172 fuel tanks...Both or one?

ScottK

Pre-takeoff checklist
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ScottK
On the fuel selector panel of the 172i I fly, it says to switch to a single tank while cruising about 5000'. What are the reasons for this? And I'm assuming they mean 5000 msl?
 

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Has to do with an AD, the referenced kit adds a vent line between the tanks I believe...

Airworthiness Directive
DEPARTMENT OF TRANSPORTATION
Federal Aviation Administration
14 CFR Part 39
Amendment 39-1415; AD 72-07-02
Airworthiness Directives; CESSNA Models 172, 172A, 172B, 172C, 172D, 172E, 172F, 172G,
172H, 172I, 172K, Airplanes
AGENCY: Federal Aviation Administration, DOT
DATES: Effective March 25, 1972.
72-07-02 CESSNA: Amdt. 39-1415. Applies to the following airplanes:

MODELS SERIAL NUMBERS AFFECTED
172 28000 thru 29999
172 36000 thru 36999
172 46001 thru 46754
172A 46755 thru 47746
172B 17247747 thru 17248734
172C 17248735 thru 17249544
172D 17249545 thru 17250572
172E 17250573 thru 17251822
172F 17251823 thru 17253392
172G 17253393 thru 17254892
172H 17254893 thru 17256512
172I 17256513 thru 17257161
172K 17257162 thru 17258855
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.
(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.
This amendment becomes effective March 25, 1972
 
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It's because of the way Cessna designed the fuel vent system, the original purpose of which was to allow the use of both tanks simultaneously. There is a single ram inlet that pressurizes the left tank and a crossover that is supposed to equalize pressure between the two tanks. Apparently, for whatever reason, it' didn't always work the way it was supposed to and just because there was an AD doesn't necessarily mean that it was common for it to misbehave, maybe it only happened once or twice but this AD was followed a few years later by another one that required replacement of the non vented filler caps with new vented ones so obviously something was going on.

Because of the intricacies of balancing fuel tank vent pressures in flight many airplanes simply do not have a BOTH position on the fuel selector.
 
The reason that it flows unevenly is because of the reason you specify, but that has SQUAT to do with the AD or why airplanes don't have BOTH positions.

The AD is because of vapor lock possibilities when drawing decreased rate through both lines at the same time that is alleviated with the higher flow rate through a single line.

The reason many planes (nearly ALL low wings) don't have both is because they're not gravity fed which means that if one tank were to run dry before the other it wouldn't pump fuel (take two straws, put one in a glass of liquid and the other in free air and try to suck through them together). Navions are a bit unusual in that they have the Cessna 150 style ON - OFF selector because both tanks gravity drain into a common smaller tank and things are pumped from there (in fact, there's only one filler for both main tanks as well).
 
Thank you folks. I didn't think to look there for an explanation.
 
The AD is because of vapor lock possibilities when drawing decreased rate through both lines at the same time that is alleviated with the higher flow rate through a single line.

I'm not going to say you're wrong but it's hard to fathom vapor lock in a gravity feed system in which all lines and the selector valve itself are behind the firewall. It's also curious as to why only 172 models A-K? Why not 150's or 177's or 182's? The AD being based on altitude however leads me to believe you may be right, what is the source of your information?

There are plenty of gravity fed aircraft such as high wing Pipers and Luscombes that do not have a BOTH selection and there are documented cases of people installing Cessna selector valves or, in the case of the Luscombe, operating with both shut-off valves open and running out of gas due to un-noticed fuel transfer and loss that occurs because there is no cross vent to equalize vent pressure in the two tanks. I know that if you put a vented cap on a Champ or early Citabria the same thing will happen even though they only have an ON/OFF selector valve and always run on BOTH.
 
I'm not going to say you're wrong but it's hard to fathom vapor lock in a gravity feed system in which all lines and the selector valve itself are behind the firewall. It's also curious as to why only 172 models A-K? Why not 150's or 177's or 182's? The AD being based on altitude however leads me to believe you may be right, what is the source of your information?

There are plenty of gravity fed aircraft such as high wing Pipers and Luscombes that do not have a BOTH selection and there are documented cases of people installing Cessna selector valves or, in the case of the Luscombe, operating with both shut-off valves open and running out of gas due to un-noticed fuel transfer and loss that occurs because there is no cross vent to equalize vent pressure in the two tanks. I know that if you put a vented cap on a Champ or early Citabria the same thing will happen even though they only have an ON/OFF selector valve and always run on BOTH.

Cessna 177 series is quite a bit different than all the other cessnas you mention, which have two vents and two fuel pumps along with a firewall shutoff valve separate from the fuel selector. The selector has a "Both" position, yet even with two vents they don't feed evenly.
 
