Many pilots base the go-no-go decision on ceiling and visibility. Yet the accident record shows there are other considerations of equal or greater importance. For example, how many times have you delayed or canceled a flight because of forecast or reported severe turbulence enroute or at your destination?
The hazard of thunderstorms is obvious and can be visualized. However, high and low level clear air turbulence is often treated casually. When clear air turbulence is encountered above 15,000 feet, it is referred to as turbulence encountered outside of convective clouds. At lower altitudes it is simply mechanical or low level turbulence.
Low level turbulence often takes the form of a mountain wave in mountainous or rolling terrain. It is the phenomenon meteorologists identify as a potentially severe form of orographic or gravity wave turbulence. Many pilots and passengers have lost their lives due to ignorance of the mountain wave phenomenon.
Interestingly, all it takes to form a mountain wave is wind across the peaks of 15 knots or better, at an angle of 30 degrees or more. The hazard is twofold: severe turbulence that results in airframe overstress or failure, and down drafts that exceed the climb rate of the airplane.
In one classic example, FSS had briefed a Mooney pilot to expect clear skies and a strong northwesterly flow over the mountains. He was specifically warned of the possibility of severe turbulence. Surface winds in the mountain passes were estimated at 40 to 60 mph.
The pilot was flying from Cheyenne, Wy., to Albuquerque on an IFR flight plan. Out of Pueblo, along Victor Airway 210, he entered La Veta pass, with an MEA of 13,500 feet. Shortly afterward he transmitted: May Day, May Day, May Day; Mooney 977M. When Pueblo Approach control responded, the Mooney pilot transmitted Pueblo, this is Mooney 977M in La Veta pass and Im losing altitude at one zero thousand, squawking 7700. I cant break the downdraft. Shortly thereafter the pilot again transmitted May Day. This was his last transmission.
The aircraft impacted at the 9,000 foot level on a steep, 50-degree gradient slope of the Sangre de Cristo Mountains. Fourteen minutes later a professional pilot with more than 6,000 hours was flying his Cessna 411 along the same route. He reported that, while transiting La Veta, he was using full power at 120 knots and still being forced down at about 400 fpm. After a 2,000-foot altitude loss the aircraft began maintaining altitude.
Although there was no turbulence, he reported, I have never experienced such an intensive and long-lasting downdraft situation especially in smooth flight conditions.
But such encounters arent only the province of the Rockies. In a mishap on the east coast, a 13,000-hour commercial pilot was transferring a patient on a non-emergency medical flight in a Piper Seneca.
The pilot had flown south from Washington, Pa., south through West Virginia to Greenville, S.C., on the west side of the Appalachians to avoid a cold front to the east. He told briefers he would return via this route since his flight down had been smooth.
The patient was being moved from Greenville to Philadelphia. During the preflight weather briefing the Greer FSS specialist informed the pilot of the AIRMET Foxtrot 2, which forecast winds along his route of 30 knots or more within 2,000 feet of the surface. He added that along the proposed route several pilots had reported moderate to severe turbulence with intense up- and down-drafts 20 miles north of Sugar Loaf Mountain near Busic intersection.
The Seneca pilot then rerouted his flight slightly southwest of the airway, filing direct Greenville to Holston Mountain VOR. The WAC chart showed the MEA along this direct route as 7,000 feet. After departing Greenville at 13:29 EDT, the flight was routine to a point near the north end of a ridge line that forms Mount Mitchell in the Great Smokies. The WAC chart notes, Severe turbulence may be encountered in the vicinity of Mt. Mitchell. His weather briefing showed severe turbulence had already been reported by other pilots, subsequent to his trip down a couple of hours earlier.
At 13:35 EDT the flight was turned over to Asheville Tower and the pilot checked in, climbing to 7,000 feet. The controller advised that he could remain at 7,000 and be vectored through the valley or climb to the MEA of 9,000 feet on the airway to go over the mountains. He elected the mountainous route. The higher altitude was approved and the pilot climbed to 9,000 feet.
