Piper Cherokee Fuel confusion

mandm

mandm

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We’re doing the fuel calibration and the numbers for the fuel. The Piper Arrow has 2 x 25gal tanks, I was not sure what the unusable fuel was, I always thought it was 2gal. And I wasn’t sure if that meant I had 48 gallons or 50 gallons of fuel. I bought a fuelhawk to measure the fuel and for the PA28, the only one I could find is a 24 gallons usable measuring stick.

Now, we pulled out the AFM and found that there is 25 gallons usable on each side for a total of 50 gallons, with an unusable fuel of 3 pints, part of the original weight and balance calculation from 1969.

Does anyone know how accurate the Fuelhawk 24gal sticks are? And does anyone know of a fuel stick for the Piper Arrow?

I learned something new today. I understand the concept of usable and unusable fuel but not having the unusable placarded and additionally the fuelhawk stick having a 24 gal usable stick did bring about some confusion.
 
Can't vouch for the Arrow, but when I did the generic one for the Navion, I punted on knowing what the unusable fuel was (or even if I drained it all out). I added fuel a gallon or so at a time until the tanks were full than extrapolated back from that. Full-39.5 (spec usable) gallons = whatever is left is unusable.
 
My '73 Archer, with hershey bar wings, likely the same as your '69 has 50 total with 48 useable. You sure you're looking at the POH?
 
Guess that was back in the day where they didn't lift the intake a bit so that you didn't suck any crap that was on the bottom of the tank into the fuel pump or fuel system.

When I first started flying you'd hear about engines quitting because they sucked something in from the tank. Much more frequently than now days.
 
The ‘63 Cherokee 180 that I fly is the same thing. It’s 2gal unusable, 48gal usable.
 
I wonder if there's some terminology confusion in that W&B sheet between "unusable" and "undrainable".
 
AV8R_87 said:
I wonder if there's some terminology confusion in that W&B sheet between "unusable" and "undrainable".
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I looked at the TCDS and there are a few older models that list 2.2lbs of unusable. None of those are Arrows, but they all share the same wing/tank construction. Maybe a lawyer said that a few pints of fuel is a bit tight for "unusable"
 
I thought I remember reading 24 gallons usable per tank.

The problem I have with using the fuel hawk for absolute accuracy is the dihedral of the wing.
 
However the placarded number must be the usable amount of fuel, right?

The fuel tank does say 25 gallons.

Anyone here work for Piper or know anyone who works at Piper? (If you want to be geeky / nerdy)
 
mandm said:
However the placarded number must be the usable amount of fuel, right?

The fuel tank does say 25 gallons.

Anyone here work for Piper or know anyone who works at Piper? (If you want to be geeky / nerdy)
Click to expand...
Go to the TCDS - It's the correct source. They are 25 gallon tanks with 24 useable in yours. Some had a larger useable # that corresponds to what's in your W&B, but they are 25 gal tanks. See post #6.
 
This happens a lot… what is presented to pilots AINT the whole story. It’s useful for understanding, it’s safe, it’s usually close…

From small pipers to big Airbus, from ppl aero to ATP type rides.

This drives those who REALLY need to know bonkers. It prevents the mechanically inept from having to get a phd in aero to get a ppl.

It’s good enough, that’s all. That’s how ALL this works, to a good ENOUGH standard. Flying AINT safe, we make it safe ENOUGH.

If you’re flight planning to a gallon, uh… If you just gotta know the whole truth, someone mentioned this, go to the original engineering team.

Personally, I operate GA in quarter tank increments and a watch. In my old Piper literally a spring wound egg timer velcroed to the floor, and some rubber bands on the flap handle. I start full, I move rubber bands when I hear a clanky bell. No clue, nor do I care capacity, usable, unusable, any of that. I memorize stuff for checkrides…

Don’t believe me, compare a POH to a AMM.
 
I always use the most pessimistic of:
1. The fuel gauges.
2. My preflight planning/watch.
3. The fuel flow gadget in my engine monitor.

Any of them tells me I'm unexpectedly low on fuel, I'm gonna land.

Years ago we were bringing the plane out of annual. The IA had to go somewhere, but her other A&P was there and we put the plane back together. We test flew it (guy was an aspiring private pilot so he was glad to come along). Opened up the cowl afterward and checked for leaks and stuff and all was good. Next day we're loaded up and departing for Oshkosh. Margy is flying. A short time in to the flight I note that the fuel is well down from full. Hmmm.... tell Margy I was pretty sure I topped the tanks (there's a quirk in the Navion that you can have them appear to be full and not, but I've not made that mistake since I bought the plane). I tell her I'll keep an eye on it. All through this she's wrangling a class B transit and flight following and had just got it when I tell her something is wrong. We land at the next airport.

