FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsMedium
A pilot studying the height-velocity diagram ('dead man's curve') for a specific helicopter notes a shaded avoid area at low altitude and low forward airspeed. What is the primary reason this combination is hazardous?
- AGround effect at this height causes excessive lift, leading to loss of control
- BThere is insufficient altitude and/or airspeed to successfully complete an autorotation if the engine fails
- CDynamic rollover risk is highest at this combination of altitude and airspeed
- DThe rotor system is most susceptible to retreating blade stall in this speed and altitude range
Show answer & explanationAnswer & explanation
Correct answer: B. There is insufficient altitude and/or airspeed to successfully complete an autorotation if the engine fails
The height-velocity diagram identifies combinations of altitude and airspeed in which, if the engine fails, the pilot lacks either enough altitude to gain the necessary airspeed and rotor energy, or enough forward speed to cushion the landing, making a safe autorotative landing very difficult or impossible.
Why the other options are wrong
- A. Ground effect is generally beneficial for lift, not a hazard causing the avoid curve.
- C. Dynamic rollover relates to ground contact with a pivoting skid/wheel, not airborne height-velocity combinations.
- D. Retreating blade stall occurs at high forward airspeed, unrelated to the low-altitude/low-airspeed shaded area.
Height-Velocity Diagram
A performance chart showing combinations of altitude and airspeed where a safe autorotative landing following engine failure may not be possible, often called the 'dead man's curve.'
- Shaded areas represent unsafe altitude/airspeed combinations for engine-out landings
- Low altitude/low airspeed is dangerous because there's no time to build rotor RPM or gain airspeed
- High altitude/low airspeed is also hazardous because it takes time to accelerate to a safe autorotation speed
Memory trick: Dead man's curve: low and slow leaves no room to recover.