What is Windshear: How does it affect aircraft Operation
Windshear: Why a Sudden Change in Wind Can Kill an Approach
Windshear has destroyed more aircraft on final approach than almost any other weather phenomenon. It is invisible, fast-acting, and capable of overwhelming an aircraft before a crew can respond. Understanding its mechanics — and the procedures built around it — is not academic. It is survival.
On 2 August 1985, Delta Air Lines Flight 191 — a Lockheed L-1011 on approach to Dallas/Fort Worth — flew through a microburst. The aircraft hit the ground 1,000 feet short of runway 17L. 137 of the 163 people on board were killed. The NTSB's investigation fundamentally changed how aviation understood, detected, and responded to low-level windshear. It was not the first windshear accident, and it was not the last — but it was the one that made the hazard impossible for the industry to continue minimising.
Windshear is defined as a sudden change in wind speed and/or direction over a short distance. That definition sounds benign. In practice, when that change occurs in the last few hundred feet of an approach — where airspeed margins are thin, the ground is close, and energy recovery takes time — it can be catastrophic within seconds.
What Windshear Actually Does to an Aircraft
To understand why windshear is dangerous, you need to understand what wind means to a wing. An aircraft does not fly on groundspeed — it flies on airspeed, the speed of air flowing over the wing relative to the aircraft. Wind contributes to that airspeed. A 15-knot headwind on final means the aircraft reaches flying speed 15 knots sooner in its groundspeed range, giving more margin above stall.
When wind changes abruptly — particularly from headwind to tailwind — the aircraft experiences an instantaneous loss of airspeed. The wing does not know that the groundspeed is unchanged. It only knows that less air is flowing over it. Lift drops. The aircraft sinks below the glidepath. If the crew does not respond immediately and powerfully, and if there is insufficient altitude to recover, the aircraft strikes the ground.
The insidious complication is inertia. An aircraft on approach has substantial mass and momentum. When the wind changes, the airspeed changes immediately — but the aircraft's speed through the air mass takes time to catch up. Full thrust from idle takes several seconds to develop on a jet engine. Those seconds, at 500 feet above the runway, may not be available.
The aircraft does not stall because the pilot flew badly. It stalls because the wind took away the energy margin faster than physics allowed any crew to restore it.
The Microburst — The Most Lethal Form
A microburst is a small, intense downdraft produced by a convective cell — a thunderstorm or even a virga shower (precipitation that evaporates before reaching the ground). The descending air hits the surface and spreads outward in all directions, creating a horizontal burst of wind radiating from a central impact point.
What makes the microburst uniquely dangerous on approach is the headwind-to-tailwind transition it forces along the flight path. An aircraft approaching into a microburst first encounters a strong headwind — the outflow on the near side of the burst. Airspeed rises. The aircraft climbs above glidepath. The crew reduces power to correct back down. Then the aircraft enters the downdraft core and the tailwind on the far side — airspeed drops sharply, the aircraft is already low and slow, the engines are at reduced power, and the aircraft is sinking in a column of descending air. This sequence, if unrecognised and uncorrected, is uniformly fatal near the ground.
Fig. 1 — Microburst encounter on final approach. The outflow headwind on the near side pushes the aircraft above the glidepath (②). Entering the downdraft core and transitioning to the tailwind side causes a rapid loss of airspeed and a steep uncontrolled descent (③) that may not leave sufficient altitude or time for recovery.
Microbursts are small — typically 1–3 nautical miles in diameter — and short-lived, often lasting only 5–15 minutes at peak intensity. Wind speed differentials across a microburst can exceed 100 knots in severe cases, though 30–50 knots is more typical of dangerous encounters. Vertical speeds in the downdraft core can reach 6,000 feet per minute. No aircraft on approach, with gear and flaps extended, can overcome that with available thrust at low altitude.
Types of Windshear
Microburst is the most lethal form but not the only one. Windshear occurs across a range of scales, mechanisms, and altitudes — each with different characteristics, predictability, and operational implications.
- Microburst (wet and dry) Produced by convective precipitation or virga. Wet microbursts are associated with visible rain shafts, detectable on radar and visually. Dry microbursts are more insidious: precipitation evaporates completely before reaching the ground, leaving no visible shaft. The only surface indications may be a ring of blowing dust and a virga column aloft. Dry microbursts are common in arid environments such as the southwestern United States and are essentially invisible until the aircraft enters them.
- Low-Level Windshear — frontal Passage of a cold or warm front can produce sharp wind direction and speed changes at low altitude. A cold front moving through an airfield may rotate surface winds by 90° or more and change speed by 20–40 knots within minutes. Frontal LLWS tends to be more sustained than a microburst but is usually detectable via ATIS, METAR trend reports, and ATC advisories.
