Fuel Dipstick Not Accurate?

[Bruce Fenstermacher]

You measured 10 and drained 12. That would be accurate enough for many specially since it seems you actually have a bit more fuel than was measured which is much better than the reverse. But when you measure 10 and count on 12 gallons remember 2.5 gallons on that side is unusable in flight. Did you drain some of that unusable fuel in your 12 gal sample? Draining fuel from the aft sump drain is not the same as the gravity feed to the engine. While I doubt 2.5 gallons transferred from the opposite tank if you drained the fuel in a timely manner, remember fuel is always transferring between tanks unless you have the fuel selector on either left or right side. Fuel is still transferred between tanks with Both or Off selected.

As for your 180 wing. What tank is in it? A stock 170 tank, the 180 tank or some other tank? Pretty important information to know and the only way you can start to measure the fuel contained in it accurately. I can say we've done a thorough study/survey of all 170 tanks recently but most I recall have the baffle. Makes me wonder if your 180 wing has a 170 tank.

[GAHorn]

I think Bill is probably on the right track.

I suggest you fill that 180 wing tank to determine it’s full capacity and compare it to the other tank.

[reecewallace]

You measured 10 and drained 12. That would be accurate enough for many specially since it seems you actually have a bit more fuel than was measured which is much better than the reverse. But when you measure 10 and count on 12 gallons remember 2.5 gallons on that side is unusable in flight. Did you drain some of that unusable fuel in your 12 gal sample? Draining fuel from the aft sump drain is not the same as the gravity feed to the engine. While I doubt 2.5 gallons transferred from the opposite tank if you drained the fuel in a timely manner, remember fuel is always transferring between tanks unless you have the fuel selector on either left or right side. Fuel is still transferred between tanks with Both or Off selected.

As for your 180 wing. What tank is in it? A stock 170 tank, the 180 tank or some other tank? Pretty important information to know and the only way you can start to measure the fuel contained in it accurately. I can say we've done a thorough study/survey of all 170 tanks recently but most I recall have the baffle. Makes me wonder if your 180 wing has a 170 tank.

I measured 12 but it only drained 10. This is the discrepancy I'm worried about. The FuelHawk reads more than is actually there.

Not sure if it drained any of the unuseable fuel—I only drained the wings, not from the belly.

Both tanks appear to hold the same amount, but one tank has a baffle directly under the filler neck and the other does not.

[GAHorn]

You measured 10 and drained 12. That would be accurate enough for many specially since it seems you actually have a bit more fuel than was measured which is much better than the reverse. But when you measure 10 and count on 12 gallons remember 2.5 gallons on that side is unusable in flight. Did you drain some of that unusable fuel in your 12 gal sample? Draining fuel from the aft sump drain is not the same as the gravity feed to the engine. While I doubt 2.5 gallons transferred from the opposite tank if you drained the fuel in a timely manner, remember fuel is always transferring between tanks unless you have the fuel selector on either left or right side. Fuel is still transferred between tanks with Both or Off selected.

As for your 180 wing. What tank is in it? A stock 170 tank, the 180 tank or some other tank? Pretty important information to know and the only way you can start to measure the fuel contained in it accurately. I can say we've done a thorough study/survey of all 170 tanks recently but most I recall have the baffle. Makes me wonder if your 180 wing has a 170 tank.

I measured 12 but it only drained 10. This is the discrepancy I'm worried about. The FuelHawk reads more than is actually there.

Not sure if it drained any of the unuseable fuel—I only drained the wings, not from the belly.

Both tanks appear to hold the same amount, but one tank has a baffle directly under the filler neck and the other does not.

What you are observing is the “unuseable” fuel remaining in the system. You are not accurately / fairly measuring at all.
You should FILL the empty tank completely to see how many gallons each tank takes. THEN use the FuelHawk to confirm the FULL reading.
This will tell you the actual size of the two tanks. (It sounds as if you are comparing apples to oranges.... 170 tank to 180 tank.)

AFTER you’ve determined which tank is a 21 gallon tank... that is the one which is a 170 A or B tank. The other likely will hold another 10 or so and that is the 180 tank...and the Described Fuel Hawk will not apply to that one. You’ll need a 180 Fuel Hawk for that one.

[cessna170bdriver]

I’ve never used a dipstick to try to measure remaining fuel with any degree of accuracy, as an eyeball through the filler neck, and keeping track of flight time since the last fill-up has always been adequate for my purposes. However, if my former instrumentation engineer self was given the problem of calibrating a dipstick, fuel hawk, etc. to show USABLE fuel (for legal planning purposes), I’d do the following:

1) Fill the tank, dip it, and call that level whatever the POH says is maximum usable fuel for that tank, in the case of a stock 170A or B, 18.5 gallons.

