Also the amount the fuel tank can hold is not the same as the amount that can be used by the fuel system. Thus you have "actual fuel capacity" and "usable fuel capacity".
In addition, some documents list gallons of fuel and other use pounds of fuel. Since different variants of jet fuel weigh-in differently, one most be very specific when quoting "fuel capacity".
Per NAVWEPS 01-40AVA-1 for the A-4A/B:
Total internal fuel is 810 US gallons of internal fuel. Three external drop tanks can be carried to increase total fuel quantity to 1710 US gallons. Fuel is normally delivered from any drop tanks by tank pressurization, and from the wing tanks by an air-driven fuel transfer pump.
The engine fuel system consists of a fuel pump, fuel control unit, flow dividers, a fuel primer solenoid and two fuel primers. The engine fuel control consists of two systems, a primary and a manual for use if the primary fails. The two seat variants had a smaller fuselage tank due to the second seat.
All fuel is delivered to the fuselage tank (behind the cockpit), from which an electrically driven fuel boost pump (with flapper valves) delivers fuel under pressure to the engine-driven fuel pump. The Skyhawk can sustain approximately 30 seconds of inverted flight with this system. Both the fuselage tank and the integral wing tank are self-sealing.
Drop tanks carried on the stores racks are vented, contain provisions for gravity fueling, pressure fueling (not A-4A) and pressurization to effect fuel transfer to the integral wing tank at the option of the pilot. Drop tanks may be jettisoned in the same manner as other external stores.
- Fuel Flowmeter (A-4B): is located in the right console and reads in "pounds per hour". It is marked from 0 to 12 with intermediate markings between the numerals. Determination of fuel flow in "pounds per hour" is made by multiplying the instrument reading by one thousand.
- JP-4 = 6.5lbs per gallon
- JP-5 = 6.8lbs per gallon
- JP-8 = 6.7lbs per gallon
Using the "810/1710" US gallon figure quoted above:
- 237 gallons (gravity filled) in fuselage tank (tank volume = 240gal).
- 570 gallons (gravity filled) in wing tanks (tank volume = 585gal).
- 6 gallons of unusable fuel in wing tanks.
- 885 gallons in three gravity filled 300 gallon drop tanks (tank volume = 300gal each).
- 12 gallons of unusable fuel in the three drop tanks.
- Total is 1,710 gallons of fuel per "NAVWEPS 01-40AVA-1" for the A-4A/B
- Thus fuel quantity quoted in this document is "actual quantity" consisting of useable fuel and unusable fuel when it is "gravity filled". Not tank volume, and not pressurization-filled fuel in the Bravo.
- IF using JP-5, then 810/1710 gallons is equal to 5,508lbs/11,628lbs.