What wind speed is too high to takeoff?
When the winds blow, the risks increase for light aircraft operations. The single leading cause of accidents involves loss of directional control during takeoff or landing. The AOPA Air Safety Foundation’s General Aviation Weather Accident Safety Review shows that over an 11-year period the National Transportation Safety Board identified wind as a primary cause of more than 2,800 accidents. These occurred primarily on landings, with takeoff being the second most likely phase of flight for wind accidents. Some might argue that wind isn’t «weather» in the traditional sense of the word, but rapidly moving air marked by airmets and measured in steeply packed isobars isn’t chicken soup. If there is a bright spot in all this, the accidents seldom cause more than minor injuries because the aircraft is moving relatively slowly and is generally in a landing or takeoff attitude sort of.
Crosswinds and gusts accounted for about 80 percent of the difficulty, according to the NTSB. Pilots coming out of hibernation in March and April seem to have more trouble, which is logical because these are windy months in most parts of the country. As the earth heats up, the temperature differential between cold and warm areas causes frontal systems to move. Fronts, highs, and lows bring wind. Pilots often haven’t been flying much in the winter or have flown only in stable conditions.
With reasonable proficiency and adequate runway dimensions, you should be able to handle surface winds up to 15 knots. The actual crosswind component might be around 7 or 8 knots. Defining reasonable is one of those devilish details, and it seems that pilots are frequently overconfident right up to the point of impact. Flight instructors say that one of the toughest maneuvers to teach is crosswind landings, partly because of the difficulty in scheduling the crosswind where and when you need it.
It also depends on how much of the wind is actually cross and whether it is gusty or smooth. In some parts of the world, a 25-knot wind is less challenging than 15 knots because of terrain and obstacles. A smooth wind right down the runway can help rather than hinder both takeoffs and landings. When the blow exceeds 20 knots, you must be on top of your game, and above 25 knots is for serious players only.
How much wind should you accept? There are some questions to ask yourself. How much training have you had in crosswinds? How long have you been flying, and where? How long have you been flying the particular aircraft? Have you flown it in gusty conditions before? When was the last time you flew in wind? Were you in command the whole time or just a passenger along for the ride? There is no dishonor in opting for more instruction, if needed, and for most of us it is a good investment.
The aircraft itself is also a part of the process. Some aircraft are just better in the wind than others. Traditional wisdom says that low wings handle wind better because it is less likely to get underneath the wing, and the center of gravity is lower. Statistically, this hasn’t been proven that I’m aware of. Cessna built tens of thousands of high-winged machines in Kansas, where the winds are anything but gentle. There were no particular ill effects, but their test pilots also knew how to fly.
There is no question that tailwheel airplanes are a bigger handful in the wind. The accident history seems to bear this out. There are two reasons. First, the aircraft sits at a positive angle of attack (AOA) and is more susceptible to gusts than one where the wing is parallel to the ground. An aircraft with a negative AOA will stay on the ground for as long as needed, but a positive AOA may lead to an early liftoff before enough speed is gained to ensure controllability.
The ground loop is not as common an occurrence with tricycle-gear aircraft, but anyone who has checked out in tailwheels knows all about it or will shortly. The center of gravity is aft of the landing gear, so the aircraft is directionally unstable on the ground. Get sloppy on landing, correct too slowly with rudder as the aircraft starts to swerve, and become a passenger while centrifugal force takes over and slings the tail sideways. With luck and a slow speed, the outboard wing may not contact the ground. The tricycle gear has done wonders in reducing wind-related accidents, and yet the encounters continue.
Demonstrated crosswind component is a favorite test question for examiners to ask. It is the highest wind observed during certification testing of the airplane, not what it is theoretically capable of handling or a limitation governing the aircraft’s operation. As a guideline, particularly for new pilots, consider it limiting. To find the number, look in the pilot’s operating handbook under «speeds for safe operation.» Some of the numbers are impressive: for the Cessna Skylane RG, 18 knots; Beech Sierra, 17 knots; Bonanza V35, 17 knots; Cessna 172, 15 knots.
If your aircraft was built before 1975, you probably won’t find the speeds in the POH. But there’s an alternative. A 1965 copy of the FAA’s Flight Training Handbook recommends using 20 percent of the stall speed as the maximum allowable, so an aircraft that stalls at 60 knots can handle a direct crosswind of 12 knots.
Some aircraft will demand less of pilots in the wind than others. This has to do with wing loading and response. A heavier airplane generally will ride better in turbulence because it doesn’t respond as quickly to the gusts and eddies of air. We frequently make matters worse by overresponding and are often late on corrections, so just as the natural stability of the airplane is about to kick in, the pilot adds a full load of control input. It’s enough to make you reach for the barf bag. A light touch generally does the job.
The runway length, width, and alignment are considerations as the wind gets strong. If it’s right down the slot, then life is generally good except in cases of wind shear. We’ll get to that in a moment. A wider runway obviously leaves more margin when it comes to avoiding runway lights, rougher terrain, and other obstacles. In extreme cases, pilots have angled the aircraft across the runway to reduce the crosswind component. That is pushing the limits.
