AIM ¶ 7-4-3 — Wake Vortex Strength
AIM 7-4-3: Why wake vortices are strongest behind heavy, clean, slow aircraft, plus induced roll hazards for short-span aircraft. Study guide for checkrides.
In Plain English
Wake turbulence is generated by every aircraft producing lift, but its strength depends on a few key factors of the generating aircraft:
- Weight — heavier aircraft create stronger vortices
- Speed — slower flight produces stronger vortices
- Wingspan and wing shape — these govern the geometry of the vortex
- Configuration — flaps and other devices change vortex characteristics; a "dirty" configuration hastens wake decay
The worst-case scenario — the strongest vortex — occurs when the generating aircraft is HEAVY, CLEAN, and SLOW. Memorize that phrase; it directly drives why takeoff and landing phases behind large jets are the highest-risk wake encounter scenarios.
The primary hazard from a wake encounter is induced roll — rolling moments that can exceed your aircraft's roll-control authority. Flight testing showed:
- Aircraft tend to be ejected from the vortex naturally rather than stay trapped in it.
- Counter-control is generally effective when your wingspan and ailerons extend beyond the vortex's rotational flow field.
- Short-span aircraft (relative to the generator) have the hardest time countering induced roll, even high-performance types.
In rare cases a wake encounter can cause catastrophic structural damage, but induced roll is the typical threat. Small-aircraft pilots must be especially vigilant behind larger traffic.
AIM Source Text
FAA AIM ¶ 7-4-37-4-3. 7-4-3. Vortex Strength
Weight, speed, wingspan, and shape of the generating aircraft's wing all govern the strength of the vortex. The vortex characteristics of any given aircraft can also be changed by extension of flaps or other wing configuring devices. However, the vortex strength from an aircraft increases proportionately to an increase in operating weight or a decrease in aircraft speed. Since the turbulence from a “dirty” aircraft configuration hastens wake decay, the greatest vortex strength occurs when the generating aircraft is HEAVY, CLEAN, and SLOW. Induced Roll In rare instances, a wake encounter could cause catastrophic inflight structural damage to an aircraft. However, the usual hazard is associated with induced rolling moments that can exceed the roll-control authority of the encountering aircraft. During inflight testing, aircraft intentionally flew directly up trailing vortex cores of larger aircraft. These tests demonstrated that the ability of aircraft to counteract the roll imposed by wake vortex depends primarily on the wingspan and counter-control responsiveness of the encountering aircraft. These tests also demonstrated the difficulty of an aircraft to remain within a wake vortex. The natural tendency is for the circulation to eject aircraft from the vortex. Counter control is usually effective and induced roll minimal in cases where the wingspan and ailerons of the encountering aircraft extend beyond the rotational flow field of the vortex. It is more difficult for aircraft with short wingspan (relative to the generating aircraft) to counter the imposed roll induced by vortex flow. Pilots of short span aircraft, even of the high performance type, must be especially alert to vortex encounters. (See FIG 7-4-2 .) FIG 7-4-2 Wake Encounter Counter Control
Oral Exam Questions a DPE Might Ask
Q1Under what conditions does a generating aircraft produce the strongest wake vortices?
Per AIM 7-4-3, the greatest vortex strength occurs when the generating aircraft is HEAVY, CLEAN, and SLOW. Vortex strength increases with operating weight and decreases with speed, and a clean configuration delays wake decay (a dirty configuration hastens it).
Q2What is the primary hazard of a wake vortex encounter, and what determines an aircraft's ability to counter it?
Per AIM 7-4-3, the usual hazard is induced rolling moments that can exceed the encountering aircraft's roll-control authority. The ability to counteract this roll depends primarily on the wingspan and counter-control responsiveness of the encountering aircraft.
Q3Why must pilots of short-span aircraft be especially alert to wake vortex encounters?
Per AIM 7-4-3, counter-control is usually effective when wingspan and ailerons extend beyond the vortex's rotational flow field. Short-span aircraft — even high-performance types — cannot extend beyond that flow field as easily, making it more difficult to counter the imposed roll.
Related Paragraphs in AIM Chapter 7