How much fuel did I get?

Jaybird180

Jaybird180

Final Approach
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Jaybird180
I pay a wet rate on airplane rental, so fuel usage is only important to me in that I have enough to safely complete my flight.

This weekend I flew with my family. I used a clear tube dipstick and measured 27 gals in the extended tanks of the Skyhawk (52/56 gals? -NS on this airplane). I landed 1.5 hrs on the Hobbs. I then added another 0.5 Hobbs getting back into night currency. Afterwards, I measured 18gals (9 each side) total and ordered 20 gals of fuel (10 per side). I went inside to check weather, and file my flight plan. I paid the bill and took my family out to the plane. On my preflight, I measured +11 gals. I measured each wing twice and did the math in my head (+5 on left, +6 on right). I went inside where I purchased fuel and adjusted the bill to 11 gals. I continued my preflight still a little bothered by the discrepancy, but determined that at 29 gals I had plenty reserve to complete the flight, but I still re-measured, and came up with 31 gals.

Enroute to home base, I had to make a weather diversion. This was 1.0 hour on the Hobbs. After waiting an hour on the ground, we departed and landed home, adding another 0.5 Hobbs. This is the receipt I turned in when I turned in my rental ticket 4.0 Hobbs time. I did not measure fuel after tying down the airplane.

Today, the place where I purchased fuel calls me to say they stand behind their original assertion that I received 20 gals of fuel. They say their pumps are calibrate and certified and they are inspected by the FAA.

I called the FBO manager at home base, but was only able to leave a voicemail. Any advice on resolving this situation? Again, path of least resistance for me is to simply allow the FBO to pay the fuel company for the other 9 gals they claim the put in the airplane. However, this could reasonably happen if it were my own airplane.

I'm now going to set back, listen to the experienced opinions here and learn.

Feel free to ask questions about any pertinent details -no need to say where this is, but I think I have it all covered.
 
Jaybird180 said:
measured 27 gals
1.5 hrs on the Hobbs.
another 0.5 Hobbs
measured 18gals (9 each side)
ordered 20 gals of fuel (10 per side).
measured +11 gals (29 gals)
re-measured, and came up with 31 gals
1.0 hour on the Hobbs
another 0.5 Hobbs
Click to expand...

So... You flew for two hours, and measured 9 gallons burned. Doubtful, unless you spent a goodly portion of those 2 hours (like, half of it) taxiing.

You also had two measurements that came up 2 gallons apart without the quantity of fuel in the tanks or the position of the aircraft changing... As in, pretty much nothing changed.

Even once you know how much fuel was added back at the home base to top you off, you won't really know for sure how much fuel you got, because without an accurate fuel flow meter in the plane, you're relying entirely on the dipstick measurements you took, since the plane was not topped off or empty at any time. Those measurements, as you can tell by the fact that two of them came up 2 gallons apart with nothing else changing, are not entirely accurate, and generally the lower the amount of fuel in the tanks, the less accurate the measurements will be.

Also, the fuel pumps do get calibrated quite accurately. IIRC when I worked at an FBO and the state came out to check up on us, each truck had to fill an exact 100-gallon tank and be within 0.1. So, you're not going to end up with a 50% error. If it was an accounting error, maybe - But I'm sure the FBO checked their records before calling you. In addition to the "how many gallons was this fill" meter, all the trucks have a small "how many gallons has this truck ever pumped" meter that was used to ensure that there were no accounting errors.

Pay the bill. :yes:
 
I would trust their dispensing equipment before trusting your little glass tube. Those things are worthless. Does the bottom of the tank slope? Were you on the downhill side or the uphill side? That's probably a gallon difference in the measurement right there.

As others have pointed out, there is no way that you only burned nine gallons in two hours of flight.

