Captain Judy’s Corner: What is turbulence?
What is the definition of turbulence?
The definition of turbulence is a sudden, violent shift in airflow. It creates up and down currents caused by irregular atmospheric motion.
We were in the clouds, Instrument Meteorological Conditions (IMC), and flying in an area without radar coverage. My first officer (FO) was busy programming the Citation jet’s system with the latitude and longitude coordinates instructed by Bangladesh controllers. I was focused on leveling the Citation at FL390 (39,000’). Suddenly the FO pointed upward exclaiming, “Oh my!”
I looked up in the direction he had been pointing right into the belly of an Airbus. The mammoth jet was so close that we could see the rivets on the underside of the plane. Immediately, our Crew Resource Management (CRM) training went into action, anticipating what was likely next: wake turbulence. Knowing turbulence could cause structural damage, I slowed the jet below maneuvering speed. The FO monitored gauges with verbal callouts, “Airspeed 116, crosschecked.”
The relief pilot had been napping in the cabin when the turbulence hit. Within moments, he was in the cockpit. While trying to keep his footing he shouted, “What the heck?!” The situation was explained quickly enough. The next few minutes seemed like hours while the turbulence forced our small jet into anything but upright. Once the Citation was through the Airbus’s vortices, the remainder of our flight to Hanoi was uneventful.
What causes turbulence?
Turbulence can be caused by many different conditions – wind, storms, jet stream and objects near the plane, such as mountain ranges. These conditions can range from the surface up to all flight levels, and there is no real way to forecast turbulence or see it on a map. During my world flight in the Citation, I experienced wake turbulence at altitude. Years prior, I experienced wake turbulence during take-off in my small, 2-seat Grumman.
On this particular day, I had been holding short while a large cargo jet was cleared for departure. The jet was departing with a light quarterly tailwind on the cross runway. The air traffic controller cleared me for departure as the large jet cleared the runway. Then the controller followed with, “Caution: wake turbulence.” I had been taught wake turbulence avoidance and knew the hazards. I followed the proper procedures as practiced during my initial training and as published in the Aeronautical Information Manual (AIM) Chapter 7, section 3. I recalled the least favorable conditions were a heavy, clean aircraft with a light quarterly tailwind.
The opportunity to climb did not present itself. Less than 500’ above the runway, the vortices pushed my Grumman down. Already at full power, the best I could do was keep the wings level and fly the airplane. The critical lesson I learned from this experience was the least favorable wind conditions and to factor in cross runways.
Student pilots learn that air is a fluid. Air can be measured and has weight. Turbulence is caused when an airplane flies through waves of air that are irregular or violent, which cause the aircraft to bounce around yawing, pitching, or rolling. Turbulence can be compared to two oceans meeting. Both oceans have large waves and currents that as the collide create even larger waves and currents.
What are the different types of turbulence?
- Wake turbulence forms behind an aircraft as it passes through the air creating wingtip vortices. These vortices can remain in the air or over the ground for up to three minutes after the passage of an aircraft. The greatest vortex strength occurs when the generating aircraft is HEAVY, CLEAN, and SLOW.
- Clear air turbulence (CAT) occurs at altitudes above 15,000 feet MSL and is caused by strong wind shears in the jet stream.
- Thermal turbulence develops when warm air rises in ‘pockets’ then slowly descends as the air cools. Under normal atmospheric conditions, air temperature usually decreases with altitude. When these conditions are reversed, cool air under a narrow, warm layer forms temperature inversion turbulence. Turbulence occurs in the surrounding boundaries of the warm layer.
- Mechanical turbulence is created when the air near the surface of the Earth flows over obstructions, such as hills, mountains, or buildings. The normal horizontal wind flow is disrupted causing swirls and irregular air movements.
- Frontal turbulence is caused by the abrupt wind shift between warm and cold air masses. Fast-moving cold fronts are the most severe in this type of turbulence.
- Air flowing over the tops of mountains can travel down the leeward side causing a flow to form with strong air current waves changing at all altitudes. Mountain wave turbulence can extend for hundreds of miles downwind of a mountain range.
- A storm cloud is the visible portion of a turbulent system in a thunderstorm. Updrafts and downdrafts often extend outside the storm with severe turbulence extending as much as 15 to 30 miles.
What are the effects of turbulence?
When an aircraft experiences turbulence, the plane can drop or change altitude suddenly. This is why pilots always caution passengers to buckle up and stay seated when they are experiencing flight turbulence. The sudden movements put passengers at risk.
How can pilots avoid turbulence?
Understanding the causes of turbulence is one of the best methods for avoiding it. Because there are different types of turbulence, knowledge is critical. Avoidance or inadvertently entering turbulence are best practiced through becoming a knowledgeable pilot so you can anticipate turbulence and be prepared to respond. Understand how and where turbulence forms and the procedures for your aircraft. Like all seasoned pilots, I can testify to the power of turbulence!
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