Section 2, Chapter 4 - Windshear

Question 1

What is windshear, and why is low-level windshear particularly concerning for aircraft?

Answer 1

• Windshear Definition: A sustained change in wind velocity along the aircraft flight path that happens faster than the aircraft can accelerate or decelerate.
• Low-Level Windshear: Occurs from the surface to about 1,500 ft and can significantly affect aircraft control during departure or final approach phases of flight.

Question 2

What conditions should alert controllers to the possibility of windshear?

Answer 2

• Frontal/Squall/Thunderstorm Activity: Presence in the vicinity of the aerodrome.
• Low-Level Inversions: Surface wind significantly different from wind a few hundred feet above ground.
• Local Terrain or Buildings: Large obstructions causing windshear, turbulence, and gusts related to wind speed and direction.

Question 3

How does low-level windshear affect an aircraft’s airspeed and lift during flight?

Answer 3

• When an aircraft traverses a windshear line, its inertia maintains the ground speed, but the change in wind vector causes a sudden fall in airspeed.
• This reduction in airspeed results in reduced lift until the aircraft regains its original airspeed.
• This effect can be particularly hazardous at critical climb-out speeds.

Question 4

What happens when an aircraft on final approach encounters a windshear line, and what is required to maintain safety?

Answer 4

• Encountering a windshear line can cause a sudden loss of airspeed and an increase in the rate of descent.
• A rapid application of power is required to prevent the aircraft from descending to a dangerously low height.

Question 5

What happens when an aircraft passes through a windshear line from a tailwind to a headwind, and what are the pilot’s required actions?

Answer 5

• Transitioning from a tailwind to a headwind increases airspeed and causes a deviation above the glidepath.
• The pilot’s instinctive power reduction can leave the aircraft short of power with a high rate of descent as it approaches the glidepath.
• A rapid increase of power is necessary to prevent the aircraft from sinking below the glidepath at a dangerously low altitude.

Question 6

Why must sudden changes in airspeed be countered rapidly, and what might the flight crew do if a windshear alert is generated during a turn?

Answer 6

• Rapid Response: Sudden changes in airspeed must be countered quickly to maintain a safe margin above stalling speed and a defined flight profile, especially during climb-out and approach phases.
• During a Turn: The crew may level the wings to maximize the climb gradient unless a turn is needed for obstacle clearance.
• High Workload Response: The reply ‘Standby’ to ATC instructions is common due to high flight deck workload during such events.

Question 7

How do onboard windshear detection systems work and what are the priorities during windshear recovery?

Answer 7

• Detection Systems: Modern airliners have onboard windshear detection systems that provide audible warnings to the crew. Alerts can be predictive (before encountering windshear) or reactive (after penetration).
• Windshear Recovery: Pilots fly recovery maneuvers until the alert ceases, possibly deviating from their clearance.
• Priority Over TCAS: Windshear alerts take precedence over TCAS, and the TCAS display may not be monitored as closely during recovery.
• Recovery Priorities: Maintain aircraft control and ensure terrain clearance. High rates of climb during windshear recovery, using maximum thrust, may exceed those of normal missed approaches, potentially leading to level-off altitude exceedance and reduced separation from other aircraft.

Question 8

What actions should ATC take in response to windshear reports and missed approaches due to windshear?

Answer 8

Missed Approach Response:
* Be prepared for the aircraft to exceed the missed approach altitude.
* Provide enhanced traffic information and appropriate instructions/advice to ensure safety.

Relaying Windshear Reports:
* Relay windshear information to subsequent inbound and outbound aircraft until confirmed that the condition no longer exists.

Required Pilot Reports:
* Warning of windshear presence.
* Height or height band where windshear was encountered.
* Time of encounter.
* Details of windshear effect on the aircraft (e.g., speed gain/loss, vertical speed tendency, change in drift).

Question 9

How should windshear information be handled in ATIS broadcasts?

Answer 9

• Windshear information may be included in ATIS broadcasts at aerodromes where ATIS is available.
• Controllers should provide additional details and amplify the information for individual aircraft if necessary.