Switching to a Single Tank After Shutting Down the Engine on a C172S
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Switching to a Single Tank After Shutting Down the Engine on a C172S
Afternoon all,
Earlier this month I flew an LPC with a very experienced CAA examiner out in Florida to complete my 2 year revalidation (I only had the old FAA paper licence so couldn't legally fly as PIC to complete the required hours for revalidation by experience
).
It was thoroughly enjoyable, a great refresher & gave me experience of some new procedures to boot.
On shutting down the engine I selected the left fuel tank (Fuel Selector LEFT or RIGHT in the Securing Aircraft Checklist). When asked why I had switched to a single tank the best I could offer was that I was following the procedure detailed in the after shut down check list.
I am now struggling to understand the logic of this procedure - the examiner observed & I agree that while it increases the risk of the next pilot running a tank dry in half the expected time by ommitting to check the fuel selector position before or during flight, it has no obvious benefit.
Can any ppruners clarify whether there is a benefit to selecting a single fuel tank on a C172 after shut down that outweighs the increased risk of it being missed during subsequent checks?
Regards all, enq.
Earlier this month I flew an LPC with a very experienced CAA examiner out in Florida to complete my 2 year revalidation (I only had the old FAA paper licence so couldn't legally fly as PIC to complete the required hours for revalidation by experience
).It was thoroughly enjoyable, a great refresher & gave me experience of some new procedures to boot.
On shutting down the engine I selected the left fuel tank (Fuel Selector LEFT or RIGHT in the Securing Aircraft Checklist). When asked why I had switched to a single tank the best I could offer was that I was following the procedure detailed in the after shut down check list.
I am now struggling to understand the logic of this procedure - the examiner observed & I agree that while it increases the risk of the next pilot running a tank dry in half the expected time by ommitting to check the fuel selector position before or during flight, it has no obvious benefit.
Can any ppruners clarify whether there is a benefit to selecting a single fuel tank on a C172 after shut down that outweighs the increased risk of it being missed during subsequent checks?
Regards all, enq.
Interesting question.
My personal preference would be to never operate on "both", because of inevitable uneven feed badly indicated by the semi-useless Cessna gauges, and because if you do have an empty tank on one side, then selecting "both" will draw air from that side, and probably not fuel from the other. Controlled switching between left and right is more likely to give me balanced tanks, in my experience.
After shut-down, I'd normally select "off" ! The worst that happens then is that the next person to try and fly can't start the engine until they've done their checks properly.
G
My personal preference would be to never operate on "both", because of inevitable uneven feed badly indicated by the semi-useless Cessna gauges, and because if you do have an empty tank on one side, then selecting "both" will draw air from that side, and probably not fuel from the other. Controlled switching between left and right is more likely to give me balanced tanks, in my experience.
After shut-down, I'd normally select "off" ! The worst that happens then is that the next person to try and fly can't start the engine until they've done their checks properly.
G
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If you park it across a slope with both tanks selected then fuel will run from the higher tank to the lower one leaving you with a fuel imbalance for the next flight (or even in the worst case fuel coming out through the lower tank vent).
Dave
Dave
Probably to avoid fuel flowing from one tank to the other if the aircraft is parked on a cross slope, and if the tanks were near full the fuel could eventually run out of the vents.
However this is an opinion, I do not have much experience of C172's but I believe some older cessna 172 fuel valves are not on/left/right/both, but just a biased valve which if selected left or right just allows more fuel to flow from the selected tank, so it might not actually stop cross filling on the ground.
However this is an opinion, I do not have much experience of C172's but I believe some older cessna 172 fuel valves are not on/left/right/both, but just a biased valve which if selected left or right just allows more fuel to flow from the selected tank, so it might not actually stop cross filling on the ground.
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It is to stop the fuel cross flowing into the opposite wing. If I park mine left wing down and don't turn the tank off then the fuel flows across and out the vent. I can tell you from practical experience you lose rather a lot of fuel if you don't......
You don't fly 172's switching between tanks, they are always flown with BOTH to on unless there is a need to settle a fuel imbalance and that is only done in cruise flight.
You don't fly 172's switching between tanks, they are always flown with BOTH to on unless there is a need to settle a fuel imbalance and that is only done in cruise flight.
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Thank you all, the prevention of draining explanation makes sense.
I believe some check lists I've used mention selecting the tank with the least contents which makes me wonder if there is also a possibility that the selected tank is more easily able to leak than the other (hence least contents = least loss of fuel)?
Regards, enq.
I believe some check lists I've used mention selecting the tank with the least contents which makes me wonder if there is also a possibility that the selected tank is more easily able to leak than the other (hence least contents = least loss of fuel)?
Regards, enq.
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Why has it gone quiet ?
"because if you do have an empty tank on one side, then selecting "both" will draw air from that side, and probably not fuel from the other"
On a gravity fed system - I don't think so !
But flying out of balance may cause a fuel feed line to become uncovered.
On a gravity fed system - I don't think so !
But flying out of balance may cause a fuel feed line to become uncovered.