2) Drain whatever increment of fuel you want to calibrate the stick to, call that level whatever the previous was, minus the increment.

3) Rinse, lather, and repeat until you’ve drained what the POH says is the maximum usable fuel, and call that zero. It doesn’t matter how much fuel is left in the tank, as it’s unusable for planning purposes.

I’ve never tested or done the math to determine the effect of aircraft attitude on this type of calibration but given the shape of the tank and inboard location of the filler, I suspect that it could be significant, especially with respect to roll. It might be prudent to repeat the calibration at a couple of angles just to get a feel for it.

[reecewallace]

I’ve never used a dipstick to try to measure remaining fuel with any degree of accuracy, as an eyeball through the filler neck, and keeping track of flight time since the last fill-up has always been adequate for my purposes. However, if my former instrumentation engineer self was given the problem of calibrating a dipstick, fuel hawk, etc. to show USABLE fuel (for legal planning purposes), I’d do the following:

1) Fill the tank, dip it, and call that level whatever the POH says is maximum usable fuel for that tank, in the case of a stock 170A or B, 18.5 gallons.

2) Drain whatever increment of fuel you want to calibrate the stick to, call that level whatever the previous was, minus the increment.

3) Rinse, lather, and repeat until you’ve drained what the POH says is the maximum usable fuel, and call that zero. It doesn’t matter how much fuel is left in the tank, as it’s unusable for planning purposes.

I’ve never tested or done the math to determine the effect of aircraft attitude on this type of calibration but given the shape of the tank and inboard location of the filler, I suspect that it could be significant, especially with respect to roll. It might be prudent to repeat the calibration at a couple of angles just to get a feel for it.

This is solid advice working backwards so you can ensure you don't touch the unusable fuel. I'll do this then calibrate my Fuel Hawk dipstick if I need to. Thanks

[edbooth]

Since everyone appears to be splitting hairs on this, do any of these "dipsticks" have built in temperature compensation ???

[GAHorn]

Since everyone appears to be splitting hairs on this, do any of these "dipsticks" have built in temperature compensation ???

My C-172 19-gal FuelHawk does.... when the fuel temp decreases and causes increased density...so does the FuelHawk.

[n2582d]

... remember fuel is always transferring between tanks unless you have the fuel selector on either left or right side. Fuel is still transferred between tanks with Both or Off selected. ...

Bruce,
That may be true for the Weatherhead valve on s/n's prior to 20285. On the later style machined aluminum block valve all ports are closed in the off position as discussed in this thread.

[hilltop170]

I would be really surprised to see a 180 wing on a 170. For starters, the early 180s have always had bladder tanks and are either 27.5 or 32.5 gallons on each side. The 27.5 gallon tanks have a mechanical wing root gauge like the 170s, the 32.5 gallon tanks have electric float level indication.

A 170 tank cannot be installed in a 180 wing without major butchering of the wing as the 180 bladder is rolled up and installed thru the filler neck hole. There is no removable top cover skin on a 180. The struts are another difference, 180 struts have a greater chord.

However, if it is indeed a 180 wing on a 170, there will not be a tab inside, the bladders do not have any internals whatsoever other than vent, drains, and gauge.

32.5gal C180 fuel tank

[GAHorn]

Those are excellent comments Richard. Also the bladder upper surface is held to the “roof” of the tank-compartment with clips. If those clips release and allow the bladder to collapse, the fuel level may not be accurately checked with a dipstick anyway. One of the oddities I’ve seen is increasing indications of fuel quantities while in-flight due to a collapsing bladder. I would imagine the same erroneous reading might occur using a dipstick if a bladder did not return to it’s proper shape when the vent/vacuum is relieved.

[reecewallace]

I'd say it's very unlikely I have 180 tanks.

That said, my fuel tanks are clearly different inside. One has baffles directly below the filler neck, the other does not. They are both metal, not bladders.

[GAHorn]

I'd say it's very unlikely I have 180 tanks.

That said, my fuel tanks are clearly different inside. One has baffles directly below the filler neck, the other does not. They are both metal, not bladders.

Gotta wonder what the basis of approval is if it’s not a 170 tank.

[reecewallace]

I'd say it's very unlikely I have 180 tanks.

That said, my fuel tanks are clearly different inside. One has baffles directly below the filler neck, the other does not. They are both metal, not bladders.

Gotta wonder what the basis of approval is if it’s not a 170 tank.

Suppose it doesn't matter. I'm just going to have to drain them from full and measure the quantity on both tanks separately. Now to find 37gal worth of jerry cans!

[hilltop170]

Don’t drain them, go fly! Just land with enough!

Be sure to use a calibrated pump/meter when you fill them back up, cans are not accurate enough and too much is left to interpretation. If you are going to all that trouble, make sure it is done accurately.