Managing fuel burn on x-country w/C172

[UncleNobby]

Re: post 20 -- 1/4 tanks doesn't give you much fuel when you consider reserve requirements. I presume the final leg is usually a short one?

Also, why stop twice to see how fuel is burning and run the tanks close to dry? Wouldn't you just make it non stop if that is the case or stop once and fill up for the second leg? Seems like the 2 stops on one tank is over complicated - unless of course it is a food/rest stop.

[B2N2]

I always left the fuel selector on both in a C172.
Slightly off topic maybe, but be very aware if you take a club aircraft or other rental for a long trip.
30+ year old aircraft (even with a moderately new engine) have been flown (read abused) by so many people that the fuel burn may be way outside of what the POH tells you.
This is what I used to do with an airplane if I was not familiar with that particular one:

• Leave with full tanks
• Fly a two hour leg ( not one 172 will run out of fuel in two hours)
• Note altitude powersetting and approximate mixture setting
• Refuel and calculate fuel burned on this leg
• Fly a three hour leg at the same altitude and powersetting
• Refuel and calculate fuel burn.
• If you feel comfy fly a four hour leg.

You now know the fuel burn for this particular aircraft including fuel needed for the climb.
I would have a timer set to 4 hours and would start it at engine start.
Prior to the timer reaching zero I would be on the ground at an airport.
The airplane that I flew on one of those trips burned 7.3 gallons/hr, rounded up to 8 and with 40 gallons could safely fly a 4 hr leg with an hr reserve.
Would otherwise never have thought about flying 4 hrs in something like a c172.

If you change aircraft tail numbers for the next trip, do the whole thing again.

[pistongone]

Many years ago i was taking a C150 accross the channel and approaching the coast we noticed the left gauge was well low So we landed at Lydd to have a look. Some helpfull chap in the club told us he knew what the problem was. He just moved the fuel vent so it was behind the wing strutt and all was well! The vent was out in the full flow of air and was pressurisng the left tank more than normall. But as has been said it would have leveled itself when the fuel got to a certain level, but better safe than sorry

[Tinstaafl]

There's another way to get accurate fuel consumption data. In an aircraft with a L - R - OFF fuel selector. Start with the tanks filled to a known, easily seen point eg the tabs or the filler neck or a convenient scratch or whereever. Start, taxi, take off & climb to your typical cruise altitude (better slightly higher than lower) then set *leaned* cruise power on one tank. Note the time and change to the other tank and cruise for an hour or, better, two. Note the time & change back to the start tank for descent & landing.

Refuel to the original level & note what each tank used. You now have the data to calculate fuel consumption from start to landing excluding cruise and, separately, leaned cruise. If you apply the calculated leaned fuel rate to the entire flight duration you will have a figure that represents how much fuel would be used if the aircraft had started above the airfield at cruise altitude/configuration/power setting, and arrived above the destination the same way. Compare that extrapolated fuel burn with the total fuel burn for the flight to find the additional amount of fuel that is used from start to top of climb and top of descent to landing.

Extending the topic a bit:
If you have multiple tanks eg mains and auxillaries and are without an accurate fuel flow system it's possible to compensate - slightly - and have at least two points where the amount of fuel on board is known accurately. First point is prior to departure with full tanks or filled to a known level. The second point is running the alternate tank dry or very nearly dry. The principle is that if you have two or more containers (instead of a single large container) you will have a known quantity remaining in the remaining, unused containers if consumption is confined to a single container at a time until it is empty.

Provided the time from start to when you switch to the alternate tank is relatively short then what's left in the mains will be very close to the calculated amount after subtracting estimated fuel burn. Over such a short period of time the margin of error is small.

If you have a good idea of cruise fuel flow eg the proving fuel consumption flight discussed above, then the time to empty on the Aux can be anticipated to within a few minutes. At the point where the Aux runs dry or you decide it's as close to empty as you're willing to go, you have a pretty accurately known quantity of fuel on board in the remaining tank(s). If not run dry there will be a margin of error but it doesn't affect the accuracy of the known fuel in the other tanks.

If you have to switch between pairs of aux's to maintain lateral balance then for this purpose you would consider the pair to be a 'single' tank and known fuel on board accuracy will be when both aux. tanks are empty.

[IO540]

Very clever, Tinstaafl

I sometimes wonder if I should run a tank dry but the place one would be doing this always happens to be over some inhospitable place and rescue might take time I run mine down to about 6USG, which is clear enough on the fuel gauges (which are accurate on my plane). Deakin advocates running a tank totally dry but ...

[Tinstaafl]

I've done it. And not done it. There are competing reasons for each.

