Aviation Safety Corner
GREETINGS! For this month's topic, I would like continue our discussion
on, "Wind at Your Back." The record shows that pilots should not discount the effects of a tailwind on
takeoff and landing performance. This is Part II of IV.
Now let's start our discussion on, "Missing the Mark." Besides the increased rollout distance we would encounter after touching down with a tailwind, it is likely that the higher ground speed would cause us to over-shoot the intended touchdown point on the runway.
If we push the nose over to touch down near the normal point, airspeed increases, and so does lift. We may land hard, bounce, balloon or float a long way down the runway. The result is a touch down a lot farther down the runway than we intended. If the landing attempt is on a short runway, we may not have room to stop. If we don't recognize what is happening soon enough to go around, the aircraft may run off the end of the runway. Then, the outcome depends on what lies beyond the paved surface. Ideally, there should be a long over-run area; but many airports have "airplane-catchers" (such as drainage ditches, fences and perimeter roads) just off the ends of their runways.
Now let's discuss, "Shaky Subject." One of the interesting things I discovered while researching this article is that many pilots don't really understand how significant tailwinds can be on takeoff and landing. I asked a sample of local pilots and flight instructors what they thought about tailwinds and if they knew what the correction factors are for a 10-knot tailwind on takeoff or landing for the airplanes they commonly fly. Almost all of them simply indicated that they would never attempt a takeoff or landing with a tailwind if they could help it.
They said it was just common sense not to. I then asked if they had ever done it unintentionally. Most had at one time or another, and a couple of the pilots indicated that they had, had real close calls because of it.
When asked what correction factors need to be applied to their performance data, only one of the pilots had the right answers. The others thought that a tailwind would have some effect, but that it probably would be similar to the effect of a headwind, but in reverse.
While obviously it was not a scientific study, it does provide some insight into what a variety of pilots know about tailwinds. A review of several of the more common aeronautical training and reference publications reveals part of the problem: Outside of the aircraft flight manuals, there is little mention of the effects of tailwinds on takeoff and landing performance.
There is nothing in the AIM, and most training manuals do not specifically state the effect that a tailwind has on takeoff or landing performance. The FAA's Flight Train-ing Handbook does mention the change in performance, but understates it.
One of newer training books states that a pilot should, "takeoff upwind if possible" and that, "taking off into the wind is good airmanship." One of the reasons given is that an upwind takeoff results in the shortest ground run. The book also advises that, "taking off downwind is not recommended." That's fine, as far as it goes. But what happens if you do takeoff or land with a tailwind?
Now, let's discuss, "Unexpected Encounters." Most pilots seem to dismiss tailwinds as a factor on takeoff and landing performance with the conviction that, "I'd never do it, so I don't need to worry about it."
Unfortunately, there are circumstances in which proper understanding of that factor is critical to proper Decision-Making, and our ability to deal effectively with the situation.
Although the most likely situation in which a pilot would encounter a tailwind on takeoff or landing is at an uncontrolled airport, it is also possible at controlled airports. My experience with the sudden wind shift is an example. Also, while most of us would assume that controllers would use a runway that has a headwind or no wind, that is not always the case.
The FAA's Air Traffic Controller Handbook provides controllers with the following guidelines for selecting active runways:
1. Whenever the wind speed is greater than five knots, use the runway most nearly aligned with the wind.
2. The "calm wind" runway should be used whenever the wind is less than five knots. This runway is specified by the airport management and contained in facility directives, and is chosen to maximize arrivals and departures while minimizing noise impact on local dwellings.
3. The local controller can use any other runway when it is operationally advantageous to do so.
One would hope that the first guideline would always prevail however, although not a common occurrence, it is possible that an airport could experience wind of less than five knots, allowing use of the "calm wind" runway.
To provide maximum noise abatement, for example, the use of an active runway with adverse light winds could produce a situation in which aircraft taking off and landing could have up to a five-knot tailwind component.
This might not seem like a lot of wind, but if the aircraft is a Cessna 172, the correction factor for takeoff and landing per-formance is a 25 percent increase in the respective distances over the no-wind values.
Now, in conclusion, remember to practice "GOOD AIR-MANSHIP", at all times while you are flying. Don't miss next month's Aviation Safety Corner, when we'll discuss, "On the Record", & "Phases of Flight," in Part III of IV.
Larry G. Harmon
FAA AVIATION SAFETY COUNSELOR
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