Page 39 - Twin & Turbine May 2017

Stevensville Airport (32S)
substantial tailwind was a significant factor in this mishap. Beginning a trip to the southeast, the pilot chose to take off in that direction regardless of the environmental factors. It’s possible also that the owner’s hangar or the fuel facility was closer to the approach end of Runway 12 than the reciprocal, downhill runway. Either or both, taking off on Runway 12 was a direction of convenience, not of operational necessity.
Very luckily (and it may have been luck), the aircraft’s occupants did not suffer serious injuries or death when it went out of control and impacted terrain alongside the runway.
How Much, Actually?
Convention has it that we take off and land into the wind. We learn from very early in our training that taking off into the wind helps get us aloft sooner, and that landing into the wind permits us to stop in a shorter distance. But how much does it matter, actually? Does it hurt to try to take off with the wind at your back, or land with a tailwind? Is there enough of a difference that, if the pattern is otherwise completely empty of traffic that you should still conform to the standard and takeoff or landing into the wind, even if that doesn’t make sense for your direction of flight? Well yes, it does.
Most Pilot’s Operating Handbooks (POHs) carry at least some caution about tailwind takeoffs and landings. We usually must go back to very basic training-type airplanes to get any suggested rules of thumb. Combine the recommendations of a few and you can derive some good rules of thumb about tailwind takeoffs and landings you might apply to flight in your twin or turbine, to decide if it’s worth the risk.
For example, the Cessna 172S POH gives some fairly precise guidance on the relative effects of a tailwind versus the “conventional” headwind takeoff. Note 3 from the Takeoff Distance performance chart tells us that we should decrease the takeoff distance we derive from using the chart by 10 percent for every 9 knots of headwind. But it also tells us to increase takeoff distance by 10 percent for every 2 knots of tailwind component.
Put another way, a tailwind component has almost five times the performance effect as a comparable headwind component. If we normally take off into the wind to improve takeoff performance, we really want to avoid taking off with a tailwind because the performance will be significantly impaired.
Cessna gives us similar guidance for landings with a tailwind. The Landing Distance chart contains a similar nearly five-to-one difference between landing distance improvement with a headwind component and increased landing distance with a tailwind.
Now let’s look at performance information for a light twin with which I’m very familiar, the Beech Baron 58. The folks at Beechcraft don’t give us any general rules for adjusting the takeoff distance for head- or tailwind components. They do, however, provide Takeoff and Landing Distance charts to let us determine the effect of head- or tailwinds on computed performance.
May 2017
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TWIN & TURBINE • 37