- Temperature inversion shear A temperature inversion traps warm air above cooler surface air, creating a stable boundary layer. Wind speeds and directions above and below the inversion can differ significantly. Aircraft climbing or descending through the inversion experience a step change in airspeed. Common overnight and in calm anticyclonic conditions, often with no convective weather in the vicinity.
- Mountain wave and terrain-induced shear Airflow over ridges and mountains creates standing waves, rotors, and violent downslope winds such as the foehn, chinook, or bora. Lee-side windshear can produce rapid and violent airspeed changes at low altitude well downwind of the terrain. The hazard zone can extend tens of miles from the mountains and affect approach corridors at airports that appear unrelated to nearby high terrain.
- Jet stream shear and clear-air turbulence At high altitude, the edges of jet stream cores produce horizontal shear layers that cause clear-air turbulence (CAT). While typically not a safety-of-flight issue for the aircraft structure, severe CAT has injured unsecured cabin occupants and occasionally caused structural damage. Unlike low-level windshear, jet stream CAT primarily affects comfort and cabin safety rather than aircraft controllability.
The Performance Sequence — What the Numbers Show
The physics of a windshear encounter can be quantified, and understanding them makes clear why altitude and time are the resources that matter most. Consider a jet on final approach at 1,000 feet AGL, 140 knots indicated, gear and flaps extended, engines at approach thrust. The aircraft encounters a 40-knot headwind-to-tailwind shear transition over 10 seconds.
| Phase | Wind Change | Airspeed Effect | Aircraft State | Altitude Remaining |
|---|---|---|---|---|
| ① Entry — headwind | +20 kt headwind increase | +20 kt momentary gain | Climbs above G/P; crew reduces thrust to correct | ~1,000 ft AGL |
| ② Core penetration | Headwind drops to zero | −20 kt from peak | Back at approach speed; thrust still reduced | ~700 ft AGL |
| ③ Tailwind transition | +20 kt tailwind builds | −40 kt total; now 20 kt below Vref | Go-around initiated; engine spool-up lag 3–8 sec | ~400 ft AGL |
| ④ Downdraft core | 500–2,000 fpm added sink | Airspeed still recovering | Stick shaker possible; max thrust still developing | ~150–250 ft AGL |
The margin between that last row and the ground is measured in single-digit seconds. The lesson is not that the crew responded incorrectly — it is that the event timeline, at low altitude, may leave no room for even a correct response to succeed. Prevention and early recognition are the only reliable defences.
Detection Systems
The aviation industry's response to accidents like Delta 191 was a systematic effort to detect windshear before aircraft entered it, and to warn crews who had already encountered it. The resulting detection architecture operates on three levels: ground-based infrastructure, airborne systems, and crew-sourced reporting.
Ground-Based Detection
The primary ground infrastructure in the United States is the Terminal Doppler Weather Radar (TDWR), installed at 45 major airports following the post-DL191 regulatory push. TDWR uses Doppler radar to detect the radial velocity of precipitation — including the horizontal outflow signature of a microburst — at ranges up to 60 nautical miles, with update cycles of approximately one minute. When TDWR detects a microburst alert on or within 3 miles of a runway, ATC is required to issue a windshear alert to all arriving and departing traffic.
The Low-Level Windshear Alert System (LLWAS) complements TDWR at many airports. LLWAS is a network of anemometers placed around the airport perimeter and on the field. The system compares wind readings across the network and generates an alert when differences exceed defined thresholds. LLWAS cannot detect shear along the approach path away from the field boundary, but it is effective at identifying shear on or near the runway surface itself.
Airborne Detection
Modern transport aircraft carry onboard windshear detection and warning systems — either reactive (triggered by the aircraft actually experiencing the shear) or predictive (using forward-looking sensors). All large transport category aircraft certificated after 1994 are required to carry reactive windshear systems as a minimum.
- Reactive windshear systems Process real-time air data — inertial vertical speed, airspeed, and flight path angle — to detect the performance degradation signature of a shear encounter already in progress. When the combination of airspeed loss and sink rate exceeds defined thresholds, the system activates. The standard aural warning is: "WINDSHEAR, WINDSHEAR, WINDSHEAR." The critical limitation is that reactive systems warn after the aircraft has entered the shear — energy recovery time is already diminished.
- Predictive windshear systems (PWS) Use the aircraft's own weather radar in forward-looking Doppler mode to detect the velocity signature of a microburst up to 3 nautical miles ahead — giving approximately 30–60 seconds of warning before the aircraft enters the hazard. The aural call is: "GO-AROUND, WINDSHEAR AHEAD." PWS is now standard on most modern transport aircraft and represents a fundamental improvement over reactive-only protection. The key limitation: PWS requires precipitation to function. It cannot detect dry microbursts.