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As being said before, C172s are always flown with fuel selector in BOTH position, unless the fuel imbalance indicated by fuel quantity gauges requires balancing the fuel. I also don't recommend using OFF switch EVER (unless the aircraft is being stored for a longer period - say a month or longer).
Regarding the "flying with one tank empty" topic: is it really safe to let one fuel tank dry out? Isn't fuel system on most aircraft designed with redundancy in mind shall there be a vapour lock (average C172 doesn't have aux fuel pump) or any kind of blockage in fuel line?
I kind of miss the PA38's fuel system in most aircraft - L/R/OFF fuel selector colored red placed in the middle of the panel - it does its work perfectly and if taught properly, it's almost impossible to forget to change the fuel tanks. And also, no complications about "slushing fuel", refuelling and other problems developed by C172's fuel system...
Regarding the "flying with one tank empty" topic: is it really safe to let one fuel tank dry out? Isn't fuel system on most aircraft designed with redundancy in mind shall there be a vapour lock (average C172 doesn't have aux fuel pump) or any kind of blockage in fuel line?
I kind of miss the PA38's fuel system in most aircraft - L/R/OFF fuel selector colored red placed in the middle of the panel - it does its work perfectly and if taught properly, it's almost impossible to forget to change the fuel tanks. And also, no complications about "slushing fuel", refuelling and other problems developed by C172's fuel system...
Moderator
is it really safe to let one fuel tank dry out? Isn't fuel system on most aircraft designed with redundancy in mind shall there be a vapour lock
The PA38 fuel system, like all low wing systems which come to my mind, does not have a "both" selection available, because a certification requirement for aircraft which are to have a "both" selection available, is that the airspaces of the wing tanks be interconnected with a vent line, and that is just not possible. A vent line would have to connect the outboard upper area of the tanks to each other, and then not have a dip in the middle, where fuel would sit, and prevent proper venting. Not physically possible.
A few high wing aircraft do not have a both selection (200 series Cessna come to mind) I don't know why Cessna designed them this way, but I'm sure they had their reasons. There certainly have been crashes because 100 series Cessna pilots flying 200 series Cessnas forgot to use the other half of the fuel they were carrying!
Cycling the fuel selector through the selections from time to time is a good idea. It keeps things from seizing up. If taking off with fuel selected to "off" is a concern, only allow pilots who preflight check fuel selector positions to fly!
The original poster noted he was flying a C172S. This model has the fuel injected Lycoming IO360, therefore there are considerable differences between this aircraft and the earlier carb equiped lycoming powered C 172's (1968 to 1985) and care should be taken when applying procedures applicable to the old models to the new build (1998 to present) varients.
To answer the original question, the fuel selector should be placed on left or right particularly if the the parkign surface results in the aircraft tilted to the left as fuel will migrate to the low wing and then out the fuel vent. I takes very little slope for this to happen if the tanks are full.
It is also a good idea because it exercises the fuel valve. I was in one privately operated C172 whose owner always left the valve in the "both position" It was so stiff I had real doubts about getting it to the off position without breaking it....not a good thing in the case of an engine fire or forced approach
To answer the original question, the fuel selector should be placed on left or right particularly if the the parkign surface results in the aircraft tilted to the left as fuel will migrate to the low wing and then out the fuel vent. I takes very little slope for this to happen if the tanks are full.
It is also a good idea because it exercises the fuel valve. I was in one privately operated C172 whose owner always left the valve in the "both position" It was so stiff I had real doubts about getting it to the off position without breaking it....not a good thing in the case of an engine fire or forced approach
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Just out of curiosity, since I don't fly the 172: Do you also select either left or right when refueling, or do you leave the selector on both?
I'm thinking about the situation where both tanks are nearly empty. You start to fill one tank up, but with the fuel selector on "both". When one tank is full, you'll need a minute to move the hose and the stairs to the other side to start filling up the other tank. How much fuel can migrate from the full tank to the empty tank in that time, leaving you with a not-quite-full tank on one side?
I'm thinking about the situation where both tanks are nearly empty. You start to fill one tank up, but with the fuel selector on "both". When one tank is full, you'll need a minute to move the hose and the stairs to the other side to start filling up the other tank. How much fuel can migrate from the full tank to the empty tank in that time, leaving you with a not-quite-full tank on one side?
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Pilot DAR: Seems I was a bit unclear... What would you do if you had fuel flow obstruction on the full tank and the other tank was totaly empty? Isn't it more safe to leave fuel in both tanks in case anything similar to that happens?
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Glide...
But seriously, though it is remotely possible for there to be a fuel tank obstruction, I rate that as being so very unlikely, that it does not even register, compared to my possibly wanting to establish the quantity of fuel used for a very long flight. Even that circumstance is rather rare though. The tanks have "finger strainers" whose purpose is to prevent obstructions to fuel flow in the fuel lines.
Certainly operating a 172 with "both" selected as a normal operation is fine, as long as the fuel selector is exercised reguarly, and positioned appropriately for parking or fuelling, as suggested by the flight manual.
