Windshear is quite simply rapidly changing wind… it could be horizontal (such as from a low level jetstream) or vertical (such as from convective weather). The term windshear literally means a “tearing” of the wind as it changes abruptly.
What many pilots cannot easily define though is how much tearing is required for windshear to be present? Flight Safety Foundation Briefing Note 5.4 from their Approach and Landing Accident Reduction Toolkit defines windshear as unexplained:
▫ Airspeed deviation of +/- 15 knots or more
▫ Vertical Speed excursion of 500 fpm or more
▫ Pitch Attitude excursion of 5 degrees or more
▫ Glideslope deviation of 1 dot or more
▫ Heading variation of 10 degrees or more
▫ Unusual power requirements
▫ Groundspeed variations
The dangers of windshear are well known and the accident list regretfully long. One common theme across many windshear accidents is unfortunately that the crew did not recognize it early enough to carry out an effective recovery.
Recognizing windshear is a critical first step to recovering from it. From personal experience, I’ve seen many pilots be their own worst enemy and initially mask the windshear event. For example, a pilot may be so focused on flying their target approach speed, that they may not notice they have adjusted their pitch attitude or vertical speed significantly in order maintain the airspeed. They may still have the target speed but the vertical speed has come to zero and the pitch attitude has changed by more than five degrees – and in some cases they have had to pull the power to idle to do so.
Another “gotcha” is heading variations – many pilots forget that windshear could come from a lateral position such as thunderstorm cell located on your left or right; that’s why heading and groundspeed are in the above list too. As you climb and descend, you can expect a normal amount of veering of backing of the wind… but an abrupt change in direction or speed can be an early indicator of windshear. With so many of today’s aircraft equipped with advanced avionics and complete GPS packages, monitoring groundspeed should not be cumbersome.
When it comes to recovery, the general rule of thumb (barring any specific directive in the POH/AFM or company standard operating procedures) is to climb away from the ground at the best lift/drag ratio. For smaller aircraft this is Vy, for larger aircraft V2 or Vga.
While flying at the correct speed is important, clearly understanding when you’ve entered windshear may be even more important. Below are two representative recovery profiles taken from a windshear encounter in a simulator.
Figure 1 – Comparative windshear recoveries.
In both cases, the performance decreasing shear started as the aircraft was climbing through 400’ AGL. In the first case, the pilot quickly recognized that the aircraft speed should not be decaying and initiated recovery immediately. In the second case, the pilot waited until speed had decayed to nearly V2 before initiating recovery – approximately ½ NM later along the departure path.
It should be noted the second pilot flew the recovery more effectively – they were much more accurate at maintaining the target speed of V2. However, the first pilot, even though they flew at V2 +20 didn’t get a sink during the manoeuvre like Pilot #2 did. Why? It may be worth considering the extra energy the aircraft was carrying when the recovery was initiated in Case 1.
Ultimately, both recognition and recovery skills are critical in any windshear encounter. But as pilots, we may want to increase the focus on improving our recognition skills.