Turbulence and Wind Shear in Practice - ATPL 050 — Meteorology

The wind module introduced turbulence and the jet streams as physics; this lesson returns to them as hazards. What the bumps actually do to an aircraft and its occupants, how the intensity is reported, why the forecasts disappoint and the pilot reports matter, and where clear-air turbulence waits. Then the second half turns to wind shear — the change of wind that ambushes an aircraft at the two moments it can least afford surprises, just after takeoff and on final approach.

How turbulence is reported

Like icing, turbulence in reports and forecasts comes on an ICAO ladder of three qualifying terms:

Light — slight, erratic changes of attitude or altitude; coffee trembles, seat belts stay comfortable;
Moderate — larger changes of attitude and altitude with airspeed variations, but the aircraft remains in control throughout; occupants feel the strain of their belts, and unsecured objects move;
Severe — abrupt, large changes of attitude and altitude, large airspeed variations, the aircraft may be momentarily out of control; occupants are forced violently against their belts, and unsecured objects are tossed about the cabin.

What the bumps do to an aircraft

The effects are mechanical and human at once: disturbed attitude and altitude, fluctuating airspeed, g-loads that stress the structure, difficulty controlling the aircraft and even reading the instruments — and, most consistently of all, injuries to unbelted occupants and cabin crew, the commonest turbulence harm by far. The airmanship response inside the bumps: slow to the recommended turbulence-penetration speed, which protects the structure from gust loads while keeping a margin above the stall. Avoiding them in the first place is the usual two-step: in planning, the weather briefing (SIGMETs and the significant weather chart's convective and turbulence areas) and a track and altitude chosen away from them; in flight, the radar kept working around convective cells and a level or track change out of any rough layer.

Cruising at FL 310 some 870 miles east of Tokyo on 28 December 1997, a United 747 bound for Honolulu felt what the captain called wave action. He switched the seatbelt sign on again as a precaution, and announcements went out in English and Japanese asking passengers to fasten their belts; the flight attendant translating chose not to render all of it, so as not to alarm a cabin that was overwhelmingly Japanese-speaking. The radar showed light green and no red, the Doppler turbulence mode showed nothing, and the SIGMET for severe turbulence between FL 290 and FL 350 did not cover this piece of ocean. Within a minute or two the aircraft hit clear-air turbulence hard.

The flight data recorder logged a positive 1.8 g, then six seconds later a negative 0.8 g, and a roll to 18 degrees; the altitude barely moved. In the cabin this section's commonest harm did its work on the people it always finds: none of the passengers killed or seriously hurt had a seatbelt fastened. The passenger who died, in seat 46F, was unbelted, and suffered a fatal spinal injury; others described being lifted from their seats by the third negative excursion and striking the ceiling. Eighteen people were seriously injured and 171 slightly, out of 393 aboard. The crew returned to Narita and landed without further incident.

The probable cause was the inadvertent flight into adverse weather and the difficulty of forecasting turbulence over the ocean, with the clear-air turbulence itself and a safety advisory not being issued by the flight attendant as factors. The airline's bulletin afterwards commended the crew and reinforced two things: communication between the flight deck and the cabin, and its policy that passengers keep belts fastened whenever seated, sign or no sign. Forecasts lie and reports save, as the section below puts it, but over an empty ocean there may be neither — and the only defence that needs no warning at all is a buckled belt.

Turbulence, gustiness, wind shear: three words

The syllabus wants the vocabulary separated, and the wind module's definitions carry straight over:

Turbulence — irregular, chaotic air motion, eddies superimposed on the mean flow: what actually shakes the aircraft;
Gustiness — the anemometer's version: rapid fluctuations of the measured wind speed and direction at a point on the surface;
Wind shear — a change of the wind vector between two points in space: a gradient, not a motion.

The three are related by cause and effect: strong shear rolls the flow up into eddies, so shear is a producer of turbulence — which is exactly how clear-air turbulence is made.

