Respect the Sign

Turbulence is a daily reality in aviation, but that doesn’t stop all pilots from doing their best to steer clear of it. The possible absence of visual clues where turbulence resides forces flight crews to rely on their knowledge of its causes as well as reports from other nearby aircraft to avoid it. Although a recent trend of turbulence related injuries might substantiate a dose of fear in the mind of some passengers, the likelihood of aircraft damage resulting in a crash is highly remote. However, unexpected encounters of rough air with passengers and flight attendants not strapped in their seats is cause for concern and certainly bares reason to respect an illuminated seatbelt sign.

By definition, turbulence is an unsteady movement of air. Because of the uneven heating of our planet by the suns rays, resulting temperature variations give birth to wind as well as our global weather patterns - all having their hand in the creation of turbulence. Disruption in the flow of wind caused either by obstacles in its path or different wind velocities interacting with one another can create undetectable turbulence better known as clear-air turbulence (CAT). Although turbulence in-of-itself is invisible, inclement weather in the form of thunderstorms or where two or more weather fronts meet often produces obvious signs of bumpy air ahead. Minus these visual clues however, CAT demands a knowledgeable crew to anticipate its likelihood.

Surface heating causes turbulent pockets of rising warm air called thermals, especially in the summer months. Thermals can be found all over the earth’s surface where the sun has a pin-pointed and dramatic impact on objects such as pavement and mountain sides compared to lesser heat absorbed surroundings. Particularly during departure and arrival phases of flight, these often unavoidable turbulent spots are cause for annoyance with potential for nausea!

Couple rising warm air with moisture and cumulous clouds (tall and puffy appearance) can form. These moderately turbulent clouds can eventually grow into thunderstorms (cumulonimbus clouds), often higher than 60,000 feet. Within a thunderstorm, rising moist air cools and condenses into water droplets large enough to fall to the surface as rain. The combination of rising and falling air within the core of a storm is cause for severe turbulence and is to be avoided – often by more than 20 miles. Thunderstorms in the vicinity of airports are a major threat to departing and approaching aircraft due to their associated downdrafts as a result of heavy rainfall. These downdrafts impact the ground and disperse in all directions causing dramatic wind shear called a “microburst”. Microbursts have been linked to numerous aircraft accidents, spawning a plethora of both onboard and ground-based technology to detect and help aircraft avoid their presence.

Turbulence related to a disruption in wind is usually the most difficult to avoid due to the absence of visual cues, i.e., CAT. At higher altitudes CAT can be found in numerous locations, most notably near the jet stream and mountainous areas. The jet stream is a high altitude river of air that flows from west to east. Pilots use the jet stream to their advantage when flying east, as these winds can provide well over a 200mph tailwind. Flying west is clearly a detriment, but either way the price to pay with the jet stream is the turbulence it can cause when flying near its edge. Just like a fast flowing river tends to create “eddies” of turbulent water where it meets a stationary shore, the jet stream also creates turbulent eddies where it meets relatively still air.

Wind related turbulence is also often imparted by interference with stationary objects. As wind forces air up the side of a mountain, gravity attempts to restore the risen air back down the other side. This gravitational pull alongside the horizontal push of wind at a mountaintop causes a phenomenon called “mountain wave turbulence” and can be quite severe. The presence of saucer shaped “lenticular” clouds occasionally lends a visible clue of existing mountain wave turbulence, often spanning more than 200 miles downwind of a mountain range. Closer to the ground, buildings and trees tend to disturb surface winds, creating turbulence not nearly as severe as mountain wave turbulence.

Rough air also tends to reside where air masses converge to form frontal boundaries. Air masses are large volumes of temperature and moisture-constant air, often spanning thousands of square miles. Where two different air masses meet i.e., the frontal boundary, winds have a tendency to suddenly change in both direction and speed, resulting in turbulence of varying severity. One obvious clue of a frontal boundary is a defined change in weather, such as a sharp and drastic line where thunderstorms and/or cloud formations begin and end.

Avoiding turbulence begins on the ground with thorough pre-flight planning. Prior to each flight, pilots digest an array of meteorological data at their disposal and enact a strategy of steering clear, or at the very least, coping with areas of bumpy air. To paint a big picture of the weather issues facing a particular flight, crews plot their route on a map that simultaneously displays weather radar returns, frontal boundaries, and wind data. Where threats are pinpointed, a decision may be coordinated with a flight planner to alter a route or cruising altitude in an effort to deviate around expected turbulence.

Once in flight, flight crews use onboard weather radar as well as their own eyes to avoid potential rough air. Additionally, radio communicated turbulence reports between air traffic control (ATC) and other nearby aircraft can provide a “heads-up” solution for other aircraft seeking a smooth ride. For example, an aircraft experiencing turbulence at 36,000’ might report their experience and request a “ride report” from ATC. ATC in turn may inform the pilots that an aircraft 50 miles ahead of them is reporting smooth conditions at 32,000’. At the pilot’s request, ATC would likely permit a descent to the affected aircraft in search of smoother air.

When juggling decisions, pilots must always prioritize safe outcomes, occasionally at the detriment of passenger comfort. Consequently, aircraft or fuel endurance limitations may prevent flight crews from desired turbulence deviations. In this case, or anytime the Captain determines turbulence is a threat, extinguishing the seatbelt sign may not be in the cards. Considering a rising trend of passenger apathy toward an illuminated seatbelt sign, my hope is that this information provides a newfound respect to an often unpredictable danger.