Tuesday, May 10, 2011

In this pic (taken from Aviation Explorer), an unfortunate pilot is encountering three different types of turbulence, and presumably encountering turbulent conditions in his underwear as well. I'm not actually sure if that last part makes sense, but I'm gonna go with it.
What the hell is turbulence anyway? Here's the short answer: it's when the air around your airplane changes direction and/or speed quickly. It's a fact of life for a pilot, so I'm gonna yammer on about it for a little bit more.

Turbulence is caused by 4 basic building blocks, which can act separately or join forces in an orchestra of suck (as per the picture above). The components are Mechanical, Thermal, Wake and Wind Shear.

With mechanical turbulence, the wind travels along the surface of the earth and hits various objects, like trees, buildings, mountains and whatnot. The air moves around the objects and becomes turbulent. As you fly over (or behind, relative to the wind) these objects, you get bumped about.

A description of thermal now: On a warm day, the ground gets hot. The hot ground causes the air above it to heat up, and that air rises. The rising air screws up the smooth ride for airplanes flying through it. One rule of thumb that works is if there is a layer of puffy clouds on a hot day, the thermal turbulence will only rise as high as the cloud layer and the ride above the clouds will be thermal-free.

Wake turbulence is caused by other aircraft. While an aircraft flies to and fro burning fuel and money, the lifting surfaces displace a whole lot of air and the engines blow air out the back end at high speed. All this air takes a little while to settle down, and if you fly close behind another aircraft you could be exposed to its wake. Depending on the size and configuration of the offending aircraft, it can be nearly imperceptible or it can be the aviation equivalent of being in a canoe out on the water as a supertanker goes by at high speed. Generally an aircraft that has flaps lowered and is at a low airspeed generates the most wake turbulence. Boeing 757's and 767's are nice planes, but are notorious for their unpleasant wake characteristics.

Wind Shear is caused by an angry god who wants to smite you with thunder and lightning, and by other stuff too. When the direction or speed of wind changes significantly within a short horizontal or vertical distance, the boundary areas will be bumpy. A rough ride caused by flying through a thunderstorm or crossing through a jet-stream boundary area would both qualify as wind shear turbulence.

Back to that pic at the top for a second, hopefully now it makes a little more sense - first the pilot encounters turbulence in the forms of thermal, then mechanical, then wind shear. Good thing no other planes were in the area or he coulda had 4/4, a perfect score (if you hate your airplane's wings).

In Canada, we report turbulence based on 3 levels of intensity. The following is shamelessly stolen from the Canadian AIM:

Occasional: Less than 1/3 of the time. Intermittent: 1/3 to 2/3. Continuous: More than 2/3.

Light - Turbulence that momentarily causes slight, erratic changes in altitude and/or attitude (pitch, roll, yaw). Report as “Light Turbulence”.
Turbulence that causes slight, rapid and somewhat rhythmic bumpiness without appreciable changes in altitude or attitude. Report as “Light Chop”.

Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. Food service may be conducted and little or no difficulty is encountered in walking.

Not a big deal for most people, but it gets bumpier ;)
Moderate - Turbulence that is 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. Report as “Moderate Turbulence”.
Turbulence that is similar to Light Chop but of greater intensity. It causes rapid bumps or jolts without appreciable changes in aircraft altitude or attitude. Report as “Moderate Chop”.

Occupants feel definite strains against seat belts or shoulder straps. Unsecured objects are dislodged. Food service and walking are difficult.

When it's moderate, you know you are flying through the air in a small metal tube. White-knucklers will be having a bad day, and the occasional beverage might end up on the floor or walls.

Here's a video of moderate chop that I took a little while ago - you can really only tell it's a rough ride 'cause the camera is shaking.

Severe - Turbulence that causes large, abrupt changes in altitude and/or attitude. It usually causes large variations in indicated airspeed. Aircraft may be momentarily out of control. Report as “Severe Turbulence”.

Occupants are forced violently against seat belts or shoulder straps. Unsecured objects are tossed about. Food service and walking impossible.

You'll notice there is no "Severe Chop" - at this level it's all just lousy.

We do our best to avoid turbulence - at cruising altitude sometimes all it takes is a climb or a descent of a couple of thousand feet to change a rough ride into a smooth one, or we might alter our route laterally to avoid an area of widespread unrest. If we have no choice but to fly through a zone of turbulence, we can also slow down. As our airspeed increases, our lifting surfaces are capable of generating more lift. This is good, but only to a point. Airplanes have an airspeed called 'maneuvering speed' (Va), below which a sudden intense gust of wind will cause the lifting surface to stall (and unload) rather than producing so much lift as to cause structural deformation. Depending on the situation, you might prefer to have a stalled wing or tail instead of having them depart the aircraft, but then you are dealing with a stalled airplane and that can have its own set of complications. Anyway, maneuvering speed decreases with weight and if you wanna totally nerd out, as a general rule the reduction in Va will be half the percentage reduction in aircraft weight. Another important note is that Va is also only predicated on full deflection of the controls on a single axis, and some forms of turbulence (ie thunderstorms) can cause intense stresses in multiple directions, which can easily result in catastrophic structural damage at speeds below Va.

What prompted this post?

This is a Citation that I was flying for another company a while back when we got a taste of severe turbulence. The skies were clear, but a strong frontal passage had caused lots of wind shear and mechanical turbulence, and it had been reported as severe by other pilots just ahead of us. I slowed down to our maneuvering speed, cinched up my seat-belt and shoulder harness, told the pax to hang on, and waited for it. Going through about 8,000 feet on the descent, we hit the wall. I was hand-flying (I prefer to hand-fly if it's gonna be really bumpy, so I can tune into what the plane is doing a little better) and the plane started to act like she'd been tasered. The severe bumps were only for a few seconds, but during that time, the oxygen masks in the back deployed uncommanded. The above pic is of a nice mechanic packing them back into the ceiling. The plane was fine - we were nice and slow when we hit the bumps and there wasn't any structural damage or anything, but it kinda got my mind on the subject.

I hope all your airflow is smooth, and your oxygen masks only deploy when you ask them to :)


Brendan said...

Do they have the 'extreme' class of turbulence in Canada?

Anonymous said...

I hope that you spend some of your time instructing. That was a masterful account of a tricky subject.
Smooth skies!