iwebslinger

How hard does it have to be blowing for planes not to take off?

number_6

Varies a lot by plane type and runway and wind direction, but 40 knots is marginal, 60 knots closes most airports and 100 knots is about the operational maximum (but no airline would be willing to fly in those conditions). Luckily airports have cleverly built runways in the prevailing wind direction, which is often quite predictable, to optimize this.

cblaisd

Every airplane in the United States has been certified for a maximum crosswind component (which is a calculation involving the direction of the wind and speed -- e.g., a 50 knot headwind 5 degrees off the nose will be less of a crosswind component that a 90 degree crosswind at 20 knots).

For a pilot to attempt a takeoff (or landing) when the crosswind component is known to be higher than the certificated maximum means the pilot is in violation of the plane's airworthiness certificate, would be subject to FAA action, has become a "test pilot," and is really stupid.

Each airline may well have its own procedures, though, that mandate no takeoffs or landings at crosswind components less than the certificated maximums.

Palal

In which direction?

Most of the time, cross-winds are the problem. cblaisd pretty much said it all. Usually, runways are designed, based on past wind data, to be open at least 95% of the time, due to crosswinds.

If you get heavy tailwinds and the runway is not long enough for a particular plane going to a particular place, then the plane won't take off. (the same is true for landings)

Not sure if there's a limit for headwinds, but I imagine there must be.

cblaisd

There are lots of stories about Piper Cubs actually flying backwards, relative to the ground. Here's one from the advertisement page for a book of aviation memoirs/funnies:

...The battle wasn’t over yet. I reduced the power, raised the nose and extended full flap. I trimmed the Cub to hang from its propeller at 45 mph. Then I turned the airplane 180 degrees and flew into the wind.
On a heading of 230, the Cub floated slowly backward over the ground. The controller’s radar would show that I was tracking 050 degrees at five mph. This went unnoticed for several minutes before someone in the tower must have picked up the binoculars.
“Charlie Uniform Bravo,” the controller barked, “what are you doing now?”
“Uniform Bravo is flying downwind, slowly,” I said.
There was a long pause. “Charlie Uniform Bravo, you’re number four following the Learjet.”
“Negative contact, Uniform Bravo.”
Of course I couldn’t see the traffic. It was still ten miles away and I was flying with my back to it.
“United Express 233, cleared to land 24 Right.”
“Cleared to land 24 Right, 233.”
“Charlie Uniform Bravo, what airspeed can you give me on final?”
“Anywhere from 30 to 85 knots, Uniform Bravo.”
That translated from a minus five to a plus 50-knot forward ground speed. The other traffic would be approaching at 100 to 120 knots over the ground.
“This is not going to work, Charlie Uniform Bravo.”

www.happylanding.com/you'd/You'd%20Fly%20Laughing%20Too.htm

rar indeed

How is this calculated? Is there a distinct formula for each type of aeroplane (let's say v=10cosT+5sinT where T is the angle off the nose of the aeroplane) to allow forcalculations over an entire continuous cylindrical coordinate system or is it some sort of discrete table (10 kt at 90 degrees, 20 kt at 45 degrees, 50 kt at 0 degrees)?

cblaisd

Well, speaking as a humanities major let me say, huh?

I assume it's simply physics, and the amount of force in one direction is attentuated by that force not being entirely from that direction.

Here's an online calculator:
http://www.aeroplanner.com/calculators/avcalcdrift.cfm

And here's one similar to the one I used to use in piloting my little Cessna and later Cherokee:
http://sportys.com/acb/showdetl.cfm?...oduct_ID=10238

If your question is about how is a particular aircraft's maximum crosswind component determined, I believe it is a matter of both wind tunnel testing and test flying to the point at which the pilot needs Depends.

Iirc from my now-lapsed pilot days, low-wing planes generally have a higher crosswind component than high wing ones (e.g., most Cessnas and most Pipers, respectively). The highest I ever landed in was a 90 degree 15 knot crosswind in a Piper Cherokee; I believe that was almost at the maximum. It was a puckery ride but the airport had only a north-south runway.

