Specific Range
Distance flown per unit of fuel; maximizing it gives best fuel economy.
Formula
Variables
Example
Flying at the speed that maximizes specific range gives the most distance per kilogram of fuel.
How it works
Specific range tells you how far an aircraft travels for each unit of fuel burned. It is found by dividing ground speed by fuel flow because the numerator measures distance covered per hour and the denominator measures how much fuel is used per hour, so the result is distance per unit fuel. In practice, a higher ground speed helps, but only if the extra speed does not increase fuel burn too much; the best specific range occurs at the speed where that ratio is largest.
When to use it
Use this when planning long flights, comparing cruise speeds, or choosing an operating point that gives the greatest distance for a given fuel load. It is especially useful in performance calculations for flight planning, endurance, and fuel-saving strategy.
Worked example, step by step
Given: Ground speed, GS = 420 kt Fuel flow, \dot{f} = 7.0 lb/min Step 1: Make sure the units match the desired answer. Since 1 knot = 1 nautical mile per hour and fuel flow is per minute, convert fuel flow to lb/hr: 7.0 lb/min × 60 min/hr = 420 lb/hr Step 2: Apply the specific range formula. SR = GS / \dot{f} SR = 420 nm/hr ÷ 420 lb/hr Step 3: Simplify. SR = 1.0 nm/lb Final answer: The aircraft’s specific range is 1.0 nautical mile per pound of fuel.
Common mistakes to avoid
- Using indicated airspeed instead of ground speed when the aircraft is affected by wind
- Forgetting to convert fuel flow into matching time units before dividing
- Assuming the highest speed always gives the best range, even though fuel flow rises with speed
Did You Know?
Specific range — distance per unit of fuel — is an airliner's equivalent of miles per gallon.
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