The Danger of Windshear

Writen by: David W. Thornton
Aviation Examiner

The copilot accepted the approach clearance and I turned the Lear 45 toward the initial fix for the approach.

We were flying into Victoria, Texas to pick up two members of the family that owned the airplane for a trip to sunny Florida. Sunny was something Victoria was not today. The ATIS, the recorded weather broadcast for the airport, reported low ceilings, heavy rain, gusty winds and limited visibility. Our airborne weather radar indicated a band of rain just south of the airport. We had been enveloped in clouds for the entire short flight from Houston.

We descended to 2,000 feet to start the instrument approach, a GPS-based approach to runway 31 right. We configured the airplane for landing and began the descent along the electronic glide path toward the runway.

We had only descended about 200 feet when the airspeed went crazy. First, the speed increased by 30 knots, almost reaching the maximum speed for the flaps. Then, just as quickly, it reversed and dropped rapidly into the red, low-speed tape on the primary flight display.

“STALL… STALL,” the electronic voice of the Learjet’s warning system called out.

I advanced the throttles to maximum thrust and called out, “windshear, flaps eight,” to begin the missed approach procedure. We “cleaned up” the airplane from landing configuration, retracted the flaps and landing gear, and climbed to the missed approach altitude. Or rather, we climbed through the missed approach altitude. With the airspeed in safe territory but still fluctuating, we ballooned several hundred feet high. Even with reduced power and pointing the nose down, the Learjet did not want to descend.

Shortly after, the controller assigned us a climb and vectored us around for another try. With the weather rapidly moving through the area, we were able to fly behind the squall and land successfully and without experiencing anything abnormal other than rain so heavy that it partially obscured the view through the windshield.

What we had experienced was the worst windshear of my career. Windshear is any sudden change in wind speed or direction over a short distance. It can occur either horizontally or vertically. While windshear can occur at any altitude, it is more dangerous when the aircraft is close to the ground.

In 1985, Delta Air Lines Flight 191 crashed when it encountered windshear on approach at Dallas - Fort Worth International Airport (KDFW). The Lockheed L1011 was flying through the rain shaft of a thunderstorm while on an ILS approach when the headwind suddenly decreased by 25 knots and ultimately increased to a 30 knot tailwind while the downdraft increased from 18 to more than 30 feet per second according to an FAA analysis. In spite of applying full power to all three engines, the aircraft hit an open field just short of the runway then became airborne again to strike a light pole and car on a road near the approach end. The airplane ultimately hit two water towers on the airport property and exploded. The accident killed 136 of the 163 passengers and crew plus the driver of the car.

An analysis of the accident showed that the plane likely flew through a microburst, a very intense downdraft that is localized to an area about two miles in radius. The airplane first encountered increasing performance as it flew through the updraft at the periphery of the microburst, causing it to go high on the glideslope. Then, as the airplane entered the downdraft, the airspeed slowed dramatically and the airplane went below the glideslope, triggering an alert from the plane’s ground proximity warning system (GPWS). In spite of going to full power, it was too late for the airplane to successfully fly out of the windshear.

Although windshear was known in 1985, it wasn’t well understood. According to the FAA, the Delta 191 crash resulted in changes to training to help pilots better recognize the danger of windshear and make a decision to use an escape maneuver early rather than continuing the approach. New technology, such as airport low level windshear alert systems (LLWSAS), better radar, and enhanced GPWS with a windshear protection mode have contributed to safety as well. There are limitations though. In my windshear incident, the GPWS windshear alert was not triggered because the airplane was above 1,500 feet, outside the danger zone for takeoff and landing.

The windshear escape maneuver can vary from airplane to airplane, but is typically similar. The flying pilot should advance the throttles to maximum power and pitch the airplane nose up sharply. It should go without saying that the autopilot should be disconnected for this. No configuration changes, such as retracting or extending flaps and landing gear, should be made while the airplane is in windshear. Once the airplane has gained the safety of altitude and airspeed, flaps and gear should be retracted.

If windshear is likely on takeoff, the pilot can choose to delay rotation. Delayed rotation means extra airspeed in the early stages of the climb that can be very helpful if the airplane encounters windshear shortly after takeoff. Rotation speed (Vr) can generally be increased by 10 percent to a maximum of 20 knots (check your aircraft guidelines and limitations). This speed should be briefed, but should not be set with an airspeed bug. Consider the extra runway that will be needed due to the longer acceleration time as well.

Similarly, the landing reference speed (Vref) can be increased as well. An old rule of thumb is that landing speeds can be increased by half the gust factor. If the wind speed is reported as “10, gusting to 20 knots,” Vref can be increased by five knots. Stable approaches typically require that the airspeed on approach be no more than 20 knots faster than Vref, so this should be considered a limit for increasing the landing speed. Again, consider the runway since a higher landing speed means that more stopping distance is required.

Ultimately, avoidance is the best tool for surviving windshear. The Flight Safety Foundation identifies several warning signs for windshear. These include thunderstorms, gusty frontal passages, blowing dust, rings of dust, whirlwinds, mountain waves, and, of course, warnings from airport windshear alert systems or pilot reports. If conditions are ripe for windshear, be prepared to go around. In some cases, a delay or diversion might be the best course of action. Microbursts often only last for about 15 minutes so a short delay can make a big difference in safety.