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Terrain Flying AC 91-15, Part 2

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Over the years the FAA has published many outstanding advisory circulars. One in particular stands out in my mind. It's titled Terrain Flying (AC 91-15) and was last revised in 1967. It's no longer in print. It's the best accumulation of information I've ever seen on mountain flying route selection.

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I present Part 2 of this information knowing full well that you recognize it was last updated in 1967. Since terrain doesn't move (not even in shaky California), you can be pretty sure that comments regarding terrain are still valid. Any reference to cities and their populations are probably out of date. The information here is presented as a preliminary step in preparing a flight via any of the listed routes. It's your job to familiarize yourself with all the other available information necessary to conduct a flight safely (I sound like the FAA don't I?). In other words, you'll need to check and make sure these listed routes don't penetrate special use airspace as well as Class B, C, and D airspace. Hey, you're all big boys and girls and can figure this out easily. So, be careful. Additionally, don't run while holding sharp objects because if you fall you might hurt yourself. I think you see what I'm saying, right?

Of course, even though I'm showing the text of AC 91-15 in its original form, I can't help but make a few clarifications. Some of the syntax in this AC is strange and I can't help but comment and clarify, but I've tried to leave the document in its original form, unmodified, as much as possible. Any comments I make are included in square brackets [e.g.].

Excerpts from Terrain Flying (AC 91-15)

San Diego to Oregon State Line, Via Los Angeles

Pilots will normally experience good flying conditions along the coastal area in both summer and winter. Coastal fogs, prevailing between March and July, usually burn off by 10 a.m. or noon, and there are occasional high fogs during winter months [they're referring to mountain fog or upslope fog], suggesting a careful weather check. Smog may be encountered in the Los Angeles area [no kidding!]. Crossing the Tehachapi Mountains can be accomplished most any time during the day and most any time of the year. However, there are occasional thunderheads over these mountains in the summer and fairly heavy snow in the winter. Ideal flying conditions usually prevail in the San Joaquin-Sacramento Valley during the summer and winter, with a few low fogs in the winter [those fogs will also occur in the summer too!]

Experienced pilots who know the northern Rockies, the valleys and passes of the Cascades, the Siskiyous and the Bitter Roots give a general description of the outstanding topographic features of the Northwest States and explain the bearing this has on flying in general. [Here are the descriptions given by these experienced aviators.]

On the western edge of Oregon and Washington (Figure 1) there is an irregular, not too high, range of mountains known as the Coastal Range, topped to the north on the Olympic Peninsula by the very rugged and sparsely settled Olympic Mountains [remember, this AC was last updated in 1967!]. These are between Puget Sound and the Pacific Ocean just south of the Strait of Juan de Fuca. Through this Coastal Range between Oregon and Washing flows the mighty Columbia River. In southern Oregon this Coastal Range is fused with the Cascade Mountain by the Siskiyou Mountains.

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East of the Coastal Range from Vancouver, B.C., south along the edge of Puget Sound to Tacoma, Centralia, and Portland is a fairly wide valley known as Western Washington Valley, through which arterial highways and railroads run. South of Portland to a little below Eugene is the Willamette Valley. These two valleys vary in elevation from practically sea level to approximately 400 feet, except where the fringes of the Cascade Mountains project toward the west near Kelso and Castle Rock, Washington, and just northwest of Portland where the Coastal Range projects towards the Columbia River. South of Eugene to Medford and the east-west Siskiyou Mountains, the foothills of the Coastal Range and Cascades intermingle, which in some instances make for hazardous flying in bad weather. East of the valleys mentioned above is the north and south range of mountains, rugged and sparsely settled for most of its distances, known as the Cascade Mountains. This range of mountains has many prominent high peaks. Just south of the Canadian border is Mt. Baker, 10,778 feet, and east and southeast of Mt. Baker are many mountain peaks that rise up to an elevation of more than 8,000 feet. One of these, Glacier Peak, has an elevation of 10,568 feet. South of Mt. Baker and Glacier Peak near Seattle is Mt. Rainier, 14,410 feet, and south of Mt. Rainier are Mt. St. Helens 9,677 feet [I don't think it's quite as tall nor as wide now!] and Mt. Adams, 12,307 feet.

