Mountain wave hazards

Downdraft severity (AIR 1.5.7):

• Worst near the mountain, at about summit height.
• Up to about 83 ft/s (5,000 ft/min) lee of high ranges.

No-warning danger:

• Wave can ride smooth despite severe up- and downdrafts.
• Night, overcast, or no distinctive cloud: no warning, worse (AIR 1.5.7).

Altimeter errors:

• Smooth downdraft: altimeter and rate-of-climb may briefly show a climb (AIR 1.5.7).
  • Cause: local pressure drop lags true altitude loss.
  • Descent unnoticed until aircraft passes back through original pressure level, closer to ground.
• Pressure drop not local: extends throughout the wave, downwind and well above mountains.
• With non-standard temperature, altimeter may overread by up to 3,000 ft (AIR 1.5.8).

In-flight indications (RAC 11.7.12):

• Altitude excursions and airspeed fluctuations, with or without felt turbulence.
• Pitch and trim changes needed to hold altitude.
• Turbulence light to severe, by wave magnitude.

Trap: smooth ride or clear sky with no lenticular cloud does not mean no wave or downdraft.

Practice