Twin Proficiency: A Decision of Convenience

Twin Proficiency: A Decision of Convenience

What effect does a tailwind have on your takeoff performance? More than you might think.

The National Transportation Safety Board (NTSB) released an unusually detailed Preliminary Report on the crash of a turbocharged piston airplane:

The aircraft impacted terrain shortly after takeoff from Stevensville Airport (32S), Stevensville, Montana. The private pilot and his passenger received minor injuries. Visual meteorological conditions prevailed. According to the pilot, he based the airplane at 32S, and he and his wife planned a final destination of New Orleans. The takeoff was conducted from Runway 12.

The pilot “accelerated” the airplane to “80” [knots] and then lifted off. Shortly after, the airplane “couldn’t climb or accelerate.” The pilot stated that he was unaware of the cause of the problem, and that it felt like there was a “rapid decrease in power.” He reported that he ensured that the engine controls were in their appropriate positions for full takeoff power, but that the airplane “would not lift” any further. The pilot reported that he had insufficient time to scan the engine power instruments or diagnose the problem, due to the airplane’s proximity to the ground. The pilot did not retract the landing gear during the event. He did not report his maximum altitude, but he did report that he had previously experienced problems with the turbocharger system.

The airplane struck level terrain adjoining the south side of the runway. The airplane came to rest upright about 300 feet from the runway centerline, approximately 3,500 feet along the runway from the Runway 12 threshold end, or about 300 feet prior to the Runway 30 threshold end.

A pilot-rated eyewitness who was situated on the northeast side of the runway, about 2,400 feet from the 12 end, reported that the engine sounded normal. The airplane just broke ground as it passed abeam of him, and he then mentioned to a person who was with him to watch the airplane, because its takeoff appeared to be unusual. The airplane achieved a maximum altitude of about 50 feet above the ground, and then began a “steep right descending turn.” The right wingtip struck the ground first; it exhibited a brief flash of fire which quickly disappeared. The witness stated that runway 12 has a “substantial” uphill slope, and the terrain and trees also rise in that direction. He also reported that at the time, there was a “quartering tailwind” from the “northwest” of about 15 knots. The 32S automated weather observations were recorded as being from 340 and 350 degrees, between 9 and 12 knots, with numerous gusts to 16 knots. The observations also reported visibility 10 miles, temperature 2 degrees C, dew point minus 6 degrees C, and an altimeter setting of 29.98 inches of mercury. 

I call this event and others like them a “crash of convenience.” Of course, there is nothing convenient about crashing an airplane. What I mean is that attempting takeoff uphill with a 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.

Stevensville Airport (32S)

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.

On the Baron Takeoff Distance chart I plotted ground roll distance (zero obstacle height) for a 20-degree C day at a
2,000-foot elevation airport. The airplane is at maximum gross weight (5,500 pounds). Note that this calculation assumes the pilot adheres to the Associated Conditions technique at the upper left of the chart, and uses the liftoff and 50-foot speeds tabulated for the airplane’s weight.

In this example, a zero-wind takeoff would require approximately 2,800 feet from the beginning of the takeoff roll to clear a 50-foot obstacle. Factor in a 10-knot headwind component and the computed takeoff roll distance is 2400 feet, a roughly 14 percent improvement. Make that 10-knot breeze a tailwind, however, and the computed 50-foot obstacle distance is 32 percent longer than the zero-wind takeoff – the tailwind’s detrimental impact is more than twice the amount per knot as the positive effect of a takeoff headwind.

From either the simple Cessna’s tailwind warnings or the Beech twin’s performance charts, we can confirm the wisdom of taking off into the wind in all but the most unusual cases.

Coming Down

Let’s look at the performance change on landing when comparing a headwind component to a tailwind. Cessna’s 172S POH has already told us a knot of tailwind is worth nearly 5 knots of headwind. The Beechcraft Baron 58 POH gives us a sample calculation below.

On a 20-degree day at 2,000 feet and assuming a maximum gross weight 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.


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-degree 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 for 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.

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