Usable fuel

[Rebel Lord]

Hello, I'm still learning my new Piper Cherokee 235. One thing that I need some help with is the usable fuel. In the POH, Checklist and owners handbook they all state that there is only 1 pint of fuel unusable in each tank, about half a gallon total. Placarded on each tank says 3 gallons unusable.

I always err on the side of caution and I'd never get that low on fuel to find out but why is there such a big discrepancy?

 

[steingar]

My Cherokee had about 2 on either side according to everything I could check. Your's is mine's big brother. Might even be using the same Hershey bar wing. Id go conservative and assume 3.

 

[Rebel Lord]

My Cherokee had about 2 on either side according to everything I could check. Your's is mine's big brother. Might even be using the same Hershey bar wing. Id go conservative and assume 3.

Yeah its pretty much the same bird just bigger engine and tip tanks

 

[Witmo]

My Cherokee had about 2 on either side according to everything I could check. Your's is mine's big brother. Might even be using the same Hershey bar wing. Id go conservative and assume 3.

What does the TCDS for your model and serial number have to say? I would go with that number. I've glanced at the TCDS for Piper Cherokees and none of the numbers listed seem to match what either of you are saying your aircraft placards say.

 

[Capt. Geoffrey Thorpe]

Run a tank dry and find out. Real world data trumps documentation every time.

 

[Rebel Lord]

What does the TCDS for your model and serial number have to say? I would go with that number. I've glanced at the TCDS for Piper Cherokees and none of the numbers listed seem to match what either of you are saying your aircraft placards say.

Thanks! this is a really cool resource I didn't know about. It says only 2.3lbs of fuel is unusable. So why would it be placarded as 3 gallons unusable? Should I go dig though the log books?

 

[Witmo]

Run a tank dry and find out. Real world data trumps documentation every time.

Unusable fuel is normally arrived at by determining how much fuel can be used in every "normal" flight attitude. In some airplanes you could run the tanks bone dry in level flight but the engine would quit long before that if your attitude was a climb or descent or bank. The Cessna 150 has a different unusable fuel figure than the 152 because they added a second fuel pickup in the 152 tank so that more fuel could be used in a climb/descent when the pitch attitude uncovers the single fuel pickup in a 150 tank. If I were worried about my airborne fuel state, I'd try and keep the airplane as level as possible and head for the nearest fuel stop hoping I'd be able to use the "unusable" fuel and avoid have to 'splain an off airport landing to the feds (of course this would never happen to me in real life as I would always land with my reserve long before needing to use up unusable fuel )

 

[Capt. Geoffrey Thorpe]

Unusable fuel is normally arrived at by determining how much fuel can be used in every "normal" flight attitude.

In a Cessna 120 that would have to be be about 1/4 tank unusable because you could unport the "lower" tank outlet in a slip - which is a "normal" thing in an airplane with no flaps...

 

[Witmo]

In a Cessna 120 that would have to be be about 1/4 tank unusable because you could unport the "lower" tank outlet in a slip - which is a "normal" thing in an airplane with no flaps...

I slip often and I have 40 degrees of flaps available. I've seen some aircraft placarded that certain maneuvers require a minimum of some amount of fuel to preclude engine stoppage from occurring because of fuel starvation. You'd have to research what the manufacturer/FAA/CAA considered a normal vs an extraordinary flight attitude at the time of airframe certification.

 

[Dan Thomas]

I slip often and I have 40 degrees of flaps available. I've seen some aircraft placarded that certain maneuvers require a minimum of some amount of fuel to preclude engine stoppage from occurring because of fuel starvation. You'd have to research what the manufacturer/FAA/CAA considered a normal vs an extraordinary flight attitude at the time of airframe certification.

In a high-wing airplane, tanks in the wings, and the selector on Both, the fuel system will continue to get fuel even in a radical slip as long as the higher tank still has some fuel in it. Airplanes with no Both provision are more likely to suffer an engine stoppage with unporting. All the pilot has to do is slip with the low-fuel tank on the low side and the selector turned to that tank.

The 210 is a bad one for unporting due to the minimal wing dihedral.

 

[sourdough44]

Yeah, I’d say flight profile/attitude etc. factor in. I’ll bet with low fuel one could dream up a profile to ‘unport’ one of the fuel pickups.

Just think of all the liability costs factored in to aviation. It’s just another example.

 

[GRG55]

Thanks! this is a really cool resource I didn't know about. It says only 2.3lbs of fuel is unusable. So why would it be placarded as 3 gallons unusable? Should I go dig though the log books?

I'd venture a guess the 2.3 lbs of unusable fuel is applicable to the main (inboard) tanks, and the higher total is a result of your having 4 tanks on the plane.

The tip tanks and wing extension are an "add on" to the original wing and fuel tank configuration after Piper decided to hang a thirstier 6-cyl engine on the front of the Cherokee. It could be the tip tanks can't be drained to the same degree as the original main tanks, and the cumulative effect is 3 gallons unusable.

I have 188 gallons in 4 tanks including the factory tip extensions on my Piper Aztec, and there's 11 gallons unusable in total.

Piper went back to the simpler 2 tank fuel system with the semi-taper wing Dakota.

 

[dmspilot]

Hello, I'm still learning my new Piper Cherokee 235. One thing that I need some help with is the usable fuel. In the POH, Checklist and owners handbook they all state that there is only 1 pint of fuel unusable in each tank, about half a gallon total. Placarded on each tank says 3 gallons unusable.

I always err on the side of caution and I'd never get that low on fuel to find out but why is there such a big discrepancy?

Simplest explanation is someone put a wrong placard on, does it look factory or aftermarket? I would be looking at the maintenance logs to try to determine when it was installed.

The PA-28-236 unusable fuel is 2.5 gallons per tank, perhaps someone mistook that number for the wrong aircraft, and rounded it up to the nearest whole number.

 

[kgruber]

My Cessna 185 had "header tanks." you could unport the wing tank for probably a minute/more before the engine would quit. I bet the 210 has them as well.

 

[Skip Miller]

Hello, I'm still learning my new Piper Cherokee 235. One thing that I need some help with is the usable fuel. In the POH, Checklist and owners handbook they all state that there is only 1 pint of fuel unusable in each tank, about half a gallon total. Placarded on each tank says 3 gallons unusable.

1 pint unusable in each tank is 2 pints or 1 quart.

Thanks! this is a really cool resource I didn't know about. It says only 2.3lbs of fuel is unusable. So why would it be placarded as 3 gallons unusable? Should I go dig though the log books?

2.3 pounds of 100LL is about 1/3 gallon.

I agree that something seems wrong with the placard but I also think you need to spend some time with a conversion table and a calculator. In general if you are measuring fuel in pints you are likely to run out of fuel in flight. There is enough variability in fuel consumption for a variety of reasons that you need a better handle on fuel remaining, usable or unusable.

 

[chemgeek]

Well, it is important to know the unusable fuel because it is normally included in the empty weight of the aircraft. Only the usable fuel would be used to calculate W&B. What does the W&B table say in the POH? Or is that where you are getting your info?

If the actual unusable fuel is 1/8 gallon per tank it is insignificant for either W&B for fuel endurance planning.

 

[kgruber]

Easy...............The TCDS is correct.
An idiot installed the wrong decals on the wing.............which wear out fast and get replaced often.