"Wind Shear, Wind Shear"

In contemplating my recollection of today’s events, I can’t help but recall distant memories of Delta 191, the Lockheed L10-11 that crashed at Dallas, Ft. Worth International Airport (KDFW) in August 1985 after encountering wind shear on final approach to what was then runway 17L. I was in high school at the time and on a summer camping trip in the Colorado Mountains with a group of church friends when news of the tragedy reached me. My Dad was an L10-11 First Officer and unfortunately for me, the news I received did not include any information about my Dad. We spent another two days hiking through the mountains that week before I was able to confirm that my Dad was not involved. As it turns out, Dad was safe at home that day, but his friend and colleague Captain Ed Connors was in command of Flight 191.

There are a number of videos and audio clips available on YouTube and other sites that accurately depict the sequence of events that day at DFW, but what is rarely discussed is how little we really knew about wind shear at the time and more importantly, how poor the detecting and reporting was of such events. Today, most major airports are equipped with LLWAS (Low-Level Wind Shear Alerting System), a system comprised of 6 to 33 anemometers placed on and around an airport sensing wind speed and direction within 2 1/2 miles of the airport and our airliners have both reactive and predictive wind shear equipment to warn pilots of possible threats via aural and visual means. I feel certain, if Captain Connors had the information and warning systems then that we have today, Delta 191 would have been another flight that everyone forgot.

Today’s events come at the tail end of an otherwise uneventful three day trip. Uneventful is almost always a good thing when you’re talking about airplanes. Our day started with an early morning flight back home followed by a quick trip to Colorado Springs and back before we could call it a day. As we prepared for our flight to Colorado Springs, the Captain commented that the winds, currently blowing from 270 at about 30 knots, gusting to over 40 were too strong for us to land. The runways in use at Colorado Springs are oriented north/south, so a wind from 270 would be a direct crosswind. The MD80 we were flying boasts a max demonstrated crosswind limitation of 30 knots on a dry runway with good visibility (if you‘ve been following along, this may sound familiar). The runway was dry and there were no visibility issues complicating our arrival, but with winds gusting to 40 knots, we were going to need a backup plan. We noted that the forecast called for diminishing winds that should be below our limitations by scheduled arrival time and took off for the Springs with Denver International as an alternate.

As we approached the top of descent, we retrieved a current weather report for the airport and learned that the forecasts were correct and that the wind at the airport was just below the landing limits of the aircraft. We asked our flight attendants to prepare the cabin early and take their seats in anticipation of the kind of turbulence that typically accompanies strong westerly winds over the mountains and started to prepare for landing. The ride definitely did not disappoint. Sometimes we make a big fuss and predict a rough ride only to have perfectly smooth and comfortable conditions all the way to touchdown. I know we lose some credibility with the cabin crews when this happens but in this case, our prediction for turbulence was spot on. I cooled the cabin down a few degrees and hoped no one in the back would start the chain reaction that usually follows the first person who gets sick to their stomach. I won’t elaborate…

Let me add a few words about wind shear…The best course of action is almost always avoidance. But the truth is, airlines operate into cities every day that are prone to low level wind shear. If they cancelled every flight to Colorado Springs and Denver that intersected with a wind shear report, then air travel to those cities would be practically non-existent. With these cities in particular, it seems to be the combination of their location up against the eastern edge of the Rockies and the common nature of strong westerly winds that produce turbulence and wind shear events. Pilots are taught to avoid areas of severe wind shear at all costs, but intermittent or lesser events can be navigated with good piloting techniques and extra caution.

It is commonly held that pilot reports (PIREPS) of wind shear in excess of 20 knots or 500 feet per minute climb or descent within 1000 feet of the ground are all good indications of severe conditions and should be avoided at all costs. Given the fact that these conditions develop, change and dissipate rapidly, a pilot would be wise to consider the amount of time since the report was made.  Keep in mind that the aircraft just a few miles in front of Delta 191, flew the same approach to the same runway with no indication of threat and landed safely. The most dangerous form of wind shear and the most likely cause of the Delta 191 accident is a convective microburst. Some microbursts have been documented with wind changes in excess of 150 knots. Also, since microbursts intensify for several minutes after they first impact the ground, the severity may be up to twice that which is initially reported. It is very important to remember that the aircraft ahead of you on the approach may experience vastly different conditions than you will encounter in the same airspace.

