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Owner's Corner

OWNER'S CORNER
Lloyd Stephens, Aircraft Owner WVFC lgs@qnet.com  

Mountain Winds





Lenticular clouds indicative of a mountain wave near Bishop, CA. The mountains in the photo are about 10,000 feet, and the cloud bases are about 13,000 feet.

 

As most of us know, March and April are usually good months for kite flying because the days are frequently windy.  Here in the Bay Area we don’t think too much about wind.  Oh, there are those days when the wind isn’t right down the runway, so you might need some crosswind skills, or is strong enough that it can cause some pretty good turbulence, but generally not too many days when the wind makes flying a small plane almost impossible, unless you literally want to risk your life to do so. 

For the last 15 years I have lived part time in the Eastern Sierra, between Mammoth Lakes and Bishop, and have flown extensively out of both airports, and I want to tell you that wind is a considerably different factor when you add in some mountains.  Winds in the Eastern Sierra frequently are strong enough to blow over trees, overturn semi trucks and trailers, and blow portions of the roofs off of buildings.  These are not winds that you want to fly in, particularly in a small plane.

I was once asked by a club member to tell him about the route that I use going to Bishop (essentially along V 230 and over Mammoth Pass, just south of Mammoth Mountain).  He was planning on going to Las Vegas that afternoon.  When I told him that I wouldn’t advise that because the forecast was for wind gusts to 65 mph over the mountain ridges, he said that wouldn’t bother him because he would be flying at 13,500 feet and would be considerably above the ridges.  I don’t think he quite got the concept of how strong the winds can be and how high they can reach.

I was thinking about this the other day when I was reading an article about a recent fatal crash in New Mexico where a Mooney M20E pilot from Texas took off from the Angel Fire airport in winds gusting to 55 mph with a crosswind that exceeded the published limit of the aircraft, after being warned by the airport manager about the winds.  Now the Angel Fire airport is located just to the east of Taos in a mountain valley between peaks that rise to more than 10,000 feet.  The airport elevation is 8380 feet.  There were four people in the plane headed back from a ski trip, so it probably didn’t have a lot of excess power at that altitude.  I don’t think this pilot got the concept of how dangerous this was either.  An article about this can be found at:

http://www.mysanantonio.com/news/local_news/article/Pilot-warned-about-flying-Sunday-4326894.php

Although the article focuses on the fact that the crosswind exceeded the capability of the aircraft, it didn’t appear to be a takeoff problem.  It appeared to me that the pilot just wasn’t able to handle the wind conditions once he got into the air.

So....a little discussion about mountain winds:  Winds generally flow west to east.  This is called zonal flow.  Most of the mountain ranges in California are oriented generally north-south.  That means that the airflow generally is perpendicular to the mountains.  Air flows like water.  It is smooth on the upwind side of an obstacle, and becomes considerably turbulent on the downwind side.  As the air flows upward over a ridge, some of the air continues on up (a mountain wave condition), and some drops sharply downward, forming rotors below the level of the ridge and significant downdrafts on the lee side.  In the Sierra, this flow pattern is enhanced by the fact that the western side rises gradually, while the eastern side drops off sharply.  The air also accelerates when forced between two obstacles, like mountain peaks, and through mountain passes, causing gusty wind conditions.  Even when the winds are light the turbulence can be moderate on the eastern side of the mountains, which is where the Mammoth airport is located.  In these conditions don’t plan to descend just after passing over the ridgeline. Maintain altitude until you are a distance away from the mountains. 

High or low pressure centers can change that flow.  The winds tend to rotate counterclockwise around a low center and clockwise around a high.  The air also tends to be pulled inward toward a low and away from a high.  So winds are not always zonal.  They tend toward southwest before a front passes and then may go east to northeast if the low pressure area drops southward and the winds wrap around the low back toward the west.  And as the low then moves further east, the flow becomes northwest.  Flow that parallels the mountain ridges can cause turbulence, but the turbulence is generally not as significant as that caused by a strong zonal flow on the eastern side of the mountains.  If the wind is form the east or northeast, though, turbulence and downdrafts can be expected on the western side of the mountains.

So far we’ve pretty much been talking about the winds over the ridges.  If you check the winds aloft forecast it can tell you a lot.  Look at both the force of the winds and the direction (generally given in true--not magnetic--direction).  I generally don’t want to fly over the Sierra if the winds aloft at 12,000 feet are in excess of 40 knots, and I like them to be under 30.  Look also at the winds at 18,000, though.  If there is a large discrepancy between the two--winds, for instance, at 30 knots at 12,000, but 65 knots at 18,000--don’t assume it will be fine at 12,000.  That would tend to show a pretty good mountain wave development.  Also, be aware that flight service (now located solely in Prescott) may not be aware of local conditions.  One of the pages that I frequently visit is the NWS page at:  http://www.wrh.noaa.gov/mtr/forecast.php  For trips to or over the Mammoth area, or to Tahoe, read the Reno area forecast discussion (AFD) and the appropriate zone forecast (Zone) for a better discussion than you will get from flight service.

Absent a storm condition, the surface winds are pretty predictable in the Eastern Sierra.  It is generally calm in the morning, but as the day heats up the surface winds start to pick up and become gusty in the late morning or early afternoon, reaching a peak about 4:00 pm before tapering off in the evening.  The best flying over there is in the morning as you avoid both the winds and the considerable thermal turbulence that develops on warm days.

I was once on a hike in the Little Rock Creek canyon on a pretty windy day and was able to observe a rare phenomenon.  The temperature was just right so that when the wind swirled around with the gusts that were hitting us, little tornado-like clouds, about 4 to 5 feet long were continually developing and disintegrating.  It was quite amazing, and a little scary, to be able to see exactly what the wind was doing.  I have never seen that before or since.

When flying, it is always a good idea to know from which direction the winds are coming and to visualize what is happening to the air, even if there are no clouds to make it visible.  Being able to do this when you are flying in the mountains is especially important.  This article doesn’t tell you all you need to know to fly safely in the mountains.  You are responsible for learning that.  It’s just something to make you think about winds in the mountains.  Your training and experience is what counts.  Check the forecasts, look carefully at the wind conditions, and evaluate whether you, and the plane, can handle those conditions, before you go.  Fly smart, and make good decisions.  This is a very appropriate quote from the airport manager at the Angel Fire airport after the accident there: “We assume that these pilots are smart enough to realize that they're not God. They can't do everything; the plane's only designed to do so much.”


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