By Joel Stoller
The Learjet captain who had just landed reported "moderate rain... smooth ride all the way down final approach" to the tower. An ominous thunderstorm was bearing down on the airport, just two miles to the northwest. The aircraft following the Learjet was a Boeing transport. The monitor onboard the airliner, assigned the task of alerting the crew to possible impending wind shear, began its aural alert with a sense of urgency. "Wind shear! Wind shear!" exclaimed the phantom crew-member.
The two pilots, now with a heightened sense of urgency, immediately responded to the electronic voice and flashing red "wind shear" warning lights by rapidly advancing the throttles while calling out "maximum power!" and increasing the pitch of the aircraft to 20 degrees nose-up. The aircraft stall warning stick shaker began clattering like machine-gun fire as the aircraft descent rate increased to more than 5,000 feet per minute a mere 400 feet above the ground. The flight data recorder later revealed that a positive rate of climb for recovery was regained at a height of only 20 feet, the jetliner barely achieving lift, hanging in the air by a thread. The aircraft strained to gain altitude out of the danger zone before finally climbing to a safe altitude, where the crew sighed with relief. This was a realistic scenario in a simulator to practice wind shear recovery techniques where nobody in the air or on the ground got hurt. Other crews in actual conditions, unfortunately, have not been so lucky.
The meteorological phenomenon known as wind shear has been a hazard to aviators since the pioneering days of the Wright brothers. However, it was not until the mid-1970s that a positive link with thunderstorm activity was discovered through the research studies of Dr T.T. Fujita. Before the Fujita studies, it was believed that the leading edge of rain-cooled air associated with a thunderstorm, known as a gust front, was the prime threat to aircraft taking off or landing. You have probably observed and felt this characteristic wind shift, rapid temperature drop, and a big gust of wind that precedes a thunderstorm, often covering a wide area miles ahead of the thunderstorm cells that produce them.
Fujita studied two wind-shear-related crashes that occurred in 1975. He analysed weather data and wind damage patterns in the aftermath of Eastern Flight 66 at John F. Kennedy International Airport, which showed a strong concentrated downdrafts, or what he termed a down-burst, of damaging winds in the approach path to the runway. He observed similar conditions in the analysis of Continental Flight 469 at Denver's Stapleton International Airport. Fujita discovered that more than just gust-front activity was involved in bringing these aircraft down. Fujita coined the term "microburst" to describe small-scale, intense downdrafts that, on reaching the surface, spread outward in all directions from the down-draft centre. Interestingly, as described in the NOAA publication Microbursts: A Handbook for Visual Identification
(Fernando Caracena, 1990), the author describes how a court investigating a wind-shear-related accident during a thunderstorm at Kano, Nigeria, in 1956 concluded that "the accident was a result of a loss of height and airspeed caused by the aircraft encountering, at approximately 75 m (250 feet) after take-off, an unpredictable thunderstorm cell which gave rise to a sudden reversal of wind direction." Caracena concluded, "This ruling, perhaps because of its tentative wording, does not seem to have alerted the rest of the aviation world to the dangers of thunderstorm downdrafts in take-off or landing." Twenty years after that accident, the aviation world finally did take notice when the 1976 Fujita studies were published. Six more scientific papers on wind-shear phenomena were published through 1986.
How do these weather studies based on airline accidents relate to the single-engine pilot and wind shear avoidance? As always, the type of aircraft flown has no bearing on the fact that weather phenomena are something which that aviator must respect with knowledge, training, and sound decision making to safely complete each and every flight.
The Aeronautical Information Manual provides a valuable overview of the types of wind shear hazards that may be encountered, and the various sources for alerting pilots of these hazards. Wind shear is generally defined as a sharp loss or gain of airspeed at low altitude. As mentioned earlier, wind shear activity usually accompanies thunderstorm activity in the form of gust fronts or microbursts. However, wind shear can also be associated with strong high- or low-pressure systems producing strong and gusty winds (closely spaced isobars), sometimes made worse by buildings or tall trees near the landing threshold of the runway, causing the wind currents to become even more chaotic. This type of wind shear would be more typically encountered in no convective (no thunderstorms) VFR conditions.
Many larger airports across the country have Low-Level Wind Shear Alert Systems (LLWAS), Terminal Doppler Weather Radar (TDWR), or the newest technology - Weather System Processor (WSP) equipment-to advise controllers and hence pilots of impending microbursts, gust fronts, and wind shear. The equipment comprises a series of strategically placed sensors around the airport and near runway approach and departure ends. These are coupled to computers analyzing wind data, which then present an advisory or warning to controllers to immediately relay to pilots. The airport ATIS (automated terminal information service) will include in its remarks that LLWAS advisories are in effect, giving the pilot some advance warning of what to expect on arrival.
