Changing tanks on the PA 28
Readability5, thanks for the correction and my apologies - I had not noticed that!
However, my basic comment stands that we should be consulting the POH/Flight Manual FIRST, rather than relying on hearsay or other peoples "opinions" on how the fuel system (or anything other system on the a/c come to that) should be operated.
My experience is that some FTOs do not emphasise the importance of the POH/Flight Manual (and there is a legal requirement that, as pilots, we are aware of these documents AND any amendments prior to flight), indeed such documents are often hidden away somewhere!
Some of the so-called "checklists" on the market are also, in my opinion, quite abysmal and full of inacurracies!
However, my basic comment stands that we should be consulting the POH/Flight Manual FIRST, rather than relying on hearsay or other peoples "opinions" on how the fuel system (or anything other system on the a/c come to that) should be operated.
My experience is that some FTOs do not emphasise the importance of the POH/Flight Manual (and there is a legal requirement that, as pilots, we are aware of these documents AND any amendments prior to flight), indeed such documents are often hidden away somewhere!
Some of the so-called "checklists" on the market are also, in my opinion, quite abysmal and full of inacurracies!
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I agree re generic checklists sold by pilot shops - avoid them.
One should always read the actual POH as a first place to look.
However, there is more to the electric fuel pump than meets the eye. Most low wing planes have it after the fuel tank selector valve. I therefore don't see why turning it on (as I have been religiously doing for 5 years) around changing tanks should make any difference. It isn't going to pressurise the pipework in the valve; in fact it will reduce the pressure at that point.
One should always read the actual POH as a first place to look.
However, there is more to the electric fuel pump than meets the eye. Most low wing planes have it after the fuel tank selector valve. I therefore don't see why turning it on (as I have been religiously doing for 5 years) around changing tanks should make any difference. It isn't going to pressurise the pipework in the valve; in fact it will reduce the pressure at that point.
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I don't have a PA28 fuel diagram to hand, but IIRC one of the key aspects is that the electric fuel pump is on a loop that circumvents the mechanical fuel pump.
So its providing redundancy in case the mechanical pump fails and blocks the system. Don't know about other types.
WRT to POH, yes, BUT (a very big but), they are only as good as the updating process.
Older PA28 POHs may not have updated - e.g. SB753 giving expanded spin recovery techniqes for the PA28-140.
Also, are you entirely happy with the POH checklists, e.g. the approach to using carb heat in some sircraft? A bland "carb heat check" downwind recommendation is ambiguous and may mean (in Arizona) a quick look, whilst in Florida it may mean the sustained application in high RH and elevated temperatures. Slavishly following the POH and leaving the carb heat on long enough to see rpm drop and recovery could be a pretty bad thing to do.
So, yes I do agree that the POH should be consulted, but equally believe that good airmanship dictates that the pilot ensures that POH configuration management is effective and that the POH is interpreted intelligently.
I don't have a PA28 fuel diagram to hand, but IIRC one of the key aspects is that the electric fuel pump is on a loop that circumvents the mechanical fuel pump.
So its providing redundancy in case the mechanical pump fails and blocks the system. Don't know about other types.
WRT to POH, yes, BUT (a very big but), they are only as good as the updating process.
Older PA28 POHs may not have updated - e.g. SB753 giving expanded spin recovery techniqes for the PA28-140.
Also, are you entirely happy with the POH checklists, e.g. the approach to using carb heat in some sircraft? A bland "carb heat check" downwind recommendation is ambiguous and may mean (in Arizona) a quick look, whilst in Florida it may mean the sustained application in high RH and elevated temperatures. Slavishly following the POH and leaving the carb heat on long enough to see rpm drop and recovery could be a pretty bad thing to do.
So, yes I do agree that the POH should be consulted, but equally believe that good airmanship dictates that the pilot ensures that POH configuration management is effective and that the POH is interpreted intelligently.