Now I go off to find a mechanic. When I come back, Margy is sitting on the tarmac taking the access panel off for the boost pump (this we had overhauled during the annual so it was a prime suspect). The mechanic asks "how do you get her to do that." I point out she just finished doing the owner-assist annual. Anyhow, turns out that one of the fuel lines to the engine driven pump was loose (the other end from the one that went to the boost pump, but we had replaced those as well, They were date stamped 50 years ago (plane was only 48 years old at that point)). We didn't see the leak after the test flight as 100LL vaporizes pretty quick. Tighten things up and refuel. I calculated we were "burning" 60 GPH for the 20 minutes we were in the air.
 
I
mandm said:
View attachment 138289

The AFM, this is right? it’s old.
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I see your AFM shows pints indeed. The POH and TCDS are not as accurate as the AFM, which is because each plane may have a variation. It looks like your plane left the factory as certified with a lower Unusable Fuel than most PA-28’s. My former PA-28-140 had the identical AFM page, so this looks legitimate and the most correct answer. However, if your plane had it’s fuel tanks changed/modified/repaired (possibly as shown in a 337) in the logbook history, then maybe you are at the “standard” unusable fuel now after all those years.
 
The only way to tell for sure is to measure the fuel in the tank.

Fly one on tank until the engine sputters. Switch to other tank, which you made sure had enough fuel.

Easy Way:

Land. Measure the fuel, if possible, with your Fuel Hawk. Put in one or two gallons. Measure again, right down what the Fuel Hawk says and the amount you put in. Repeat until you fill the tank to the top (it helps to have the airplane as level as possible).

Harder Way:

After landing, drain the remaining fuel. Then add in the unusable fuel (whatever that might be) and do the Easy Way starting with the book unusable fuel

The Easy Way is not totally accurate as your plane may feed to below the book unusable fuel because the book number is for all normal flight (like climb of decent), not just level flight.
 
mandm said:
However the placarded number must be the usable amount of fuel, right?

The fuel tank does say 25 gallons.

Anyone here work for Piper or know anyone who works at Piper? (If you want to be geeky / nerdy)
Click to expand...
The placard on the wing near the filler will (or should) show the total fuel. The placard on the fuel selector should show the usable fuel. Thew difference is the unusable fuel.
 
Just to add, un-useable fuel is a regulatory defined term and included in the certified empty weight as shown in the TCDS with most found in Note 1 of the TCDS. The aircraft fuel quantity indicating system is required to show the usable fuel remaining onboard. Total fuel qty is simply a reference value.
 
There are changes to things inside the airplane that are not visible, but that can (or are intended to) change the useable fuel numbers. This is the RH fuel tank from an early Cessna 172M:

1740073710254.png

You are looking at the inboard end of the tank. The front is on the left. The red plug is in the fuel outlet, so you can see that with the airplane's nose well up (Vx climb, say), fuel sloshes to the rear and there will be some that can't reach the outlet. With the nose well down (full flaps, throttle closed), there is a LOT of fuel that won't reach that outlet. That number would be the unusable fuel.

By the time the 172N came out, things had changed (I think in the later M models it changed):

1740073995661.png

TWO outlets, one at each end, that are teed together lower down in the airplane. This reduces the unusable fuel by a whole bunch.

The system looks like this:
1740074131295.png
 
I get that you are trying to better understand you aircraft, and that is commendable.

But if I ever get into a situation where 1 or 2 gallons of gas is making a difference, it will be time to question my decision making paradigm.
 
I’m close to buying a Cessna 175 (fingers crossed for the prebuy this weekend). It holds 51 gallons, but Cessna says that at Vx above 167 horsepower, only 42 gallons is usable.
 
midwestpa24 said:
But if I ever get into a situation where 1 or 2 gallons of gas is making a difference, it will be time to question my decision making paradigm.
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Over the years I've read too many reports about people running out of fuel, often after flying over a number of good fuel stops. I usually start getting the pucker factor if I get below 12ish gallons as it is just not worth it to me.

Stopped once per my usual planning on a X-US flight and I had noticed a bit more of a headwind than planned on that last leg. The next leg the winds really started picking up and I dipped further into my reserve than I would have like to, but even with the winds I still had a reserve. If I had only taken on the fuel that should have worked, I could have easily been one of those people that came up short because of the much higher than forecasted headwinds. (In reality I would have landed for more fuel. But you get my drift. :cool: )
 
M20K said:
Over the years I've read too many reports about people running out of fuel, often after flying over a number of good fuel stops. I usually start getting the pucker factor if I get below 12ish gallons as it is just not worth it to me.

