Wind significantly affects an aircraft in flight. Because aircraft fly suspended in the atmosphere, movement of air will ultimately cause movement of the aircraft in the same direction. This forces pilots to adjust their flight paths to accommodate the effects of wind in order to get their aircraft to their planned destination. This is a routine task for pilots and much emphasis is placed on correcting for wind in flight training.
When an aircraft is cruise flight, the effects of wind are easy to cope with. The pilot can adjust heading left or right to apply a crab angle to keep the aircraft heading where it needs to. This is a fairly easy procedure because flying from point to point does not require a great deal of accuracy. This is not the case on approach and landing. Because the pilot needs to touch the aircraft down on a runway which may only be tens of feet wide, they must control the aircraft’s flight path with a great deal of precision. Even small changes in wind velocity can have a relatively massive effect on the aircraft’s flight path. This makes landing in gusty winds excessively difficult.
The purpose of this post is to provide you with some insight on the dangers associated with high wind landings and how to mitigate those dangers to operate as safely as possible. This is primarily oriented towards small, light general aviation aircraft and does not reflect the best procedures for larger aircraft. The purpose of this is not to teach you how to land an aircraft.
Aircraft limitations
Crosswind: There is certainly a limit to the amount of wind an aircraft can fly in. The primary limitation is the amount of crosswind present on the landing runway. If the primary direction of the wind is not oriented directly in line with the landing runway, you must apply a crab in order to keep a ground track with the runway center line. Crabbing is accomplished with the ailerons and will cause the longitudinal axis of the aircraft to form an acute angle with the runway center line. This means that at some point before touchdown, you will need to apply some rudder to yaw the aircraft to align it with the runway center line. This will cause the aircraft to fly in a slip for a short period of time. As the crosswind increases in angle and intensity, crab angle must be increased in order to keep the aircraft’s flight path aligned with the runway center line. This will require you to increase the amount of rudder pressure to align the aircraft with the runway center line before touchdown. Eventually, a point will be reached where there will not be enough rudder available to counteract the force generated by the ailerons. This point is called the maximum crosswind component and its value can usually be found in the POH.
Headwind: In theory, if the max cruise speed of an aircraft is 100 knots, then the maximum headwind that aircraft could land in is 99 knots (I figure you need at least one knot to fly to the runway). In practice however, this is not the case, here’s why.
Wind shear: Wind shear is simply defined as a change in wind direction and speed over a given distance. Most pilots experience wind shear every time they land. Wind shear varies greatly in intensity and can range from changes in wind speed of a few knots to shifts of over a hundred knots. Most often, wind shear is caused by surface friction. As fast-moving air flows over the ground, it encounters trees, houses, buildings, etc. This causes the smooth flowing air to form eddies and shift direction and speed. This is one of the causes of gusty winds. High wind velocities will almost always bring gusts, and therefore will always bring some wind shear. The biggest concern wind shear places on you is its effect on your approach speed.
For example: let’s say that you are on approach to an airport. You are flying at 60 knots and your aircraft’s stalling speed is 50 knots. The winds are aligned with the runway at 20 knots gusting to 30 knots. This means that a 10 knot wind shear would be expected on final. If the airspeed indicator reads 60 on final and you are flying in the gust, when the gust disappears, your airspeed will drop to 50 knots and you are in danger of stalling.
Because of this, wind shear is the greatest risk when you land in high winds. In theory, the maximum amount of wind shear an aircraft could land in is a value equal to its maximum cruise speed subtracted by its stalling speed. I.e. if max cruise is 100 kts and stalling speed is 50 knots, you could land the aircraft with 50 knots of wind shear. Obviously this would be a foolhardy endeavor at best.
Adjusting the approach
As the velocity of the wind increases, the approach needs to be adjusted to prevent stalling form wind shear. A rule of thumb is to add half of the gust factor on to your final approach speed. For example: if the winds are 10G14, add 2 knots to your final approach. This rule works fairly well for low wind conditions where full flaps can be used. As the wind speed increases, a new mentality must be sought.
