The International Cessna 170 Association

Fellow 170 Fans,

I'm a former low wing pilot and didn't have any Cessna time before I bought my 170A earlier this year. In the past I had to switch fuel tanks between left and right on the low wing planes. Now that I have fuel tanks above me and a "Both" option, why would I ever burn out of just one tank during normal operations?

I can think of a couple maintenance related reasons to burn out of one tank, but I haven't thought of a good reason to run off one tank on a normal average flight.

Doug

On a trip long enough to push the range of the airplane, some folks run one tank way down, if not dry, so to have most of the usable fuel in one tank, making it less likely to unport a tank in uncoordinated flight in the landing pattern. I used to play that game, but hardly ever fly more than 3-3.5 hours at a time any more. Also, most of the time, one tank will feed faster than the other, and you might want to run on one tank to even the load. The only time I select a single tank anymore is to prevent cross feed during refueling.

To refuel without cross feeding is simply put the cross feed selected to one tank only . I always fly on one tank on long haul flights as it shows me a better idea of range even using the fuel computer. The tank I use is always the one closer to me for weight distribution.some nervous passengers often ask how much fuel we have on board and I always say I have 2.5 hrs reserve on board. That’s the tank I am not using .

Doug, MIles and David have pretty much covered it. There is almost no reason to fly in anything other than Both. It is nice to have the option of either tank for those special occasions you think it necessary.

Thanks, Guys.... I assume everyone switches the tanks to OFF when parked? Any reason not to?

Well....I used to do that, turn it off after every flight. I’m not sure doing that amounts to anything other than wearing out an expensive item. After having a leak requiring disassembly to replace the seals, I’ve converted to leaving it alone. Occasionally I’ll check its s/o function from the front gascolator.

I virtually leave my valve in the BOTH position for the flight regime. The few times I’ve switched to one tank or the other in-flight has been for the purpose of equalizing FAR-different fuel levels or the (almost never, of course, heh-heh) times I’ve been so low on fuel that I wanted to use the other for a “last-ditch” reserve.
(I still .... er, I mean... I WOULD HAVE still ...landed on BOTH.)

I disagree with the belief that using the fuel selector valve “wears” it out. Use it...or lose it... is my belief.

Those who rarely use the valve are the ones most surprised when they find it frozen and unworkable. Further, if you do not occasionally at least, turn the valve off... How do you know you CAN? ..for example in the event of an engine fire?
If you leave it in the ON posiiton, any failure of the carburetor float valve, gascolator, or hose and fittings may lead to the complete draining of connected tanks and possible flooding of the ground or hangar floor. Obviously that could lead to disaster.

These valves are properly lubricated with a “fuel lube” (fuel-resistant grease) to prevent damage to the components when the valve is operated. If the valve is not excersized regularly then the grease is not spread and may tend to lose effectiveness. That grease also lubricates O-rings and extends their lifetime. If that does not occur from lack of use the O-rings can dry out, become brittle, contract, and leak. The metal surfaces of the components become dry and subject to damage from lost lubrication.
If the valve were not intended to be utilized then there would be warnings and cautions issued to that effect.

Would you not use the carb-heat, or mag switch, or ??? for fear of “wearing it out”? Use it or lose it. That’s my opinion, for what it’s worth.

dstates wrote:I haven't thought of a good reason to run off one tank on a normal average flight.

In the past 10 years I have run on one tank a couple of times to equalize weight. I went through a period where one tank was drawing fuel more than the other, but it hasn't been a problem for a while, so I don't even remember which tank it was.

Early 172s have a flight manual note (or AD?) to run on one tank when cruising above 5000msl, presumably because of vapor formation. Not sure if the problem applies to the 170.

I always select one tank while fueling to prevent cross-feeding.

DaveF wrote:

dstates wrote:I haven't thought of a good reason to run off one tank on a normal average flight.

In the past 10 years I have run on one tank a couple of times to equalize weight. I went through a period where one tank was drawing fuel more than the other, but it hasn't been a problem for a while, so I don't even remember which tank it was.

Early 172s have a flight manual note (or AD?) to run on one tank when cruising above 5000msl, presumably because of vapor formation. Not sure if the problem applies to the 170.

I always select one tank while fueling to prevent cross-feeding.

It wasn’t just “early” 172s to which that applied. It’s a complicated/long story but the bottom line is:

Some 172 operators had experienced engine roughness and Cessna investigated and found it caused by a fuel vapor anomaly.
Despite the fact that the problem only existed with Lycoming-equipped 172s, (therefore not “early” airplanes) the fuel placard was (inappropriately) issued for all 172s. The standard after-market placard-kit sold by supply houses include the instruction to switch to single-tank-operating “immediately” upon reaching 5,000 feet.

NO CONTINENTAL ENGINED 172 experienced the problem and it was a mysteriously evasive, difficult to duplicate matter even with Lycomings.

That placard appears to have been installed in some 170s despite it’s non-applicability to that aircraft.

AD 72-07-02 applies to all Continental-powered 172s, as well as some Lycomings. For whatever reason, it does not apply to any 170s. I almost never fly below 5000 msl, but have never had a vapor problem in my Avcon 170B, nor did I ever have a vapor problem in my T210, even though that was a well-known problem. Vapor happens when it wants to.


Compliance: Required as indicated, unless already accomplished.

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, 0509021-2 or 0509021-3 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.

DaveF wrote:AD 72-07-02 applies to all Continental-powered 172s, as well as some Lycomings.

Not exactly correct, IMO... the AD applies to all 172 models from 172, 172A thru 172K. (As opposed to stating it applies to all Continental-powered models....)

I fly on both, and park on either L or R (mainly to address cross feeding).