Fuel Grades, Additives and Capacity

Two wing tanks, two approved grades, and a refuelling technique that decides whether you actually get the capacity the book promises. The numbers here are the ones you plan a flight against; the technique is the one that makes the numbers true.

At a glance

• Approved grades100LL (blue) and 100 (green)
• Total fuel capacity56.0 U.S. gal
• Total usable fuel53.0 U.S. gal
• Capacity of each tank28.0 U.S. gal
• Usable fuel in each tank26.5 U.S. gal
• Maximum isopropyl alcohol additive1%
• Maximum DiEGME additive0.10% to 0.15%

The two approved grades

Only two aviation fuels are approved: 100LL grade, dyed blue, and 100 grade, dyed green. The colour is how you confirm at the pump and again in the sampler cup that you have been given what you asked for.

WARNING — Using a fuel that is not on the approved list may damage the engine and the fuel system components, and that damage can end in engine failure.

Anti-icing additives and their ceilings

Two additives are permitted, each with a concentration ceiling. Isopropyl alcohol must not exceed 1% of the fuel, and diethylene glycol monomethyl ether — DiEGME — is held between 0.10% and 0.15%. These are limits, not targets: the ceiling exists because more additive does harm of its own.

Capacity, and the three gallons you never see

Each wing tank holds 28.0 gallons, giving 56.0 gallons total. Of that, 53.0 gallons is usable, so three gallons — one and a half in each tank — is fuel the system cannot deliver. Plan against the usable figure and treat the total as a servicing number only.

Getting the full 56 gallons in

Capacity is only theoretical if the aeroplane is filled in the wrong attitude, because fuel crosses between the tanks through the system while the filler cap is off.

Refuelling for maximum capacity

1. Attitude — park the aeroplane wings-level in the normal ground attitude.

2. Fuel selector — place it in the Left or Right position, which minimises cross-feeding between the tanks while you fill.

Check yourself

• Name both approved fuel grades and the dye colour of each.

• What is the difference between total capacity and total usable fuel, and how many gallons is that difference?

• State the maximum permitted concentration for each of the two approved fuel additives.

• What two things must be set before refuelling if you want the full capacity in the tanks, and why?

• How much usable fuel does a single tank hold?

Study aid — always fly your aircraft's current AFM/POH.