Terrain Flying AC 91-15, Part 1

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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.

I present this information knowing full well that you'll 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 Any reference to cities and their populations are probably out of date.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. Nevertheless, I've tried to leave the document in its original form, unmodified, as much as possible. Any comments I make are included in brackets [e.g.].

Excerpts from Terrain Flying (AC 91-15)

Denver to Salt Lake City

The direct route is over high terrain with very few service points as shown in Figure 1 [this was written before EFAS, HIWAS, TWEBS, etc., which are available along many of these routes]. Better go by way of Cheyenne. Remember that the high turbulence and high wind velocity effects increase during the day thoughout this region. Plan this flight, as well as most others described hereafter, for early morning.

Avoid all summer thunderheads in and around Salt Lake valley. Sometimes these produce hailstorms which can cause serious damage to all-metal airplane and which could be fatal to fabric-covered airplanes.

During the winter months, smoke from factories in the Salt Lake valley area reduces visibility considerably, especially in the early morning hours.

Reno to San Francisco

Follow the airway. It takes 8,000 to 9,000 feet to get over Donner Pass [see Figure 2]. In summer months thunderheads will build over this area during the day, dissipating in the late afternoon. Terrific snowstorms, freezing temperatures and high winds are encountered in winter months.

Salt Lake City to Los Angeles, Via Las Vegas

There are some mountains along the route which call for the usual care in mountain terrain flying. Stick to the airway between Salt Lake City and Las Vegas. Follow the highway west of Las Vegas. If Mint Canyon is closed in, take an alternate route from Victorville though [the] Cajon Pass via San Bernadino.

Winslow, Arizona to Daggett, California, via Needles, California

This route is suggested only for planes with higher horsepower and long range, because of the distances involved, high altitude, winds, thunderstorms and snow [Figure 3]. There is much snow between Winslow and Kingman during winter months. Gusty and high surface wind conditions usually prevail during afternoon hours. There are usually strong westerly headwinds. Pilots should follow Highway 66 and take all the precautions advised for both mountain and desert flying.

El Paso to Phoenix, Via Tucson

This route is suggested for the itinerant pilot whose trip would terminate in southern Arizona and California [Figure 4]. The ranges are lower, weather conditions better, service points more numerous and generally good flying conditions prevail during early morning and late afternoon. Take mountain and desert precautions.

Phoenix to Los Angeles, Via Blythe, California

Here the flight had best be made in the early morning [Figure 5]. In San Gorgonio Pass, flanked by San Jacinto Peak (10,804 feet) and San Gorgonio Mountains (11,502 feet), high winds and turbulence can become violent in the afternoon. The peaks make excellent landmarks, however. The same conditions prevail as on the Sale Lake-Las Vegas-Los Angeles flight.

Tucson to San Diego

One suggested route is vial Blythe and Los Angeles [Figure 6]. A careful check of the coastal weather between Los Angeles and San Diego should be made before continuing on the last leg. Another suggested route is via Gila Bend, Yuma and El Centro since there are good airports along this route. However, one word of caution on this latter route. Pilots are warned that is a good practice to stop [and refuel as well as making] a thorough check of the weather ahead before going over the mountains to San Diego [I recommend that you stop at Imperial County airport (Figure 7A) or Calexico airport (Figure 7B). The Navy at one time required all transient aircraft from the east to [stop at El Centro before] overflying El Centro, [because they] were getting lost or running out of fuel in the coastal fog or stratus clouds.

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