But seriously, though it is remotely possible for there to be a fuel tank obstruction, I rate that as being so very unlikely, that it does not even register, compared to my possibly wanting to establish the quantity of fuel used for a very long flight. Even that circumstance is rather rare though. The tanks have "finger strainers" whose purpose is to prevent obstructions to fuel flow in the fuel lines.
Certainly operating a 172 with "both" selected as a normal operation is fine, as long as the fuel selector is exercised reguarly, and positioned appropriately for parking or fuelling, as suggested by the flight manual.
If I recall the C172 I flew, I think it was a K had a requirement to select left or right when operating above 5000ft, unless some mod had been made to the vent system.
So as usual, check the POH for your particular aeroplane, as it would seem not all C172 can be flown with the fuel valve on 'both' all the time.
So as usual, check the POH for your particular aeroplane, as it would seem not all C172 can be flown with the fuel valve on 'both' all the time.
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Just out of curiosity, since I don't fly the 172: Do you also select either left or right when refueling, or do you leave the selector on both?
However there is a quirk in the 182 fuel system whereby it only vents on one side and then is a vent connection between each wing. When the tanks are fuelled to full then there is a tendency for fuel to flow through this as well.
flyingmac
The fuel schematic you provided the link for refers to a C172 fitted with a carbourated Lycoming. The original poster specified he was flying a C172S . The fuel system for this aircraft has some significant differences due to the fact it is fitted with a fuel Injected engine.
The fuel schematic you provided the link for refers to a C172 fitted with a carbourated Lycoming. The original poster specified he was flying a C172S . The fuel system for this aircraft has some significant differences due to the fact it is fitted with a fuel Injected engine.
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There is another system on Cessna 172's not mentioned yet, and that is the Fuel Cut Off Valve. Yesterday I flew a Cessna 172R (2000 model with steam gauges) this had a Fuel Selector for "Left", "Both" and "Right". The traditional "Off" position was not available.
The Pre-start Checklist called for the Fuel Cut Off Valve to be closed (pushed in), and the post shutdown checks called for it to be opened (it comes out about 3"). The valve is located just above and to the right of the Fuel Selector switch.
It is not shown on the diagram posted above.
The Pre-start Checklist called for the Fuel Cut Off Valve to be closed (pushed in), and the post shutdown checks called for it to be opened (it comes out about 3"). The valve is located just above and to the right of the Fuel Selector switch.
It is not shown on the diagram posted above.
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C172S POH: To ensure maximum fuel capacity and minimize cross-feeding when refueling, always park the airplane in a wings-level, normal ground attitude and place the fuel selector in the left or right position.
I dont see how selecting L or R on the fuel selector will prevent fuel from cross-flowing from one tank to the other through the vent line that connects both tanks. Can anyone elaborate? I mean, the line will remain open and fuel will be free to migrate from tank to tank regardless of fuel selector position right?
Selecting either L or R depends on which tank is fullest or will be serviced first? I fly a 152 that usually pours out a quite a lot of fuel through the vent on the left wing when its parked on the grass. It only has a "on/off" fuel selector. Would it make a difference to set it to off while refueling or when its parked?
I dont see how selecting L or R on the fuel selector will prevent fuel from cross-flowing from one tank to the other through the vent line that connects both tanks. Can anyone elaborate? I mean, the line will remain open and fuel will be free to migrate from tank to tank regardless of fuel selector position right?
Selecting either L or R depends on which tank is fullest or will be serviced first? I fly a 152 that usually pours out a quite a lot of fuel through the vent on the left wing when its parked on the grass. It only has a "on/off" fuel selector. Would it make a difference to set it to off while refueling or when its parked?
Vent lines are intended to vent air not fuel. Fuel will only be in the vent lines if the tank is so full the only way for excess fuel to escape is through the vent line.
If the fuel valve is left in the both position the fuel will travel from the high tank through the fuel selector valve to the low tank for the system in carbourated versions of the C172. The new build C172R and C172S have a small fuel reservoir tank between the fuel selector valve and the engine with the fuel shut off valve between the collector tank and the engine. Selecting left or right will stop fuel from the other tank from draining into the reservoir tank and then across to the other side. Unlike the earlier carb versions of the C172 pulling the fuel shut off on a C172R or S will not stop fuel from transfering from one tank to the other.
Sorry to be blunt but the fuel system is not complex and frankly you should not be flying the aircraft without understanding how it works.
If the fuel valve is left in the both position the fuel will travel from the high tank through the fuel selector valve to the low tank for the system in carbourated versions of the C172. The new build C172R and C172S have a small fuel reservoir tank between the fuel selector valve and the engine with the fuel shut off valve between the collector tank and the engine. Selecting left or right will stop fuel from the other tank from draining into the reservoir tank and then across to the other side. Unlike the earlier carb versions of the C172 pulling the fuel shut off on a C172R or S will not stop fuel from transfering from one tank to the other.
Sorry to be blunt but the fuel system is not complex and frankly you should not be flying the aircraft without understanding how it works.