mypaks
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How Does Crosswind Landing Works?

Fri Mar 16, 2007 2:12 am

http://www.airliners.net/open.file/1095135/M/

How does the a/c after landing turns 90 degree?
WHat does the pilot have to prepare for such landings?
 
KLM772ER
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 2:21 am

It actually don't have to turn 90 degree, just maybe 30 degrees in this case... It is done by steering with the rudder which has still enough efficiency at this speed... so normally the pilot steers the aircraft shortly before touchdown or just right after onto the course of the RWY

Normal procedure is: during approach the pilot is correcting the wind component by turning the nose slightly into the wind
than as mentioned above shortly before or right after touchdown the heading is corrected with the rudder... and afterwards it is the normal procedure as on every other landing..

But I think this thread should be moved to the Technical/Operation forum..

[Edited 2007-03-15 19:27:42]
 
flyf15
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 3:03 am

There are two methods of handling crosswind landings, and they are usually used in some sort of combination.

Crab: The aircraft is pointed into the wind at the required amount to keep it flying towards the runway. This is done purely by trial and error.... try out something and if its not enough or too much (drifting to one side or the other), correct appropriately. The advantage to this is the airplane is flying completely normally other than the fact that the airplane is pointed in a different direction than the runway.

Slip: The aircraft is pointed down the runway and aileron is applied in the direction of the wind (banking into the wind) but rudder is applied in the opposite direction to keep the airplane from turning. The advantage to this is that the airplane and the runway are pointed in the same direction.

Depending on the aircraft, pilot, situation, etc.... you will choose the appropriate method. Crab is definitely the preferred option as slips are uncomfortable for passengers, more difficult, have you flying uncoordinately low and slow, and do not provide a wings level touchdown. You will see slips most commonly in light general aviation aircraft. In my aircraft, the CRJ-200, we could not really even slip if we wanted to, the wings are so close to the ground that any bank on touchdown puts us dangerously close to striking a flap.

Most often, a combination of the two methods is used. A crab is done all the way down final due to its more stable nature and then sometime before touch down, the aircraft is transitioned into a slip to allow for touchdown with the aircraft pointed down the centerline. Once again, when this transition occurs depends on the pilot, aircraft, and situation. If a general aviation pilot is doing this, most likely it will be somewhere around 100ft above the ground to allow time to get stabilized into the slip and be ready for touchdown. In the CRJ, due to the fact that we cannot add much bank, once we start to point the nose down the runway with rudder, we immediately start to drift sideways (of course, depending on how strong the crosswind is). Due to this, we have to do the transition from crab to "slip" (we never actually enter a full slip) at the last possible moment. I usually kick the rudder to point the aircraft down the runway while entering or already established in the flare... a second or two before touchdown.

In the picture posted, the pilot did not take the aircraft out of the crab before touchdown, hence not being pointed down the runway. Almost instantly in touchdown, rudder is used and the airplane is steered to go the correct direction. Some aircraft have their gear designed to be able to handle massive loads like that... not touching down "straight" and it is actually the proper technique for that kind of plane.
 
KELPkid
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 3:04 am

Well, usually you bank a few degrees (3-5) into the wind, and then use the rudder pedals to keep the aircraft pointed straight down the runway (which puts the plane in a condition known as a side slip)...and touchdown is usually one wheel at a time  Smile This is pretty basic-that's how we do it in General Aviation planes. There is more than one X-wind technique, and I'm sure some airlines/airline pilots use differen ones for different reasons...airliners with wing mounted engines have maximum bank angles that can be maintained before an engine pod will scrape the ground  Wink
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
 
kearney
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 3:48 am

I thought slipping was to lose altitiude without increasing speed... you crab into the wind and keep your windward wing slightly lower to maintain your course... crossing the rudder with the ailerons (side slip) is one way to spin some aircraft (if you keep the nose up rather then down).

Keeping the wind ward wing lower will reduce the tendancy of the aircraft to roll away from the direction the wind is coming from. This is only important in a strong crosswind approach

[Edited 2007-03-15 20:55:32]
 
AirWillie6475
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 4:55 am

Quoting Kearney (Reply 4):
I thought slipping was to lose altitiude without increasing speed.

You're thinking of a forward slip, that's when you use full rudder and opposite aileron.
 
bio15
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 4:57 am

Quoting Bjoernvenghaus (Reply 1):
It actually don't have to turn 90 degree, just maybe 30 degrees in this case

The photo was taken far from the touchdown zone, perspective can fool you. There is NO chance to land at a 30° angle from the centerline. A landing gear couldn't pull that off unaffected.

Say you had a 40 knot crosswind component and land at 130 knots, a crabbing approach would point your nose around 17° away from the centerline. That being considering the extreme: massive crosswind and slow approach speed for a jet under such wind circumstances. Usually an aircraft crabs no more than 10° during crosswind landings.


Alfredo
 
KLM772ER
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 5:41 am

Quoting Bio15 (Reply 6):
The photo was taken far from the touchdown zone, perspective can fool you. There is NO chance to land at a 30° angle from the centerline. A landing gear couldn't pull that off unaffected.
http://www.airliners.net/open.file/0223248/L/

this one has at least 30 degrees.. though it is not a crosswind landing..