In the picture to the left, you an see that as an airplane enters the leading edge of a microburst, the first indication is an increased headwind. As the aircraft continues into the microburst, the headwind shifts to a downdraft then a tailwind. It is the down draft and tailwind portion of the microburst that is intensely dangerous to airplanes of all shapes and sizes. Additionally, a microburst viewed from above is round and shaped like an upside down mushroom. It is possible to fly through the edges of a microburst and never encounter the downdraft or the tailwind.



Back to my flight…

As we approached the airport from the south, I made every effort to be prepared early for the approach and landing. Arriving from the south into Colorado Springs when the airport is landing north can be a handful even on a normal day. Approach control usually instructs us to cross the FSHER intersection at 14,000 feet on the arrival. FSHER is roughly 25 miles south of the airport, so with a touchdown zone elevation of 6,118 feet above sea level, we only have 25 miles to lose just under 8,000 feet. This isn’t a problem and can be handled without too much effort if you have already slowed the aircraft by the time you reach FSHER. The gotcha here is that a typical descent speed for the MD80 would be around 300 knots, plus or minus depending on a number of factors, and the pilots are not required to slow to 250 knots until descending below 10,000 feet. So if the pilot crosses FSHER at 14,000 feet indicating 300 knots, then getting down is going to be next to impossible since slowing from 300 to 250 will add about an extra 5 miles to the distance needed to descend.

Maybe that’s too much information, but I want to emphasize that this arrival can be a challenge even without the threat of wind shear. I crossed the FSHER intersection at 14,000 as instructed with the airspeed steady at 250 knots in preparation for our descent for runway 35R. We were then cleared to continue our descent and were given a heading to intercept the final approach course for the runway. There weren’t any aircraft ahead of us on the approach to relay reports on the airborne conditions, but Tower and Approach Control were reporting gains and losses near 20 knots on the ground surrounding the field. We elected to continue our approach and make a final assessment of the conditions at 1,000 feet on final.  We wouldn't ge that far.

I slowed the aircraft and configured for landing a little early to ensure a stabilized approach and was on final approach speed, glide slope and fully configured for lading at 1,500 feet above touchdown when we got a yellow wind shear light on the glare shield. The yellow light is an indication of an “increasing performance” wind shear and is yellow to emphasize caution. Increasing performance means that we went from a tailwind to a headwind or a headwind to a stronger headwind in a short distance. Neither of these is particularly dangerous, but is a possible indication that we were entering the leading edge of a microburst. A go-around is not required for this indication, but great caution is advised and we were prepared for a possible missed approach and escape maneuver.

About 200 feet later in the descent and just about the time I had processed the yellow light flashing in my face, the light extinguished and was replaced by a red flashing wind shear light and aural “wind shear, wind shear“ warning from the speaker above my head. In anticipation of a possible wind shear event, I had already increased our approach speed by 20 knots and was intentionally flying slightly high on the glide path. I knew the runway ahead was long and sloped upward from the touch down zone, so I was confident that dissipating the extra speed would not be a problem. I announced “escape” and pressed the TOGA (Take-Off, Go-Around) buttons on the throttles which automatically commanded go-around thrust on both engines.  I followed the commands of the flight director to just under 20 degrees nose up, but for a short time, the aircraft would not climb.

It is difficult for me to believe, but with 24 years of flying experience under my belt, this was my first actual encounter with “decreasing performance” wind shear outside of the simulator. This is something I train for every single time I enter the sim, but I had never actually seen one in real life, and this one was pretty mild. With the nose pointing to the sky and two JT8D-219 engines producing nearly 40,000 pounds of thrust, the jet would not climb. We were well over a thousand feet above the ground with power to spare in case the event became more severe, but it was an incredibly long 3-5 seconds before we began to exit the wind shear and started to climb away from terra firma. Once we began to climb, I asked the Captain to raise the gear, reduced thrust on the engines and cleaned up as we would on a normal flight.

We departed the area to the east where the ride was a little less turbulent and entered a holding pattern to give the conditions a little time to improve. We also wanted a few other airplanes go in and land without incident before we elected to try again. On the second approach, we added in the same safety margins as before, but encountered a relatively smooth ride down final and landed uneventfully. As we taxied to the gate, I recalled images of Delta 191. I couldn’t help but wonder if Captain Ed Connors would have made a different decision about his approach and landing that day had he been graced with the knowledge and equipment that we had on this approach. Maybe he would have executed a missed approach earlier. Maybe he wouldn’t have started the approach at all. I guess we’ll never know for sure, but I am confident the outcome would have been much different.