FAA Flight Instructor Airplane (FIA)Performance, Weight and BalanceMedium
A flight instructor is explaining engine performance to a student. The airplane's normally aspirated engine is rated at 180 hp at sea level under standard conditions. Using the general rule of thumb that a normally aspirated engine loses approximately 3% of its rated horsepower for every 1,000 ft increase in density altitude, approximately how much horsepower is available for takeoff at a density altitude of 6,000 ft?
- A171 hp
- B147.6 hp
- C126 hp
- D162 hp
Show answer & explanationAnswer & explanation
Correct answer: B. 147.6 hp
Applying the rule of thumb, power loss = 3% per 1,000 ft × 6 (thousands of feet) = 18% loss. Available power = 180 hp × (1 − 0.18) = 180 × 0.82 = 147.6 hp.
Why the other options are wrong
- A. Uses an incorrect 1%/1,000 ft loss rate, understating the power loss.
- C. Uses an incorrect 5%/1,000 ft loss rate, overstating the power loss.
- D. Uses an incorrect 2%/1,000 ft loss rate instead of the standard 3%.
Density Altitude and Engine Power Loss
A normally aspirated (non-turbocharged) reciprocating engine loses power as density altitude increases because thinner air reduces the mass of the fuel/air charge available for combustion.
- Rule of thumb: approximately 3% power loss per 1,000 ft increase in density altitude.
- Reduced power available directly increases takeoff and climb distances.
- Turbocharged engines can maintain rated power up to their critical altitude, unlike normally aspirated engines.
Memory trick: Think in thousands: 3% gone for each thousand feet you climb.