AC 90-23G

Aircraft Wake Turbulence (AC 90-23G)

Advisory Circular 90-23G provides guidance on avoiding wake turbulence from large aircraft. It explains how wake vortices are generated, their behavior (descent, outward drift, persistence), and pilot techniques for avoiding wake encounters during takeoff, landing, and enroute flight.

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Why This Document Matters

Wake turbulence is an invisible killer in aviation, and this AC explains the aerodynamics behind it and how to avoid it. Wake vortices are generated by every aircraft producing lift, but large, heavy aircraft at slow speeds and clean configuration produce the most dangerous wakes. The vortices descend at about 400-500 feet per minute and drift laterally with the wind. This is heavily tested on knowledge exams and is critical practical knowledge when operating near airports that serve large aircraft.

Study This Document in One Loop

What is the Study Loop?

A 30-60 minute scenario-first session that replaces hours of passive reading.

Stage 1

Scenario

Departing a Class C airport in a Cessna 172. You're #3 for takeoff. #1 is a Boeing 737 departing runway 13, #2 is a King Air 350 cleared to position and hold. Wind is 140° at 6 kt. Tower clears #2 and then says "Skyhawk 1234X, position and hold behind the King Air." What's your wake separation, and where will you rotate?

Stage 2

Decision

How far behind the 737 can you safely rotate? Where are the 737's wake vortices likely to be now, and where will they drift in the next 60 seconds? Can you refuse or delay the clearance?

Write your answer before you open the handbook. That exposes the gap.

Stage 3

Targeted Learning

Open only these sections of the AC 90-23G:

Chapter on Wake Generation (heavy/clean/slow = worst wake)
Chapter on Vortex Behavior (descent 400-500 fpm, lateral drift with wind)
Chapter on Takeoff and Landing Avoidance (rotate before the heavy's rotation point, climb above)
Chapter on ATC Wake Separation (3/4/5/6 NM based on weight categories)

Stage 4

Debrief

Compare your Decision to what the handbook says:

?Wake from 737: where does the worst wake sit 90 seconds after their rotation? Descending at ~400 fpm and drifting with the 6-kt quartering wind?
?Rotate BEFORE the 737's rotation point (their vortices start where they pulled back). Can you see their rotation mark from your position?
?Climb ABOVE their climb path as soon as safely possible. What climb gradient does your 172 have at gross weight and today's DA?
?ATC separation may or may not meet your safety margin. When is it right to say "Unable, wake turbulence"?

Stage 5

Reinforcement

Turn your biggest miss into fast-recall rules:

Wake descends ~400-500 fpm and drifts at ~5 kt laterally. Stay ABOVE and UPWIND of the preceding heavy's flight path.
Takeoff: rotate BEFORE the heavy rotated, climb ABOVE their climb path.
Landing: stay ABOVE the heavy's approach path, touch down BEYOND their touchdown zone.

What Order to Read the AC 90-23G

Don't read by chapter number. Work the four phases. Start with whichever you're weakest in.

Survival Thinking

“What can hurt me?”

•Wake vortex encounter recovery (fly through, don't fight it — it passes in seconds)
•Takeoff and landing avoidance geometry (above and upwind)

Interpretation

“What am I looking at?”

•Wake strength factors: weight, clean config, slow airspeed = worst
•Vortex behavior (descent rate, drift, persistence)

Prediction

“What will happen?”

•Predicting where the preceding aircraft's wake is now vs. in 2 minutes

Checkride Mode

“Can I explain it under pressure?”

•DPE will ask wake separation when departing or arriving behind a heavy. Know the rules.
•"Caution wake turbulence" from tower does not shift responsibility — still your call.

Chapter-by-Chapter Guide

What each section covers and the key topics to study

Wake Vortex Generation and Behavior

How wake vortices are created, their strength factors, descent rate, and dissipation characteristics.

Key Topics

Vortex generation (wingtip vortices)Heavy, clean, slow = strongest vorticesDescent rate (400-500 fpm)Persistence (up to 3 minutes)Crosswind effects on vortex drift

Avoidance Procedures

Specific procedures for avoiding wake turbulence during takeoff, landing, and enroute operations.

Key Topics

Landing behind a large aircraft — stay above glidepath, land beyond touchdown pointDeparting behind a large aircraft — rotate prior to their rotation pointParallel runway operationsIntersection departures

Study Tips

Memorize the key wake turbulence avoidance rules: when landing behind a large aircraft, stay at or above the large aircraft's glidepath and land beyond its touchdown point.
Understand that wake vortices are worst when the generating aircraft is heavy, clean (no flaps), and slow — this is typically on approach or departure.
Know how crosswind affects wake vortices: a light crosswind (5 knots) can keep the upwind vortex on the runway while moving the downwind vortex away. No wind means both vortices stay near the runway.

Frequently Asked Questions

How long do wake vortices last?

Wake vortices can persist for up to 3 minutes in calm wind conditions. They begin to dissipate when they contact the ground or are broken up by wind. A crosswind of 5 knots or more is generally sufficient to move vortices away from the runway.

When are wake vortices most dangerous?

Wake vortices are most dangerous when following a large, heavy aircraft that is in a clean configuration (no flaps) at slow speed — typically during approach or departure. Light aircraft are most vulnerable due to their lower wing loading.

Quick Facts

Document ID: AC 90-23G
Last Updated: 2014
Cost: Free
Publisher: FAA

Applies To

StudentPrivateInstrumentCommercialCFIATP

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