The Mountain Wave on the Lee Side
A pilot is flying a light single at 11,500 ft over a north-south mountain range with peaks at 9,000 ft. The wind at 10,000 ft is 35 kt from 270°, and the air is stable with a moist layer near the summit. The pilot notices a smooth, lens-shaped cloud downwind of the ridge, a roll cloud beneath it, and the aircraft begins to experience a steady updraught followed by a downdraught. The pilot's destination is 60 nm downwind of the range, and the aircraft has 2.5 hours of fuel remaining. The pilot's altitude is 11,500 ft and the minimum safe altitude over the destination is 5,000 ft.
1. Identify the phenomenon and explain the conditions that produce it.
Read the worked answer
The lens-shaped cloud, roll cloud and alternating updraught and downdraught are the signature of a mountain wave. The conditions are a strong wind (35 kt) blowing across the ridge at a significant angle, a stable layer near the summit that forces the air to oscillate rather than mix, and sufficient moisture to make the wave visible as lenticular cloud. The wave extends far downwind, often 50–100 nm or more, with the strongest downdraughts on the lee side of the ridge. The pilot should expect severe turbulence in the rotor zone beneath the roll cloud, and smooth but strong vertical currents in the wave itself. The wave can also produce a standing lenticular cloud that appears stationary while the wind blows through it.
2. What are the specific hazards and how should the pilot manage altitude and route?
Read the worked answer
Hazards include severe turbulence in the rotor, strong downdraughts that can exceed the aircraft's climb capability, loss of altitude control, and icing if the aircraft is flown into the lenticular cloud where supercooled water may exist. The pilot should avoid the rotor zone entirely, either by crossing the ridge at a safe height well above the rotor or by routing around the wave. If the aircraft enters a strong downdraught, the pilot should maintain a safe speed, accept the altitude loss, and turn away from the ridge rather than attempt to out-climb the downdraught. The pilot should also allow for the fact that the destination 60 nm downwind may still be within the wave, so the approach and landing could encounter turbulence and wind shear.
3. The pilot has 2.5 hours of fuel and the destination is 60 nm downwind. What factors determine whether to continue or divert?
Read the worked answer
The pilot should assess the strength and extent of the wave, the aircraft's ability to maintain altitude, and the fuel reserve. If the downdraughts are strong and the aircraft is losing altitude, the pilot should turn back or divert to an aerodrome upwind of the range where the air is smoother. Continuing downwind into a wave with limited fuel and a possible need to hold or divert is risky. The pilot should also consider that the destination may be experiencing the same wave, with turbulence on approach and a possible crosswind. A diversion to a nearby aerodrome with better conditions and a longer runway is preferable to pressing on. The decision should be made early, while fuel and altitude margin remain.