AINsight: Who’s Responsible for Wake Vortex Avoidance?

 - July 22, 2022, 11:39 AM

Wake turbulence can be a threat on any flight. Every aircraft, both large and small, generate wake turbulence as a function of creating lift. Wake turbulence vortices can vary in strength, duration, and direction and if encountered can cause a loss of control inflight event or accident. The trick to surviving a wake turbulence encounter is to avoid it altogether.

Under IFR flying, wake turbulence avoidance is accomplished by air traffic controllers applying minimum separation standards based on each aircraft’s class, as determined by size or aerodynamic characteristics. Separation may be accomplished by assigning specific speeds (distance and time) or altitudes to be flown. Pilots are expected to fly the speed and altitude assigned by controllers to maintain this minimum separation. 

A pilot accepting a clearance to visually follow a preceding aircraft accepts the responsibility for traffic separation and wake turbulence avoidance. This is a common scenario for a wake turbulence encounter when pilots accept a clearance for a visual approach behind landing traffic. In this case, the pilot must maintain separation both vertically and horizontally from the preceding aircraft.

According to the Aeronautical Information Manual (AIM), the most common hazard of a wake turbulence encounter is associated with induced rolling moments that can exceed the roll control authority of an aircraft. In rare cases, the wake encounter can cause catastrophic in-flight structural damage.

An in-flight wake turbulence encounter close to the ground is almost always fatal. Wake turbulence encounters at higher altitudes can only be mitigated through proper and appropriate upset recovery training. 

Business jets are not immune from wake turbulence encounters. Notable examples include two fatal wake turbulence encounters—where separation was lost behind a larger airliner during the approach phase of flight—and a more recent wake turbulence encounter during cruise that resulted in a serious in-flight upset. 

In the first example, an Israel Aircraft Industries 1124A Westwind on a visual approach lost wake turbulence separation with the preceding aircraft, a Boeing 757-200. Both aircraft were executing visual approaches to the John Wayne Airport (KSNA) in Santa Ana, California.

Prior to the accident, the pilots of the Westwind were instructed by ATC to follow the Boeing 757, reduce speed to follow, and were cleared for the visual approach to Runway 19R. At this point, the Westwind was 3.3 nm behind the 757 on a converging course. 

Roughly, 20 seconds later, the first officer told the captain, “Eh, he’s pretty close.” The captain responded, “Okay, I’m, ah, let's go flaps twelve,” followed by, “I got him, okay we can do it—no problem.”

Approximately, 35 seconds after being cleared for the visual approach the Westwind was instructed to contact the tower. Upon initial check-in with the tower, the tower controller advised the Westwind that the 757 was indicating 30 knots slower. At this point, the Westwind had closed to about 2.2 nm behind the 757 passing through 3,700 feet. This exchange with ATC was the last recorded radio transmission by the accident aircraft.

Over the next 30 seconds, the Westwind’s CVR recordings indicated the completion of the landing checklist and the first officer’s concerns about the glide path of the 757 stating, “A little too high on the ah…” and the distance between the two aircraft saying, “I don’t know looks kinda close.”

The last radar returns from the Westwind indicated that it was at 1,100 feet and 2.1 nm behind the 757. Witnesses reported seeing the aircraft on final approach and then suddenly pitching down and rolling about its longitudinal axis and crashing. The two pilots and three passengers were killed. 

According to the NTSB, the probable cause of the accident was the pilot in command's failure to maintain adequate separation behind the 757 and failure to remain above its flight path, which resulted in a wake turbulence encounter. 

Another wake turbulence encounter involved a Bombardier Learjet 45 that crashed on approach to the Mexico City International Airport (MMMX), killing all nine aboard the super-light jet and seven on the ground. The accident report from this event determined the crash was the result of the Learjet flying too closely behind a Boeing 767-300ER.

At the time of the accident, the Learjet was 4.1 nm behind the 767. But the minimum allowable distance for the lighter aircraft to follow the “heavy” jet is 5 nm.