In Oregon south of the Columbia River, are Mt. Hood, Mt. Jefferson and Three Sisters, all of which extend above 10,000 feet and Mt. Thielsen and Mt. McLoughlin, both of which are above 9,000 feet in elevation. Through the Cascade Range of mountains are several passes that are used by railroad, highway and flyways: Stevens Pass, between Wenatchee and Everett; Snoqualmie Pass, east of Seattle to Yakima and Wenatchee; the gorge following the Columbia River; and the McKenzie River Pass, east of Eugene.

East of the Cascades in Washington, the Columbia River Basin extends eastward almost to the Washington-Idaho line. To a larger degree this area is agricultural and flat open country. East of the Cascades in Oregon the land is much higher, with some mountainous areas extending northeast to the Wallowa and the Blue Mountains of eastern Oregon.

In northern Idaho and western Montana there are many mountain ranges running generally northwest-southeast that make up the great Rocky Mountain chain, and consist of some rugged terrain and sparsely settled areas. Through these chains of mountains there are few passes. The main pass follows the Coeur d'Alene, St. Regis, and Clark Fork River Valleys and takes you over Mullan Pass, Superior, and Missoula Valleys. In south central Idaho there are some very rugged mountains that branch off from the Bitter Root Range and cover a vast area. Included in the general "primitive" area are some desert and lava-bed areas. In the Bitter Roots near Butte, Montana, and to the north is the Continental Divide. From the Divide eastward, the territory generally sweeps out into high plateaus except for scattered high mountains such as those north of Yellowstone Park, southeast of Great Falls and south of Billings where the Big Horn Mountains extend from Montana into Wyoming.

In southeastern Idaho the mountains are offshoots of more extensive mountains of western Wyoming which include the Tetons and Wind River Range in Wyoming, and the Wasatch Mountains of northeastern Utah.

These mountains make for rather easy navigation. But the weather is strange there and the rugged and sparsely settled areas make it necessary that the pilot be versed in more than just piloting ability. Knowledge of topography is very important.

The pilot flying in either summer or winter in this area must know the fundamentals of meteorology. The topography has a lot to do with the variable weather conditions.

There can be several types of weather prevalent in these regions at the same time say the experienced pilots. There can be valley or coastal fog west of the Cascades, even rain, and clear weather east of the Cascades, with a still different kind of weather in the mountains of Idaho and Montana. Many times during the summer in the mountainous regions, particularly those of Montana and Idaho, there can be severe turbulence from thunderstorms often accompanied by hail; and it is not always possible to accurately forecast hail. Icing conditions can be found either in summer or winter and this is of special importance to any pilot.

The greatest problems for the inexperienced pilot in this Northwest Country are those associated with the weather, and the pilot must finally make his own decision. He [she too] should be willing to wait for improvement in conditions where he feels unsure, and must learn to turn back a little sooner than necessary if he encounters worse weather than anticipated enroute.

Experienced pilots warn against what they call a fairly prevalent practice in this area of flying "on top." This leads to numerous incidents and a few accidents each year when pilots either become lost or are unable to get down VFR. Conditions should be carefully evaluated before venturing on top in this mountainous area for extended flight. Don't permit yourself to get into a situation where you run "fresh out of experience." Weather at [your] destination can change quite rapidly or the tops can easily build to heights beyond the altitude performance of the aircraft and the pilot. Extended flights above 10,000 feet without the use of oxygen should be avoided. With the loss of an engine while flying on top, even in a light twin-engine aircraft may be unable to maintain an altitude above the clouds and thus force the pilot into IFR conditions.

Here is a list of eight flyways that are usually followed in this area.

