Flying High in High Terrain

The second group consists of the rest of us. When we head west, we often have to fly hundreds of miles out of the way through regions like New Mexico and Northern Arizona to avoid the towering peaks, only to discover that these routes are littered with rugged peaks, vast high deserts, and even airports at higher elevations. Back east, the Appalachians can also surprise us with their high-altitude elements. Our friends in the U.S. Air Force encountered some of these issues more than 50 years ago, so let’s start by examining what they learned. 

Back in 1964, the U.S. Air Force found itself in a quandary. The high-flying B-52 Stratofortress was being tasked with flying well below 1,000 feet to evade radar detection. Soon, B-52 crews began reporting concerning turbulence encounters. So, the Boeing Company “borrowed” a B-52H from the active fleet, equipped it with sensors, and began the search for turbulence. 

It didn’t take them long to find it. The flight crew began their day in Kansas, flying at 500 feet. As they entered Colorado, they eventually turned north and began climbing from the local elevation of 6,500 feet to 14,000 feet, about 1,000 feet higher than nearby East Spanish peak. There, they encountered a powerful mountain wave-induced rotor caused by 60-knot winds. In a matter of seconds, the vertical stabilizer was torn from the aircraft. Only the quick thinking of the crew saved the aircraft, which landed safely in Arkansas. 

This incident, along with the subsequent research, helped the entire pilot community understand the challenges posed by mountain ridges and strong winds. So, what can we learn from their experience? 

Many believe that mountain wave formation is a phenomenon unique to the Rocky Mountains, thanks to the tall and rugged peaks. However, some of the strongest wave formations are encountered in the Appalachians along the Virginia/North Carolina border near Mount Rogers. Pilots flying through this region during the late fall through early spring routinely report experiencing powerful up and downdrafts. Head out to the West Coast, and the Los Angeles basin is another prime wave zone. Strong winds off the Pacific Ocean are not uncommon, creating clear days in the greater Los Angeles area while simultaneously generating powerful wave turbulence over the mountain ranges and passes that separate Los Angeles from the deserts to the north and east and the coastal valleys to the north. So, what might this look like? 

Consider a Cessna 172 pilot flying eastbound out of San Diego at 8,000 feet. The pilot notes that the sky is clear and the groundspeed on the GPS shows a 50-knot tailwind. So far, so good. As our pilot approaches the first set of 5,000-foot peaks, the aircraft begins to be pushed upward by the wave action. At this point, our savvy pilot, aware that what goes up must eventually come down, calls to ATC to request clearance to ride the wave up to 9,000 feet. Once on the lee side of the peak, the wave pulls the aircraft back down to 8,000 feet. Thankfully, the ATC controllers in these regions are aware of the effects and are usually eager to assist. However, every pilot must be aware that this kind of passage can result in an encounter with strong up and downdrafts, complete with severe turbulence and high structural loads. 

Now, what about flying westbound into the wind? For that, let’s go a bit further north to the Tehachapi Pass area that divides the central valley of California from the high desert, which begins just west of Edwards Air Force Base. Crossing the pass from east to west in strong winds, the unsuspecting pilot may encounter robust downdrafts that are stronger than the maximum climb rate of their airplane. If these winds are encountered, a 180-degree turn back toward lower terrain is likely the best course of action. In any case, always have a way out! 

While Tehachapi and the Los Angeles Basin are places where mountain wave formation is common, they are by no means the only locations where these conditions occur. Anywhere where the winds are above 20 knots and the peaks are within a thousand feet of your aircraft, especially in areas where lenticular or rotor clouds are present, should be approached with extreme caution or avoided altogether. 

Before we leave the topic of mountain wave turbulence, let’s review the behavior, or rather the misbehavior, of many two-axis autopilots. If a pilot is approaching a ridgeline and encounters a strong updraft, the autopilot will attempt to maintain altitude. This dogged persistence in holding the altitude steady can increase the indicated airspeed well beyond the turbulent air penetration speed. 

It also reduces the altitude cushion available when the inevitable downdraft appears on the far side of the peak. Conversely, if the airplane is approaching a ridgeline into a strong headwind, the autopilot will try to maintain altitude by increasing pitch (thereby reducing airspeed). This loss of energy can mask the severity of the encounter until it is too late to turn around. So, when in doubt, take control and make a turn. 

The airport in Sedona, Arizona, is nestled within the stunningly beautiful red rocks that dominate the northern part of the state. The airport sits on a butte approximately 5,000 feet above sea level at the western terminus of the Mogollon Rim. This is Zane Gray country for all you western novel enthusiasts, and it is a popular destination for pilots from the greater Phoenix area, which is 85 miles to the south and nearly 4,000 feet below. Sedona Airport offers amazing views of the red rocks, a delightful airport restaurant, and more than its fair share of general aviation accidents. 

In the summer, locals who are familiar with the area fly in and out in the early mornings or late afternoons when the air is denser, the winds are lighter, and everything is just right. However, by mid-afternoon on a summer day, the density altitude can easily exceed 8,000 feet, and the wind gusts can be quite strong. Pilots usually land on either Runway 3 or 21, but if the winds permit, they often depart to the southwest on Runway 21 due to the high terrain to the north. However, strong winds can descend from the San Francisco peaks in nearby Flagstaff and create strong downdrafts at the immediate southern edge of the airport. These downdrafts and the high-density altitudes can exceed the rate of climb of arriving or departing general aviation aircraft. So, enjoy the amazing views of Sedona, but plan your trip carefully to avoid the pitfalls. 

Night flying in the foothills of the east and the ridgelines of the southwest presents its own set of challenges. While it is extremely tempting to savor the cool night air and gentle winds that are typical during evening hours, there are a few hazards to be aware of. In the Appalachians, airports are fewer and farther apart and are usually nestled in a deep valley between two parallel forested ridgelines. 

In the daytime, this is a visual delight. At night, it can be a bit more challenging. In the southwest, the airports are located 50 to 75 miles apart, separated by 5,000 to 6,000-foot ridges, and often unattended at night. On moonless nights, the sparse desert ground lights and the starlight above can blend together, making it difficult to determine which way is up. And if the pilot sees a large dark area ahead, it is usually a mountain. 

So, if our pilot is descending toward a nearby city and it suddenly disappears, it is time to climb immediately to avoid the granite peak just ahead. In fact, on dark nights, it is often best to remain at altitude until you are over the last mountain range and then descend when over the lights of the destination. It makes perfect sense, in fact, to fly the airways even when VFR and flying direct and maintain their recommended minimum altitudes. 

Some basic rules of thumb might be helpful. First, if the winds are forecast to be above 20 knots within 1,000 feet of the ridgelines, be prepared for the presence of mountain wave activity and always have a Plan B. Second, if we see those uniquely shaped lenticular clouds on top of the ridges, our suspicions are confirmed—it’s time to consider and execute an alternate plan. Third, in the high desert, consider flying in the early mornings and later afternoons when the temperatures are cooler, the winds are lighter, and our airplanes have more pep and vigor. 

Oh, and if a night flight is on the agenda, consider the risks it presents in high terrain or sparsely populated mountain regions. And finally, when in doubt, check with the local CFI or the destination airport to see if there are any peculiarities, preferred routes, or hazards. We have the best seat in the house to view some of the most beautiful terrain in the world. With a little preparation, we can make it a safer journey. Fly safe! 

Do you want more pro tips? Check out “Deadly Attitudes: Know How To Spot Risky Thinking.”