What is the maximum angle between the airplane's heading to the runway when an airplane is being landing sideway due to crosswind?
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asked Jan 8, 2019 at 3:54
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$\begingroup$ The exact angle is not what the performance specifications state. Instead, the maximum crosswind component (What is the maximum crosswind component for large airliners?) is specified, from which you can roughly work out the angle for a specific type of aircraft if you're interested. Each aircraft model is different. $\endgroup$– Greg HewgillCommented Jan 8, 2019 at 4:01
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1$\begingroup$ 90 degrees, if the crosswind's strong enough :-) $\endgroup$– jamesqfCommented Jan 8, 2019 at 19:50
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$\begingroup$ I don't think 90 degree will be possible as with angle, both rear landing gears will be parallel to the runway. With that position, the force will be strong enough to damage it. $\endgroup$– AirCraft LoverCommented Jan 9, 2019 at 1:35
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4 Answers
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A B777 (as in your photo) would generally have a maximum crab angle of about 16 degrees when approaching and landing at the Maximum Demonstrated Crosswind Component of 38 knots.
A 38 knot crosswind is not limiting and can be exceeded provided you apply a sideslip or crab correction at touchdown. It is however limiting if you plan to touchdown maintaining the crab.
E6B Heading, Ground Speed, And Wind Correction Angle
By using a sideslip or crab correction at touchdown, Airliners will sometimes approach and land when the wind exceeds the Maximum Demonstrated Crosswind Component. If the crosswind component was 50 knots and the aircraft was had a lower approach speed (due to light weight), the crab angle during approach could be as much as 25 degrees.
answered Jan 8, 2019 at 14:18
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$\begingroup$ Thank you for your answer. Very informative. $\endgroup$ Commented Jan 9, 2019 at 1:30
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$\begingroup$ What is that "calculator"? Is that available in cockpit during flight? $\endgroup$ Commented Jan 9, 2019 at 1:31
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$\begingroup$ The E6B is type of circular slide rule which all pilots learn to use while in basic training. Very few airline pilots use them as modern airliners have Flight Management Computers that make the E6B mostly redundant. $\endgroup$ Commented Jan 9, 2019 at 2:08
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There isn't a maximum crab angle (you aren't supposed to touch down crabbed to the extent you can prevent it) and there usually isn't a maximum crosswind component either on most airplanes as a "legal" limitation.
Usually the aircraft's AFM limitations give a maximum crosswind component (a lateral vector derived from wind direction and speed you get from using a chart) with a statement that it is the maximum "demonstrated" component and is not considered limiting. It's simply the maximum component that was able to be tested during cert testing.
For crosswind components beyond that, it's not "illegal" to land from an exceeding aircraft limitations standpoint, but airlines may designate the demonstrated maximum component from the AFM as a hard limit for crews to observe as a policy.
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1$\begingroup$ As far as I can tell, the landing technique for most airliners these days is to keep the crab all the way to touch down and only add rudder to correct it once the mains are on the ground—see also Terry's comment on the other answer; unlike me he speaks from personal experience. $\endgroup$ Commented Jan 8, 2019 at 20:09
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2$\begingroup$ That is brutal on landing gears. With really large airplanes with a lot of mass and inertia, you will see a technique of holding a wings level crab into the flare, then aligning the airplane with rudder just before touchdown. With all the mass you have a few seconds before the crosswind starts to drift you across the runway. On smaller airplanes this doesn't work so well and it works better to land with sideslip, that is wing down with opposite rudder. When I was flying RJs I landed in crosswinds using sideslip (couple degrees of wing down and opposite rudder), just like a light plane. $\endgroup$– John KCommented Jan 8, 2019 at 20:15
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No airplane touches down sideways, some may approach at and angle but generally the goal is to touch down with the airframe in line with the orientation of the runway. What you see is a crabbed approach and the aircraft will straighten out prior to touching down. As Greg notes in the comments this angle is defined (and variable per aircraft) by the crosswind component. The maximum angle desired is 0 as anything above that will add side loads to the gear that are generally not a good thing. As the FAA notes in their Airplane Flying Handbook
...The crab method is executed by establishing a heading (crab) toward the wind with the wings level so that the airplane’s ground track remains aligned with the centerline of the runway. This crab angle is maintained until just prior to touchdown, when the longitudinal axis of the airplane must be aligned with the runway to avoid sideward contact of the wheels with the runway.
Practically an aircraft can touch down a bit off the longitudinal axis and with proper control you can straighten it out but its generally not great for the airframe or gear. Poor longitudinal control in a tail wheel can also lead to ground looping.
The exception to this is the B52 which can actually touch down sideways although the gear is in line with the runway.
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5$\begingroup$ For 747s at least through the -400 series, you are allowed to touchdown in a crab up to the max demonstrated crosswind. Personally I never liked doing that and kicked out to align with the runway with a slight slip to the upwind side, but touching down in a crab was what was encouraged at the two 747-100/200 carriers I flew for; they really didn't want to drag a wing. Take a look at the copy of the Boeing Flight Crew Training Manual available at volarenargentina.com/descargas/_747-400_FCTM.pdf for the -400, page 6.29 for max demonstrated crosswind and 6.30 for touching down in the crab. $\endgroup$– TerryCommented Jan 8, 2019 at 6:57
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1$\begingroup$ @Terry Interesting! You may want to add that in an answer as I did not know that and its pretty pertinent. Were side loads on the gear ever an issue in that situation. $\endgroup$– DaveCommented Jan 8, 2019 at 15:13
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1$\begingroup$ I never heard of any side load issues, but I didn't follow maintenance concerns and history closely. I'm guessing that there wasn't any problem because it was permissible to operate with inoperative body gear steering (we did it often) which meant that every time you made taxi turn of more than just a few degrees, you were dragging the body gear sideways. Really hard on the tires. $\endgroup$– TerryCommented Jan 8, 2019 at 21:49
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The answer varies according to the plane's design. Some planes are constructed with landing gear so robust and flexible that significant crosswinds will not overstress them during a crabbed landing (Ercoupe). Other planes are constructed with swiveling mains so the plane lands at a crab angle but the gear tracks straight down the runway (Lockheed C-5A).
answered Jan 8, 2019 at 4:04
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$\begingroup$ The Ercoupe's trailing link gear was robust but the term stout might make ppl think it was super rigid, It was designed to flex under side loads so it could tolerate crabbed landings. $\endgroup$– John KCommented Jan 8, 2019 at 4:09
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$\begingroup$ I meant stout in the sense of "robust" i.e., it didn't break at touchdown in a crabbed attitude. Should I edit? -NN $\endgroup$ Commented Jan 8, 2019 at 4:38
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$\begingroup$ Maybe use "stout, yet flexible" or something like that. $\endgroup$– John KCommented Jan 8, 2019 at 4:58