The Challenges of Flying the Mountain States and the Mountain Wave Phenomenon – What Pilots and Passengers Should Know

Multi-State Geological Formation

The central mountain range of the United States, often called the Rocky Mountains, stands as a massive iconic spine-like feature stretching more than 3,000 miles from Canada to New Mexico, shaping the landscape and even driving the nation’s identity. This huge multi-state geological formation emerged from tectonic discord and clashes millions of years ago. It is home to a rich tapestry of ecosystems.

Influencing Weather Patterns

The region boasts alpine meadows and dense towering forests, fostering unique flora and fauna found nowhere else in the country. The Rockies act as a natural barrier, dividing the continent and influencing weather patterns while providing the perfect staging area for outdoor enthusiasts. It is also home to indigenous peoples whose cultures have intertwined with the rugged terrain for a very long time.

Towering Spires of Intimidating Granite

Within this impressive range are world-famous landmarks such as Yellowstone National Park, a unique geothermal feature brimming with geysers and hot springs, and the Grand Tetons, which are towering spires of intimidating granite that pierce the sky. The Rockies are known for some of the tallest peaks in North America, including Colorado’s majestic summits like Mount Elbert and Pikes Peak, attracting climbers and mountaineers in search of the ultimate challenge.

The Gold Rush Era

Beyond their powerful physical presence, the Rockies have symbolic significance, conveying the truly pioneering and steadfast spirit of the western United States. This mountainous divide witnessed a westward expansion of society, the gold rush era, and the laying of railroads that connected a young and emerging superpower. Today, they continue to inspire awe and reverence while also presenting unique aviation-related challenges for both pilots and aircraft.

Strong Shearing Winds

One phenomenon associated with mountain flying is the mountain wave. The potential for a mountain wave to cause a severe inflight disruption is very real, and in fact, they have even been responsible on rare occasions for structural break-ups in inflight aircraft. These “waves” can produce strong shearing winds that are present even many thousands of feet above a mountain peak or ridge. In short, the hazards posed by mountain waves should be respected and avoided whenever possible.

The Destructive Turbulence Found in Thunderstorms

Along with powerful wind shear effects, these “waves” also generate strong turbulence that is inherent within a typical mountain wave rotor or spinning area of turbulence. In some cases, mountain wave rotor turbulence can be far more severe than the destructive turbulence in thunderstorms. As most pilots discover, even in their earliest stages of training, avoiding thunderstorms is paramount for that very reason. The NTSB (National Transportation Safety Board) has shown mountain wave turbulence to have been a major contributing factor in several aircraft accidents over the years.

Turbulence Causing Phenomena

Getting caught in a severe mountain wave rotor can typically result in total loss of aircraft control and a flight path upset. In the case of large mountain ranges like the Rockies, the mountain wave rotor effect can be experienced sometimes dozens or even hundreds of miles from the actual mountain peaks and for several thousand feet above the highest ridge. This means that this turbulence-causing phenomenon can be found at the cruising altitudes of commercial and business jets.

Altered Flight Path

Exercising extreme caution with mountain wave turbulence when reported by FAA weather reporting stations is always the best course of action. Pilots may even choose to alter their flight path and accept an ATC reroute to avoid the potential for encountering this dangerous, severe turbulence.

Clear Air Turbulence and Mountain Waves

The FAA’s Airplane Upset Recovery Training Aid has the following recommendations regarding loss of aircraft control at altitude, “Turbulence, when extreme, can lead to airplane upsets and/or structural damage. These turbulence incidents can cause large airspeed, altitude, or attitude deviations. The aircraft may be momentarily out of control. Severe or extreme turbulence can be associated with clear air turbulence and mountain waves.” It also states, “Moderate turbulence will be experienced 150-300 mi. downwind on the leeward side when the wind component is 25-50 kt. at ridge level. Severe turbulence can be expected in mountainous areas where wind components exceed 50 kt. are perpendicular to and near ridge level.”

Advanced Instrumentation

Aircraft flying transcontinentally from coast to coast should be aware of the possibility of severe turbulence when crossing the front range. Keep in mind that the very large nature of the Rocky Mountain range is indicative of severe rotor-type turbulence. Recent research, aided by advanced instrumentation, has helped experts understand that rotors associated with mountain waves do indeed rise to much higher altitudes.

High Jet Aircraft Cruising Altitude

Aviation researchers with the National Center for Atmospheric Research (NCAR) in Boulder, Colorado, have studied and experimented with mountainous airflows for several decades with specially instrumented aircraft that effectively measure higher altitude turbulence. In a 1992 experiment, an NCAR Sabreliner jet aircraft experienced severe turbulence at 20,000, 30,000, and even 39,000 ft. This is significant findings that support the claim that the hazards of mountain wave turbulence can be found a high jet aircraft cruising altitude.

High Altitude Lenticular Clouds

Finally, the jet stream, a high-altitude river of rapid-moving airflow that circles the globe, also interacts with mountain wave-generated turbulence to further aggravate the effects on aircraft. High-altitude lenticular clouds are sometimes a good way to detect potential high-altitude rotor turbulence, although in many cases, the danger can be present in completely clear skies. Lenticular clouds get their name from their resemblance to a lens shape.

PIREPS or Pilot Reports

The good news is that modern advanced onboard weather radar that is fitted into most modern commercial jet aircraft can detect areas of extreme turbulence. This, combined with real-time weather reporting, means that pilots and airline dispatchers have virtually all the tools they need to avoid this type of inflight hazard. Another great tool that helps pilots avoid severe turbulence is something known as PIREPS or Pilot Reports. These are simply reports by other aircraft at a similar altitude and on a similar route. The reports alert others headed into the same airspace to potential disruptive flight hazards. All said, modern aviation is remarkably extremely safe, even considering these unseen hazards.