pump when switching.

Not needed in my experience but it’s nice to feel like you’re  doing something while you wait…

Edited July 3, 2021 by Shadrach

[Shadrach]

14 hours ago, Ulysse said:

My POH recommends switching boost pump On before changing fuel tank. I did that for the initial flights after I acquired the plane. However, on a downwind leg, I switched the boost pump to select the fullest tank, but before I had time to change, the engine stumbled. I immediately stopped the pump which restored normal operation after a few seconds. 

Since then, I do not switch ON the boost pump anymore and just change tanks watching closely any variation of FF.

What is the correct procedure ?

 

 

14 hours ago, Ulysse said:

My POH recommends switching boost pump On before changing fuel tank. I did that for the initial flights after I acquired the plane. However, on a downwind leg, I switched the boost pump to select the fullest tank, but before I had time to change, the engine stumbled. I immediately stopped the pump which restored normal operation after a few seconds. 

Since then, I do not switch ON the boost pump anymore and just change tanks watching closely any variation of FF.

What is the correct procedure ?

 

The boost pump may be incidental to the issue.  If I run a tank down to the point of pressure drop (just before the engine sputters)  I have on occasion had a slight hiccup when switching back to that tank after filling.  Before takeoff I always try to check that the selector is free to move and that I have fuel continuity from each tank.

Don’t have much experience with Continental Injection systems. Is it conceivable that turning the boost pump on caused a pressure drop aft of the pump and a boiled a little fuel?

[Ulysse]

I think I found the answer in the POH (I should have read it all before posting. Apologies). In downwind, I was running the engine at low power and the low boost pump might have induced an overrich condition as mentioned in the emergency procedures section of the POH. I think this caution notice should have been included in the normal operations procedures as well since it says to use the boost pump in that section. 
Not switching tanks during downwind is new to me. It has been part of my training for PPL and I have done that for many years: we have an equivalent of the american GUMPS which includes switching tank. I guess I will do like many have suggested: switch at top of descent.