Page 42 - Twin & Turbine May 2017

Baron 58, the landing distance over a 50-foot obstacle (i.e., from about over the runway threshold to the point the airplane stops, assuming maximum braking is applied) is 2,750 feet in zero wind. Add a 10-knot headwind component and the total landing distance is 2,500 feet, a roughly 10 percent improvement. Land under those conditions with a 10-knot tailwind, however, and the total obstacle-clearance landing distance is 3,400 feet – a 24 percent increase in landing distance.
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Accelerate/Stop
Multi-engine pilots are correct to be aware of accelerate/stop distance before takeoff as well. What is the effect of a tailwind on accelerate/stop distance? Again, on a 20-dgree day at a 2,000-foot field elevation, computed accelerate/stop distance using “book” (Associated Conditions) technique, the maximum gross weight, zero-wind accelerate/stop distance is 3,400 feet. A 10-knot headwind component shaves 200 feet, or about 6 percent, off the runway requirement. Take off under the same conditions with a 10-knot tailwind component, however, and the accelerate/stop distance is 4,400 feet – a 30 percent increase in runway requirement compared to that for zero wind.
Long-Winded
Here are some general rules of thumb:
• Each knot of headwind component on takeoff improves takeoff performance by roughly 1 percent, while each knot of tailwind component degrades performance by 3 to 5 percent. Tailwinds are three to five times as detrimental to takeoff as headwinds are an improvement.
• While each 1 knot of headwind component improves landing performance by about 1 percent, each knot of tailwind component degrades landing distance by about 3 to 5 percent. Tailwinds are roughly three to five times as effective at altering landing performance thank headwinds...and the alteration is not in your favor.
• In almost all cases, then, there is very good reason for avoiding tailwind takeoffs and landings, even if it makes more sense for the direction of flight on departure or arrival.
• Our discussion has centered on changes in terms of percentages. Takeoff, landing and accelerate/stop distances are already long in twins and turbine-powered airplanes. Increasing runway requirements by (in some cases) roughly one-third is significant.
• The heavier the airplane, the more dramatic the performance loss when taking off or landing with a tailwind.
• Tailwinds drastically reduce performance and margins for error that make it even more challenging to survive a power loss, total engine failure or other abnormal or emergency condition.
Do some similar performance calculations with the handbook for the airplane you fly to see what impact tailwinds have on your takeoff, landing and accelerate/stop distance.
Some runways are “one-way” because of extreme runway slope or very high obstacles close to one of the runways. In those cases, you’ll need to investigate further to determine the best runway for landing and takeoff. Often that means a phone call ahead to talk to local pilots in addition to reading any notes from the FAA Charts Supplement (formerly Airport/Facilities Directory).
Don’t fall into the trap of setting yourself up f•or a crash of convenience. Take the extra minute or two needed to taxi to the appropriate runway, take off in the appropriate direction, and only when airborne turn to proceed on your route. T&T
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Thomas P. Turner is an ATP CFII/MEI, holds a Masters Degree in Aviation Safety, and was the 2010 National FAA Safety Team Representative of the Year. Subscribe to Tom’s free FLYING LESSONS Weekly e-newsletter at www.mastery-flight-training.com.
40 • TWIN & TURBINE
May 2017