View Full Version : C172 Fuel Tank Dipping
The Cessna 172R holds 56 gallons (53 gallons of which is usable). If you dip the tanks are you measuring the total fuel or only the usable fuel - i.e. if the dip stick shows 40 gals, is this 40 usable or 37 usable. I can't find the answer in the POH.
Tolka
Mike Cross
2nd Aug 2005, 13:40
None of the 172's I've ever flown have had a Cessna supplied dipstick documented in the POH.
The dispstick will show whatever the person who made/calibrated it made it to show.
What I am really trying to determine is whether the unusable fuel is in the tank and would therefore be recorded when the tank is dipped or is it in the fuel lines or some other such inaccessible place where no amount of dipping of the tanks will record it.
Tolka
Mike Cross
2nd Aug 2005, 17:00
I don't profess to be an expert but:-
The engine needs a relatively air-free supply of fuel at a reasonable rate of delivery. What you can drain from the tanks when the machine is on the ground is not the amount that you will be able to use when in the air. You may not be wings level so the fuel in one tank might be in a big pool away from the outlet where you can't get at it and the rate at which fuel is flowing may not be enough to meet the engine's demands (which in turn will be dependent on throttle setting) added to which vibration may well be causing a mixture of fuel and air to be flowing down the pipes.
I was in a powerboat race once where the tank was about a third full but we lost power as a result of aeration due to the rough conditions.
If the POH shows 56 usg of which 3 usg is unusable then full is 56 usg. As the POH says, not all of that is usable and the piping gascolator and carburettor will not hold 3 usg between them so there will still be fuel visible in the tanks even when you have no usable fuel left.
You might find this (http://www.faasquadron.org.uk/essenceoflife.html) useful. Also this (http://sss.sd54.bc.ca/aviation/cdroms/avsafe2/docs/0373E.pdf) shows how fuel can remain in the tanks even when the outlet is at the lowest point.
FlyingForFun
2nd Aug 2005, 17:17
I don't know the answer, it will depend on your dipstick. However:The Cessna 172R holds 56 gallons (53 gallons of which is usable). Presumably 28 gallons in each tank, 26.5 useable gallons in each tank. So is the top mark of your dipstick 28 gallons, or 26.5 gallons? That should answer the question.
(BTW, all 3rd-party supplied dipsticks I've seen for Cessnas show useable fuel, but I don't promise that will be true for yours too.)
FFF
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steve_moate
11th Aug 2005, 10:49
My dipstick shows 26.5 gallons at full, and the tanks are indeed 28 each side. It also warns you to 'offset' the dipstick when dipping to avoid the ridge at the bottom of the tank immediately below the fuel cap.
Purchased at Transair some time ago, and marked as 'C-172 Skyhawk'. I assume that other 172 variants have different tanks and therefore differently calibrated dipsticks.