Windshear and Microbursts: The Hazard That Begins by Helping You

Of all the phenomena I briefed during my service, the microburst is the one I considered most deserving of respect, for a reason which is not immediately obvious: the encounter begins with the aircraft performing better than expected.

Windshear is defined as a change in wind speed or direction over a short distance. It may occur horizontally or vertically, and it may be encountered at any altitude. At cruising level it is largely a matter of comfort and, in severe cases, of structural loading. Near the ground, during approach or immediately after take-off, it becomes a question of whether sufficient performance remains available.

The microburst

A microburst is a concentrated, rapidly descending column of air emerging from the base of a convective cloud, typically less than four kilometres across, striking the surface and spreading outward radially in all directions.

Downdraught speeds may reach 6,000 feet per minute. The outflow may produce horizontal wind changes exceeding 80 knots across the width of the feature. The entire event commonly lasts between five and fifteen minutes, which means it may develop and dissipate between one observation and the next.

The encounter sequence

An aircraft flying an approach through a microburst experiences three distinct phases in rapid succession, and the danger lies in the order in which they occur.

Phase one: increasing headwind

The aircraft enters the outflow on the near side, where the air is moving towards it. Indicated airspeed rises. The aircraft balloons above the glidepath and performance appears to improve.

The instinctive response — and the trained response, under normal circumstances — is to reduce thrust and lower the nose to regain the profile. This is precisely the wrong action, and it is the reason the phenomenon has proved so lethal. The aircraft arrives at the most demanding portion of the encounter having just given away its energy.

Phase two: the downdraught

The aircraft passes into the descending core. The headwind component disappears, replaced by a strong downward flow. Rate of descent increases sharply and altitude is lost rapidly.

Phase three: increasing tailwind

Emerging on the far side, the aircraft encounters outflow now moving away from it. The headwind becomes a tailwind. Indicated airspeed falls abruptly, lift decreases, and the aircraft is descending, low, slow and with thrust which was reduced two phases earlier.

The recovery margin at this point is measured in seconds and in a few hundred feet.

Recognition

The following indications warrant immediate consideration of a go-around or a delayed departure.

• A ring or curl of dust or debris at the surface, particularly beneath a convective cloud base
• Virga — precipitation visibly falling from a cloud base and evaporating before reaching the ground
• A localised rain shaft with a distinctly curled or flared base
• Rapid, unexplained airspeed fluctuation on approach
• Marked disagreement between reported surface wind and the wind the aircraft is experiencing
• Reports from preceding aircraft of airspeed loss or gain on the same approach

Response

Every operator publishes a windshear recovery procedure and that procedure governs. The common elements are consistent across types:

• Apply maximum available thrust without delay
• Rotate towards the pitch attitude specified in the procedure, typically around 15 degrees, and adjust as required to avoid ground contact
• Follow the flight director windshear guidance or the stall warning system as applicable
• Do not change configuration — retain existing flap setting and, on approach, do not retract the landing gear until clear
• Accept the loss of airspeed and, if necessary, operate at the stick shaker while maintaining the climb
• Do not attempt to regain the original flight path until clear of the shear

The instinct to protect airspeed by lowering the nose must be resisted. In a microburst recovery, altitude is the resource in shortest supply.

Prevention

The most reliable protection remains avoidance. I would recommend the following as standard practice during convective conditions.

Delay rather than attempt. A microburst lasts perhaps ten minutes. Holding for fifteen is almost always the better decision, and it is a decision far more easily made before commencing the approach than during it.

Treat any convective cell within five nautical miles of the aerodrome as a windshear threat, whether or not it lies on the approach path. Outflow spreads radially and travels well beyond the parent cloud.

Add a margin to approach speed where windshear is suspected, in accordance with the operator's procedures, and consider the longest available runway.

Brief the recovery before it is needed. The response must be immediate and it must be automatic. There is no time in which to consider options.

A note on detection

Doppler weather radar detects microbursts through the divergent velocity signature they produce near the surface, and many major aerodromes now operate low-level windshear alert systems. Modern airborne predictive windshear systems provide warning ahead of the aircraft.

These systems are valuable and have unquestionably saved lives. They are not, however, universally available at every aerodrome in the region, and they do not remove the requirement for visual recognition and sound judgement. The dust ring at the surface remains, in many circumstances, the earliest warning available.