Page 20 - July 2015 Volume 9, Number 7

Ask the Expert
That Pesky Fuel Arm
by Tom Clements
Throughout my many years of conducting initial and recurrent King Air training courses, a topic that consistently stirs some consternation is the fuel calculations associated with the Weight and Balance form. The problem arises because the fuel tanks in a King Air do not all reside at the same location in the wing. Hence, there is not one location – one “arm,” the distance aft of the Weight and Balance (W & B) datum point – that applies to all of the fuel.
In fact, the W & B section of the POH does not even list a fuel arm! It lists arms for every other cockpit and cabin option – pilots, passengers, baggage, cabinet contents – but no fuel arm is shown. Huh? Then how do we do our W & B calculations?!
Not to worry. We need an arm to be able to calculate the fuel’s moment, using the familiar formula, Moment = Weight x Arm. Instead of giving the Arm, Beech simply
provides the associated Moment for each different Weight of fuel on board. If we wish to actually know the Arm – and there is really no need to know that, other than for curiosity’s sake – then we can rearrange the formula to see that Arm = Moment/Weight. Let me give you a couple of examples, using the fuel in a King Air model 200 as an example.
A chart in section six of the POH shows that 200 gallons of Jet A fuel, with a density of 6.7 pounds per gallon, gives a weight of 1,340 pounds and a moment of 2,437. However, there is a little gotcha here: 2,437 is not the moment, but is the moment divided by 100.
Back in the day when none of us carried around a calculator in our smartphone, and we preferred to avoid the laborious task of doing longhand multiplication and division, Beech did the work for us by providing charts that gave the Moments for different Weights and the
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18 • KING AIR MAGAZINE
JULY 2015