Many nursery rhymes express fear, suffering and disaster. Ring Around the Rosie was about the bubonic plague; Peter Peter Pumpkin Eater couldn’t keep his wife, so he imprisoned her; a farmer’s wife amputated the tails of three visually impaired mice and the parenting techniques of The Old Woman Who Lived in a Shoe would today warrant the intervention of Child Protective Services. Of course, we all know that when it’s raining, it’s pouring, and the old man is snoring, that we’ll bump our head (on the cockpit ceiling, no doubt) and won’t get up in the morning. King James and Mary of Modena’s baby-napping scandal notwithstanding, we’ve also been taught that if it’s windy and the bough breaks, the baby will fall – likely due to a royal microburst.
Perhaps for us valiant aviators, these were a childhood introduction to the potentially traumatic and unforgiving effects of weather. The list of atmospheric monsters has lengthened since we were kids and pilots can’t outgrow or ignore them. No longer a fairy tale, turbulence demons live in the heart of our flying territory.
Our Non-Terrestrial Territory
Weather and turbulence are the result of uneven heating of the Earth by the sun. Combine this with pressure, temperature, moisture differences, planetary rotation with surface friction and you have the ingredients for a changing and sometimes turbulent atmosphere. The troposphere is the lowest layer of the atmosphere, 3.7 to 6.2 miles (19,500 ft. to 32,500 ft.), and it’s where nearly all weather conditions take place. The top of the troposphere varies with latitude (it is lowest over the poles and highest at the equator) and by season (lower in winter and higher in summer). It can be as high as 12 miles or 65,000 feet near the equator, and as low as four miles or 23,000 feet over the poles in winter. It contains approximately 75 percent of the atmosphere’s mass and is by far the wettest layer of the atmosphere, containing 99 percent of the total mass of all water vapor. And it’s normally this water vapor that causes a bumpy ride. But there are other monsters lurking in our non-terrestrial territory.
The Turbulence Tempests
- Clear Air Turbulence.
CAT normally occurs outside of clouds at altitudes above 15,000 feet MSL, and it’s caused by strong wind shears in the jet stream.
- Thermal Turbulence.
Localized columns of convective current that result from surface heating or cold air moving over warmer ground. For every rising current, there is usually a compensating downward current also causing turbulence.
- Temperature Inversion Turbulence.
Even though a temperature inversion produces a stable atmosphere, inversions can cause turbulence at the boundary between the inversion layer and the surrounding atmosphere.
- Mechanical Turbulence. When the air near the surface flows over obstructions, such as trees, plateaus, mountains or structures, the normal wind flow is transformed into swirling eddies and currents.
- Frontal Turbulence. Frontal turbulence is caused by the lifting of warm air or the abrupt wind shift between warm and cold air masses. The most severe cases are associated with fast-moving cold fronts.
- Mountain Wave Turbulence. As air flows over mountains and down the leeward side, a standing mountain wave is formed and air currents oscillate between altitudes. It can extend for hundreds of miles downwind of the mountain range.
- Thunderstorm Turbulence.The storm cloud is only the visible portion of a turbulent system. Updrafts and downdrafts often extend outside of the storm, with severe turbulence possible as much as 15 to 30 miles downwind and 5,000 feet above the storm.
It’s Thunderstorm Season
While turbulence is normal and happens often, it can be dangerous and each season has its weather challenges. Spring has fast-moving fronts and high winds causing severe squall lines. Summer has thunderstorms and tropical storms. Autumn is usually the quietest season. Winter has higher winds, blizzards and more frequent clear air turbulence. Visible moisture (cloud) is typically indicative of a bumpy ride, but often, air movement that causes bumps is invisible and occurs unexpectedly. In April, the Jetstream and its associated clear air turbulence have shifted north into Canada, usurped by convective turbulence that peaks April through August across North America.
From April 16 through June 15 of 2018, data from my carriers TAPS equipped aircraft (Turbulence Auto PIREP System) reported that there were 200 incidents of moderate or severe turbulence. Approximately 76 percent of the moderate or greater turbulence was below FL200, and 61 percent of all encounters were below 15,000. Take a look at an actual TAPS report from one of my flights at work. It’s included with this article and shows both the positive and negative “g” values that we encountered while flying through smallish, “popcorn,” or “cotton ball” cumulus clouds. A cumulonimbus monster would have been much worse.
