Good weather briefing of vital importance


Mountain flying brings up visions of very high peaks, mostly in the west of Canada or the United States. Mountain flying techniques should also be used when flying around any area where there are large hills or valleys anywhere in North America.

Weather is the biggest hazard while flying in hilly or mountainous areas. Studying the weather is therefore of vital importance when planning to fly in these areas. Flying techniques are important, but if our weather knowledge is inadequate, good technique may just get us into more trouble.

Too many of us rely just on TAFS and METARs for our weather information. TAFs and METARs only give us information about the five nm radius of the airports where we are interested in flying into or out of.

Before flying over mountainous terrain or in valleys on bad weather days, special care must be taken to get a good weather briefing, which includes an Area Forecast. Graphic Area Forecasts read from a computer, or full briefings by a Flight Information (FIC) Specialist, are the only ways pilots can find out what might occur on the route of flight and find out what to be prepared for.

Possible frontal systems, thunderstorms, low ceilings and showers along or near the route of flight are items to watch for in a briefing. When flying valleys, approaching frontal systems and thunderstorms may not be seen until they are very close, leaving little time to turn around.

Low ceilings may initially leave room below for safe flight, but often drop gradually, without us noticing, reducing the manoeuvring room available. Ceilings and visibility’s can drop suddenly when it starts to rain or snow. A sudden shower in a valley can cut off all exits. Ceilings can drop 2,000 feet and visibilities can drop two miles or meteorological conditions) instantly.

The CARs tell us it is legal to fly in uncontrolled airspace with 500 feet vertical distance from cloud and with one mile visibility above 1,000 feet and with two miles visibility below 1,000 feet. As we can see, being legal when flying under low ceilings with the possibility of showers around, is not necessarily safe.

Winds cause multiple problems in the mountains. They tend to follow valleys, but they can’t bend as quickly, causing turbulence. As valleys narrow, the wind speed increases (Bernoulli’s Principle) causing turbulence. Winds that do cross valleys cause downdrafts on the upwind side of the valley, updrafts on the downwind side of the valley and often turbulence in the centre of the valley.

Again, it is normal practice to fly on the right side of a valley. If there are strong winds however, fly on the side of the valley where the winds are going up the mountains and watch carefully for opposite direction traffic.

Winds of 25 kts or greater in the mountains can cause strong downdrafts, updrafts and turbulence from the bottoms of the valleys to more than 2,000 feet above the tops of the ridges.

The downdrafts may exceed the climb capability of even medium size aircraft.

It is therefore recommendedthat ridges be crossed with at least 2,000 feet clearance in strong wind conditions. It is also recommended that when climbing over a ridge in windy conditions, the ridge be approached at a 45-degree angle or less so that a turn away from the ridge may be quickly made if a downdraft is encountered.

Winds of 50 kts or greater can produce mountain waves, although mountain waves have been known to develop with wind speeds of as low as 25 kts. Aircraft caught in mountain waves will climb and descend with the rising and descending air. The mountain wave activity may reach to very high altitudes, and may extend as far as 100 nm downwind of a mountain range. Mountain waves may or may not be turbulent.

The downdraft side of the mountain wave may reach speeds of 5,000 feet per minute. The increase in wind speed in a mountain wave downdraft will result in a significant pressure drop causing the altimeter to over read by as much as 3,000 feet. An aircraft may therefore descend a considerable distance (3,000 feet) before either the altimeter or the vertical speed indicator indicates the descent.

It may be possible to climb out of a mild mountain wave. Most often, it is not. Fly the aircraft attitude and pay attention to the airspeed to avoid a stall and to avoid over speeding the aircraft.

Telltale signs of mountain wave activity are lenticular, rotor and cap clouds.

We normally fly on the right side of the valley to avoid other aircraft that may be in the same area. This doesn’t always allow us to see around the next bend in the valley. If a right turn is coming up, we should move to the left side of the valley as far as is comfortable and peer around the corner before making the turn to avoid running into an unseen cloud bank.

Care must also be taken when operating from runways in the mountains, as wind shears, turbulence, updrafts, or downdrafts may be present near the runways and the surrounding mountains may obscure rapidly approaching frontal systems and thunderstorms.

Flying is more fun than studying the weather. A good weather watch however, will help you avoid those “I wish I was on the ground” moments and make flying more fun.

Dale Nielsen is an ex-Armed Forces pilot and aerial photography pilot. He lives in Abbotsford, B.C., and currently flies medevacs from Victoria in a Lear 25. Nielsen is also the author of seven flight training manuals published by Canuck West Holdings.