- EGPWS / TAWS integration Enhanced Ground Proximity Warning Systems incorporate windshear algorithms as part of their broader approach-phase monitoring suite. Some manufacturers integrate windshear warning logic with flight envelope protection, providing not just a warning but active pitch and thrust guidance toward the escape attitude.
Fig. 2 — Layered windshear detection architecture. TDWR provides wide-area ground surveillance and routes alerts through ATC; LLWAS monitors surface-level shear on and near the airport; PWS gives the aircraft a forward-looking warning 30–60 seconds before the hazard; reactive systems trigger once the aircraft has entered shear. PIREPs from preceding aircraft fill coverage gaps in all automated systems.
PIREPs — The Human Layer
Pilot reports (PIREPs) remain one of the most operationally immediate sources of windshear information. An aircraft that encounters windshear on approach should file a PIREP immediately after executing the go-around — before the next aircraft reaches the same segment of the approach. ATC is required to broadcast windshear PIREPs to all traffic in the terminal area. In several historical accidents where the event was foreseeable, the preceding aircraft's report either did not reach the crew in time or was never issued.
Crew Recognition — What to Look For
Systematic windshear recognition requires awareness of both external cues — environmental conditions that suggest windshear is possible — and in-flight performance deviations that indicate the aircraft is already being affected.
Environmental Pre-Indicators
These cues should elevate crew alertness before descent and hold it there through the approach:
- Thunderstorms, convective activity, or virga within 20 miles of the airport
- SIGMET, convective SIGMET, or AIRMET Sierra active for the terminal area
- ATIS or METAR reporting gusty, variable, or shifting surface winds
- Windshear PIREPs from preceding aircraft on the same approach
- Visible precipitation shafts or virga near the extended centreline
- Dust rings, blowing dust, or surface weather visible near the runway
- Significant difference between reported surface wind and winds aloft
In-Flight Performance Cues
Once these appear, the aircraft may already be in shear. Immediate go-around execution, without hesitation, is the correct response:
- Unexplained airspeed fluctuations exceeding ±15 knots from target
- Unexplained deviation from the glidepath not corrected by normal inputs
- Sink rate increasing without a corresponding increase in pitch-down attitude
- Thrust required significantly different from normal for the flight condition
- Stick shaker (stall warning) activation
- Windshear warning annunciation from onboard reactive or predictive system
The Windshear Escape Manoeuvre
The windshear escape manoeuvre differs from a standard go-around in one critical respect: it prioritises energy recovery over aircraft configuration management. In a normal go-around, the crew follows a sequenced procedure — thrust, positive rate, gear up, flap schedule. In a windshear escape, the immediate requirement is to arrest descent and restore airspeed with maximum available thrust, accepting that the aircraft may remain in approach configuration for several seconds.
- 1THRUST — Maximum immediately. Both thrust levers to the takeoff/go-around (TOGA) detent without delay or assessment. In a reactive windshear warning or a predictive warning with the hazard confirmed ahead, maximum thrust is the correct initial action in every case. Engine spool-up lag means every second of hesitation is altitude lost that cannot be recovered.
- 2PITCH — Rotate to escape attitude. Pitch up to the windshear escape pitch attitude — typically 15–20° nose up, varying by aircraft type. Some flight mode annunciations command a specific pitch target. If the flight director is commanding a descent, do not follow it — fly the published escape attitude instead.
- 3DO NOT RETRACT FLAPS OR GEAR. Configuration changes during the escape manoeuvre reduce lift and consume crew attention that is needed for aircraft control. Flaps and gear remain as configured until the aircraft is in a positive climb and clear of the hazard. The urge to "clean up" is instinctive and must be actively suppressed.
- 4ACCEPT STICK SHAKER if necessary. If stall warning activates during the escape, maintain the escape attitude and do not lower the nose to silence it. In a windshear encounter, stick shaker may activate because the energy state of the encounter has temporarily reduced effective angle of attack margin — not because of a classical aerodynamic stall. Lowering the nose in this context reduces the climb gradient at the worst possible moment.
- 5DO NOT ATTEMPT TO LAND. There is no windshear encounter — including one that begins below 200 feet AGL — where continuing to land is the correct response. The escape manoeuvre is executed regardless of height above the runway. Contact with the runway while in a microburst downdraft is not a controllable event.