Stopped once per my usual planning on a X-US flight and I had noticed a bit more of a headwind than planned on that last leg. The next leg the winds really started picking up and I dipped further into my reserve than I would have like to, but even with the winds I still had a reserve. If I had only taken on the fuel that should have worked, I could have easily been one of those people that came up short because of the much higher than forecasted headwinds. (In reality I would have landed for more fuel. But you get my drift. :cool: )
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The Comanche has either 4 hours (mains) or 6 hours (mains plus aux) of fuel on board. I always plan a minimum of one hour reserve, so either 3 hours or 5 hours. Usually my bladder or my spouse wants out of the plane long before the 5 hour mark. The airplane also has a digital clock in the panel, with a stop watch that automatically starts from time of start up. If the running time plus the ETE on the GPS exceed 3 or 5 hours, a fuel stop is planned. Like you, I refuse to become one of the statistics for allowing too much air in the fuel tank.
 
Just make your own fuel stick, empty tank, add 2gal, measure c etc
 
Roller said:
Just make your own fuel stick, empty tank, add 2gal, measure c etc
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But start with the specified unusable fuel in the tank, not an empty tank. We had a 150 in which someone had made a dipstick calibrated from empty, and a student and instructor ran out of fuel on final with full flaps. They figured they still had four gallons or so, more than the 30-minute minimum reserve, but that airplane has an unusable fuel level of 3.5 gallons, and they flew into it and the engine quit. No damage, but a heads-up on what unusable fuel really means.
 
Personally, I think you’re (GREATLY) over- thinking this. Have you seen anything anywhere to suggest the unusable is over 2 gallons? No? It’s 2 gallons. Done. In an Arrow, at an optimistic 10gph the difference between 2 gal and 2 pts is, like, single-digit minutes.

My stick for my Warrior looks like this (sorry - don’t recall the brand - it’s been over 15 years). I dip on the outboard side to get the lowest value. Has usable on one side, actual on the other. And notice it’s not a linear scale: the spacing is designed to accommodate the shape/slant of the fuel tank. Seems accurate based on refilling the tank. Also notice I put a piece of vinyl tubing on the top to keep me from dropping it in the tank.

IMG_7424.jpeg
 
The other question is whether you dip stick can even get down to the end of the usable fuel. On my Navion if the tip of the dipstick touches fuel, then you've got some useable. The unusable fuel is also unreachable from the filler neck.

Somewhere I've got excel spreadsheets and plots of the depth measurements and the digital counts from the EI fuel gauges for every gallon from full to empty for my plane. I used these to calibrate the MVP-50 and make the accompanying chart for the fuel dipstick.
 
I rented a Cessna 182RG for my CFI checkride in 2001. It was February in Minnesota, and I had to fly from Superior, Wisconsin to Saint Paul Downtown Airport, close to one hour. The checkride was 1.7 hours, I think, and the examiner was not someone I enjoyed spending time with. (That's a story for another time.) As soon as I got the temporary certificate, I could not wait to get out of there, and it was only after takeoff that I started to think about fuel. Oh... OK. I have flown nearly three hours, and I have another hour to go. Better stop to get some fuel. But the first and second airports along my route had not plowed the runways, and I was very apprehensive about landing, so I continued.

If you have flown in the smaller Cessnas, you know how the fuel gauges look. They get down below the one quarter mark and start to bounce back and forth between E and 1/4. By the time I got to Superior, the needles were no longer bouncing. They were steady, just slightly above E. I am thinking about how this is going to look on the NTSB report - ink still wet on the CFI certificate and the pilot runs out of gas!

I have no idea how much fuel it took after I landed. But I do remember there's not much but trees on that last half hour south of Superior.
 
Pretty sure there are only 3 defined fuel measurements on an Arrow: below tabs, tabs, and full. Anything finer than that and IMO you're just guessing, and why would you do that? If you want to get an accurate fuel burn calibration, take the fuel to exactly full, or exactly tabs, fly it around, fill each to the same point and note how many gallons you put in it. The gas pump is going to be pretty accurate. Watch the gauges in case you have a leak and as a double check to swapping tanks from time to time. Carry extra gas. The risk/reward thing definitely favors carrying extra gas. Enjoy the beautiful airplane!
 
I have a fuel stick for the Cherokee that I bought from AS. It agrees very well with the totalizer and what goes in when I refuel. Once you can't see fuel at the bottom of the tank, under the filler neck, you're at 4-5 gal total in the tank. If both tanks are like that and you decide to take off, the joke's on you.
 
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