7.4 Wind Shear, Microbursts, Turbulence, and Mountain Waves

ACS PA.I.C.K3g is turbulence. PA.I.C.K3h includes microbursts. Chapter 6 introduced wind shear and lenticulars as wind-system theory. This section is the hazard: what the airplane does in a microburst, how intense the bumps are allowed to be called, and how to stay out of a heavy jet’s wake.

Wind shear and the microburst

Wind shear is a sudden change in wind speed and/or direction over a short distance. Low-level wind shear (LLWS) is the same thing close to the ground, where you have no altitude to trade. PHAK’s usual sources: fronts, thunderstorms, temperature inversions, and strong winds (greater than 25 knots) above that inversion. AIRMET Tango can flag non-convective LLWS potential below 2,000 feet AGL.

Performance rule you must be able to say out loud:

• Tailwind → headwind: indicated airspeed and performance increase.
• Headwind → tailwind: indicated airspeed and performance decrease.

The second case, near the ground, is how airplanes hit the overrun.

A microburst is the most severe low-level shear. AIM 7-1-24: a small-scale intense downdraft that hits the surface and spreads outward in all directions, producing both vertical and horizontal shear. Parent clouds can be ordinary convective cells, not only a spectacular CB. Microbursts occur in heavy rain and in benign-looking virga. A ring of blowing dust may be the only surface clue when little rain reaches the ground.

Published dimensions — cite them as handbook/AIM values, not as a single invented “always 1.5 miles” fact:

AIM Figure 7-1-14 is the PAR storyboard. On takeoff or approach the airplane first meets a headwind (performance increases — airspeed jumps, you feel “rich”). Then the headwind dies, a downdraft hits, and a tailwind arrives (performance decreases). A trainer that rotated fat on the first gust is now slow, sinking, and out of runway. The correct PAR decision is not to be there: hold short or go around and leave when virga, a blowing-dust ring, a thunderstorm, or a microburst alert is on the departure or arrival path. There is no published private-pilot “add 20 knots and poke through it” technique.

Jordan’s takeoff into virga from 7.1 is this item. The rain that never reaches the runway is evaporating into dry air under the convective base — PHAK’s microburst setup. Increasing performance on the roll is not a gift. It is the first half of the shear.

Turbulence by source and by intensity

Name turbulence by what made it, then by how hard it hits.

• Convective / thunderstorm — updrafts and downdrafts in unstable air and in every thunderstorm. Any Convective SIGMET implies severe or greater turbulence. AIM: expect turbulence near convective activity even in clear air.
• Thermal (convective current) — sunny-day bubbles over pavement and bare ground; sink over water and vegetation. Bumpy below a few thousand feet on a summer afternoon; usually light to moderate unless a tower is building.
• Mechanical — wind over hangars, trees, ridgelines, and city blocks. Worse with stronger wind and bluffer obstacles. The lee side of a hill in a stiff wind is a sink-and-rotor problem at pattern altitude.
• Mountain wave — stable air and a strong wind blowing nearly perpendicular to a ridge. The air oscillates downstream. Standing lenticular clouds (including ACSL) mark the wave crests. They can look smooth and still. Air is moving through them at high speed. Rotor clouds under a crest, on the lee side near or below ridge level, mark breaking, possibly severe turbulence. Do not fly the rotor “to see.” Wave energy can extend far above the peaks (the handbook notes effects sometimes above 60,000 feet) and many miles downwind. The lee-side downdraft when you fly toward the ridge into the wind is PHAK’s terrain-collision setup.
• Clear-air turbulence (CAT) — often near the jet stream and tropopause, with no cloud warning. More of a high-altitude story, but the name belongs on the list.
• Wake turbulence — man-made, treated below.

PIREP intensity (AIM / Aviation Weather Handbook) is the vocabulary the test uses:

Report what you flew, not what the radar painted. ATC radar does not detect turbulence (AIM). Rain intensity is only a clue, and a bad one next to a thunderstorm.

Wake turbulence — geometry, not folklore

Every airplane that is producing lift trails two counter-rotating wingtip vortices (AIM 7-4). Strength increases with weight and decreases with speed; a “dirty” configuration helps the wake decay. The greatest vortex is therefore heavy, clean, and slow — a heavy jet just after rotation or just before landing, flaps not yet doing you any favor as a follower.

Behavior you must visualize:

• Vortices exist from rotation to touchdown. Note those two points.
• They sink at several hundred feet per minute and drift with the wind.
• Near the ground (within about 100 to 200 feet) they move laterally at about 2 or 3 knots in calm air.
• A light cross-runway component of 1 to 5 knots can hold the upwind vortex on the runway and shove the downwind vortex toward a parallel runway.
• A tailwind can push vortices forward into the touchdown zone. AIM’s all-caps line: the light quartering tailwind requires maximum caution.

AIM 7-4-6 avoidance (same-runway picture):

• Landing behind a larger aircraft: stay at or above its final-approach path, note the touchdown, land beyond it.
• Departing behind a larger aircraft: note its rotation point, rotate before that point, and climb above its path until you turn clear. Do not cross below and behind later.
• Landing behind a departing larger aircraft on the same runway: land well before its rotation point (so you never fly through the wake that begins at rotation).
• After a larger aircraft’s low approach, missed approach, or touch-and-go: vortices can sit on the runway. AIM: wait at least 2 minutes before takeoff or landing.
• Helicopters: stay outside about three rotor diameters of a slow hover taxi or hover; treat arriving and departing helicopters like other strong vortex generators.

ATC may say “caution — wake turbulence.” Whether or not you hear it, the pilot owns the interval. Accepting a visual follow is accepting the wake.

Sam lands a 172 after a heavy jet on a calm-to-light quartering tailwind day. The jet touches down long. Sam does not plant on the numbers in the jet’s leftovers. Sam flies the glidepath above the jet’s path and lands beyond the jet’s nose-gear marks — or waits. Rotating after the jet’s rotation point on departure would put Sam into the sinking pair. Rotate before, climb above.

Do not invent a complete unpublished wait-time table for every weight class. Use AIM’s 2-minute missed/low-approach interval and the rotate-before / land-beyond geometry. Controllers apply additional time or radar wake separation; that is their minima, not a substitute for seeing the vortices in your head.