Operational Decision-Making — Before the Approach
The most effective windshear defence is not the escape manoeuvre. It is the decision not to begin an approach into conditions where the escape manoeuvre may become necessary. This requires applying a lower threshold for action than most crews apply to routine weather deviations.
| Condition | Recommended Action | Rationale |
|---|---|---|
| Microburst alert on approach corridor (TDWR / ATC) | Do not commence approach. Hold or divert. | Alert indicates a confirmed hazard. Microburst intensity can exceed aircraft escape performance. |
| Windshear PIREP from preceding aircraft on same approach | Treat as a firm go-around condition. Delay minimum 15–20 min or until alert cancelled. | Microbursts are short-lived but intensify rapidly. The PIREP reflects conditions several minutes ago — not current state. |
| PWS alert triggered on approach | Immediate go-around. Do not continue approach. | Verified Doppler return consistent with microburst is 3 NM ahead. This is a confirmed, not precautionary, warning. |
| Thunderstorm within 3 NM of airport | Delay approach. Request updated TDWR / LLWAS status from ATC. | Microburst footprint can reach the approach corridor even if the cell is not directly overhead. |
| Unexplained ±15 kt airspeed deviation on approach | Go-around immediately. | A deviation of this magnitude is the shear signature. A stabilised approach is no longer achievable. |
| Reactive windshear warning in flight | Execute escape manoeuvre immediately. No further assessment required. | The aircraft is inside the hazard. Every second of delay is altitude lost. |
Historical Accidents That Shaped the Rules
The regulatory and procedural framework for windshear did not emerge from theory. It was written in the wreckage of specific accidents, each of which exposed a gap in understanding, detection, or procedure that was subsequently closed.
- Eastern Air Lines 66 — JFK, 1975 A Boeing 727 on approach to JFK in a thunderstorm environment encountered windshear and struck approach light stanchions 3,000 feet short of the runway. 113 of 124 occupants were killed. The NTSB's investigation introduced the concept of the microburst into aviation awareness and triggered the first serious research into low-level windshear detection — more than a decade before TDWR became operational.
- Pan American 759 — New Orleans, 1982 A Boeing 727 encountered windshear during takeoff through a microburst associated with a thunderstorm directly over the airport. The aircraft lifted off normally and then immediately lost performance, striking trees and houses in a residential area. All 145 on board and 8 people on the ground were killed. This accident drove the initial requirement for onboard windshear warning systems.
- Delta Air Lines 191 — Dallas/Fort Worth, 1985 The accident that catalysed the entire modern windshear regulatory framework. The NTSB found that the crew had visual and radar indications sufficient to warrant avoiding the approach but continued into a microburst associated with a growing convective cell. The resulting FAA mandate drove TDWR deployment, LLWAS network upgrades, and eventually the requirement for airborne predictive windshear systems on all large transport aircraft.
- USAir 1016 — Charlotte, 1994 A Douglas DC-9 struck trees on approach after a microburst encounter in deteriorating conditions. This accident occurred after TDWR had been mandated but before the system was installed at Charlotte. Windshear was reported by a preceding aircraft but did not reach the crew of USAir 1016 in actionable time. The accident accelerated TDWR installation timelines and reinforced requirements for timely PIREP dissemination by ATC.
Each of these accidents was survivable — in the sense that a different decision, made earlier, with a lower threshold for action, would have placed the aircraft somewhere other than the ground. The regulations that followed were attempts to make those better decisions the default, not the exception.
Regulatory Requirements
| Requirement | Applicability | Governing Standard |
|---|---|---|
| Reactive windshear system | All turbine-powered transport aircraft certificated after 1994; retrofit required on existing fleet | FAR 121.358 / EASA CS-25 |
| Predictive windshear system (PWS) | Required on new-build transport aircraft; widely adopted ahead of mandate | FAR 25.1303 / AC 120-41B |
| Windshear training | Required for all Part 121 and 135 crew members; scenario-based simulator training mandated | FAR 121.409 / AC 00-54 |
| TDWR at major airports | Deployed at 45 US airports with highest convective weather risk; ATC required to issue alerts to all traffic | FAA Order 7110.65 |
| Windshear escape procedures | Required in AFM and operator SOPs for all certificated transport category aircraft | FAR 25.1585 / AC 120-41B |
The Correct Mental Model
Windshear is not a condition that skilled crews manage through technique — it is a condition that well-prepared crews avoid entering, and escape from immediately when they do. The margin between a successful escape and a ground impact is measured in hundreds of feet and single-digit seconds. There is no version of "let's see how this develops" that ends well.
The mental model that produces good outcomes is a simple threshold, applied without negotiation: any credible indication of windshear on the approach path is a go-around condition until the hazard is confirmed absent. Not a monitoring condition. Not a "keep the approach and advise when able" condition. A go-around condition. Every system, procedure, and training scenario built around windshear since 1975 exists to move that threshold earlier — from the moment of impact, back to the top of descent, back to the dispatch decision.
Windshear kills approaches that were continued into it. It has very rarely killed approaches that were abandoned at the first sign of it. The asymmetry is not subtle — it is absolute.
