Fuel management on a C172
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From: EU
Fuel management on a C172
I flew C172s in South Africa which had selectable Left, Right and Both fuel tanks.
The C172s I've flown in the UK have the same switch-gear but were either only used on Both or had the other two options blanked out.
Why the discrepancy? And which system is better for the C172 considering its high-wing design?
Any thoughts appreciated...
The C172s I've flown in the UK have the same switch-gear but were either only used on Both or had the other two options blanked out.
Why the discrepancy? And which system is better for the C172 considering its high-wing design?
Any thoughts appreciated...
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From: In my own little world
I only ever fly with the C172 set to both as well, but then I've never flown one to the absolute max range either.
Last time I refuelled one, happened to be before my PPL renewal flight, and the instructor said to set it to the side that is being fuelled at the time, as you get a bit more fuel that way.
Last time I refuelled one, happened to be before my PPL renewal flight, and the instructor said to set it to the side that is being fuelled at the time, as you get a bit more fuel that way.
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From: UK,Twighlight Zone
I only ever fly with the C172 set to both as well, but then I've never flown one to the absolute max range either.
Last time I refuelled one, happened to be before my PPL renewal flight, and the instructor said to set it to the side that is being fuelled at the time, as you get a bit more fuel that way
Last time I refuelled one, happened to be before my PPL renewal flight, and the instructor said to set it to the side that is being fuelled at the time, as you get a bit more fuel that way
Uh! That I really need explaining to me.
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From: Londonish
Again, I always fly on BOTH. It's advisable (and, IIRC noted in the POH) to set on one tank whilst refueling - you don't need to change to the tank that's being refueled.
The reason is: When on both the tanks will cross-feed / cross drain (obviously when selected to one side that won't happen).
Start with empty tanks, fill one. Until the second one is filled, the first one is cross draining into the empty one. Dependant on how fast it's draining, you could be loosing a significant amount of fuel from the tank you first filled. They will level out over a time, but you might find you're a good few ltrs down on 'FULL' tanks.
The reason is: When on both the tanks will cross-feed / cross drain (obviously when selected to one side that won't happen).
Start with empty tanks, fill one. Until the second one is filled, the first one is cross draining into the empty one. Dependant on how fast it's draining, you could be loosing a significant amount of fuel from the tank you first filled. They will level out over a time, but you might find you're a good few ltrs down on 'FULL' tanks.
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From: UK,Twighlight Zone
When being initially checked out on the C172 my instructor advised running on both tanks unless approaching a critical low fuel situation where one tank should be exhausted before switching to the other.
The Cessna fuel system is joined in the roof well down the line from the tank outlets and is gravity feed. All Cessna tanks drain unevenly and it is common to empty one tank before another. Switching the empty tank off has no effect.
I am getting a feeling off an aweful lot of old wives tales going on here. My 'instructor said it so it must be right'.
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From: Londonish
Not intentionally being a smart-arse, but for some reason this stuck in my head a while back, so I just looked it up to be sure I didn't imagine it:
POH, Cessna 1976 Model 172M, Section 1 (General), Page 1-4, Underneath the Fuel Capacity table:
That's not to say it's correct, but it's definately in the POH, and not something my instructor ever mentioned.
I don't have your knowledge of how it's plumbed, and there may well be imperfections in the way it crossfeeds, but surely it doesn't matter how low the join is? Fuel would still cross-feed even if you put a pipe from the bottom of one wing, draped it across the tarmac and up to the bottom of the other - it's just finding a level.
Can't really comment on the almost out of fuel scenario however..
POH, Cessna 1976 Model 172M, Section 1 (General), Page 1-4, Underneath the Fuel Capacity table:
NOTE: To ensure maximum fuel capacity when refueling, place the fuel selector valve in either LEFT or RIGHT position to prevent cross feeding
I don't have your knowledge of how it's plumbed, and there may well be imperfections in the way it crossfeeds, but surely it doesn't matter how low the join is? Fuel would still cross-feed even if you put a pipe from the bottom of one wing, draped it across the tarmac and up to the bottom of the other - it's just finding a level.
Can't really comment on the almost out of fuel scenario however..
Last edited by Mark1234; 19th March 2008 at 09:13. Reason: spelling
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From: Dublin
Could the fuel critical situation be so that you'd have a good idea how much fuel was left on board, when one tank ran dry?
Of course for this to work, you'd have to be running on a selected tank for the whole time....ie at no point on the flight be running on "both" thanks. Similar to the way people fuel plan in a low wing aircraft.
dp
Of course for this to work, you'd have to be running on a selected tank for the whole time....ie at no point on the flight be running on "both" thanks. Similar to the way people fuel plan in a low wing aircraft.
dp
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From: not where I want to be
I think you're at cross-purposes with each other!