Run dry:

No reason the engine shouldn't continue again once fuel supply is restored. If you're on the ball the engine doesn't completely shut off. You can have any standby pump on and hand on or near the selector in readiness. If you're logging fuel usage adequately.

If, for some dreadful reason (an emergency), I'm *really* down to only minutes of fuel then I'd rather have those few minutes available without having to change tanks at some critical moment eg short final.

Don't run dry:

Even a small amount of fuel in another tank provides an alternative if something should cause a blockage from the current supply tank.

If you won't be refueling for a while then some fuel in left a bladder type tank is beneficial to its preservation.


I often run the Aux tanks dry in PA31s that I fly. I have sufficient time from the low pressure warning to change tanks without the engines missing a beat. It also has crossfeed available so even if a feeding tank becomes blocked that engine can access fuel from the other side instead of the (empty) aux.

One concern in the PA31 is that pax don't like seeing fuel gauges on empty - especially if they know there's still another hour or two to fly! I alternate which engine will run dry its aux, while the other one is supplied from its main. After the 1st engine is back on its main supply then I select the 2nd engine to use the rest of its aux fuel. This means the pax always see at least one fuel gauge with plenty of fuel in it.

[tmacar]

Pull from both or only one tank at a time in a C172 Xcountry flight

OK, way late for the original question, but most answers seem to be overly specific or skip around the main issue, which is the aircraft's fuel feed system. In an aircraft with gravity feed like the C172, the only good reason I can see to pull from just one tank is to balance out the weight in each wing if you want to. Also, it's possible that something might possibly block or restrict flow from one tank. If that happens, and you've run the other one dry, you better hope you can find a place for a nice dead stick landing.

Staying in the air until landing time requires not just thinking about what will or likely will happen, but also thinking about what COULD happen. Reasons not to run one dry then switch over include the chance of sucking in some sediment you didn't know was present, accidentally running the first tank completely dry and having to restart and maybe having to recover from a stall, and imbalancing the aircraft which will, at least a bit, affect handling.

The reason folks do run one tank at a time as mentioned in the Piper is that it's a low wing aircraft, fed by a pump that draws from one tank or the other, not both. And even then they switch back and forth, not just run one dry or nearly dry before going to the full tank, to avoid all the possibilities mentioned above.

I think this falls under the old saying about there being old pilots and bold pilots, but no old, bold pilots. In my mind, staying unnecessarily on one tank until it's nearly empty is being a pretty bold pilot. And switching back and forth in an aircraft that can gravity feed from both at the same time just adds another thing to do (in other words creating an unnecessary potential point failure) and, if nothing else, puts extra wear on the valve.

[Pilot DAR]

the only good reason I can see to pull from just one tank is to balance out the weight in each wing if you want to.

That is a good reason, and I have done it. Another good reason is if you've lost, or have a leaking fuel cap, you can get as much fuel out of that tank into the engine as possible, before you lose the fuel anyway. 'Done that a few times too!

staying unnecessarily on one tank until it's nearly empty

I used to do this in several aircraft I flew, as the fuel quantity indication was either not at all available, or poor, so running the tank dry told you exactly how much fuel you had in that tank - none! (but at least you knew!)

puts extra wear on the valve.

It's nice to know that they work though!

[ChickenHouse]

As always, the first place to find answers is - the POH. C172 is run on both tanks all the time below 5000ft DA, single tank higher.

Beware of the "fly on one tank until running rough, switch and you have the same endurance as on the first tank", it is wrong for a C172, due to cross feed from the overhead venting connection between the tanks.

Uneven use of tanks IS a common "feature" of C172s, get used to it. A lot has been written and tried, after Cessna had the strange idea with the venting construction. Keep in mind they abandoned the "one side vented fuel cap" and non-vented caps are no longer sold for a long time.

Know your fuel burn from refilling and hours flown. Do realize C172 gauges are often digital - if they fidget, there is fuel in the tank, if they don't, they may be empty (or the gauge broken, but you know that from refilling and hours flown).

[bingofuel]

I have a vague memory that an Engineer once told me the fuel selector valve on a Cessna 172 is not a true, both/left/right, but more a bias valve where even selecting left or right it still draws fuel from both tanks but draws more from the tank selected with the fuel valve. Of course this would mean you could never actually run one tank dry and have an engine stoppage with fuel still available in the other tank.

Anyone have any knowledge of this ?

[9 lives]

fuel selector valve on a Cessna 172 is not a true, both/left/right

That is not correct. When you select the right or left tank, the other tank supply to the engine will be turned off, and isolated. If, while fueling, you want truly full tanks, select left or right, so the fuel does not crossfeed through the valve while the second tank is being filled.

[bingofuel]

Interesting? Thanks for that!

