The Mountain Pass And Ridgeline Venturi Effect

Mountain flying is one of the more challenging but rewarding types of aviation available to most pilots. The scenery is magnificent, landing on remote strips affords access to some of the least-trammeled locations on Earth and its rare to find much of a crowd. Whole books-and careers-have been based on the skills and knowledge needed to fly mountainous terrain safely. Indeed, the hazards of mountain flying in personal aircraft go far beyond what is normally taught to "flatlander" pilots in ground school. Most mountain flying caveats dwell on the vagaries of leeward hazards such as downdrafts, waves, rotors and resulting turbulence. The effects of temperature and altitude on aircraft performance also are part of this equation, as is the reduced power available when a normally aspirated engine is taken up to around 10,000 feet msl and the pilot forgets proper leaning. This is not one of those articles.

Key Takeaways:

• Traditional mountain flying discussions often overlook the specific hazards encountered on the windward side of ridges and mountain passes.
• The Venturi effect causes wind-driven air accelerating over these geographic features to experience a significant decrease in pressure and air density.
• This pressure drop results in a false high altimeter indication, making the aircraft fly lower than indicated, and critically reduces engine power due to decreased air density.
• Approaching a pass or ridge with a tailwind amplifies these dangers, leading to unexpectedly shallow climb angles, limited abort options, and significantly higher impact energy in the event of a crash.

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