Mentor Matters: Slip-sliding away

The reason comes down to energy, and the relative difficulty a jet experiences in shedding it during the approach to landing and rollout. Consider a largish piston single that crosses the numbers at 80 knots at a 3,000-pound landing weight, then contrast it with a light jet doing so at 100 knots and 10,000 pounds. The jet is carrying more than five times the kinetic energy to the runway compared to the piston—and to make matters worse, experiences less drag to help shed it. Many jets in the landing configuration can’t descend much more than 1,000 feet per minute and lose airspeed at the same time, even at idle thrust.

All the single-pilot jets currently in production lack a reverse thrust capability, so wheel brakes and speed brakes (if installed) are solely responsible for shedding the energy present at and after touchdown. The job is manageable when variables are aligned in favor of the jet—meaning that the approach is flown in the proper configuration, on glidepath, at the proper airspeed, and braking is performed on a dry runway. Under these conditions jets can turn in downright impressive landing numbers. Many light jets landing on a dry runway need as little as 1,500 feet from main gear touchdown to a complete stop.

Unfortunately, pilots have proven adept at presenting the aircraft with more energy than the airplane can shed. They frequently come in high and fast from an unstable approach, or land on a contaminated surface less than optimal for stopping—or deal with a mix of the two.

Boeing has extensive research on jet overruns and found in one survey that 80 percent of air carrier overruns happened on wet or contaminated runways. Boeing also learned that the majority of overruns couldn’t be attributed to a single factor, but rather a mix of runway condition, poorly flown approach, or improper pilot landing and braking technique. Furthermore, Boeing discovered that often, if only one of these multiple factors was changed, a successful stop would have occurred.

Pilots have known for years the importance of flying a stabilized approach, and for decades the industry has promoted a set of stabilized approach criteria that attempts to remove judgment calls from the cockpit during the approach phase of flight—when workload is high and the pilot’s ability to self-assess is low. Rather than attempting to judge if the approach will work out or not, a half-dozen parameters must fall within specific windows, or the pilot must perform a go-around.

For example, the main conditions for a stable approach call for an airspeed no more than 20 knots above target, and the glide-
slope needle’s deflection within one dot of center, by no later than 1,000 feet above the airport elevation when in instrument meteorological conditions, or 500 feet above the airport in visual meteorological conditions.

Unfortunately, one study found that approximately 4 percent of approaches flown by turbine aircraft do not meet these parameters, yet in 97 percent of these unstable approaches the pilots continue the approach to landing instead of performing the go-around. What’s more, pilots are 30 times more likely to press on in the face of an unstabilized approach than go around. And since they are far more likely to get away with it than suffer an excursion, each landing leads to a normalization of the belief that an unstable approach can end successfully.

These poorly flown approaches are most likely to lead to excursions when flown to wet, slush- or snow-covered, or otherwise slippery runways. And it doesn’t help that the industry has found it is exceedingly difficult to predict exactly how much runway will be needed to stop an airplane when conditions are other than dry. The Boeing study found that in more than half of non-dry runway excursions, the runway was wet only—that is, not contaminated by standing water or frozen precipitation. Yet in 75 percent of these wet-only accidents, the braking action experiences were more consistent with snow—or even ice—than with accepted models of stopping on wet runways.

The FAA echoed this alarming fact with the publication of Safety Alert for Operators 15009, “Turbojet Braking Performance on Wet Runways.” It states that “several recent runway landing incidents/accidents have raised concerns with wet runway stopping performance assumptions. Analysis of the stopping data from these incidents/accidents indicates the braking coefficient of friction in each case was significantly lower than expected for a wet runway.”

There are similar difficulties predicting braking performance on snow. In an attempt to refine performance planning during winter conditions, some Canadian airports report a Canadian Runway Friction Index (CRFI) value for snow- or ice-covered runways. This is a vehicle-measured coefficient of maximum braking available that the pilot can use to adjust their dry landing distance calculations to an expected required distance based on the contamination present. Somewhat depressingly, one of the most commonly experienced contaminants—a thin layer of dry snow—experiences the largest range in braking degradation. An aircraft normally stopping in a total distance of 2,400 feet could, when landing on a mere 3 mm of dry snow, require from 4,000 feet to as much as 6,300 feet to come to a stop. And without a recent measurement a pilot has no idea where in the range their performance right now will fall.

Given the frequency of runway overruns, and the difficulty in accurately predicting stopping distance on contaminated runways, how can pilots minimize their risk—short of ruling out anything but landing on dry runways?

First, when approaching a non-dry runway, the pilot must be extra vigilant about meeting stabilized approach criteria, and be ready for and committed to a go-around if all parameters are not met. Next, they should realize that non-dry performance numbers given in their airplane flight manual or pilot’s operating handbook are estimates only—the manufacturer will clearly state they are for advisory purposes and cannot be counted upon. Given this, if the runway length is close to the published required runway, little to margin for error likely exists, and the pilot should consider looking for another, longer runway. Hoping to meet a manufacturer’s landing performance numbers is never a good strategy, but it’s especially risky on contaminated surfaces.

Finally, after landing out of a well-flown, stable approach on a conservatively long runway, the pilot should use immediate and full stopping aids until at taxi speed. Many excursions have resulted from a pilot’s delaying full brake application until it’s too late. Remember: Braking is not likely to be consistent during the rollout. If good braking is found right after touchdown, trusting that it will persist for the length of the runway has often proven to be trust misplaced.AOPA

Neil Singer is a Master CFI with more than 9,500 hours in 15 years of flying.