A pilot is departing from a low-elevation airport on a cool day, then later departs from a high-elevation airport on a hot day. To achieve effective translational lift (ETL) at the high, hot airport compared to the low, cool airport, the helicopter will need:
- AThe same indicated airspeed, since ETL is a fixed IAS value
- BA higher true airspeed to compensate for the thinner, less dense air
- CA lower indicated airspeed because thinner air accelerates ETL onset
- DNo forward airspeed at all, since ETL depends only on power setting
Show answer & explanationAnswer & explanation
Correct answer: B. A higher true airspeed to compensate for the thinner, less dense air
ETL occurs at roughly the same indicated airspeed regardless of density altitude because IAS reflects the same dynamic pressure and mass airflow through the rotor disc. However, because the air is thinner at high density altitude, a higher true airspeed is required to achieve that same dynamic pressure and airflow, meaning the helicopter must move faster through the air to get the same aerodynamic effect.
Why the other options are wrong
- A. IAS for ETL stays roughly the same, but the question asks about the underlying true airspeed needed to reach that IAS.
- C. Thinner air does not lower the indicated airspeed at which ETL occurs; if anything, TAS must increase.
- D. ETL is airspeed-dependent, not purely a function of power setting.
ETL and Density Altitude
Effective translational lift occurs at a fairly consistent indicated airspeed, but in thinner air (higher density altitude) a greater true airspeed is required to reach that same indicated airspeed.
- ETL typically begins around 16-24 knots IAS
- IAS reflects dynamic pressure, which depends on air density
- Higher density altitude requires more TAS to produce the same IAS and airflow effect
Memory trick: Thin air makes you 'work harder and faster' (more TAS) to feel the same lift boost