Fasten Your Seat Belts!

Published July 30, 2014
By MSgt Julie Meintel
445th Airlift Wing Wright-Patterson AFB, OH

TMF Fall 2014 --

We've all been there. It's a great day for flying: a perfectly bright and sunny sky, cruising at altitude, visibility is clear and a million, blue sky above and clouds below ... and then it comes out of nowhere. Turbulence. It probably won't ruin your day or your mission, but it could if you aren't paying attention.

Turbulence is defined by AFH 11-203, Vol II, Weather for Aircrews, as "a weather disturbance caused by abrupt, small-scale variations in wind speed and direction." Sounds pretty straightforward and easy to understand. But every person will likely experience turbulence a little bit differently based on many factors, and so the aviation community, military as well as civilian, has categorized it like this:

Light turbulence: Momentarily causes slight, erratic changes in altitude and/or attitude (pitch, roll, yaw).

Light chop: Slight, rapid, and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude.

Moderate turbulence: Similar to light turbulence but of greater intensity. Changes in altitude and/or attitude occur, but the aircraft remains in positive control at all times. It usually causes variations in indicated airspeed.

Moderate chop: Turbulence similar to light chop but of greater intensity and which causes rapid bumps or jolts without appreciable change aircraft altitude or attitude.

Severe turbulence: Causes large abrupt changes in altitude and/or attitude. It usually causes large variation in indicated airspeed, and the aircraft becomes very hard to control.

Extreme turbulence: Aircraft is violently tossed about and crew is incapable of controlling it. Large sudden changes in altitude and/or attitude take place. Extreme turbulence can and often does cause structural damage to aircraft.

You can come across turbulence anytime, anywhere. There is a relationship between turbulence and wind variations in the jet streams, which are wide swaths of concentrated winds at high speeds that encircle the earth. Jet streams tend to be more pronounced during the winter months when there are two, and sometimes three, major jet streams: the polar jet stream, the subtropical jet stream, and the arctic jet stream. It would be nice if it were possible to avoid the jet streams altogether when we are flying, but that is just not going to happen.

A significant percentage of the world's commercial and military flight traffic travels on flight paths that go right through the mid-latitudes, which is where the polar jet stream is. So since there really isn't a way to avoid turbulence, the best thing to do is know what it is, what it feels like and how to handle it. Always go back to your regs: AFH 11-203. Vol II, Weather for Aircrews, is your best bet for any weather-related information. The rules of weather phenomena don't really change that much, so it's easy to get comfortable with your knowledge of it. But it's a good thing to just refresh the ol' memory and make sure you are up on what you need to know for the conditions you can expect to find on your mission.

There are plenty of indicators and ways to know what the weather is like up ahead on your flight path. First, look at your surroundings. Are you near terrain? You may be encountering mechanical turbulence, caused by wind flowing over terrain or other irregular objects that cause disruptions in the airflow. Depending on things like roughness of the terrain or obstruction, wind speed, and stability of the air mass, the intensity of the turbulence is widely varied. Or it might be mountain wave turbulence, the result of wind blowing perpendicular to the top of a mountain range. A fairly reliable indicator of mountain wave turbulence is the cloud formations in the area. Although turbulence can exist there with or without cloud formations, it most often happens when clouds are present. Try to avoid the cloud formations to the greatest extent possible, since they often contain severe and even extreme turbulence.

Maybe you are over open flatlands, nowhere near any kind of terrain. Then you might see convective turbulence, which is the result of alternating currents of warm and cool air at lower altitudes. The turbulence itself tends to be light to moderate, and you will generally find it beneath the clouds, with the air above the clouds being much smoother. You are most likely to encounter this condition on sunny, hot, dry days.

Another variety of turbulence--high altitude or clear air turbulence--comes from atmospheric pressure, the confluence of two jet streams, mountain waves, cold and warm fronts, and thunderstorms. You will most often find it above 15,000 feet and in conjunction with a major change in wind speed or temperature, like wind shear.

Finally, wake or vortex turbulence is created by the difference at each wingtip created by high-pressure air on the underside of the wing joining with the low-pressure air flowing over the top, resulting in a pair of vortices, rotating in opposite directions. Wake turbulence is most severe behind a heavy aircraft, especially if configured with both gear and flaps retracted and flying slow.

Pilot Reports (PIREPs) are given by transiting pilots so that others coming through the same area within a given time frame will know what the weather conditions are. They are only useful if they are given regularly and are as accurate and detailed as possible, so do your part. Use these numerical codes from AFH 11-203, Vol II, to describe turbulence, if there is any, for crews coming behind you:

0 = trace
1 = light turbulence
2 = moderate turbulence in clear air occasional
3 = moderate turbulence in clear air frequent
4 = moderate turbulence in cloud occasional
5 = moderate turbulence in cloud frequent
6 = severe turbulence in clear air occasional
7 = severe turbulence in clear air frequent
8 = severe turbulence in cloud occasional
9 = severe turbulence in cloud frequent
X = extreme turbulence

Again, this information doesn't really change that much from year to year, and it is easy to become complacent and feel like you are experienced enough that you don't need to review the weather rules.

Do yourself and your crew a favor, though, and take a few minutes to brush up while you are doing your mission planning. Make sure to give regular PIREPS along the way to help the other crews in your vicinity. Watch out for your wingman and fly safe!

KC-135 Stratotanker