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I agree; in fact I did my own checklist from the POH and then adding avionics-specific stuff.
I don't think there is (normally) a bypass on the mechanical fuel pump. Unless the mech pump is a gear pump (or some other positive displacement type) if its drive shaft should shear off then you can still push a liquid through it, and that's what I understand the electric pump would do.
However a common "selling point" of using the electric pump around tank changeovers is to pressurise the pipework and prevent the formation of bubbles. This I don't understand; the electric pump is normally in the wrong place for that.
I don't think there is (normally) a bypass on the mechanical fuel pump. Unless the mech pump is a gear pump (or some other positive displacement type) if its drive shaft should shear off then you can still push a liquid through it, and that's what I understand the electric pump would do.
However a common "selling point" of using the electric pump around tank changeovers is to pressurise the pipework and prevent the formation of bubbles. This I don't understand; the electric pump is normally in the wrong place for that.
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I don't think there is (normally) a bypass on the mechanical fuel pump.
I think that there is an the PA28, but unfortunately I don't - have the diagram to hand, so I may be wrong in my assertion.
I can visualise a schematic loop around the mechanical pump, but then again the brain does a wonderful job of convincing you that you saw what you wanted to see
The "bubbles" in the fuel line seeling point (vapour lock?) is an interesting one, but I've never encountered that in a PA28 - IIRC the boost pumps in some Beech's are mandadted for that purpose under certain conditions (high temp.)
Your point about adding avionics to the checklist is an interesting one that I hadn't considered.
As we see more advanced kit in the GA fleet, a simple and generic "radio on, transponder to standby" clearly won't cut the mustard anymore and the aftermarket nature of much kit clearly means that the POH will not reflect what sits in front of the pilot,
I think that there is an the PA28, but unfortunately I don't - have the diagram to hand, so I may be wrong in my assertion.
I can visualise a schematic loop around the mechanical pump, but then again the brain does a wonderful job of convincing you that you saw what you wanted to see
The "bubbles" in the fuel line seeling point (vapour lock?) is an interesting one, but I've never encountered that in a PA28 - IIRC the boost pumps in some Beech's are mandadted for that purpose under certain conditions (high temp.)
Your point about adding avionics to the checklist is an interesting one that I hadn't considered.
As we see more advanced kit in the GA fleet, a simple and generic "radio on, transponder to standby" clearly won't cut the mustard anymore and the aftermarket nature of much kit clearly means that the POH will not reflect what sits in front of the pilot,
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Just a couple of technial pointers. The PA28 fuel system is one long line from fuel selector through the pumps to the carb. If the mechanical fuel pump fails (it's a diaphragm pump, by the way) then the elctric pump will still feed fuel through the mechanical pump.
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In general, the electric fuel pump needs to be on at any time there will not be suficient time for the mechanical pump to fail, you to realise and turn on the electric one and for fuel to start flowing again.
Most PA28s use the figure of 1000ft AGL as a starting point.
To say that one would have the electric pump on when flying over a built up area in case the mechanical pump failed and a forced landing would be required in the built up area seems to miss the point that one must be high enough on such a flight to glide clear regardless of the reason why the engine could fail!
Having the electric pump on when changing tanks means that it is already running so if you select an empty one then switching back to the one with fuel can be done straight away cause the electric pump is already running and things should be restored quickly.
Another reason for turning on the electric fuel pump in some aircraft is the reduction in engine power at high altitudes. The higher one flies, the less power the engine produces and the less power available to drive the engine accessories such as the mechanical fuel pump. Some aircraft require the electric pump to be on at such altitudes to ensure adequate fuel flow.
As previously mentioned, read the POH.
Regards,
DFC
PS, A Local "flight manual" (for want of a better term) produced by say a club will not form part of the C of A unless the CAA (EASA) and the Type Certificate Holder agrees to the document. That is why all the CAA supplements one finds in Piper Manuals are written by the CAA but put there by Piper.