Microbursts can outperform any aircraft in the sky today. Early warning coupled with sufficient escape altitude is the only defences available for pilots to win this contest. As described in the AIM, the characteristics of microbursts include:
Size - Less than 2 km in diameter as it descends from the cloud base to a horizontal outflow of 5 km as it hits the ground
Intensity - The downdrafts can be as strong as 6,000 feet per minute with horizontal winds near the surface as strong as 45 kt, resulting in a 90-kt shear (headwind to tailwind change traversing an aircraft)
Visual signs - Near convective activity embedded in heavy rain connected to a thunderstorm, or seen in light rain in the form of virga, or with no precipitation appearing as a ring of dust
Duration - Usually a 15-minute total duration, with horizontal winds increasing in the first five minutes with the maximum-intensity winds lasting two to four minutes.
A prime source of information is pilot reports, especially from those aircraft that just landed ahead of you. To report a wind shear encounter to the tower after landing or a go-around, state the loss or gain of airspeed and the altitude at which it occurred. For example: "Wonderboom Tower, Cessna Charlie Oscar Papa encountered wind shear at 400 feet, with a loss of 15 kt, and at 200 feet, a gain of 10 kt." This report may help the next pilot in his approach speed planning on final. A loss of 15 kt would certainly warrant an increase in your final approach speed to counteract the expected airspeed loss. With an aircraft approach speed of 70 and a normal stalling speed of 55, you can see why an extra margin on final would be prudent. However, do not add speed beyond what would be appropriate to land within the calculated landing distance of your aircraft.
Your primary goal in a wind shear encounter is positive control above the stalling speed of your aircraft. Cockpit indications that you are in wind shear may include a rapid plus or minus 15-kt airspeed indication change, plus or minus 500-foot-per-minute rate of climb change, plus or minus 5-degree or more pitch change, plus or minus one-dot deflection of glideslope if using ILS (instrument landing system) course guidance, or an unusually high or low throttle/power setting to maintain normal approach speed.
Avoidance of hazardous meteorological conditions is the key to a long and safe flying career, but given that 100-per cent compliance with this goal is unlikely in a long and safe flying career, you always have the option to go around if things do not look right. Remember a clearance to land is really an option to land: You as pilot-in-command have the final say and are the final authority in such situations. You are the one solely responsible for the safe outcome of every flight. You may want to attempt another approach if you encountered a singular wind shear event that was not convective in nature but don't force the issue of landing. It may be wise to fly to the nearest airport within fuel range that is reporting more docile winds and just wait things out there.
If you do elect to go around during a wind shear encounter that degrades performance and jeopardises safety, always comply with the manufacturer's recommended procedures for wind shear encounters requiring a go-around or missed approach.
If you are fully configured for landing in a fixed-gear aircraft, this usually means the application of maximum power (go ahead and yell out "Wind shear! Max power!" as this will help motivate you to quickly respond) followed by the manufacturer's recommended flap retraction sequence. (This is usually a partial retraction initially, depending on your aircraft. Transport jets, because of their higher power-to-weight ratio and the immense thrust available, usually do not retract anything in a wind shear go-around. Light aircraft do not have such options.) Pitch to a climb attitude, even if the aircraft is descending.
Stated bluntly, your goal is to maintain positive control and avoid ground contact. This may require a higher-than-normal pitch attitude, but you must respect the upper limits of pitch possible without stalling the aircraft. If the stall warning is sounding but you have positive control and are climbing, you are at the upper limits of pitch. An analogy to this would be the view seen when you are practising power-on approaches to stalls at various flap settings. With full power in an increasing pitch-up attitude, you will eventually hear the stall warning as airspeed decays, but from the onset of the warning up to the actual stall you are still experiencing positive performance, albeit for a relatively short time.
If you hear the stall warning or recognise the aerodynamic indications of a stall during the wind shear go-around procedure, lower pitch just enough to silence the horn while maintaining positive control. And do your best to keep climbing! There seems nothing worse than that sinking feeling in a wind shear go-around with full power and a climb attitude established.
As it's so often said, try to exercise your superior judgement so as not to get yourself in a situation where your superior airmanship skills will be needed to get you out of trouble. May all your wind shear encounters be simulations of the learning kind.