AC 00-24C

Thunderstorms (AC 00-24C)

Advisory Circular 00-24C provides guidance on thunderstorm hazards and avoidance. It covers the lifecycle of thunderstorms (cumulus, mature, dissipating stages), associated hazards (turbulence, hail, microbursts, lightning, icing), and recommended avoidance procedures for VFR and IFR pilots.

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Why This Document Matters

Thunderstorms are among the most dangerous weather phenomena for aviation, and this AC is the FAA guide to understanding and avoiding them. It covers the three stages of thunderstorm development, the hazards each stage presents, and practical guidance for avoidance. The AC is particularly important for instrument pilots who may need to deviate around thunderstorms, but VFR pilots must also understand when convective weather makes flying unsafe. Thunderstorm questions appear on every FAA knowledge test.

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

You're at 6,500 ft VFR, clear above, but a line of thunderstorms is painted on your iPad 35 NM ahead directly on route. Tops to FL380. ATIS at your planned fuel stop reports BKN040, thunderstorm passing 5 NM W. You have 45 min of VFR daylight left. Detour adds 30 min of fuel you have, but the nearest non-storm airport is 60 NM SE with a 45-min ETA.

Stage 2

Decision

Continue, detour N/S around, or land immediately? At what distance is "avoid by 20 NM" actually important? What happens if you try to climb over?

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

Stage 3

Targeted Learning

Open only these sections of the AC 00-24C:

Chapter on Thunderstorm Lifecycle (cumulus, mature, dissipating — hazards per stage)
Chapter on Avoidance (20 NM clearance, never fly under, never climb over, never penetrate)
Chapter on Associated Hazards (hail 20 NM ahead, microbursts, severe turbulence)
Chapter on If You Must Fly In (aircraft config, power/attitude, do not try to maintain altitude)

Stage 4

Debrief

Compare your Decision to what the handbook says:

?Mature stage hazards: lightning, hail (up to 20 NM ahead), severe turbulence, microbursts. Which kills VFR pilots most?
?Cloud clearance: "avoid by 20 NM" is not arbitrary. Hail and severe turbulence extend that far laterally from a cell.
?Climb over: tops can be 50,000+ ft. A 172 has a service ceiling of 14,000. Why is this even a tempting thought?
?Microbursts on departure or approach: what are the signs? What do you do if you encounter one?

Stage 5

Reinforcement

Turn your biggest miss into fast-recall rules:

Thunderstorm avoidance: 20 NM minimum lateral clearance. Never under. Never over. Never through.
Mature stage is the killer — lightning, hail, microbursts, severe turbulence. Avoid, do not explore.
If you are already in one: fly attitude, maintain heading, accept altitude loss. Do not fight it.

What Order to Read the AC 00-24C

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

Survival Thinking

“What can hurt me?”

•Avoidance procedures (before you need escape tactics)
•Microburst recognition and escape (max performance climb, accept altitude loss)

Interpretation

“What am I looking at?”

•Thunderstorm lifecycle and hazards per stage
•Associated weather (hail, tornadoes, gust fronts, wind shear)

Prediction

“What will happen?”

•When storms will develop (airmass, frontal, squall line triggers)

Checkride Mode

“Can I explain it under pressure?”

•DPE will ask "How close can you fly to a thunderstorm?" Answer: 20 NM, and why.
•Know the three lifecycle stages and their primary hazards

Chapter-by-Chapter Guide

What each section covers and the key topics to study

Thunderstorm Formation and Lifecycle

The three stages of thunderstorm development and conditions required for thunderstorm formation.

Key Topics

Cumulus stage (updrafts only)Mature stage (updrafts and downdrafts, most dangerous)Dissipating stage (downdrafts only)Three requirements: moisture, lifting action, unstable air

Thunderstorm Hazards and Avoidance

Specific hazards including microbursts, hail, lightning, and recommended avoidance procedures.

Key Topics

Microburst wind shearHail potential (can be thrown miles from storm)Lightning and St. Elmo's Fire20 NM lateral avoidance minimumNever fly under a thunderstorm anvil

Study Tips

Memorize the three stages: cumulus (updrafts only, building), mature (most dangerous, both updrafts and downdrafts, precipitation begins), dissipating (downdrafts only, rain without lightning).
Remember the 20 NM rule: avoid thunderstorms by at least 20 nautical miles laterally, especially anvil clouds where hail can be thrown well clear of the visible storm.
Understand microbursts: a localized column of sinking air that can produce wind shear of 45 knots within 2-4 miles. Microbursts last 5-15 minutes and are the most dangerous near the ground during takeoff and landing.

Frequently Asked Questions

What are the three stages of a thunderstorm?

The three stages are: (1) Cumulus — characterized by updrafts only, building vertically; (2) Mature — the most dangerous stage with both updrafts and downdrafts, precipitation reaching the ground, and maximum turbulence; (3) Dissipating — characterized by downdrafts only, with the storm weakening.

How far should I stay away from a thunderstorm?

The FAA recommends staying at least 20 NM away from thunderstorms, especially severe ones. Hail can be thrown several miles from the storm cell, turbulence can extend well beyond the visible cloud, and gust fronts can spread far ahead of the storm.

Quick Facts

Document ID: AC 00-24C
Last Updated: 2013
Cost: Free
Publisher: FAA

Applies To

StudentPrivateInstrumentCommercialCFI

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