In the Maw of the Beast

Avoid it. That's the only advice pilots need to remember about wind shear, one of Mother Nature's more schizophrenic mood swings. Sometimes, though, avoidance is easier said than done, and that's why it's nice to have a backup plan just in case. For pilots of jet aircraft, this means training in specific ways to survive a severe wind shear encounter, training best accomplished in the simulator. To appreciate — from a safe vantage point — what it's like to tangle with some especially nasty wind shears, Pilot visited Continental Airline's Flight Training Facility at Houston's Intercontinental Airport (IAH) for some practice in wind shear recovery procedures.

Wind shear is any sudden change in the direction and/or velocity of the existing wind.

Strong shears that occur close to the ground pose dangerous hazards to aircraft taking off or landing nearby. Microbursts, which are highly localized, rapidly moving downward flows of relatively cold air often associated with thunderstorm activity, are the most treacherous of all. But other kinds of wind shears can be problematic for aircraft, too, including those associated with frontal activity, temperature inversions, or fast-moving airflows over irregular terrain.

Wind shear avoidance, an aviator's first line of defense, begins and ends with good situational awareness. There are numerous weather clues — predictors of wind shear, if you will — that pilots can use to help determine the likelihood of a serious wind shear. Continental advises its crews to use a weighted system of risk assessment. The more predictors that exist, the greater the probability that wind shear might be present, and the more reason there is to delay takeoff or landing. For example, the presence of convective weather in the general area should immediately create a presumption that wind shear is possible. If virga, lightning, or heavy rain is observed, the probability increases. If a temperature/dew point spread of 30 to 50 degrees also exists, or if moderate or severe turbulence is reported, the potential for wind shear becomes higher still. Pireps of an airspeed gain or loss of 15 knots or more or low-level wind shear alert system (LLWAS) reports of wind velocity changes in excess of 20 knots are prima facie evidence that wind shear is present. But such telltale clues aren't always available to the pilot in a timely manner, which makes wind shear prediction an inexact science.

Advances in wind shear detection technology have helped to remove some of the guesswork. At airports equipped with terminal Doppler weather radar, a wind shear alert or a microburst alert is reason enough to abandon takeoff or landing plans until the alert has abated. Continental, the first major airline to equip its fleet with a predictive wind shear detection system, mandates that a takeoff or landing should not be attempted if the aircraft's on-board system sounds a wind shear warning. Predictive wind shear systems can provide crews with as much as a 90-second warning of microburst activity ahead of the aircraft.

But despite all reasonable precautions, wind shears still catch pilots by surprise, which is the reason for our visit. We're here to sidle up to the wind shear beast, poke a stick in its eye, and learn how to get away in one piece.

Prior to entering the Boeing 757-200 simulator, instructor Ken Quick offers some prudent advice on precautions to take if there is even a slim possibility of wind shear existing after departure. We first need to select the longest suitable runway. "Suitable" takes into consideration not only length, but also such things as slope, wind component, and the location of any convective weather that may lurk in the vicinity of the airport. We consider the variables involved in the first scenario that we will fly, a departure from Los Angeles International Airport, and choose Runway 25R.

We next ponder our takeoff technique and flap setting. Jet aircraft rotation speeds vary by weight and flap setting. Lower flap settings and higher weights translate into faster takeoff speeds (and consequently longer takeoff rolls). By using the lowest available flap setting and pretending that the aircraft is at the heaviest possible weight for the runway and conditions (even though it isn't), pilots can determine the fastest rotation speed that still satisfies other takeoff performance criteria. The aircraft will then have an increased energy state once airborne, compared with a lower-speed takeoff. The extra speed can be traded for altitude should a wind shear encounter take place immediately after takeoff.

But there is a downside to this technique as well. If a severe wind shear late in the takeoff roll causes a rapid increase in tailwind component, the faster groundspeed combined with the sudden tailwind component may jeopardize a successful abort. In that scenario, a higher-flap-setting, lower-speed takeoff would provide a greater stopping margin on the available runway. The 757 is more susceptible to tail strikes during takeoff at lower flap settings, something which complicates the equation further. All things considered, Quick recommends that we use flaps 20, the normal takeoff setting for the aircraft.

