Windshear & TCAS

070 Operational ProceduresChapter 4

Windshear characteristics

Key fact

Windshear is a rapid change in wind speed or direction. Vertical shear causes turbulence — horizontal vector changes with vertical distance. Horizontal shear — headwind/tailwind/crosswind component changes with horizontal distance.

Effects on approach energy

Change	Effect
Headwind increase	Energy increases
Tailwind increase	Energy decreases
Downdraught	Energy decreases

Key fact

Microburst: downdraughts in a 4 km area for 1–5 minutes. Winds diverge from centre (high pressure area). Wind speed zero at centre. Wind shifts from headwind to equal and opposite tailwind. Range rule: headwind/tailwind change × 2 gives windshear magnitude in knots (e.g. 40 kt headwind loss = −80 kt windshear).

Windshear go-around procedure

Key fact

1. Aggressively apply full throttle
2. Pitch up to stick shaker
3. No configuration changes (speedbrake OK)

Report windshear to ATC. PWS (Predictive Windshear System) measures horizontal movement of precipitation — OFF during take-off, ON during landings.

Wake turbulence

Key fact

Created when generating lift (nose wheel lifts). Tip vortices separate upward around the tip and spread outward from the runway. Left wing = clockwise; right wing = anticlockwise. Travels horizontally at ½ wingspan height. Worst when aircraft are heavy, clean, and slow in light crosswinds. Avoid by flying above and upwind of the preceding aircraft.

Wake categories

Category	Mass
Heavy	over 136,000 kg
Medium	7,001–135,999 kg
Light	up to 7,000 kg

Radar wake separation (nm)

Leading	Following	Separation
Heavy	Heavy	4 nm
Heavy	Medium	5 nm
Heavy	Light	6 nm
Medium	Light	5 nm

Key fact

Time-based separation: departure, opposite, and displaced — 2 min; intermediate point — 3 min for light behind medium/heavy; arrival — 2 min ( 3 min for light behind heavy/medium). Wake dissipation time approximately 3 minutes.

ACAS / TCAS

Key fact

ACAS required: MCTOM over 5,700 kg AND MOPSC over 19. Provides traffic advisories (TA) and resolution advisories (RA). Crew must follow RA unless visually acquiring traffic and manoeuvre would compromise safety.

Exam trap

Do not chase the glideslope in windshear — fly attitude/power for airspeed. No configuration changes during windshear escape. Separation not required if VFR traffic has preceding heavy/medium in sight and is cleared to land after. TCAS uses Mode S squitters (1 s interval) for air-air awareness.

MNPS / NAT related windshear restrictions

Key fact

Restricted conditions for noise abatement and NAT operations include: crosswind (inc. gusts) over 15 kts, tailwind (inc. gusts) over 5 kts, contaminated runway, adverse weather (windshear), take-off visibility under 1 NM, landing ceiling under 500 ft AGL.