WX BRIEF

Mountain flying hazards

Clear air turbulence, severe wind conditions, rime and clear ice can accompany spectacular views.

By Karsten Shein
Comm-Inst, Climate Scientist


Airbus ACJ pilot transitions to a higher altitude above the Swiss Alps. Mountain waves and other alpine weather can affect the atmosphere thousands of feet above the tops of the highest peaks.

An aerial view of rugged mountain terrain is an amazing sight. Whether it’s the raw granite faces of the Dolomite Alps, a smoking caldera in the Cascades, glaciers high in the Andes, or even the hazy and forested ridges of the Smoky Mountains, the view from altitude is often far more magnificent than from any hiking trail or mountain parkway.

But a flight over the mountains may also evoke a feeling of primal frontier exploration—facing unknown dangers hidden behind stalwart peaks, with little chance of rescue should anything go wrong.

Not least among such worries is the mountain weather. Those who fly the mountains regularly know very well how quickly the weather can turn and bite an unprepared pilot. Some of the more common weather with which a mountain-flying pilot must contend are fog, windshear, thunderstorms and icing.

Even relatively low-altitude mountain chains can generate weather conditions that can overpower a small aircraft. Unfortunately, every year a few pilots fail to give mountain weather the respect it deserves.

At the least, they may end up in a forced landing or running off the runway—but in too many cases the flight ends in a CFIT accident that may claim the occupants’ lives. For example, on May 12, 2008, the pilot of a Piper PA32-260 took off from FQD (Rutherfordton NC) in the predawn hours for a flight to ORK (North Little Rock AR).

About 30 min into the flight, over the undulating Ap­palachian mountains of western North Carolina, the AVL (Asheville NC) radar indicated the aircraft passing over the airport at 9200 ft. Over the next 10 minutes the Piper exhibited rapid fluctuations in altitude, at one point descending 1000 ft in just 10 sec.

Before contact was lost, the radar showed the aircraft at 5800 ft—near the tops of several peaks in the area. The wreckage of the Piper was found on the south side of Cold Mountain (the same one of movie fame) at 4700 ft elevation.

While the pilot had checked the weather on the Internet the night before the flight, he had neither obtained an approved weather briefing nor filed a flightplan, even though IFR conditions prevailed over much of the area.

Most important for the pilot, however, was the fact that the winds—observed at AVL around takeoff time—were from 330° at 22 kts with gusts to 31, and the altimeter was 29.67 in of mercury.

The current surface analysis chart from the National Weather Service showed a very strong air flow from the northwest over the region, and an Airmet had been issued the day prior for moderate turbulence and high winds over the area.

Furthermore, model estimates of the 7000-ft winds aloft placed them out of the northwest at about 60 kts. Had the pilot been trained in obtaining weather information over the Internet, he might have been able to obtain all this information for himself.

But much of this information is not readily accessible or interpretable except by meteorologists or dedicated flight briefing services such as DUATS—which is part of why an approved briefing is so important.

A look at the map of western North Carolina reveals that the mountains track from the northeast to the southwest. A strong flow from the northwest would flow perpendicular to the axis of the mountain range and set the stage for significant mountain wave development.

A review of the weather maps for the day of the accident clearly shows a relatively strong late spring midlatitude cyclone spinning away to the north of the flight. The wind flow around the low is counterclockwise and the low pressure at AVL indicates that the pressure gradient—the change in pressure over a distance and the source of wind—was strong.

Forecasts for the movement of the system also showed that the storm would move out later that day and that the winds would diminish over the mountains. Had the pilot talked to a briefer, he might have delayed his departure by a few hours and possibly avoided the crash.

Mountain waves

The weather phenomenon that brought down the PA32-260 is one that has brought down scores of aircraft over the past century. It is one of the most dangerous weather-related aspects of flying in proximity to mountains.

When a strong wind flow encounters a mountain, it is forced to rise up the windward side of the terrain. In order to accommodate the additional air molecules, the converging air must accelerate.

The result is fast-moving air flowing over and around the peaks and ridges of the mountains. On the lee side of the mountains, the flow exhibits a behavior very similar to the flow of a river downstream from a rock around which it has had to flow.

Stack of lenticular clouds shrouds the peak of Mount Jefferson in Oregon, while ragged rotor clouds surround the lower slopes. The pattern of a standing mountain wave is clearly apparent in the overlying cloud deck, and is a visible indicator of potentially extreme turbulence in the vicinity.

In the immediate vicinity of the lee slope, the air, which became much denser as it was compressed around the obstacle, sinks down the lee slope. This immediate downdraft can be very strong and is often accompanied by a turbulent eddy as the flow surrounding the peak wraps around the lee face.

Further downwind, the air that has been displaced going over the mountains is set up into a wave pattern that dissipates gradually as the flow departs the range. These mountain waves are often called standing waves because the crests and troughs do not migrate downwind with the flow, but rather appear stationary with respect to the mountains that generated them.

This is a result of the mountain being in a fixed position, so the first wave will always crest right above it. Often there are flattened, lens-shaped (lenticular) clouds stacked up around the wave crests that give an indication of the presence of a standing wave.

There may be a ragged ball of cloud beneath the wave crests as well. These rotor clouds form when the pressure in moist air is lowered as a result of an eddy vortex occurring due to the windshear of the standing wave flow above it. A rotor cloud is a clear indicator of severe to extreme turbulence beneath the wave.

In many locations, the air flowing over a mountain range is too dry to form clouds. As a result, the standing waves and any rotors beneath them may be invisible to aviators. This isn’t to say that the flow doesn’t contain the same potential dangers, and caution must be exercised. In addition, depending on the slope and elevation of the terrain, and the speed of the air flow, a standing wave can crest many thousands of feet above a major obstacle.

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