When driving the beast keep the fuel on BOTH, as has been noted it won't make any difference switching it if you run out of fuel on one tank 'cos it will just continue to feed from the other anyway.
However if you want to refuel to the maximum potential level if you switch the fuel tap to the tank being refueled inititially that should prevent it cross-feeding to the other tank (since it's then just a direct path from that tank to the motor). If it cross-fed while you were moving from that tank to the other with the re-fueling nozzle you'd lose a little from the initial tank into the other wing tank (ie the one you're just moving to). In practice I don't think it really makes an enormous difference. Arguably if you left the tap on BOTH and were to fill the second tank up, wait for a bit and then top up again it's probably almost as good as you'd get - and with less potential for forgetting to change your tanks in the air
FP.
When driving the beast keep the fuel on BOTH, as has been noted it won't make any difference switching it if you run out of fuel on one tank 'cos it will just continue to feed from the other anyway.
However if you want to refuel to the maximum potential level if you switch the fuel tap to the tank being refueled inititially that should prevent it cross-feeding to the other tank (since it's then just a direct path from that tank to the motor). If it cross-fed while you were moving from that tank to the other with the re-fueling nozzle you'd lose a little from the initial tank into the other wing tank (ie the one you're just moving to). In practice I don't think it really makes an enormous difference. Arguably if you left the tap on BOTH and were to fill the second tank up, wait for a bit and then top up again it's probably almost as good as you'd get - and with less potential for forgetting to change your tanks in the air
FP.
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From: Dubai
I would assume that his thinking is based upon exhausting one tank in order to determine that you at least have an idea what is left in the other and hence how long you have before the buttock clenching panic ensues.
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From: Here and there. Here at the moment but soon I'll be there.
The Cessna fuel system is joined in the roof well down the line from the tank outlets and is gravity feed. All Cessna tanks drain unevenly and it is common to empty one tank before another. Switching the empty tank off has no effect.
When you fly the 172 selector should be set to both. When refueling it should be set to left or right to prevent cross-feeding.
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From: Here and there. Here at the moment but soon I'll be there.
For info. Both the left and right tanks have two supply pipes from them, which are routed down infront and to the rear of each door. They all meet near and at the fuel selector.
There is also a crossover line which connects the two tanks this is for venting purposes. It is not designed for the transfer of fuel between the tanks but this transfer can occur due to various reasons (pressures, flying attitude, etc). The only external vent pipe is on the left hand tank.
There is also a crossover line which connects the two tanks this is for venting purposes. It is not designed for the transfer of fuel between the tanks but this transfer can occur due to various reasons (pressures, flying attitude, etc). The only external vent pipe is on the left hand tank.
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From: UK
It's a while since I flew the C172 but I recall a note in the POH/AFM that if cruising about 5,000 ft the fuel should be selected to LEFT or RIGHT in case of vapour lock in which case the opposite tank would then be selected.
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From: UK,Twighlight Zone
It's a while since I flew the C172 but I recall a note in the POH/AFM that if cruising about 5,000 ft the fuel should be selected to LEFT or RIGHT in case of vapour lock in which case the opposite tank would then be selected.
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From: Here and there. Here at the moment but soon I'll be there.
What's the effect on handling of flying on one tank for too long? Is it dangerous or merely noticable?
It's a while since I flew the C172 but I recall a note in the POH/AFM that if cruising about 5,000 ft the fuel should be selected to LEFT or RIGHT in case of vapour lock in which case the opposite tank would then be selected.
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From: Euroland
There are a number of reasons for uneven flow from each tank.
Fuel vent not 100% perfect
Slight leak at the cap
Not flying with the wings level and ball in the middle (which also wastes fuel).
Since few pilots fly perfectly wings level and in balance there will always be some uneven feed. Therefore I teach pilots to select either L or R at top of the climb (first cruise FREDAI check) and then use the 30 minute / 1 hour left/right swap. Noting accurately the times each tank is used for.
Thus at any stage the pilot should be able to say how much fuel is in each tank and the tanks will be close to balanced (more important on ones with long range tanks and even more important if flying long range).
Selecting Both again pre-descent or if time available with engine failure.
Regards,
DFC
Fuel vent not 100% perfect
Slight leak at the cap
Not flying with the wings level and ball in the middle (which also wastes fuel).
Since few pilots fly perfectly wings level and in balance there will always be some uneven feed. Therefore I teach pilots to select either L or R at top of the climb (first cruise FREDAI check) and then use the 30 minute / 1 hour left/right swap. Noting accurately the times each tank is used for.
Thus at any stage the pilot should be able to say how much fuel is in each tank and the tanks will be close to balanced (more important on ones with long range tanks and even more important if flying long range).
Selecting Both again pre-descent or if time available with engine failure.
Regards,
DFC
Last edited by DFC; 19th March 2008 at 21:43.