Single tank operation above 5,000?

[russfarris]

Does anyone know why the single tank only above 5,000 feet restriction for early 172s doesn't apply to the 170? The engine and fuel system/vents are identical, as far as I know. Just curious, Russ Farris

[N1478D]

There was a very informative article about the Cessna fuel vent systems and changes in a recent LPM article. The article described how the fuel system started out fairly simple and changed over the years in to a very complex system of tubes, hoses, etc. Getting more and more complex as changes were introduced at different times.

[zero.one.victor]

The wording is ambiguous--is it:

1) single tank,only above 5,000'
or
2) single tank only,above 5,000' ???

I don't see why either would make sense.

Eric

[russfarris]

Eric, I guess what they mean is don't have the fuel selector on Both above 5,000 feet!

Joe, I read the LPM article; it was very informative, but I still don't have a clue why the early 172 has this restriction. I believe there is a vent mod that does away with it. My original question stands...why the early 172 and not the 170? (I'm not looking for trouble, just inquiring minds gots'ta know!) Russ Farris

[zero.one.victor]

Russ,not knowing any better,I would interpret that as "single tank,only above 5,000'. I assume that the perceived danger is of a single tank running dry,the engine quitting,and not having enough altitude under ya to re-start. If both tanks run dry,I guess altitude ain't gonna affect your chances of a re-start!

Eric

[russfarris]

I'm up entirely too late doing this! The 172 Type Data sheet says...

required fuel selector placard: s.n. 28000 to 17258855 (1970 172K)

"SWITCH TO SINGLE TANK OPERATION IMMEDIATELY UPON REACHING CRUISE ALTITUDES ABOVE 5,000 FEET"

Airplanes with Cessna service kits SK-172-31B or SK172-32 can have the limitation removed. Interesting!

I notice this affects Lycoming powered 172s as well; the engine change was made in 1968. Maybe the all-seeing, all-knowing Karnak the Magni...uh, I mean George Horn can shed some light on this burning issue! Russ Farris

[wa4jr]

I don't have my 170 owners manual with me at work, but seems I remember the manual saying to switch to single tank operation upon reaching cruise altitudes above 5000 feet. I have a 54' 170B. Now my curiousity is up and will have to go back and re-read the manual and my placard around the fuel selector. I do know I have used single tank operation below 5000 feet to balance the tanks, and have also used both tanks above 5000 for cruise. Engine continues to run fine either way. I do like the logic that says be on both tanks below 5000 feet so that you don't inadvertantly run one dry at low altitude. Actually I think all this was started some years ago by someone with too much time on his hands and nothing productive to do....now we should thank him for giving us another rag to chew on while the WX is too bad for flying this winter

[Dave Clark]

Gee guys I take off at over 5000 ft so do I do that on one tank? HeeHee.

I'll just put the selector to whereever it needs to be to deliver fuel and then if the tanks have fuel so will the engine. Pretty simple really.

[GAHorn]

I'm up entirely too late doing this! The 172 Type Data sheet says...

required fuel selector placard: s.n. 28000 to 17258855 (1970 172K)

"SWITCH TO SINGLE TANK OPERATION IMMEDIATELY UPON REACHING CRUISE ALTITUDES ABOVE 5,000 FEET"

Airplanes with Cessna service kits SK-172-31B or SK172-32 can have the limitation removed. Interesting!

I notice this affects Lycoming powered 172s as well; the engine change was made in 1968. Maybe the all-seeing, all-knowing Karnak the Magni...uh, I mean George Horn can shed some light on this burning issue! Russ Farris

KARNAK, THE MAGNIFICENT SAYS:
The ANSWER to the question is:
I'm Sa-Damn sorry I started all this!

THE QUESTION WAS:
How did the president of Iraq get his first name?

(I always loved Johnny Carson's shows.)

The ANSWER to the question is: Single tank operation above 5,000 feet only applied to the 172, per AD 72-07-02. (72-07-02; Title: CESSNA Models 172, 172A, 172B, 172C, 172D, 172E, 172F, 172G, 172H, 172I, 172K, )

Some 172 owners reported engine surging above 5K feet and Cessna went to great lengths to duplicate the problem but never could. They even installed clear fuel lines in a vain attempt to observe the vapor bubbles which never appeared. The FAA was certain there was a fuel head-pressure problem, so Cessna gave up fighting it and accepted this AD note. It only applies to 172's. It presumes that a clogged fuel vent line on one tank might reduce fuel head pressure sufficiently to cause formation of vapor bubbles which in turn might cause engine surging. Switching to one tank should either isolate or identify the offending fuel tank. Selecting the opposite tank would cure the problem. (No one seems to have noticed that if a tank's vent is fleetwide problematical, that selecting that tank would worsen the problem fleetwide in some consistently identifiable manner and therefore confirm the need for, or uselessness of the subject AD. Since no one has subsequently complained, the FAA feels they have correctly solved the non-existant problem, and the AD remains in effect.)