Sorry if this sounds harsh, but I'd seriously revisit your fuel measurement techniques before putting your family back in the airplane. It doesn't sound like your assumptions are correct and you're making decisions based off of them. Somewhere along the way you probably had 11-12 less gallons of fuel than you believed that you did. That is an hour of assumed range that just disappeared on you.
 
flyingcheesehead said:
You also had two measurements that came up 2 gallons apart without the quantity of fuel in the tanks or the position of the aircraft changing... As in, pretty much nothing changed.
Click to expand...
I'm doubtful if all were aboard during both measurements, hence if something changed or not.

flyingcheesehead said:
Even once you know how much fuel was added back at the home base to top you off, you won't really know for sure how much fuel you got, because without an accurate fuel flow meter in the plane, you're relying entirely on the dipstick measurements you took, since the plane was not topped off or empty at any time. Those measurements, as you can tell by the fact that two of them came up 2 gallons apart with nothing else changing, are not entirely accurate, and generally the lower the amount of fuel in the tanks, the less accurate the measurements will be.

Also, the fuel pumps do get calibrated quite accurately. IIRC when I worked at an FBO and the state came out to check up on us, each truck had to fill an exact 100-gallon tank and be within 0.1. So, you're not going to end up with a 50% error. If it was an accounting error, maybe - But I'm sure the FBO checked their records before calling you. In addition to the "how many gallons was this fill" meter, all the trucks have a small "how many gallons has this truck ever pumped" meter that was used to ensure that there were no accounting errors.

Pay the bill.
Click to expand...

The question of paying the bill is trivial

FlyNE said:
I would trust their dispensing equipment before trusting your little glass tube. Those things are worthless. Does the bottom of the tank slope? Were you on the downhill side or the uphill side? That's probably a gallon difference in the measurement right there.

As others have pointed out, there is no way that you only burned nine gallons in two hours of flight.

Sorry if this sounds harsh, but I'd seriously revisit your fuel measurement techniques before putting your family back in the airplane. It doesn't sound like your assumptions are correct and you're making decisions based off of them. Somewhere along the way you probably had 11-12 less gallons of fuel than you believed that you did. That is an hour of assumed range that just disappeared on you.
Click to expand...

This is definately not trivial. :eek: What could I have done differently (besides realized that 4.5GPH is just flat-out wrong)? What do I need to learn that I do not presently know?
 
Now that I think about it, the initial 27Gal measurement was without anyone on-board. At minimum, I should have ensured that following measurements were under similar conditions.
 
Jaybird180 said:
This is definately not trivial. :eek: What could I have done differently (besides realized that 4.5GPH is just flat-out wrong)? What do I need to learn that I do not presently know?
Click to expand...
Don't ignore how LONG you've flown the aircraft. Don't rely on a single reference to make your fuel decisions.

1.) Time flown
2.) Power setting during that time
3.) Fuel measured via dip stick
4.) Fuel gauge indications

Those are all the tools you have. Use all of them. Learn the limitations of each and if something starts to not make sense FIGURE OUT WHY. You were only taking into consideration #3.
 
Thank you for your valuable comment Jesse. You made me realize that I omitted important information in my original post.

1- This is the first time I've flow an aircraft without a known "full" fuel load prior to takeoff
2- My CFI flew it immediately before I flew it and told me how much fuel should be remaining based on how much he started with and his usage
3- I arrived at the same measurement of 27 Gals, which confirmed my belief that I had 27gals
4- My preflight planning told me that I should be able to execute the round-trip flight without needing to refuel at the power setting I chose in advance with sufficient reserves
5- I altered my preselected power setting in-flight, climbed longer than I thought I would and decended enroute, which changed my calculations
6- I did recompute fuel requirements based on the changs above and needing to get night recurrent prior to heading home
7- Based on my calculations, if I had received 11 gallons or 20 gallons would not have affected flight safety, which is why when offered the choice, I chose to have them bill me at 11 gals instead of adding 9 more

With all this said, I contacted the fueler in effort to pay for the 9gals. I also spoke with the FBO manager who helped me realize that:
1- the change in passengers could have made the 9 gal difference
2- the fuel dipstick I could have used may have been the wrong one (even though it was in the airplane. I did not verify if it was a standard or extended dipstick.

Based on this information, considering every areas of possible error, how much fuel do you guys think I landed with (minimum)?
 
As the saying goes: "You don't know what you don't know". I have never received training on the use of a fuel dipstick. I figured, 'how hard can it be. Even a total idiot can't screw it up.'

Meet the idiot that may have.
 