Just the Facts, Ma’am
Thunderstorm hazards include extreme turbulence, hail causing severe structural damage, severe icing, tornados, lightning, microbursts, wind gusts over 50 knots, extreme rain, engine compressor stalls, flameouts, piston engine failures, loss of lift, zero visibility and hydroplaning. Did you notice the occurrences of the words “severe” and “extreme?” If you fly through this crap, you’d better be wearing a diaper – you’re going to need one. Twin and turbine pilots operate in the same slice of the atmosphere as the Part 121 folks, so consider these facts:
In nonfatal accidents, in-flight turbulence is the leading cause of injuries to airline passengers and flight attendants.
Each year, approximately 58 people in the United States are injured by turbulence while not wearing their seat belts.
From 1980 through 2008, U.S. air carriers had 234 turbulence incidents resulting in 298 serious injuries and three fatalities.
Of the 298 serious injuries, 184 involved flight attendants and 114 involved passengers.
How to Report It
The AIM’s list of turbulence classifications can lead one to believe that the levels of turbulence progress from light chop to light turbulence to moderate chop to moderate turbulence – switching back-and-forth between chop and turbulence as the ride gets worse. Despite the “correct” order from the AIM, in practice, most pilots report turbulence as increasing in the following order:
Light Chop: Slight, rapid, often rhythmic bumps without significant changes in altitude or attitude. Depending on the type of aircraft (i.e. wing loading), the ride is tolerable and the seat belt sign is not necessary in transport category aircraft.
Moderate Chop: Slightly greater intensity, but still no appreciable changes in altitude or attitude. Typically, it’s time to turn on the seat belt sign in large aircraft, and the cabin crew will stow service carts but remain up-and-about in the cabin. Pilots will query ATC for ride reports.
Light Turbulence: Momentarily causes slight, erratic changes in altitude and attitude (pitch, roll, or yaw). Occupants may feel a slight strain against seat belts or shoulder straps. Unsecured objects may be displaced slightly. The seat belt sign is on, and the cabin crew is seated. Pilots may seek a different altitude or route to escape from the rough air.
Moderate Turbulence: Similar to light turbulence but of greater intensity, although the aircraft remains in positive control. Occupants feel definite strains against seatbelts or shoulder straps. Unsecured objects are dislodged. A different altitude or route will be necessary.
Severe Turbulence: Causes large abrupt changes in altitude and attitude. The aircraft may be momentarily out of control. Structural damage is possible. Occupants are forced violently against seat belts or shoulder straps. All unsecured objects, including heavy serving carts, are tossed around. This level of turbulence is often categorized as “uninhabitable.”
Extreme Turbulence: Causes the aircraft to be violently tossed about in altitude and attitude. The aircraft will occasionally be out of control, and structural damage is likely. Occupants are forced violently against seat belts or shoulder straps – injuries are likely. All unsecured objects are tossed about violently; some objects may break free from retainers.
All levels of turbulence are reported as occasional, intermittent or continuous. A typical report heard over the radio may sound like this: “L.A. center, Citation Six Romeo Bravo, three-niner-zero – smooth.” Or “…continuous light, occasional moderate chop” or “…intermittent light turbulence, request a ride report.”
In the 17th century, humpty dumpty was not only a drink of brandy and ale but also slang to describe a short, clumsy and dull person, like an egg. And if such a clumsy person were to fall down from a wall, it would be an irreversible event. Like the metaphorical lessons garnered from macabre nursery rhymes, our knowledge, wisdom and a healthy respect for the atmosphere will help to keep our airframes away from an irreversible event, and our tales (homonym and pun intended) from the paring knife. From our pilot-y perspective, few things are as impressive as a 200-knot jet stream, a fire and brimstone producing thunderstorm, the kidney rupturing lenticular clouds over a
mountain range or the roll cloud in front of a microburst – if we’re on the ground looking up, that is. When airborne near this turbulence-producing phenom-
enon, don’t be a clumsy egg and go to pieces by flying too close. Stay far away with radar on, eyes wide open and your tail tucked between your legs. It will likely add a couple of minutes to your ETA but may save you from a fairytale-like demon – and postpone the need