Such a local manual would normally be part of the operations manual and have the same weight.
Most PA28s use the figure of 1000ft AGL as a starting point.
To say that one would have the electric pump on when flying over a built up area in case the mechanical pump failed and a forced landing would be required in the built up area seems to miss the point that one must be high enough on such a flight to glide clear regardless of the reason why the engine could fail!
Having the electric pump on when changing tanks means that it is already running so if you select an empty one then switching back to the one with fuel can be done straight away cause the electric pump is already running and things should be restored quickly.
Another reason for turning on the electric fuel pump in some aircraft is the reduction in engine power at high altitudes. The higher one flies, the less power the engine produces and the less power available to drive the engine accessories such as the mechanical fuel pump. Some aircraft require the electric pump to be on at such altitudes to ensure adequate fuel flow.
As previously mentioned, read the POH.
Regards,
DFC
PS, A Local "flight manual" (for want of a better term) produced by say a club will not form part of the C of A unless the CAA (EASA) and the Type Certificate Holder agrees to the document. That is why all the CAA supplements one finds in Piper Manuals are written by the CAA but put there by Piper.
Such a local manual would normally be part of the operations manual and have the same weight.
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Originally Posted by Johnm
This is sound advice. The POH recommendations on my Archer 2 are as follows:
In start up phase use electric pump to pressurise the fuel system, then switch it off and prime the engine
After taxi but before power checks change tanks, this should ensure that both tanks get checked on the ground as well as the fuel cock.
Fuel pump on for take off and off not below 1000ft AGL or when settled in the cruise. If climbing high you might want to switch off in the climb because the pump is not designed for continuous use, it's an auxiliary back up pump. Obviously check fuel pressure before and after, there shouldn't be a significant drop in pressure.
I change tanks every half hour or at the half way point of a trip if it's an hour or less. If going over water I try to time things so I change tanks just before coasting out and then leave it until coasting in. Fuel pump on before change and off after with a pressure check each time.
If you are a regular PA28 pilot (or PA32 for that matter) it soon becomes second nature.
In start up phase use electric pump to pressurise the fuel system, then switch it off and prime the engine
After taxi but before power checks change tanks, this should ensure that both tanks get checked on the ground as well as the fuel cock.
Fuel pump on for take off and off not below 1000ft AGL or when settled in the cruise. If climbing high you might want to switch off in the climb because the pump is not designed for continuous use, it's an auxiliary back up pump. Obviously check fuel pressure before and after, there shouldn't be a significant drop in pressure.
I change tanks every half hour or at the half way point of a trip if it's an hour or less. If going over water I try to time things so I change tanks just before coasting out and then leave it until coasting in. Fuel pump on before change and off after with a pressure check each time.
If you are a regular PA28 pilot (or PA32 for that matter) it soon becomes second nature.
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To say that one would have the electric pump on when flying over a built up area in case the mechanical pump failed and a forced landing would be required in the built up area seems to miss the point that one must be high enough on such a flight to glide clear regardless of the reason why the engine could fail!
I didn't miss the point DFC but a lot of SE pilots do fly over built up areas areas where it is unlikely they would be able to glide clear in the event of an engine failure, the area below the Birmingham CTA around the North of Birmingham being my local example. Low flying has, and I suspect always will be, one of the most common infringements of the ANO.
I didn't miss the point DFC but a lot of SE pilots do fly over built up areas areas where it is unlikely they would be able to glide clear in the event of an engine failure, the area below the Birmingham CTA around the North of Birmingham being my local example. Low flying has, and I suspect always will be, one of the most common infringements of the ANO.
So why is the mechanical pump likely to fail at the very moment you change tanks? As long as it doesn't, even if the engine is starved of fuel, the engine will still turn, delivering fuel as soon as you select a full tank again.