This is one reason I stand by my aircraft as they are refueling the plane. I rent "wet" as well, but when on an XC I need to know how much fuel I've used. It's too important to leave to the line guys. Also, one man's "top off" is another man's "tabs". Finally, I can look with scorn as they spill fuel all over the wing. You can't do that when you're inside taking a pee.
 
Jaybird180 said:
As the saying goes: "You don't know what you don't know". I have never received training on the use of a fuel dipstick. I figured, 'how hard can it be. Even a total idiot can't screw it up.'

Meet the idiot that may have.
Click to expand...

I'm not sure about your model of Skyhawk, but the 172R has a spar of sorts right under the filling cap. You need to make sure that your dipstick is behind the spar and touching the bottom of the tank in order to get an accurate reading.

The 1979 182RG that I fly has no spar, so I think it's more of guess as to where to hold it to get an accurate reading.
 
Where I fly, the planes sometime lack a sump or fuel dipstick. They're cheap, so I bought my own set. One time, during preflight, I had to use my own fuel dipstick. Only later did I discover that the different 172s in the fleet have different tanks, and different fuel dipsticks...
 
In my 172n, my readings with the glass tube were very eratic. I could come close to guessing how much I would need by evaluating my recent flight (time, altitude, speed, #takeoffs etc). For a while I suspected the FBOs were being careless (or worse) and to them I now apologize. After I purchased a Fuel Flow Indicator I now know very closely how much I used and how much I will be buying. And the FBOs are quite good about putting in the right amount.

Pay the bill.
 
I also noticed erratic readings when dipping various 172 tanks. If anything more than local flight, or less than full tanks, I would dip until I had 3 readings the same. I noticed a lot of variation depending on where the tube bottomed out in the tank.
 
Anyone know the correlation factors between different fuel stick's calibration scales? I think this may have been the largest source of error.

A little surfing revealed that Cessna tanks varied over the years and that one needs to acquire the correct dipstick for that tank configuration.
 
Here's the way to do it:

10 Go to Home Depot or Lowe's and buy a paint stick and a permanent marker.
20 Run one tank completely dry. (Yep, in flight)
30 Land.
40 Put in 5 gallons.
50 Put the paint stick in, and mark the top of the fuel level on the stick.
60 While fuel is not overflowing go to 40
70 Put stick in flight bag.
 
Careful, Ed. Someone may take your seriously on Line 20, do it, crash and burn then the family will search PoA, find your identity and sue you.
 
Jaybird180 said:
Careful, Ed. Someone may take your seriously on Line 20, do it, crash and burn then the family will search PoA, find your identity and sue you.
Click to expand...

I said one tank. Not both tanks. I run one tank dry many times. Hell on one flight I ran 3 tanks dry.
 
Jaybird180 said:
I nominate Ed for Greatest Pilot in the World:D
Click to expand...

It was a 1000nm flight. Wanted to make sure I knew where all the fuel was, and how much I had left. Without a fuel totalizer I was off by one gallon on what I thought I had left vs what I actually did. Still an hour in the tanks when I landed.
 
EdFred said:
Here's the way to do it:

10 Go to Home Depot or Lowe's and buy a paint stick and a permanent marker.
20 Run one tank completely dry. (Yep, in flight)
25 Switch tanks
30 Land.
40 Put in 5 gallons.
50 Put the paint stick in, and mark the top of the fuel level on the stick.
60 While fuel is not overflowing go to 40
70 Put stick in flight bag.
Click to expand...

I fixed it for the lawyers
 
You can take two different pilots with the same dipstick and same aircraft and they'll come up with two different fuel totals. I've seen it happen.

As others have pointed out, it's just not that accurate of a tool.

No one here has asked the question that almost bit me, so I'll ask.

Did you remember to lean the mixture for cruise flight? The book numbers only apply for book power settings at manufacturer's recommended leaning.

There was another thread recently about fuel and I shared the story of landing with a significantly -- but not quite dangerously-so -- lower fuel level than I had anticipated.

It was because we were flying at a higher power setting than we knew, due to being around 3500' MSL under a cloud base vs. Our usual 7500' MSL and higher out here.

The aircraft (which we own and know it's fuel habits) happily sucked down an additional 2 gal/Hr, just as it should have.

