FAA Private Pilot Helicopter (PRH)WeatherHard
A helicopter pilot is planning a flight over mountainous terrain where winds are forecast to blow perpendicular to a ridge line at 30 knots. What hazard should the pilot be most concerned about on the leeward (downwind) side of the ridge?
- AAdvection fog development
- BMountain wave turbulence and strong downdrafts
- CRadiation fog formation
- DIncreased density altitude only
Show answer & explanationAnswer & explanation
Correct answer: B. Mountain wave turbulence and strong downdrafts
Strong winds blowing perpendicular to a mountain ridge can create mountain wave turbulence on the leeward side, producing severe turbulence, strong downdrafts, and rotor clouds. This is a significant hazard for helicopters operating at lower altitudes near terrain.
Why the other options are wrong
- A. Advection fog requires warm moist air moving over a colder surface, unrelated to this wind/terrain scenario.
- C. Radiation fog requires calm, clear nighttime conditions, not strong perpendicular winds over terrain.
- D. Density altitude is affected by temperature and elevation, not primarily by wind flow over terrain.
Mountain Wave Turbulence
Turbulence that develops on the leeward side of mountains when strong winds (typically 25+ knots) blow perpendicular to a ridge, causing oscillating wave patterns, severe turbulence, and strong downdrafts.
- Requires strong wind (usually >25 kt) roughly perpendicular to ridge
- Produces rotor clouds and lenticular clouds as visual clues
- Downdrafts can exceed the climb capability of aircraft
Memory trick: Wind over a mountain ridge rolls like ocean waves crashing downwind