At 13:56 the Seneca was turned over to Atlanta Center Tri City Sector. At 14:02 EDT the pilot was on V53, his originally planned route, eight miles north of Busic intersection and maintaining 9,000 feet. Suddenly he reported In a downdraft at 8,000 feet descending. The controller advised that he could have a higher altitude or deviate in any direction required to get out of the downdraft. In desperation the pilot requested assistance and advised he was now in clouds and unable to establish ground contact. His last transmission in a shaky voice, was Passing six thousand seven hundred (feet), descending at 1,000 fpm.
About an hour after the accident at 15:00 EDT, a wind recording device on a mountain 20 miles northeast of the impact site, at an elevation of 5,964 feet, documented surface winds from 270 degrees at 40K gusting to 70K.
Impact was on the east side of the north-south ridge line that forms Mount Mitchell. The wreckage was found on a 60 degree slope consisting of loose rock. Because of the dangerous terrain no on-scene investigation was attempted.
Mountain Waves
Compared to thunderstorms, not much has been written about mountain waves. Yet on occasion they can wield the force of tornadoes or microbursts from cumulonimbus clouds. Mountain waves, otherwise known as oropraphic mountain turbulence, lee or gravity waves, are well known to meteorologists. Above 15,000 feet, the upsetting characteristics are identified as clear air turbulence. At lower levels it is simply mechanical or low level turbulence. Mountain wave activity can be expected anytime winds at mountain top level are 15 knots or greater and flowing at an angle of 30 degrees or more to the range.
Mountain wave air flow is often smooth, producing updrafts on the windward side. As it flows uphill, the wind increases in velocity because of the venturi effect. Upon cresting the peaks, the flow pattern breaks down, not unlike the airflow over a stalled wing. This produces a complicated wind pattern, with downdrafts and turbulence predominating. About 5 to 10 miles downstream from the mountain peaks the air flow ascends and re-establishes an identifiable wave pattern.
In addition to the primary wave, six or more less intense waves may be present in some geographical areas. Wind speeds at altitudes above the mountain range increase rapidly for several thousand feet. The flow pattern can even reach the tropopause.
Conditions are ripe to create a wave pattern anywhere there is smooth undulating or concave terrain at the mountain crest. Another good spot is on the lee side of a pass between two prominent peaks, such as La Veta pass between Pueblo and Alamosa, Colo., where the Mooney referenced above was lost.
The length of the mountain range and sharpness of its features have significant effects. The lee side of long, sharp ridges will produce stronger waves and downdrafts than isolated peaks. Also the wave pattern may rise to greater altitudes and extend farther downwind.
In the Seneca mishap winds aloft were west south westerly, with speeds recorded at 08:00 EDT in Greensboro as 29 knots at 4,000 feet, 54 knots at 6,000 feet, 65 knots at 9,000 feet and 85 knots at 16,000 feet. It is unclear in the accident report why the FSS briefer gave winds at 6,000 and 9,000 feet as around 30-31 knots.
AIRMET Foxtrot 2 did warn of moderate turbulence below 14,000 feet with wind speeds or 30 knots or more within 2,000 feet of the surface. This in itself was a clue to probable mountain wave conditions. Undoubtedly the key element in the pilots decision to depart was that he had just flown the same route with no turbulence or clouds.
Yet more recent reports by pilots in a Cessna 320 and another Piper Seneca told of severe downdrafts along his proposed return route of flight. Another element influencing his decision was no doubt the necessity of getting his passenger, a medical patient, to Philadelphia promptly.
Contributing to the weather problem was an intense low pressure area centered over central Virginia, with high pressure in the southern Mississippi valley rapidly moving northeastward toward the middle Atlantic states. Significant clouds and weather were forecast for the eastern slope and most of North Carolina with tops 20,000 to occasional 40,000 feet in cumulonimbus buildups. The 8:00 EDT radiosonde showed stable moist air below 16,000 feet except for a layer of drier absolutely unstable air between 5,500 feet and 7,500 feet. The freezing level was 13,500 feet.