Next Quick briefs the appropriately dire-sounding wind shear escape maneuver, what we need to do if our flight meets up with a performance-robbing shear. If that occurs, the escape maneuver will be all that stands between us and an untimely return to earth. It is best described as a Hail Mary play, one performed by advancing the throttles to their mechanical stops while simultaneously increasing pitch attitude until a climb is reestablished or the stick shaker (warning of impending stall) activates. Airspeed becomes secondary to pitch and vertical speed. Since the stick shaker will activate just a few knots before the actual stall, it must be respected as the upper limit to further pitch increases. Intermittent activation is OK; continuous activation is not. Turns should not be attempted until the wind shear has been penetrated, since all lift must be dedicated to staying out of the dirt. Also, any increase in G-loading caused by banking could precipitate a full stall. A wind shear powerful enough to require the escape maneuver will probably contain moderate to severe turbulence as well, making precise aircraft control difficult. Landing gear and flaps should be left down until a positive climb is established. If the aircraft makes ground contact anyway, they may help to absorb some impact forces.

In the simulator, Quick is about to show us the consequences of throwing caution to the wind shear, so to speak. The ATIS at LAX is reporting light and variable winds, and a high temperature dewpoint spread. Towering cumulonimbus clouds are west of the field, and lightning has been observed. A glance at our weather radar shows solid red returns just 5 miles ahead. Despite the abundant clues suggesting wind shear, we make a judgment call (albeit an intentionally bad one) to depart anyway. Quick reminds us to buckle our shoulder harnesses, as the full motion simulator can readily reproduce moderately strong turbulence.

We take the runway and are cleared for takeoff. The aircraft is at its maximum takeoff weight of 255,000 pounds, and the takeoff roll is normal. Longtime Continental Captain Scott Tomlinson, performing first officer duties, calls out rotation speed and the Boeing lifts off the runway. Almost immediately, the aircraft is buffeted by moderate turbulence. Airspeed begins to drop precipitously, and the words "Wind shear!" appear in bright red letters on the electronic attitude indicator. At the same time, a computer voice blares out "Wind shear!" — leaving no doubt as to what is happening. I increase pitch several degrees, then several more as the climb rate begins to falter. We have barely reached 300 feet and the airplane's normally sprightly performance has turned to mush. Tomlinson shoves the throttles to their mechanical stops and begins to call out the vertical speed trend, which, unfortunately for us, is now negative. The ground is rising up to meet us. Finally, at about 28 degrees nose up, the stick shaker begins to sound, and I relax back pressure enough to make it stop. I find myself chasing the required pitch attitude as the turbulence rocks us. Like swimming through molasses, the aircraft responds s-l-o-w-l-y, no longer descending but not yet climbing, either. Finally, we exit the shear and begin to climb. Tomlinson radios a pirep of the wind shear to the tower.

Quick informs us that we survived a brutal 52-knot tailwind increase and resets the simulator for another try. This time we will encounter a similar tailwind shear, only it will occur somewhere on final approach, with the autopilot engaged. Unlike most commercial airliners, the 757's attitude indicator has a wind shear-enhanced flight director system, which, in conjunction with the autopilot, will command and fly the escape maneuver. The attitude indicator also includes a speed trend line, a green arrow which shows what the aircraft speed will be ten seconds in the future. An "up" arrow predicts an increase; "down," a decrease. The longer the line, the larger the speed change expected to occur. This can supply a crew with valuable warning time during certain kinds of wind shear events.

We have agreed to go around at the first indication of wind shear. Even a few seconds' head start on the escape maneuver can make a huge difference in the outcome. We've hedged our bets further by adding 15 extra knots to our landing reference speed, based on reported surface winds. We can trade those knots for altitude during a go-around if need be. The ILS approach proceeds uneventfully until we descend to 600 feet, where we are again drubbed by turbulence, and our airspeed begins to drop precipitously. The green speed trend line predicts that the alarming decay will only worsen, and we initiate an autopilot go around. The Boeing pitches up smartly, the autopilot nailing the escape maneuver more precisely than I did during my earlier attempt. Quick applies salve to my ego afterwards. "The autopilot has a distinct advantage over us," he points out. "It literally has nerves of steel — and no adrenaline, either."

We skirmish some more with various permutations of wind shear. Quick graciously selects "survivable mode" each time, except for a final "not survivable" approach scenario he picks to emphasize that avoidance is always the best policy and sometimes the only one that will work. This time we encounter another huge tailwind, combined with strong up- and downdrafts and a vicious right crosswind component, all of which conspire to overpower the twinjet. It's a sobering feeling to be chewed up in the maw of the beast and spit out as so much bones and gristle. "I don't like to use it in training," Quick says later of the crash. "It has limited value as a teaching aid, except to drive home the point that wind shear is serious stuff that should demand our total respect."