Is there any surface contamination? That’s the professional’s term for snow, ice, or standing water. They all reduce the coefficient of friction on touchdown which means that the wheels will slide sideways more easily.
A longer runway means that extra stopping distance is available to compensate for the slightly higher approach speed necessary to handle a strong crosswind. The rule of thumb is to add half the gust factor while on the approach. For example, winds at 15 gusting to 25 provides a gust factor of 10 knots, so add half that, or five knots, to your normal approach speed. Some pilots have difficulty dividing by two when in a crosswind so they add in the full gust factor plus some extra just to be sure. Then you can be confident that the landing will be entertaining. Do not expect to achieve the book numbers for a short-field landing during a strong, gusty crosswind.
What about shear? Shear occurs when the wind changes direction, speed, or both within a very short distance. It can cause an aircraft to stop flying. Although we normally think of shear near thunderstorms, it can be serious on a gusty day, depending on wind strength and terrain surrounding the airport. The aircraft’s power-to-weight ratio is a factor, as is the speed with which the pilot responds to the shear. Then it depends on how quickly the aircraft reacts. We’ll spend more time on this in a future column. For now, if there is a sudden drop in airspeed or you get that sinking sensation, add lots of power quickly and pull up. If it’s a serious encounter, go around and perhaps look for someplace else to land.
If you’d like to sharpen your skills, go out to the airport some Saturday morning when a front has just gone through, and watch for awhile. Do some Saturday-morning quarterbacking and then go fly with a CFI. With some practice and a little help from the resident pro, you’ll be handling the Zephyrs, Chinooks, and Santa Anas in no time.
Bruce Landsberg is executive director of the AOPA Air Safety Foundation. Read the foundation’s Operations at Nontowered Airports Safety Advisor, available online or as a downloadable PDF for more information on wind in the traffic pattern.
Taking off into the wind
Most aircraft accidents occur during the takeoff and landing phase of the flight. Collisions with obstacles during climb out, runway overruns on landing occur every now and then. On this section of the site we will take a look at the various factors contributing to the performance of the aircraft in this part of the flight. Hopefully we help the pilot ensuring safe operation during these phases of the flight as the rules require that of the pilot in command.
The effect of wind has on our aircraft is something we can influence to some extend. We can choose runways with the greatest headwind component (when the airport has more than one runway) and use the wind on our tails when flying to our destination. Wind speed and direction usually changes with altitude so that effect can be used too.
The last part of the flight (landings) can be the most demanding for the pilot, and during training he/she will spend a lot of time practicing these to almost perfection.
Headwinds & Tailwinds
Aircraft use the flow of wind over the wings to generate lift to be able to fly. A minimum amount is required to lift-off and usually the engine generates thrust to obtain this lift-off speed.
By taking off into the wind (the wind will generate part of the required lift) the aircraft lifts off sooner and this will result in a lower ground speed and therefore a shorter takeoff run for the aircraft to become airborne. It is therefore recommended.
Not only for safety reasons: a takeoff that is abandoned will also use less runway to stop because ground speed is lower (check the ASDA distance during preflight). Climbing into the wind will result in a steeper climb, which is ideal for clearing obstacles in your climb out path.
Landing into the wind has the same advantages, it uses less runway, ground speed is lower at touchdown (less wear and tear on the aircraft) and the runway is available sooner for the next aircraft when it gets a bit crowded.
A rule of thumb says that takeoff and landing distances are reduced 1,5 % for each knot of headwind up to 20 knots.
Takeoffs with a tailwind will result in the use of much more runway to get enough lift for flight (It takes distance to nullify the tailwind before any headwind is obtained for lift). Climb angle is also reduced. Think about obstacles! A five knot tailwind increase takeoff distance with 25% and a ten knot tailwind with about 55%.
The same can be said about landing distances. The main reasons for executing tailwind operations could be airport noise abatement procedures or commercial operations and in case of certain mountain airports where you land up slope and takeoff down slope again.
Landing with a tailwind has another problem: you will approach the runway with a higher ground speed and this could lead the pilot into lowering his airspeed because he/she is visually accustomed to a lower ground speed. At this point a stall can happen, with no room to recover from it. But the solution is simple: be aware of this problem and concentrate a bit more and fly your usual indicated final approach speed and expect to use more runway.
During landing the best thing to do is to avoid tailwinds altogether, only if sufficient runway and pilot experience is available it can be done safely.
Remember: tailwinds do not lower the indicated airspeed or stall speed, they influence the ground speed of the aircraft.
Turbulent Gusting Winds
On takeoff with gusting winds it will be required that you keep the aircraft on the ground somewhat longer to provide a higher margin from the stall. During landing you must add half the gust factor to your final approach speed. Thus if tower reports the wind 240 at 18 gusts 28 it is advisable to add 5 kts to your airspeed.
It is wise to do the same on takeoff. Add half the gust factor to your normal lift-off speed, this should provide a safe margin from the stall. Keep in mind that because of these higher takeoff and approach speeds more distance on the runway is required so with short runways this technique may not be ideal.