Work out engine power at altitude instantly with clear inputs, formula shown and shareable results.
A normally aspirated piston engine loses power in proportion to air density. The Gagg and Ferrar relation, power = rated x (sigma - 0.117)/0.883, matches test data well and shows why such engines make only about 75% of rated power at 8,000 ft and cannot maintain cruise power much above 12,000 ft.
Power lapse
P = P0 x (sigma - 0.117) / 0.883, sigma = density ratio
Aviation results are planning estimates only. Always verify take-off, landing, fuel, weight and balance figures against the certified Aircraft Flight Manual, current performance charts and applicable aviation regulations before flight.
No. A turbocharger maintains manifold pressure up to its critical altitude, so power stays roughly constant until that height and only then lapses.
Friction losses inside the engine do not fall with altitude, so the usable power falls slightly faster than air density does.