I've always operated on BOTH and never had a problem in 172's AND 170's. I personally believe insects build nests in the vent lines and create fuel vent problems that have plagued some operators who live in insect climes and/or undiagnosed carburetor ice played a part in the reports. (I have personally dug out a fine cigar made of green leaves and packed next to the fuel tank check-valve 3 FEET (!) inside from the vent opening of my 206. It had the nicest long-leaf wrapper on it and HAD to be made somewhere else and imported to my fuel tank by a very wealthy wasp! I wish I'd photographed it.
The only reason I looked so deep into the system was because of engine surging at altitude. (The fuel injected 206 had seperate, unconnected fuel tanks and no BOTH position available. Switching to the opposite tank immediately restored power.)

Our 170's are not affected by this Cessna 172 AD or operating procedure. Operating on BOTH is fine at all altitudes. (If the truth be known, 172's aren't really bothered either!)

[flyer170]

Pop quiz..... You are flying along at 6,500 ft, fuel selector is on both, you look out and see fuel streaming out of the quick drain on the right wing.
You can't set it down for awhile and the ground is 600 ft msl.
What do you do. This is in a standard 170, and lets assume you can't reach the quick drain in flight.
Bob

[GAHorn]

This actually happened to me back in the '70s while flying pipeline patrol. I was an avid cigarette smoker back then and my Zippo was out of fluid on a 6-hour flight. (C-140A with 42 gallon tanks.)
I opened the left window and pressed my Zippo insert upside down against the quick-drain on the left side for a moment. Pulled my arm back in, held my hand outside in the slipstream to dry it, reassembled my Zippo and closed the window. Flicked the little flint-wheel, burned off my mustache, and got my cigarette lit and smoked happily away for the next hour.
I was in deep west Texas in the open desert, not a landing site within many miles when something caught my eye down on the pipeline right-of-way. As I swung my gaze aft as I passed it, some motion caught my attention at the upper corner of my eye. I looked up and to my horror saw a steady stream of fuel pouring out of that quick drain. I quickly reached out and pulled the stuck valve to the closed position. The overhead fuel gauge read Zero! The right tank would slosh if I kicked the rudder. Nothing but rocks, hills and cactus almost as far as you could see.
It was 1972 and using mogas was not widely practiced at the time, but I found a dirt road near an isolated country store/gas station and landed. It looked like one of those places you see in advertisements for Jeep, or in one of those movies that involve hostages in remote places held by people born in the shallow end of the gene pool. I bought a 20 gallon load of "Ethyl".
I was concerned that my less-than-average fuel bill an hour later at the regular stop in McCamey, TX would look suspicious to my employer, so I taxied behind the hangar out into the sand and drained it all onto the ground, so I would take on a full 35 gallons like usual.
EPA wasn't invented yet or they'd probably still be looking for me.

[flyer170]

George...... Have you written a book about your flying experiences yet? If not you should.
What about the answer to the pop quiz
Bob

[GAHorn]

Does anyone know why the single tank only above 5,000 feet restriction for early 172s doesn't apply to the 170? The engine and fuel system/vents are identical, as far as I know. Just curious, Russ Farris

I just got off the phone with Cessna Technical Rep. Tim Fischer who had consulted with the retired Cessna Rep. responsible for developing this procedure that responds to the AD 72-07-02. Here's their answer:
The airplanes that experienced problems were later model Lycoming-powered 172's. Although with an altered fuel system which included short vent lines connecting the fuel delivery pipes to the fuel vent pipes, the problem continued. Although the factory was unable to duplicate all the symptoms reported by some operators, they did develop this procedure which eliminated all further complaints. (The Lycoming engine's greater fuel consumption may have contributed to a lowered head pressure in a system primarily designed for the C145/O300 engine.) Although none of the Continental-powered airplanes suffered from the problem, Cessna thought it conservative to include them in the AD note. (After carefully listening to the development of the conversation, I suspect it was to avoid casting dispersion upon their newly selected engines of choice, since Continentals would no longer be offered in the 172. In other words, they were not going to go back to the Continental as they'd bought over 4,000 Lycomings and had them in stock, so in order to not make it look like they'd selected a troublesome engine, they were anxious to make it apply to all previous aircraft. My own suspicion.-gh)
Even though the straight 1956 model 172 has exactly the same fuel system as the 170B none of the taildraggers ever had any such problems. (Nor in fact did any of the Continental 172's.) Proving this point to the FAA would cost Cessna a lot of time and money on airplanes no longer manufactured. Cessna took the easy way out with the FAA.

[russfarris]

George, once again your knowledge of all things arcane aeronautical has come through...my thanks, sir...Russ Farris

[GAHorn]

Arcane: ....does that mean "useless"? Or "unuseable"?