You are hereCrossing the Continental Divide - Part 1
Crossing the Continental Divide - Part 1
Flying the Interstate Highway System
by John T. Kounis
Planning a trip across the country can be intimidating, especially if you live in an area dominated by green on the sectional chart (indicating altitudes below 2,000 ft.). The Denver Sectional Aeronautical Chart, on the other hand, has five colors: all shades of brown, indicating terrain up to 14,433 ft. Brown is also the predominant color on the Albuquerque, Salt Lake City, and Great Falls sectionals.
Running right through these brown colors is the Continental Divide, which forms an obstacle for both rivers and air travel. Fortunately, it’s possible to cross it over terrain as low as 4,500 ft., but you have to select a good route. Flight planning is easier if you understand the effects of two forces: erosion and economics. Let’s start with erosion. For eons, water has been literally moving mountains by carving canyons and valleys, creating natural flyways. From just about anywhere between the Mississippi River and the Pacific Ocean, you can follow a river upstream to the Continental Divide. And you won’t have to cross a single ridge until you get there.
The second force, economics, results in another type of flyway: highways. Because it’s cheaper, highways usually follow the lowest and easiest terrain between two points. In fact, the easiest routes across the Rocky Mountains are along interstates. (Don’t let this fact lull you into complacency with flight planning, lest you encounter the nemesis of pilots who blindly follow highways: the dreaded tunnel.)
The following routes all follow interstate highways. We cover them eastbound from the West Coast, but you can easily follow them westbound by reversing the directions.
Easy Routes
The southernmost route is also the lowest one: along Interstate 10 that stretches from the Pacific Ocean at Santa Monica, California, to the Atlantic Ocean at Jacksonville, Florida. After climbing through the 2,200-ft. Banning Pass east of Los Angeles, it descends through the low desert toward the Arizona border at the Colorado River, and then climbs steadily through Arizona, reaching about 2,500 ft. at Tucson. It continues climbing into the mountains with elevations up to 5,000 ft. and crosses the Continental Divide in southern New Mexico at around 4,500 ft. After crossing the Divide, it descends southeastward toward El Paso, Texas. There, you need to follow the interstate to fly around the south ends of restricted areas R-5103 and R-5107 (the White Sands Missile Range).
The northern route from Seattle, Washington, along Interstate 90 crosses 3,004-ft. Snoqualmie Pass before descending into eastern Washington and then climbing into Idaho and Montana. If the ceilings are too low for you to transition Snoqualmie Pass, you can detour via the Columbia and Spokane rivers to the Spokane area. (The rivers slowly climb from sea level at Astoria, Oregon, to 1,800 ft. at Spokane.) You can then continue via Lake Pend Oreille up the Clark Fork and Flathead rivers to Dixon, Montana. U.S. 93 leads from Dixon to join the Interstate 90 route in Missoula at about 3,200 ft. From there, Interstate 90 climbs eastward toward the Continental Divide, cresting at around 6,500 ft. near Butte, Montana. You have to cross a 6,000-ft. pass east of Bozeman and 4,000–5,000-ft. terrain east of the Wind River Mountains around Sheridan and Buffalo. The final pass is at around 5,000-ft. between Gillette, Wyoming, and Black Hills, South Dakota. After that, it’s downhill all the way to the Missouri River.
Intermediate Routes
The closer you get to the middle of the country, the higher and more challenging the routes become. From Daggett, California (about halfway between Las Vegas and Los Angeles), Interstate 40 descends eastward toward the Colorado River. After crossing the river at 500 ft., it climbs rapidly into Arizona, passing Flagstaff at about 7,000 ft. and then continuing across the Arizona high desert with elevations between 5,000 and 7,000 ft. It reaches about 7,500 ft. when it crosses the Divide between Gallup and Grants, New Mexico, but the mountain flying is not over yet. After transiting high terrain at around 5,000 ft. to Albuquerque, it climbs into the Sandia Mountains toward 6,500-ft. Cedar Crest. The Sandia Mountains are breeding grounds for wind, turbulence, and storms. If you can’t continue east of Albuquerque, you could detour south to the Socorro VOR and then follow the 75-degree radial outbound along V-264 to the Corona VOR over 5,000–7,000-ft. terrain. (You can’t divert any farther south than V-264 due to the White Sands Missile Range.) The Corona VOR itself is at 6,411 ft.; from there, it’s downhill all the way to Texas.
Originally a path used by westbound wagon trains, Interstate 80 crosses several mountain ranges and remote deserts. From the San Francisco Bay area, it crosses California’s Central Valley before climbing over the Sierra Nevada, cresting at 7,239-ft. Donner Summit. The most desolate part of the journey follows across the Nevada and Utah deserts with elevations between 4,000 and 7,000 ft. At the Bonneville Salt Flats, west of Salt Lake City, the interstate threads through a corridor between restricted areas R-6406 and R-6404. Just past Salt Lake City, it climbs to around 7,000 ft. through the Wasatch Mountains and maintains 6,500–7,500 ft. elevations through western Wyoming. The Divide splits in Wyoming, so you’ll cross it twice. After the first time, you’ll fly over the Great Divide Basin, where water flows neither into the Pacific nor the Atlantic (it gets trapped in a chain of lakes). You’ll cross the Divide again before Rawlins.
If you detour slightly north of the interstate when you pass Rawlins, fly direct to the Medicine Bow VOR, and continue east on the 75-degree radial for about 30 miles, then you’ll cross terrain around 7,000 ft. Otherwise, you can remain over Interstate 80, which climbs to around 8,500 ft. before starting its eastward descent around Laramie toward the Missouri and Platte rivers in Omaha, Nebraska.
Difficult Routes
The most challenging—but perhaps also the most scenic—way through the Rockies is through Colorado. Interstate 70 starts in Utah north of Cedar City at around 6,000 ft., and quickly climbs to more than 7,000 ft. as it crosses two mountain ranges. In northern Utah, it descends near Green River, maintining around 4,000–5,000-ft. elevations until Grand Junction, Colorado. From Grand Junction, you can either follow Interstate 70 to Denver or U.S. 50 via Gunnison to Pueblo. Both routes have significant challenges, including mountain passes over 11,000 ft. and a necessary detour around the Loveland Tunnel on Interstate 70.
If you haven’t flown in the mountains before, get some dual instruction from a qualified mountain flying instructor. We’ll cover these difficult Colorado routes in greater depth in next issue’s Flying Tips.