FAA Flight Instructor Airplane (FIA)Performance, Weight and BalanceHard
A pilot compares two cruise altitudes for a normally aspirated airplane. At 6,000 ft, the airplane achieves 140 KTAS burning 11 GPH. At 10,000 ft, TAS increases to 148 knots while fuel flow decreases to 10 GPH. Which altitude provides the better specific range (nautical miles per gallon)?
- A10,000 ft, at approximately 14.8 nm/gal
- BBoth altitudes are equal, at approximately 13.5 nm/gal
- C6,000 ft, at approximately 12.7 nm/gal
- D6,000 ft, at approximately 14.8 nm/gal
Show answer & explanationAnswer & explanation
Correct answer: A. 10,000 ft, at approximately 14.8 nm/gal
Specific range = TAS ÷ fuel flow. At 6,000 ft: 140 ÷ 11 ≈ 12.7 nm/gal. At 10,000 ft: 148 ÷ 10 ≈ 14.8 nm/gal. The higher altitude provides better specific range.
Why the other options are wrong
- B. Incorrectly averages the two values instead of comparing them.
- C. This is the correct value but for the wrong (less efficient) altitude.
- D. Mismatches the correct nm/gal value with the wrong altitude.
Specific Range
Specific range is the distance traveled per unit of fuel (nm/gal), calculated as true airspeed divided by fuel flow; it typically improves at higher altitudes for normally aspirated engines up to a point.
- Specific range (nm/gal) = TAS ÷ fuel flow (GPH)
- Higher altitudes often improve TAS while fuel flow may decrease slightly
- Better specific range means more distance per gallon of fuel
- Used to select the most fuel-efficient cruise altitude
Memory trick: 'Speed over fuel burn tells you the true bargain altitude.'