Route 1: Salt Lake City to Pendleton, Via Ogden, Malad City, Burley, Twin Falls, Boise, Baker and LaGrande (Figure 2) is ordinarily a good flying route. You may want to take a lower altitude route; in this case, fly north from Ogden via Snowville, Strevell and Malta. The area in southern Idaho can be very hot in the summer, very cold in the winter and high winds often prevail. Check carefully on the weather over the Blue Mountains.

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Route 2: From Ogden through Great Falls to Cut Bank, Montana (Figure 3), is one of the most difficult airways in the United States in the winter. There is more snow at Idaho Falls than at any other airport in this area. High elevations mark this route from Ogden, Pocatello, Idaho Falls, Dubois, Whitehall, Butte, Helena, Great Falls, to Cut Bank. The airport at Butte is at 5,554 feet elevation and Monida Pass is almost 7,000 feet.

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Route 3: From Sheridan to Great Falls, via Billings and Lewiston (Figure 4) is a route with conditions similar to those already described. From Billings to Lewiston the airways go to the east of the Big Snowy Mountains. However, the easiest and lowest elevation is along a route west of the Big Snowy through Judith Gap and Buffalo. From Lewiston to Great Falls, generally follow the course of the railroad over not too rugged terrain, but at an altitude of about 4,200 feet.

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Route 4: From Butte and Helena to the west (Figure 5), go via Deer Lodge, Garrison, Drummond, Superior, St. Regis, Mullan Pass, Kellogg, Coeur d'Alene, and Spokane.

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Route 5: California to Oregon or Washington, via Red Bluff, California, into Klamath Falls or Medford, Oregon (Figure 6), depending on whether the destination is to be east or west of the Cascades. The most general route is Medford, Eugene, Portland, Tacoma, Seattle and Bellingham. In winter, bad weather often exists just south of the Oregon line over the Siskiyou Mountains and in the Willamette and West Washington Valleys. In most cases, the weather is better east of the Cascades.

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Route 6: The route from Klamath Falls to Bend, Redmond, the Dalles, Yakima and Ellensburg is generally open and flyable. In the summer the temperatures on this route can get quite high. It is generally advisable to go north between Mt. Lassen and Mt. Shasta, with Shasta and the Cascades on the left. Always check the winds at these high altitudes, especially in the summer.

Route 7: Spokane, Ephrata, Ellensburg, Easton and Seattle is a route over fairly flat country and generally flyable.

Route 8: Between Ellensburg and Seattle there are many occasions when the weather east of the Cascades will be CAVU, with fog boiling over the Cascades and dissipating on their eastern slopes. Snoqualmie Pass on this route is at an altitude of about 3,500 feet and on many occasions fog from the west makes VFR flight impossible. You can get into trouble trying to fly over or under this fog. Many pilots turn south from Ellensburg to Yakima and follow the Columbia River Gorge into Portland and thence to Seattle. Or, when Snoqualmie is closed, Stevens or Stampede Passes might be used. In Oregon, pilots flying between Eugene and Redmond or Bend use the McKenzie river Pass. These last three named passes are not on the airways and pilots should approach and fly through them with care and with sure knowledge of the weather.

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Experienced pilots have a few words of caution for those who plan to fly the Columbia River Gorge route.

They tell us that the Columbia River Gorge route is only safe when the right weather conditions exist. Many pilots have become involved in accidents while attempting flights through the gorge when ceilings and visibility were too low.

Numerous power lines cross the Columbia River. Some of them are quite high above the water. During low visibility conditions, particularly when there is precipitation, it is almost impossible to see these lines. Before a pilot attempts a flight through the gorge, he should know the location of these lines and how high they are above the river.

A large amount of traffic uses this flyway when other routes are closes. The gorge is very narrow in some areas which make it difficult to reverse course. Because of these narrow areas, it also becomes increasingly important for the pilots to remain alert for airplanes coming from other directions.

For more information on this subject, see "Mountain Flying: A Primer For High Country Aviating" and "Above It All: Flying High In The Mountains."

Rod Machado

Rod Machado

Rod Machado is a flight instructor, author, educator, and speaker.

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