Engine Performance (thrust, specific fuel consumption, ratings, derating)
What this domain tests
Engine Performance tests your ability to compute thrust and specific fuel consumption, interpret ratings, and explain how altitude, temperature, bleed, and derating change available thrust. Questions often give a short scenario and ask for the effect, not just the formula.
Core rules
- Net thrust is the usable thrust after ram drag; TSFC = fuel mass flow / net thrust. Units matter: kg/(N.h) in SI, lb/(lbf.h) in US customary.
- Takeoff thrust is flat-rated to a temperature; above the flat rating temperature, thrust falls with increasing OAT. At altitude, lower ambient pressure reduces mass flow and thrust.
- Derating reduces takeoff thrust to extend life. Assumed-temperature derating uses a fictitious higher OAT; bleeds and accessory loads also reduce available thrust.
- Ratings include takeoff, maximum continuous, climb, cruise, and idle. Maximum continuous is time-limited but not a takeoff rating.
Worked example
At a cruise point, fuel flow is 2,000 kg/h and net thrust is 20 kN. TSFC = 2,000/20,000 = 0.10 kg/(N.h). If the same engine produced 18 kN at the same fuel flow, TSFC would rise to 0.111 kg/(N.h), about 11% worse. The reasoning: SFC is inverse to thrust for fixed fuel flow, so a thrust lapse directly penalizes SFC.
Traps
Mixing gross and net thrust; using fuel flow in kg/s with thrust in kN without converting hours; assuming derating improves SFC; treating maximum continuous as a takeoff rating; ignoring bleed penalties.