Mentor Matters Jet landing distance calculations

Flying ‘by the book’ can lead to big trouble

August 1, 2013

Examining light jet accident statistics, it’s immediately clear that pilots going off the end of the runway are responsible for a lion’s share of bent metal. When considering how manufacturers derive the performance information that tells a pilot how much landing runway is needed, this fact becomes more understandable. If a jet’s aircraft flight manual (AFM) says that for given weight, altitude, and temperature (WAT) conditions, 2,500 feet of runway is required to come to a complete stop, it’s not unreasonable for a pilot to be surprised if he or she ends up off the runway where 4,000 feet of pavement actually exists. However, considering all the variables that must be aligned properly to achieve the near-mythic AFM performance, a different picture emerges.

The fundamental issue is that, as compared to takeoffs—the other event where required versus existing runway must be evaluated—landings are a much less reproducible maneuver. For takeoff a pilot need merely apply full power while holding the brakes; then, at the proper speed, begin rotating so as to increase the pitch attitude about three degrees per second. Do this, and for given WAT conditions, very little variation would be expected with regards to how much pavement is needed to reach 35 feet agl.

Contrast this with a landing—to achieve AFM performance, the pilot must be exactly at the specified approach speed (VREF), and exactly 50 feet above the runway as the threshold is crossed. No floating may occur, and once the aircraft is on the ground, maximum wheel braking must immediately be applied. Landing isn’t a case of needing to be as good as a test pilot to realize AFM performance; it’s actually a case of needing to be better than a test pilot. While certifying the airplane’s performance numbers, test pilots will execute landing after landing to get the data points needed to create the AFM. If a particular approach and landing was flown at a knot above VREF, or a bit too high across the threshold, the data simply are not used, and the landing is flown again. Thus, even a test pilot might not achieve “book numbers” on every landing.

Now consider that for a light jet with a VREF of 100 knots, the aircraft is moving roughly 170 feet per second as it crosses the threshold; every second the pilot floats or delays brake application will add 170 feet to the distance needed to bring the aircraft to a stop. If the pilot flies slightly fast or high, further penalties will be extracted—120 feet extra landing distance for every knot over VREF, 20 feet for every foot higher than the ideal 50 feet crossing the threshold.

Looking at the cumulative effects of a less-than-perfect approach, imagine a pilot crossing the threshold five knots fast and 20 feet too high. The extra speed contributes to two seconds of floating, and the pilot waits three seconds to apply brakes. While this approach and landing is certainly not desirable, all pilots will have some landings that are on the outside edge of acceptable performance—and this would not be a particularly sloppy specimen. Yet for these minor deviations from ideal, nearly 2,000 feet of extra runway will be used.

Considering these facts, the prudent jet pilot applies a safety factor to AFM performance data for landing purposes. Many pilots voluntarily follow similar guidance that charter operators are required to apply for dispatch; the airplane must be calculated to be stopped in 60 percent of the available runway length. Thus, for a 5,000-foot runway, landing would only be attempted if the AFM says that 3,000 feet or less is needed for actual WAT conditions, leaving a 2,000-foot buffer for the unavoidable excursions from perfect technique.

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Neil Singer is a Master CFI with more than 7,200 hours in 15 years of flying.