LarryJ can probably give you a better answer; maybe he'll come calling on this thread.

rar indeed

As a non-physicist (), I'd have to agree that it would have something to do with the amount of force perpendicular to the aeroplane, but then I'd imagine that some sort of adjustment would have to be made with respect to certain characteristics of the aeroplane itself - size of the vertical stabilizer, etc - that would create varying changes in yaw based upon wind direction (or is it assumed that the pilot would compensate for any such changes?).

My question is more about the actual maximum crosswind component. I'm guessing it's merely a "this aeroplane can withstand an X kt crosswind" and then doing elementary trigonometry to determine the crosswind and headwind components of a wind given its speed and direction from the referenced sites. If not, I'm definately interested as to exactly what it is and how pilots can calculate the "effective" (for the lack of a better word) crosswind given a situation.

cblaisd

If I understand you: a) discovering how the perculiar aerodynamic characteristics (stablizer, degree of flaps, etc., etc.) of the plane affect the ability to land it safely is what is done in testing, so that b) one can indeed then say this airplane -- given all of that testing -- has a maximum crosswind component of x in y (flaps up/down, etc.) configuration. Then all you have to do is calculate from wind speed/direction what the component is and you have your answer as to whether it is legal to fly the airplane.

My experience is that if you're near the line of asking about legal, you shouldn't be trying the landing, my foolish at-100-hours-I-knew-everything efforts notwithstanding

Edit: Upon re-reading

I think that nails it -- and sums it better than my feeble attempts.

AEpilot76

At our airline, operations are suspended when winds are above 50kts. Also, like you said above, we can never exceed the max crosswind component.

We have a maximum crosswind component of 30kts listed in the limitations section of our manual. We can never exceed that since it is a limitation.

Some airplanes, however, have a maximum demonstrated crosswind component. That is NOT a limitation since it is the maximum amount of xwind they experienced during certfication. I've never seen a "max demonstrated xwind" in a transport category aircraft, it's usually in light aircraft

slawecki

I think the problem is too complicated (and too dangerous for mistake) to be done with calculations. I think it is done by trial and error, with lots of hard measurements. It should not be too difficult to determine when the plane is getting remotely close to instability.

National airport has a major crosswind problem with a strong wind out of the west. the alternate east west runway was disappeared about 40 years ago. It had been used by STOL commuter planes (Shorts I think) for nyc flights to take off and turn almost immediately to the north east.

carpboy

Components are easily calculated. If the wind angle theta is measured off the nose the headwind component is simply wind speed * cos(theta) and the crosswind component is wind speed * sin(theta).

As mentioned there are actually several different crosswind limitations. It is aerodynamically related to the effectiveness of the rudder in maintaining a ground track with a wing dipped into the wind. The stronger the wind the more dip and more rudder needed for a straight ground track. A demonstrated limitation means simply that was the max crosswind a test pilot flew - it does not mean that the airframe can't handle a larger one. An operating limitation is one by an operator in which a max crosswind is stated. This is strictly procedural and may be far from any demonstrated limits.

There are other affects in limiting crosswinds as well. An airplane with wing mounted engines (and low like a 737) may have a crosswind limit so that there isn't too much bank when flaring in landing, don't want to scrape an engine.

Engines themselves have crosswind limits. In the early part of a takeoff roll the airplane doesn't have much forward speed and a strong crosswind can introduce airflow into the engine in a manner not normally seen. This can cause a compressor stall. I've experienced one once and it went KABOOM. It was loud. This was a DC-10 with about a 30 kt direct crosswind. The flight was delayed waiting for the winds to drop into the engine limitation range but either the limits were too high or the wind jump as we rolled.

AAaLot

Here are some cross wind landing tests

http://www.youtube.com/watch?v=ljOxo0s33sI

Lonely Flyer

My understanding a tail wind is just a headwind in the other direction so that is why they change the direction of takeoffs.

As others have said when airports are designed they use the historical information about wind directions

Of course there will be crosswinds from time to time

cblaisd

That is essentialy correct, but a tailwind's effect on takeoff distance is dramatic.

There are some runways, too, that are essentially one-way runways; most of these are, e.g., small airstrips in mountainous regions which, due to obstructions at one end, can only be used in one direction. If the wind is wrong, then no takeoff is possible (or prudent anyway!)