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Just a couple of technial pointers. The PA28 fuel system is one long line from fuel selector through the pumps to the carb. If the mechanical fuel pump fails (it's a diaphragm pump, by the way) then the electric pump will still feed fuel through the mechanical pump.
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Why not leave the electric pump on all the time?
Only problem is spotting whether either pump has failed, on a fuel injected engine you need to prime, so this checks the electric pump is working.
Only problem is spotting whether either pump has failed, on a fuel injected engine you need to prime, so this checks the electric pump is working.
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Not PA28 related, but about fuel pumps;
My Cardinal POH says to turn the fuel pump on during run up and check for increased fuel pressure, then turn it off. It then fails to mention it again.
Regarding fuel pressure when running both pumps, I notice on my aircraft the pressure increases by 3-4 lbs. My point is: does increased fuel pressure affect the air/fuel ratio particularly on carbureted engines? Is it possible that increased fuel pressure could overwhelm the float valve and allow a higher fuel level in the float bowl, and therefore a richer mixture. (Unless there is a pressure relief valve in the system somewhere, then of course, no fuel pressure increase would be seen).
I seem to remember a Bonanza accident that was attributed to the pilot’s failure to turn OFF the electrical fuel pump after takeoff, causing the engine to run rich on part throttle, rich enough in this case to cause the engine to die, and he crashed, killing himself. Apparently this is known issue with Bonanza’s, my question is, is it an issue with all aircraft, or only with fuel injected engines?
Regards,
White Bear.
N.B. I am very well aware of how carburettors work, so those who might feel the need to explain to me the basics, please refrain from doing so.
My Cardinal POH says to turn the fuel pump on during run up and check for increased fuel pressure, then turn it off. It then fails to mention it again.
Regarding fuel pressure when running both pumps, I notice on my aircraft the pressure increases by 3-4 lbs. My point is: does increased fuel pressure affect the air/fuel ratio particularly on carbureted engines? Is it possible that increased fuel pressure could overwhelm the float valve and allow a higher fuel level in the float bowl, and therefore a richer mixture. (Unless there is a pressure relief valve in the system somewhere, then of course, no fuel pressure increase would be seen).
I seem to remember a Bonanza accident that was attributed to the pilot’s failure to turn OFF the electrical fuel pump after takeoff, causing the engine to run rich on part throttle, rich enough in this case to cause the engine to die, and he crashed, killing himself. Apparently this is known issue with Bonanza’s, my question is, is it an issue with all aircraft, or only with fuel injected engines?
Regards,
White Bear.
N.B. I am very well aware of how carburettors work, so those who might feel the need to explain to me the basics, please refrain from doing so.
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In theory keeping both pumps on will have no effect on the fuel / air mixture. The float assembly and its valve, control the level of fuel in the bowl keeping it near constant and so the supply side does n't matter so long as it can keep up.
However (there always is one!) that depends on the float having enough bouyancy to drive the valve shut against the supply pressure so there must be some supply pressure which can overwhelm the valve - but that is probably a fair bit higher than the pumps can produce.
Of course if the float valve is worn then it may 'leak' with the extra pressure and that would raise the level in the float chamber and richen the mixture, considerably richening the mixture if the float chamber is filled, however this is probably only possible at lower power settings. ( a problem my lawnmower has at the moment when stopped - allowing fuel into the crankcase on the mower, but with an updraft carb in an aircraft onto the ground.)
However (there always is one!) that depends on the float having enough bouyancy to drive the valve shut against the supply pressure so there must be some supply pressure which can overwhelm the valve - but that is probably a fair bit higher than the pumps can produce.
Of course if the float valve is worn then it may 'leak' with the extra pressure and that would raise the level in the float chamber and richen the mixture, considerably richening the mixture if the float chamber is filled, however this is probably only possible at lower power settings. ( a problem my lawnmower has at the moment when stopped - allowing fuel into the crankcase on the mower, but with an updraft carb in an aircraft onto the ground.)