Forecasts lie, reports save

An honest limitation, stated by the syllabus itself: turbulence forecasts are not very reliable — they shade areas of probability, and cannot time or place the encounter. The corrective is human: pilot reports of turbulence are extremely valuable, the freshest real measurement the system gets, and passing one promptly helps the aircraft behind prepare (belts on, speed back) or avoid the layer altogether. The wind module's January jet encounter made the same point: the chart drew the probability, and the report from the aircraft ahead timed it.

CAT revisited: effects and escape

Clear-air turbulence is the case where all of this bites hardest, because it strikes in clear air at cruise levels, with no visual or radar warning at all — sudden sharp jolts, airspeed fluctuations, and the cabin injuries that come from belts left open in smooth-looking sky. The wind module placed it: beside the jet streams, worst on the cold, polar side, near and just below the core, and in sharp upper troughs. Avoidance follows the placement: in planning, the significant weather chart's CAT areas and a route or level clear of the jet's cold flank; in flight, belts on early whenever a jet is near, then on encounter a level change out of the shear layer (away from the core) and a prompt report.

Wind shear defined

Now the definitions the second half runs on. Wind shear is a change of wind speed and/or direction over a distance:

vertical wind shear — when the wind vector changes with height;
horizontal wind shear — when it changes along a horizontal distance;
low-level wind shear — the operationally vital subset: wind shear along the final-approach path or the takeoff and initial-climb path, shear met in the lowest few hundred feet, where there is neither height nor time to recover from what it does to the airspeed.

Where shear lives

The syllabus's list of shear factories is long, and every entry is an old acquaintance:

thunderstorms and squall lines — downdraughts and the gust front's cold outflow (the downbursts lesson takes the deep dive);
fronts — sharp cold fronts above all, with an abrupt wind change across the line;
inversions — a nocturnal inversion decouples the surface air from the wind above, leaving strong shear at its top, often with a low-level jet riding just above the lid;
land and sea breezes — the wind module's coastal circulations, changing the low-level wind within a few miles;
the friction layer itself — speed and direction change between the surface and the free wind on every flight;
relief — hills, valleys and their rotors and eddies in the lee.

Crossing the shear: what the aircraft feels

The mechanism is inertia. An aircraft crossing a shear keeps its speed over the ground for a moment while the air around it changes — so its airspeed changes suddenly:

A headwind gain — jumps the airspeed and lift: the aircraft balloons above its path;
A headwind loss, or tailwind gain — drops the airspeed and lift: the aircraft sinks below it, and near the ground that sink is the killer: undershoots, hard landings, runway excursions when the drift changes late.

That asymmetry of altitude is the whole reason low-level shear on approach and after takeoff is the dangerous case — the same shear at altitude is an airspeed excursion; at four hundred feet it is the ground.

Planning and flying around shear

On the desk: the briefing's wind shear warnings and forecasts, plus the pattern knowledge to expect shear wherever the previous section's list is in play — thunderstorms near the field, a sharp front on the chart, a strong nocturnal inversion, a coastal circulation. In the aircraft: delay the takeoff or the approach while a thunderstorm sits on or near the aerodrome (the shortest and best of all shear procedures), use the shear reports of preceding aircraft, carry a speed margin when shear is expected, and on a genuine encounter fly the escape without hesitation: full thrust, go around, and no configuration changes until clear.

What the exam asks

This lesson's questions are definitions, effects and escapes. The overview, following the learning objectives:

State the ICAO qualifying terms for the intensity of turbulence

Describe the effects of turbulence on an aircraft and how turbulence is avoided in planning and in flight

Distinguish turbulence, gustiness and wind shear

State that turbulence forecasts are of limited reliability and that pilot reports are very valuable

Describe the effects of CAT on flight and how CAT is avoided

Define wind shear (vertical and horizontal) and low-level wind shear

Describe the conditions in which wind shear forms

Describe the effects of wind shear on flight

Indicate how wind shear is avoided in planning and in flight