By MR. MATT LIPTAK, Staff Writer
Turbulence is the irregular motion of the air resulting from eddies and vertical currents. It is usually not much more than an annoyance to a pilot and their passengers, but the weather phenomena, in its more extreme varieties, can cause the pilot to temporarily lose control of the aircraft or even cause structural damage. Due to its unpredictable nature, turbulence is a constant concern for forecasters and mission planners at the 618th Air Operations Center (AOC) located at Scott Air Force Base, IL.
“Light and moderate turbulence occurs fairly regularly, and no special steps are taken to mitigate the forecast,” explained MSgt Ryan Snider, the Manager of the Weather Plans Division, 618 AOC. “However, severe and extreme turbulence must be avoided and will require flight planners to change the route or planned altitude to mitigate the threat.”
Turbulence is categorized into four levels of intensity. The first level, light turbulence, can momentarily cause slight erratic changes in attitude or altitude. On the inside of the aircraft, unsecured objects may become slightly displaced, and it is still possible to walk in the aircraft with ease. With moderate turbulence, there are variations in attitude and/or control and variations in airspeed, but the aircraft remains in positive control. In the interior, however, walking becomes difficult and objects are dislodged.
The next two levels of turbulence are of more serious concern. Severe turbulence leads to large abrupt changes in altitude and/or attitude, and variations in speed. The aircraft can be momentarily out of control. Walking in the interior of the aircraft is impossible. For extreme turbulence, the aircraft is tossed about and is impossible to control, and structural damage to the aircraft may result.
To mitigate these problems and threats caused by turbulence, the 618 AOC applies intense diligence. The Air Force weather model, Global Air-Land Weather Exploitation Model (GALWEM), produces a computer-generated 140-hour turbulence forecast every six hours, Snider said. Weather technicians at the 15th Operational Weather Squadron (OWS) continuously refine the first 30 hours of the forecast by analyzing real-time data and incorporating turbulence forecasting techniques.
“The 30-hour turbulence forecasts issued by the 15 OWS are the authoritative source for Air Force flying operations and are used by forecasters at 618 AOC to create the turbulence charts found in crew papers,” Snider explained.
Of course, not all turbulence is cut from the same cloth. According to weather.gov, there are different causes of turbulence giving the weather phenomena different characteristics. Turbulence is found to come from four different sources.
Mechanical turbulence can be defined as friction caused by the interaction of the air and the ground. Irregular terrain and man-made objects cause eddies and turbulence in the lower levels as a result. The magnitude of this turbulence depends on the strength of the surface wind, the nature of the surface and the stability of the air.
Another form of turbulence is thermal or convective turbulence. On warmer days, the earth’s surface can be unevenly heated by the sun. Some surfaces, like the barren ground or rocky and sandy areas, are heated more rapidly than are grass-covered fields and water. Isolated convective currents are then set in motion with warm air rising and cooler air descending, which can cause the aircraft to have a bumpy ride.
Frontal Turbulence is caused by the collision between two opposing air masses where the sloping frontal zone causes friction and, as a result, turbulence. This type of turbulence is most notable when warm, unstable air impacts colder air and can be very severe if there are thunderstorms present.
Finally, wind shear is the change in wind direction and/or wind speed over a specific horizontal or vertical distance. Wind shear can even occur during high altitude flights.
“It is sometimes referred to as Clear Air Turbulence [CAT] and is commonly associated with the jet stream,” explained Snider. “Low-level wind shear is a specific type of wind shear that can be especially dangerous to flight operations since it can disrupt flight performance in the critical stages of take-off and landing. Doppler radars measure horizontal wind flow and can be a useful tool for observing turbulence, especially in the lower levels of the atmosphere. Abrupt changes in wind speed or direction are often a tell-tale sign of wind shear.”
Both planners and forecasters for 618 AOC always keep a close eye on wind conditions. Safety is paramount with flying, and monitoring conditions for turbulence is one of the keys to ensuring it. Since turbulence is such a common occurrence, including severe turbulence, vigilance is required.
“Severe and extreme turbulence can cause a pilot to lose control of an aircraft and are always avoided,” Snider noted. “Moderate turbulence can be uncomfortable, especially for aircrew and passengers, so pilots will adjust the aircraft’s path and altitude as needed to find calmer air. Moderate turbulence also makes air refueling difficult since it can be challenging for the tanker and receiver to maintain a connection.”
Severe turbulence is very common in and around thunderstorms due to strong updrafts and downdrafts associated with the storm, he said. At higher altitudes near the jet stream, severe thunderstorms can occur due to the stronger winds found in that area. Also, severe mountain wave turbulence, a form of mechanical turbulence, is commonly found downwind of mountainous terrain when a strong wind flows perpendicular to the terrain.
With all this and more to consider, pilots, planners, and forecasters must know how much is too much regarding turbulence and flying aircraft safely. In very extreme cases aircrew may even turn back. After all, safety is job one.
“Pilots will coordinate with ATC [air traffic controllers] to change altitude or heading to find smoother air,” Snider explained,” In the meantime, each aircraft has power settings and airspeeds recommended by the manufacturer to relieve stress on the aircraft and maximize safety, and our mobility pilots are well versed in adapting to weather conditions to guarantee the safety of the crew.”
The presence of turbulence requires a great deal of attention from pilots, flight planners, and weather forecasters alike. Safety demands they have the best, most current data on the phenomena around the clock. It is an ever-present challenge, but one that, with the help of Air Force units like 618 AOC, can be well mitigated.
“Due to its unpredictable nature, turbulence is one of the most difficult weather hazards to forecast,” Snider concluded. “Forecasters rely heavily on pilot reports to validate and, if needed, amend turbulence forecasts to provide the best possible product.”