Fuel in W&B calculations

[anatole]

I've recently been given cause to re-evaluate what I was told about the Chieftain's fuel load with respect to weight and balance.

We have just had our aircraft re-weighted, and on the Passenger Manifest & Trim Sheet it has fuel listed as 'Main Fuel (112 USG max)' and 'Aux Fuel (80 USG max)'

In the AFM Weight & Balance Section it states there are 6 gallons not usable in mains, 4 gallons not usable in the aux's.

When the aircraft was weighed it had the weight of the above 6 + 4 gallons unusable added back after it was weighed (with all fuel drained out) to determine the Basic Empty Weight.

My question is: if one was to enter 112 USG in on the Trim Sheet, would it be counting the unusable fuel twice? Seeing it was already added back and is counted in the BEW.

If this is the case, shouldn't the Trim Sheet be stating 'Mains Fuel (106 USG max)' and 'Aux Fuel (76 USG max)'?

What does everyone else's look like?!

Thanks

[klausnw]

When I weigh aircraft, the empty weight and center of gravity includes the unusable fuel. The flight sheet W&B will use the 106 and 76 gallons in your case. If it is unclear whether or not the empty weight and C.G. include unusable fuel, operating oil, all the seat pocket items, manuals and fire extinguisher have it reweighed.

Commercial twins are required to be reweighed each 36 months. The pilot doesn't want to second guess what items where on the weigh sheet each flight, so I make it very clear during the weighing.

[klausnw]

pa31typeclub.proboards.com/thread/104/example-pa31-350-weighing-form

[anatole]

Maybe I was a bit too convoluted...

Aircraft is drained of all fuel.

Full oil, hydraulic fluids etc.. usual drill

Aircraft is weighed.

60lbs (10USG) unusable fuel is added back in as per 'Weighing Procedure' in PA31-350 AFM

In our Trim Sheet the tank capacities seem to include this 10 USG. If pilot enters the full amount (112 gal in the mains, 80 gal in the auxs) is he counting the unusable twice?

Below is attached the sample W&B from the AFM itself. As can be seen it actually has the weight of all fuel including unusable in the AUX's, but then the chart below listing capacity and weight only goes up to the useable fuel.

[klausnw]

I think you have run across another example where the 50 year old manuals have inaccuracies. Don't over study the manuals examples and go with what the last Weight and Balance report says. Very common for the manuals to have a couple revisions or not revisions, that conflict with each other.

If the aircraft is used commercially the FAA has the Operator create their own General Operation Manual (GOM) just because of these common errors in old publications. Fifty years ago making changes to a manual was complicated and expensive compared to today's digital documents.

Only use the documents created for the serial number aircraft you are flying. If the W&B document says the unusable fuel is added during the weighing then only add usable fuel to your calculations.

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