That conditions were conducive to severe turbulence should not have been a surprise. A landmark study more than 25 years ago of severe windstorms along the eastern Rockies by Douglas Lilly and Edward Zipser of the National Center for Atmospheric Research concluded that severe downslope winds are usually associated with at least moderately strong mid-tropospheric westerlies or northwesterlies, with winds of 50 knots or more at mountain top level and usually with an active weather system moving through. Frequently a strong windstorm is accompanied by development of a low pressure area on the eastern slope.
In many, though probably not all, severe windstorms, a well defined inversion exists a few thousand feet above the mountain tops, the researchers found. Although not a reliable forecasting guide, [lee-side] windstorms are always associated with large pressure differences across the mountains.
Challenging the Gusts
Deciding whether threatening gusts may lie ahead is one thing, figuring out what to do about it is another. Light twins and singles are certified to withstand gust loads of 50 fps (3,000 fpm) up to 20,000 feet. Maximum G load strength in normal category airplanes is 3.8g, however, manufacturers limit most airplanes to 3 to 3.5g. Some Beech twins and singles are certified in utility category with a 4.5G limitation. At or below maneuvering speed, Va, the structure cannot be overstressed because the wing stalls to relieve the load.
In fact if you are cruising at Va a severe gust or full control deflection will push you to exactly the designed G limitation of your aircraft and no more. But a cruise speed above Va in areas of severe turbulence can result in overstress and possible airframe damage or failure.
If your owners manual shows a turbulent air penetration speed, then this is the airspeed limit in turbulent air. Keep in mind that Va is determined by the FAA during certification, while in light twins and singles the turbulence speed is a recommended figure.
Another factor to consider is the maximum rate of climb for your aircraft. With normally aspirated engines at 10,000 feet, you have lost approximately 30 percent of the rated engine horsepower. Your rate of climb is also a function of aircraft gross weight and temperature aloft. But even turbocharged engines and turboprops would not have enough climb power to overpower a sustained 2,000-3,0000 fpm downdraft at 7,000 to 14,000 feet.
Spotting areas of potential severe turbulence depends on noticing some of the telltale clouds that mark the danger zones. While it is possible that a mountain wave pattern can develop with air too dry to identify its presence, usually very specific cloud formations mark its location. These include the cap cloud, rotor cloud, lenticular cloud, and in polar regions, the mother-of-pearl cloud.
The NTSB report does not identify the cloud type involved in the Seneca mishap, however the cap cloud would be a good bet, A cap cloud is only a few thousand feet thick and covers the mountain summit in a smooth layer of moisture. It is more extensive on the windward slope with the leeward edge being stationary. When approaching it from downwind it may appear as a wall with fingers extending down the lee slope.
The most hazardous turbulence produced by a mountain wave occurs in the roll cloud or rotor circulation, which is generated on the downwind side of the mountain. Often roll clouds are like small, boiling cumulus clouds, parallel to and about the same height as the mountain ridge. Yet they have updrafts of 5,000 fpm in the leading edge and down drafts of equal strength on the downwind side. There is a boiling motion to these clouds which becomes obvious as you approach them. It is noteworthy that general aviation aircraft are not designed for gust loads of this strength.
The most dangerous features of mountain waves are the turbulence in and below the rotor clouds and the downdrafts just to the lee of the mountain ridges, and to the lee of the rotor clouds, retired Marines pilot J. D. Simpson wrote in Aerospace Safety nearly two decades ago.
And in fact, the data bears that out. In one instrumented study, NCAR scientists recorded both horizontal and vertical gusts of 2g to 4g. One gust produced more than 7gs. Up- and downdrafts during this period were 2,000 fpm, with several instances estimated at up to 3,000 fpm.
The Visual Evidence
Lenticular (lens-shaped) clouds may be found layered above the mountain range up to 40,000 feet. When present they mark artistically the presence of a standing wave. Their brushed, wispy texture attests to the smooth laminar flow at that level. The tiered structure is due to stratified layers of atmospheric humidity.
Lenticular clouds, like the rotor, are stationary. They form in parallel bars along mountain ridges, originating on the windward side due to the lifting effect of the wave, then dissipating on the lee side.