Contributing to the accident was the flight crew’s delay in slowing the Learjet as instructed by ATC. Controllers issued a speed reduction, but it took more than a minute for the pilots of the Learjet to respond.

At the time of these ATC instructions, the Learjet was overtaking the 767 by approximately 80 knots. Likewise, the pilots during the arrival used a descent technique—stepping down rather than a continuous descent—that placed the Learjet’s vertical flight path below the path of the 767. 

Investigators determined that the Learjet entered a violent wake turbulence vortex that rolled the aircraft over and pitched the nose down at an altitude (1,700 feet agl) that was too low to recover.

Less common, but more survivable, are wake turbulence encounters during the cruise phase of flight. In January 2017, a Bombardier Challenger 604 lost control as it flew beneath an Airbus A380. Pilots of the Challenger told investigators that the wake turbulence from the A380 caused their aircraft to lose 9,000 feet and roll through “several rotations.”

As described in the report, pilots experienced a “temporary loss of control” about one minute after the A380 passed overhead about 1,000 feet above the Challenger in Indian airspace over the Arabian Sea. At the onset of the wake turbulence encounter, the Challenger rolled to the left and several flight deck displays and related systems failed, as did the left engine due to a temperature exceedance. 

After the upset, the crew declared an emergency and diverted to the Muscat International Airport in Oman. Two passengers were seriously injured and two other passengers and the flight attendant received minor injuries. The aircraft was substantially damaged, and Bombardier said the aircraft “could not be returned to an airworthy state.”

Back to the AIM, as a reminder, the wake turbulence vortex strength is determined by an aircraft’s weight, speed, wingspan, and shape of the wing. The vortex strength increases proportionally to an increase in operating weight or a decrease in aircraft speed. The greatest vortex strength occurs when the generating aircraft is heavy, clean, and slow—this is problematic in the terminal area when following a larger aircraft. 

Of importance, the AIM stresses, “Avoid the area below and behind the wake-generating aircraft. Especially, at low altitude where even a momentary wake encounter could be catastrophic.” This is important since wake turbulence vortices settle below the path of the generating aircraft.

Other wake turbulence avoidance tips from the AIM: when following a larger aircraft, it is important to stay above the larger aircraft’s final approach flight path and land beyond its touchdown point. And on parallel runways (closer than 2,500 feet), be aware of the potential for the vortex to drift over your runway. 

Other avoidance procedures relate to landing behind a larger departing aircraft—in this case, note the point that the larger aircraft rotated and land well before that point. Likewise, if departing behind a larger aircraft, attempt to rotate prior to its rotation point and climb above its flight path.

A review of air traffic wake turbulence separation requirements is a great place to develop your own minimum acceptable distance to be flown behind a larger aircraft. As an example, a small or large aircraft behind a heavy aircraft requires a minimum of 5 nm of separation.

Pilots should use tools such as TCAS displays to improve situational awareness and define the minimum distance when following a larger aircraft. Additionally, situational awareness can only be improved by listening up for the “heavy” callsigns. 

The explosion of e-commerce has added a lot of bigger cargo jets to the U.S. National Airspace System, often operating at smaller airports. Cargo airlines such as Atlas (“Giant”), Air Transport International, FedEx, UPS, and Kalitta almost exclusively operate heavy jets or Boeing 757s that have nasty wake turbulence characteristics.

Pilots should develop strategies to avoid wake turbulence and have a plan to recover from an inadvertent encounter. Developing a strategy for avoidance involves a deep understanding of wake turbulence vortex generation, strength, behavior, and operational problem areas (i.e. near the ground).

Likewise, being knowledgeable on avoidance procedures—especially those outlined in the AIM—and pilot responsibilities, including the acceptance of visual approaches, will help mitigate the risk of a wake turbulence encounter.

The opinions expressed in this column are those of the author and not necessarily endorsed by AIN Media Group.