Aviation Safety Letter 1/2005
Wind at Your Back - The Hidden Dangers of Tailwind
by Gerard van Es, National Aerospace Laboratory NLR, Amsterdam, The Netherlands.
Tailwinds are very welcome when you are flying from A to B since they help shorten your flight time. However, close to the runway they can be anything but welcome. Even a bit of tailwind can be a hazard. Tailwind conditions can have adverse effects on aircraft performance and handling qualities in the critical flight phases of takeoff, approach and landing.
Performance regulations require that takeoff and landing distance data include correction factors for not less than 150 percent of the nominal tailwind component along the flight path. This margin is used to cover uncertainties in the actual wind condition. Aircraft flying at low speeds are relatively more sensitive to tailwind with respect to airfield performance. For instance, a 10 kt tailwind increases the dry runway landing distance of a large jumbo jet by some 10 percent, whereas for a small single engine piston aircraft the landing distance increases by some 30 percent. A small piston aircraft has an approach speed that is about half of that of a jumbo jet. A 10 kt tailwind will therefore increase the ground speed of this small aircraft relatively more than for the large jumbo jet, which explains the larger impact on the landing distance. On slippery runways, aircraft are more sensitive to variations in tailwind with respect to landing distance than on a dry runway. Tailwind-related overrun accident data show that in 70 percent of the cases, the runway was wet or contaminated. Clearly, the combination of tailwind and a slippery runway is a hazardous one, which should be avoided.
History tells us that tailwind is especially dangerous during the approach and landing. When an approach is made with tailwind, the rate of descent has to increase to maintain the glide slope relative to the ground. With a constant approach speed, the engine thrust must decrease with increasing tailwind to maintain glide slope. In high tailwind conditions, the engine thrust may become as low as flight idle. Flight idle thrust during the approach is undesirable for jet aircraft because engine response to throttle input is slow in this condition, which can be a problem when conducting a go-around. It can also become difficult to reduce to final approach speed and to configure the aircraft in the landing configuration without exceeding flap placard speeds.
A high tailwind on approach in itself may also result in unwanted excessive rates of descent. All these effects can result into unstabilized or rushed approaches.
When applying normal landing techniques, pilots who land their aircraft with a higher than normal approach speed tend to bleed off the speed by floating the aircraft. Floating the aircraft just off the runway surface before touchdown should be avoided because this will use a significant part of the available runway. In case of a tailwind operation, the associated increase in ground speed will further increase the landing distance. As the aircraft comes closer to the ground, the tailwind will normally decrease. This has a temporary lift increasing effect due to the increase in true airspeed (inertial effect), making it more difficult to put the aircraft on the ground, which amplifies floating of the aircraft. History tells us that in more than half of tailwind related overrun accidents, floating took place.
Another problem is the combination of tailwind and wake vortices during the landing. The wake behind an aircraft will normally descend below the flight path the generating aircraft has flown. In a light tailwind, the wake may be blown back onto the glide slope, making an encounter more likely than under normal headwind conditions. Analysis of wake vortex incidents indeed shows that the incident probability during an approach is somewhat higher in light tailwind (1-2 kt) conditions.
Wake vortices may decay less quickly at the point of flight path intersection, when a light quartering tailwind is present. This tailwind condition can move the vortices of the preceding aircraft forward into the touchdown zone. Therefore, pilots should be alert to a larger aircraft upwind from their approach and take-off flight paths. Wake vortex incidents that are attributed to light quartering tailwind are not uncommon, but are not always recognized as such. Incident data from a European airport indeed shows that the wake vortex incident probability is significantly higher in light quartering tailwind conditions. So the next time you make a tailwind landing, watch your back!
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