[ChickenHouse]

fuel selector valve on a Cessna 172 is not a true, both/left/right

That is neither true, nor absolutely wrong. The fuel selector does indeed switch off the corresponding other lower tank outlet, so - "left" does close the right tank lower outlet and "right" does close the left tank lower outlet. BUT, due to the old crazy idea of probably Clyde Vernon Cessna, the overhead venting line, which is supposed to vent the right tank by the left tanks fuel cap and is not located above the full fuel notch, does create some cross feed when tanks are almost full. So, if you go full fuel, you may have, depending on model and sideways balance of the Cessna, several gallons to leak over the overhead venting line from one tank to the other.

[9 lives]

BUT, due to the old crazy idea of probably Clyde Vernon Cessna, the overhead venting line

Don't blame Clyde, it's a certification requirement that the vent tube interconnect the two wing tanks if there is to be a "both" selection - which is why Cherokees, and similar low wing aircraft do not have a "both" selection at all.

the overhead venting line, which is supposed to vent the right tank by the left tanks fuel cap and is not located above the full fuel notch, does create some cross feed when tanks are almost full.

Yes, the overhead venting line allows venting of the right tank through the vent space of the left tank, and then overboard. That vent line has a minimal affect on allowing crossfeeding between the tanks. Though not impossible, crossfeeding, particularly left tank filling right tank, is unlikely in any meaningful quantity, as if left fuel were trying to move right with the fuel selector not on "both", there would be no venting of the right tank to allow the fuel to enter that tank. Right fuel can move to the left tank, but unless you're flying horribly cross controlled, not enough to be of concern.

the POH. C172 is run on both tanks all the time below 5000ft DA, single tank higher.

Where in the POH is this stated? I cannot find any such reference in either section 2, 4, or 7. (POH for 172P is my reference document)

[ChickenHouse]

The 172P POH does not have the 5.000ft single tank restriction, it is an AD to get the old C172 placarded as follows.

AD 72-07-02 - Selector valve placard, effective 03/25/72.
Applies to all Cessna 172's, through 1970 172K s/n 17258855.

"To reduce the possibility of engine power interruption at
altitudes above 5000 feet caused by vapor formation in the
fuel lines, accomplish the following:
(a) Effective now, the airplane must be operated on a single
fuel tank immediately upon reaching cruise altitudes above
5000 feet.
(B) On or before April 1, 1972, install at the fuel selector
valve applicable Cessna placards P/N's 0509021-1, -2 or -3
as provided with Cessna Service Letter SE72-7, dated March
17, 1972, or any FAA-approved equivalent placard which reads
as follows:
SWITCH TO SINGLE TANK OPERATION IMMEDIATELY UPON
REACHING CRUISE ALTITUDES ABOVE 5000 FEET.
(C) Compliance with the provisions of Paragraphs A and B is
no longer required when the fuel system has been modified by
the installation of applicable Cessna Kit No. SK172-31B or
SK172-32 referenced by Cessna Service Letter SE72-7, dated
March 17, 1972, or by the accomplishment of any equivalent
method approved by the Chief, Engineering and Manufacturing
Branch, FAA, Central Region."

Cross feeding via the overhead venting line can be as much as up to 3-6 gallons.

[skyhawk_norway]

On the 172SP i fly, there will be quite a difference between the left and right tank after for example 2 hours of touch and go training, due to the air vent, when flying on both.

If I am training at a remote field, I usually fly on the right tank home.

I have not checked if the tanks will equal a bit after another 1-2 hour if I am continuing flying cruise on both tanks.

(Well, guess I have not flown TGs for two hours since my initial training

H

[ChickenHouse]

The tanks will kind of equalize, as after a certain difference gravity pays it tribute. As this is dependent on circumstances, it is always advisable to refuel both tanks. I frequently take a 172H for trips around 4 hours and difference between the tanks can be quite substantial, but I did not encounter sipping exclusively one empty, yet.

[9 lives]

C172 is run on both tanks all the time below 5000ft DA, single tank higher.

vs.

SWITCH TO SINGLE TANK OPERATION IMMEDIATELY UPON
REACHING CRUISE ALTITUDES ABOVE 5000 FEET.

I partly see your point.

I have not seen any 172's with that limitation, perhaps because the (C) of the AD was complied with, and that was terminating action.

[RatherBeFlying]

After filling the higher tank on a sloping ramp, I learned to turn the fuel selector to OFF in such cases.

Fuel coming out when you remove the cap on the lower tank does that to you

I also learned that the engine runs about 15 seconds after start with the fuel selector on OFF

[Victorian]

The one time you do need to select a tank on the 172 is if it's full, or going to be filled, and standing on any kind of sideways slope where the vent is downhill. That's why "Fuel, both" is a vital part of the checklist!