A turbofan engine manufacturer specifies that the engine is 'flat rated' to an outside air temperature of 30°C. What does this rating mean for engine performance?
- AThe engine's N1 speed is fixed at 30 percent during all cruise operations
- BThe engine produces its maximum rated thrust only when the OAT is exactly 30°C
- CThe fuel control automatically limits thrust output so that full rated thrust is available at OATs up to 30°C, after which available thrust decreases as temperature rises further
- DThe engine maintains a constant EGT regardless of altitude up to 30,000 feet density altitude
Show answer & explanationAnswer & explanation
Correct answer: C. The fuel control automatically limits thrust output so that full rated thrust is available at OATs up to 30°C, after which available thrust decreases as temperature rises further
Flat rating means the fuel control unit limits fuel flow (and thus thrust) so that the engine produces its full rated thrust across a range of ambient temperatures up to the flat-rated temperature. Above that temperature, air density decreases enough that the engine can no longer maintain rated thrust without exceeding internal temperature limits, so available thrust decreases.
Why the other options are wrong
- A. N1 is not fixed at a flat percentage; thrust output, not N1 alone, is what's held constant.
- B. Flat rating applies across a temperature range, not only at exactly 30°C.
- D. Flat rating relates to thrust vs. temperature, not a constant EGT vs. altitude relationship.
Flat Rating
A design/control feature where the fuel control limits engine thrust so rated thrust is maintained across ambient temperatures up to a specified 'flat rate' temperature, beyond which available thrust decreases.
- Protects the engine from overtemperature on cold/moderate days by limiting fuel flow
- Rated thrust is constant below the flat-rate temperature
- Above flat-rate temp, thrust falls off to protect turbine limits
Memory trick: Flat rating = thrust plateau until the heat gets too much, then it slides downhill