SECTION 2: CHAPTER 4 WINDSHEAR

Question 1

Windshear is

Answer 1

a sustained change in the wind velocity along the aircraft flight path, which occurs significantly faster than the aircraft can accelerate or decelerate.

Question 2

Windshear can occur at any level, but it is low-level windshear

Answer 2

occurring from the surface to a height of approximately 1500 feet which can cause problems of sufficient magnitude to affect the control of aircraft in departure or final approach phases of flight.

Question 3

Controllers should be alert to the possibility of the existence of windshear in the following circumstances:

Answer 3

(1) The presence of frontal/squall/thunderstorm activity in the vicinity of the aerodrome.
(2) The presence of low level inversions where the surface wind will be significantly different from that at only a few hundred feet above the ground.
(3) Local terrain or buildings considered in relation to wind speed and direction; such large obstructions can cause windshear as well as the more usual turbulence and gusts.

Question 4

Most modern airliners carry onboard windshear detection systems, which will audibly warn the crew of the presence of windshear. Such audible alerts can be either predictive, occurring before the aircraft encounters the windshear, or reactive after penetration of the windshear. Pilots will continue to fly the windshear recovery manoeuvre until the onboard system ceases to annunciate the windshear alert, and may therefore require deviation from their clearance.

Answer 4

Onboard windshear alerts take precedence over TCAS annunciations and due to the high flight deck workload during windshear recovery manoeuvres the TCAS display may not be monitored to the same extent as during normal operations. The priority of the crew during windshear recovery manoeuvres is to keep the aircraft under control whilst ensuring terrain clearance. Rates of climb during such recovery manoeuvres, which employ the use of maximum thrust, will significantly exceed those during missed approaches executed for reasons such as an occupied runway, or lack of visual contact in poor visibility. These high rates of climb, especially when associated with a missed approach which has a relatively low level-off altitude, can result in pilots exceeding their cleared level and eroding separation from other aircraft.

Question 5

In the event of a pilot announcing a go around due to a windshear alert, controllers should be prepared for the aircraft to exceed the missed approach altitude. Controllers should provide enhanced traffic information as necessary and provide instructions and advice as deemed appropriate to ensure safety.
5.2 Whenever a pilot reports windshear conditions to ATC, the information shall be relayed to subsequent inbound and outbound aircraft until confirmation is received that the condition no longer exists.

Answer 5

Reports from pilots should contain the following information:

(1) A warning of the presence of windshear;
(2) The height or height band where the shear was encountered;
(3) The time at which it was encountered;
(4) Details of the effect of the windshear on the aircraft, e.g. speed gain or loss, vertical speed tendency, change in drift.

Question 6

ATIS Broadcasts

Answer 6

At aerodromes where ATIS is available windshear information may be included in the broadcasts. Controllers should amplify the information for individual aircraft if necessary.