In higher velocity winds (much above 20 knots for most small planes), full flap landings become difficult to impossible. The reason? If the flaps are deployed, they will cause the aircraft to be increasingly affected by increasing and decreasing airspeeds. More accurately, an increase in airspeed of say 10 knots (like a 10 knot gust) will cause the wing to produce significantly more lift with the flaps down than it would with the flaps up. This means that the aircraft will get thrown around more on the landing roll out. It also will cause the aircraft to require less speed in order to touch down. This means that the rudder will be less effective on touch down and the max crosswind component will be reduced. In order to prevent this, in high winds, you must reduce the degree of flaps used on the approach.
This is where things get a little complicated. By reducing the degree of flaps on final, you will make the landing easier and make the aircraft more controllable on the rollout. However, you will increase the stalling speed which will require you to carry more speed on final. So, a decision must be made. Do you take the flaps out and make touchdown easier, or do you leave some in to decrease the stalling speed and make the approach safer? Here is my recommendation:
If landing distance is not a concern, and the wind speed is relatively high, run the plane in with the smallest safe amount of flaps. Adjust your final approach speed adding a 30% buffer to the clean stalling speed. Then add the entire gust factor or the entire forecasted wind shear value.
Example: Clean stalling speed: 50 kts, Wind 25G34kts
(50*1.3)+9= Approach speed: 74
Example: Clean stalling speed 50 kts, wind 25G34kts, WS20kts
(50*1.3)+20= Approach speed: 85
Increasing speed by this much will cause your aircraft to spend an increased time in ground effect on the roll out. You will need to use small, quick corrections to maintain centerline and alignment during this period. Using the above method will likely keep you out of danger of stalling. However, it does not mean that a larger wind shear than forecasted will not be encountered. The more speed that you add to your approach, the less likely you will be to stall. That being said, extra approach speed will increasing landing distance and will require you to work harder on the roll out to bleed off the extra airspeed.
If landing distance is of concern, run the aircraft in with the smallest amount of flaps you can to make the runway. Remember, the more flaps you add, the harder it will be to get the aircraft down, but the less runway you will eat up on the roll out. Usually however, in high winds, landing distance will be considerably shorter than normal and shouldn’t be an issue. High winds and short runways do not mix well.
Setting up, testing, and flying the approach:
If you are unsure of the wind conditions, you can test the approach. Set yourself up on final and run the aircraft in at a low cruise speed (well above the stalling speed) and watch the airspeed indicator. Note the amount it walks back and forth. That value approximates the wind shear. You can use that value to adjust your final approach (this is just for testing purposes, the actual amount of shear way be higher, add a safety factor accordingly).
Once confident, determine your approach speed based on the highest of the shift you saw and the highest forecasted value. Set yourself up on glide path a few miles out and begin your approach. You can request this from tower. This will give you the most time to set up your approach. When you near the airport, reduce from cruise and fly your calculated approach speed. I recommend holding cruise speed for as long as possible and slowing down on short final. As you get closer to the runway, you will likely encounter more shear. Keep a watchful eye on the airspeed indicator for shear and to assure you are on approach speed. Continue towards the runway, adjusting the ailerons for ground track, and power for your approach speed. As you near the runway, rollout at the appropriate height and let the aircraft bleed off speed. Use your rudder to keep the nose aligned and use the ailerons to keep centerline. Remember, if you catch a gust or drift, go around immediately. Eventually after all of that excitement, the wheels will touch. Briskly apply crosswind correction and bring the aircraft to a halt.
That’s it:
It’s not as easy as it sounds. I don’t recommend playing around in high winds, especially if you do not have a lot of experience. Using this technique should keep you safe, but it does take a lot of skill to keep the aircraft on centerline and aligned. This is where I say: Don’t try this at home…
-Mike
The Blog of Michael Soroka 