Landing with 10 gal left in 75 gal tanks, when you expected at least 13-17, and you know the aircraft well -- is not a great feeling.
 
I flew across the country and back in a T-41B, 46 tach hours and just a little over 500 gallons at 11gph avg.

I either refueled myself or stayed with the airplane while the line crew serviced the airplane. Filling is either 23 or 26 gallons per side, depending on the patience of the line crew. It helped that we always topped off, and some times filled the 17 gallon fuselage tank.

The 3 gallon per side difference on fill up is 30 min of flight time. It makes a difference when you are planning that legal VFR reserve on a super long leg through the mountains.

Point to make. I never leave the airplane until it is serviced and then I know what I got.
 
One more thing. There is dihedral. For a perfectly calibrated fuels stick to be close, the aircraft needs to be level. And no, one side does NOT cancel out the other. I haven't seen a level in a rental skyhawk for quite some time....
 
jesse said:
1.) Time flown
2.) Power setting during that time
3.) Fuel measured via dip stick (or topped off or tab referenced)
4.) Fuel gauge indications
Click to expand...

I've never met a fuel gauge that I trusted. But the first 3 items listed here make up for that.
 
bbchien said:
One more thing. There is dihedral. For a perfectly calibrated fuels stick to be close, the aircraft needs to be level. And no, one side does NOT cancel out the other. I haven't seen a level in a rental skyhawk for quite some time....
Click to expand...
Ah, a new use for my smartphone with a bubble level application!
 
bbchien said:
One more thing. There is dihedral. For a perfectly calibrated fuels stick to be close, the aircraft needs to be level. And no, one side does NOT cancel out the other. I haven't seen a level in a rental skyhawk for quite some time....
Click to expand...
Most of the Skyhawks I've seen were equipped with a nice level. It was right there on the panel and part of the rate of turn indicator. Of course some were mis-adjusted to show a tilt when the plane was level.
 
MarkL said:
I've never met a fuel gauge that I trusted. But the first 3 items listed here make up for that.
Click to expand...
I ran the right tank dry in the Cherokee on the way home today. The gauges in it actually seem pretty accurate. It quit about 5 minutes after teh gauge said 0 gallons. Throughout the whole range it seemed accurate. I was 1 minute off on my estimate of when the tank would be dry.
 
To answer the above, yes I leaned in cruise. I think I had more fuel than 27 to start with. The parking has the plane slightly nose up and the takeoff and climb performance was crap.

I went by the airport today to try and get more data for the mystery and the fuel stick that was in the back seat (presumably the same one I used, but no way to tell for they get swapped from plane to plane) is a 26.5 gal scale and the airplane is placarded on the wing for 27gal.

Then when I reported the poor climb performance I was told by a CFI that airplane "is a dog". The mechanic hearing the story told me that its heavier than the other P models due to the wing ribbing inside.

Nevertheless, I still don't know how much fuel I had but I know I need to call the fueler back...after I gave them my cc# for the other 9gals they double charged me....ah, the fun.
 
Was the Hobbs accurate?

I fueled and without checking did not notice the starting pump number, someone before me forgot to record it so I just finished and entered where I stopped. Got a bill for 27 gallons when I only pumped 8 My tank only holds 18 including the carb bowl. So they kind of took my word for it but something I will check now..
 
cgarr said:
Was the Hobbs accurate?

I fueled and without checking did not notice the starting pump number, someone before me forgot to record it so I just finished and entered where I stopped. Got a bill for 27 gallons when I only pumped 8 My tank only holds 18 including the carb bowl. So they kind of took my word for it but something I will check now..
Click to expand...
Believed to be.

Anyone would question that!!!

Welcome to PoA.
 
>> They say their pumps are calibrate and certified and they are inspected by the FAA.

That would be news to me... I'm not aware of any FAA function that inspects fuel pumps.

Paul
 
PaulMillner said:
>> They say their pumps are calibrate and certified and they are inspected by the FAA.

That would be news to me... I'm not aware of any FAA function that inspects fuel pumps.
Click to expand...

Yup - They're not inspected by the FAA. It's a state function. Here in Wisconsin, it's done by the Department of Weights and Measures, itself part of the Department of Agriculture, Trade, and Consumer Protection.

Your state may vary. ;)
 
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