Surprisingly, severe turbulence can occur both above and below the extremely smooth lenticulars. Sometimes, for unknown reasons, the entire wave pattern becomes severely turbulent throughout its vertical extent. In that case, the edges of the lenticulars become ragged and irregular, with the streamlines appearing closer together. The cloud will appear to tilt toward the mountain range as the winds ascend. On the downdraft side wind speeds increase, forming local jets which accentuate the turbulence and downdraft hazard.
Mother-of-pearl lenticular clouds occur at around 80,000 feet and remain stationary for long periods of time. Yet they can change in seconds if conditions change and a considerable amount of motion takes place in and around the clouds.
When the air is dry, a mountain wave will not be marked by a lenticular. Yet a dry wave can be just as turbulent as one with more visible evidence. Your only clues to its presence are your FSS weather briefing, pilot reports of moderate to severe turbulence, recent cold front passage, or winds perpendicular to the ridge line blowing at mountain peak level greater than 25 knots and increasing with altitude. This dry mountain wave is exactly what captured the Mooney cited earlier and drove it into La Veta pass. Sadly, the FSS briefing contained all the clues the pilot needed, if he had known what to look for.
An inversion or layer of stable air, marked by the temperature increasing with altitude, below 14,000 feet is another dry wave indicator. While temperature aloft was not provided to the Seneca pilot, there were enough other starkly evident clues to the presence of a mountain wave. Yet because he had flown the route just shortly before and experienced no turbulence, the pilot discounted the warning signs. He simply failed to realize how rapidly weather conditions can change when the atmosphere is unstable over a wide area.
Clearly mountain waves can be a primary hazard to piston engine airplanes, yet the updrafts produced in the upper atmosphere also can cause a jet to exceed its Mmo if the pilot attempts to maintain an assigned altitude.
In some aircraft this means a loss of control due to the high Mach phenomenon. Conversely, a downdraft can lead to the low speed buffet boundary or stall. Underneath or above the wave action aloft the high-flying pilot may find severe CAT that, unless he has already established the recommended speed, can damage the aircraft structure.
An established mountain wave can last from a few hours or up to two days and often extend downwind for 700 miles. Altimeter errors as great as 2,500 feet have been documented when a strong wave is in progress. Think what that means with an MEA of 12,500 to 14,500 feet.
Wicked Winds
Foehn winds, also called Chinook winds, are localized warm dry winds that descend the mountain slopes and spread out in the valleys and plains.
As the winds reach the valley floor the ambient temperature can rise 50oF and a ground cover of snow can evaporate at the rate of 2 feet per day. As the winds descend the slopes there may be a mixing of air masses coupled with a temperature rise. Since there are few telltale signs, the turbulence created by Chinook winds can be as hazardous as a mountain wave.
Ironically, in both mishaps cited, the pilots could have simply reversed course into lower terrain. This technique was used successfully for many years by Frontier Airlines pilots flying DC3s. The pilots computed a point of no return when planning to transit La Veta pass. If they encountered a severe downdraft before reaching that pre-computed point, they would turn back to the east, or dive out, as they called it. Otherwise they rode out the down flow until passing the mountain crest and reaching the updrafts on the windward side. In order to make this strategy safe, they always started through the pass with excess altitude, normally 16,000 feet.
It is important to remember that the force of a mountain wave can be as devastating as a downburst or a tornado. Very large mountains are not required to generate a wave pattern. It can occur in any hill country that has elevations of 300 foot or more. In fact, 300 foot ridges have been known to cause wave action up to 75,000 feet.
Because many pilots – private and professional alike – know little about mountain waves, they rarely delay a trip due to forecast or reports of extensive moderate to severe turbulence. As with thunderstorms, you will often find the turbulence gone when you get to the forecast area. This causes a loss in confidence in the forecasts.
Yet its best to be cautious. Although no one in his right mind would intentionally penetrate a thunderstorm, the mountain wave is not given due respect.
Remember, a pilots license is a license to learn, and as youll see, the air, like the sea, can be terribly unforgiving of ignorance or carelessness.
-by John Lowery
John Lowery, a former Air Force and corporate pilot and author of several aviation books, is an aviation safety and training consultant.