Our Maule was very similar, didn't feed evenly either. The dealer we got it from had this little bit of tubing with an angled cut that he put on one of the vent inlets and said he could adjust flow by rotating it at differing angles into the slip stream. I never could get that to work though and figured if I ever crashed the NTSB would walk up to the wreck and instantly see that little piece of tubing and go AHA!!!

So I took it off.
 
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It's because of the way Cessna designed the fuel vent system, the original purpose of which was to allow the use of both tanks simultaneously. There is a single ram inlet that pressurizes the left tank and a crossover that is supposed to equalize pressure between the two tanks. Apparently, for whatever reason, it' didn't always work the way it was supposed to and just because there was an AD doesn't necessarily mean that it was common for it to misbehave, maybe it only happened once or twice but this AD was followed a few years later by another one that required replacement of the non vented filler caps with new vented ones so obviously something was going on.

The AD that required a vented cap on the RH tank was to make sure there was adequate venting if the single underwing vent got iced over or plugged by a bug. It had nothing to do with uneven fuel flow. In most of these airplanes, uneven flow is caused by leaky fuel cap gaskets or failed vent check valves in the caps. The rubber cap gasket gets old and develops microscopic cracks that can cause slightly different pressures between the two tanks. If the cap's spring leaf that clamps the cap onto the filler opening is weak, slipstream and low pressure can lift it a bit and cause problems.

Most affected airplanes have vented caps on both sides now. The old non-vented caps aren't available.

The vented cap lets air in for emergency purposes, and its check valve prevents outflow so that a full tank doesn't let fuel get blown out by the normal venting system.

Interesting stuff here:
http://www.cessna172club.com/forum/ubbthreads.php?ubb=showflat&Number=40037

Dan
 
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I know that if you put a vented cap on a Champ or early Citabria the same thing will happen even though they only have an ON/OFF selector valve and always run on BOTH.

That's an example of why the government insists on a repairman using the manufacturer's parts catalog. Those airplanes have a venting system similar to Cessna's and installing a vented cap (which, in this case, has no check valve) will produce really wild tank pressure differentials and can cause fuel losses. We once bought a Citabria that lost a heap of fuel out of the LH filler cap because the gasket was cracked. I fixed that, then had badly uneven flow issues. Took a look at the other fuel cap and discovered that it was a vented tractor cap that appeared completely identical to the proper fuel cap. I bet the previous owner had lost a cap and went to Princess Auto or someplace like that and bought what looked like the same thing and put it on. I bought the right (non-vented) cap from the manufacturer and all the problems went away.

Think about it. Two tanks, with their airspaces plumbed together at the top, will have exactly the same air pressure in them UNLESS there's airflow through the crossover line, and that airflow will be due to leaky caps or scratched or corroded filler flanges or weak cap springs or the wrong cap or a worn-out or rotten cap gasket. If the tank pressures are the same but flow is still uneven, the pilot is flying one wing low or the aluminum fuel lines have some restriction in them (a kink) or the finger strainer in one tank is offering more resistance to flow (might have bits of dead bugs or lint or a finer mesh to it) or there are more elbows in the system on one side or the selector valve has uneven flow characteristics. Cessna's selector valves use O-rings as seals for the valve balls, and those O-rings wear or swell, and since there are two balls on each side, a smaller one operated by the cam to push the larger ball off its O-ring seat, maybe the smaller ball gets into some goofy position that affects flow on one side.

Dan
 
To be honest, although I'll have to admit to doing it a lot myself, I don't consider flying the Cessna or Maule or whatever with the selector on BOTH to be particularly good piloting technique. Running single tank for timed intervals gives you a much better awareness of exactly how much fuel is in each tank. I own a 7EC Champ that has two wing tanks but only one fuel gauge located in the right tank. I guess this was adequate as the aircraft was originally designed to hold four hours of fuel with a 2 hour seat cushion but I upgraded my cushion to one that I can literally sit on all day without discomfort so I've got to be on my toes.

What I do is dip the tanks every time I land to see how close the flow rates are and they are usually very close. The single gauge however does serve an important function. One day I was leaving an airport in the foothills to head home and had decided to take a longer route back with an additional stop so I needed to add some fuel. well this particular airport had a full serve fuel island, which is pretty rare these days. The kid came out and set up the ladder and pulled the hose out while I did the normal routine on the card reader. Problem was that my credit card had a big split in it and the reader wouldn't take it. I asked the kid if he could take cash and he said no so I re-changed my plans and told him to forget it. I had enough fuel to go straight home. So I hopped in, took off and about half way back I was wondering why the gauge was doing what it was doing, which was increasing.

So I landed short of my destination at a friends place and, you already have guessed by now, the kid never put my cap back on. I was so unaccustomed to full service that I never checked before taking off so ultimately, my bad but the fuel gauge, as inaccurate as it ultimately is, was able to tip me off that something wasn't right.

So it's not completely useless :redface:
 
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