By the way if you see the crosswind-landing testing videos, there are definitely landings with a bigger angle than 17 degrees...
I confirm that 30 degrees is to much for a normal crosswind landing.. there you are right.. but I think it can affect a bit more than 17 degrees if it has to

[Edited 2007-03-15 22:48:01]
 
bio15
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 7:46 am

Quoting Bjoernvenghaus (Reply 7):
http://www.airliners.net/open.file/0223248/L/

this one has at least 30 degrees.. though it is not a crosswind landing..

By the way if you see the crosswind-landing testing videos, there are definitely landings with a bigger angle than 17 degrees...
I confirm that 30 degrees is to much for a normal crosswind landing.. there you are right.. but I think it can affect a bit more than 17 degrees if it has to

30° is excessive, 17°-plus is too. Here's a simple calculation, which you can verify being a physics student.


  • The crosswind component of wind is the wind component that strikes the aircraft along its lateral axis, that is from the sides of the aircraft. A headwind has zero crosswind component.
  • An aircraft should have safe ground handling characteristics in 90° crosswinds equal to 20% of the stall speed in landing configuration.


Now, let's assume that we have an aircraft that will land at 140 knots, and its stall speed is 125 knots, which are not strange figures, and if so, they help your case. Stall speed could be lower.

If we took 40% of its stall speed, that's twice the safety margin, we would get 50.

Now, take a 90 degree / 50 knot crosswind into an aircraft landing at 140 knots. For it to maintain a straight path, the resultant speed vector should be vertical. That is, that the displacement angle from the runway centerline equals:

tan^(-1) [ 50 / 140 ] = 19.7 degrees.


Here I have taken into account a huge off-the-limits crosswind, striking the aircraft completely from the side, and an uncorrected approach speed. When you approach in heavy crosswinds you usually increase your airspeed to lessen the crosswind effect on the aircraft.

Take 20% of a 120 stall speed and an approach speed of 145, and you get a 9.4° crab angle, and an aircraft having safe ground handling characteristics.


Alfredo
 
roseflyer
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 8:05 am

Quoting Kearney (Reply 4):
I thought slipping was to lose altitiude without increasing speed

That is a very useful type of slip that is done often in general aviation when you are needing to make a steep approach or if you are too high close to final and want to lose altitude quickly. Slips are a very inefficient way to fly that has a lot of drag, so they are good ways to lose altitude, although they aren't the most stable.
If you have never designed an airplane part before, let the real designers do the work!
 
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Jetlagged
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 9:19 am

Quoting AirWillie6475 (Reply 5):
You're thinking of a forward slip, that's when you use full rudder and opposite aileron.

No such thing as a "forward slip". Sideslip is sideways velocity, whether you are sideslipping to reduce speed and improve visibility or sideslipping to maintain runway heading in a crosswind. Both entail forward motion too, of course.

If you use the wing low crosswind technique the aircraft is uncoordinated and drag increased due to the sideslip angle. Crabbing into wind means no sideslip so less drag. A lot of pilots de-crab into a sideslip just before landing, according to what I read on this forum at least. However if you watch the videos of Boeing test pilots doing max crosswind landings, you will see they maintain the crab almost till touchdown, after in some cases!
The glass isn't half empty, or half full, it's twice as big as it needs to be.
 
AirWillie6475
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 1:27 pm

Quoting Jetlagged (Reply 10):
No such thing as a "forward slip".

That's what it's called, you're required to demonstrate it to get your private pilot pilot's license.
 
kearney
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 1:35 pm

Quoting AirWillie6475 (Reply 11):
That's what it's called, you're required to demonstrate it to get your private pilot pilot's license.

I learned to fly where they only do forward slips, then when i did a conversion course they asked me to side slip, and i was like "whats the difference" did not really get much of an answer, so what is the difference? And in my opinion neiter is used to maintain course, you should not be crossing ailerons with rudder unless you want to lose altitude, not maintain course in crosswind.
 
AirWillie6475
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 1:45 pm

Quoting Kearney (Reply 12):
I learned to fly where they only do forward slips, then when i did a conversion course they asked me to side slip, and i was like "whats the difference" did not really get much of an answer, so what is the difference? And in my opinion neiter is used to maintain course, you should not be crossing ailerons with rudder unless you want to lose altitude, not maintain course in crosswind.

The difference is Forward slip is a altitude losing maneuver, you add FULL rudder and aileron, the airplane will feel like it's about to spin. Side slip is more of a directional maneuver, just slight rudder and aileron use.
 
flyf15
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 1:59 pm

Quoting Kearney (Reply 12):
I learned to fly where they only do forward slips, then when i did a conversion course they asked me to side slip, and i was like "whats the difference" did not really get much of an answer, so what is the difference? And in my opinion neiter is used to maintain course, you should not be crossing ailerons with rudder unless you want to lose altitude, not maintain course in crosswind.

Its really two different terms for the same thing used in two different situations. In both of them you are inputting aileron and rudder in opposite directions to induce an uncoordinated condition. This does a few things, including increasing drag and creating a sideways direction of movement. I don't really like calling them "forward" or "side"... they're both just slips. In one you're travelling a certain direction and pointing the nose somewhere else, in the other you're pointing a certain direction and travelling somewhere else. Just depends on if you want to line your eyes up with the aircraft's centerline or the direction of travel. Same thing, different perspective.  Smile
 
rwessel
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 2:09 pm

Quoting Kearney (Reply 12):
I learned to fly where they only do forward slips, then when i did a conversion course they asked me to side slip, and i was like "whats the difference" did not really get much of an answer, so what is the difference? And in my opinion neiter is used to maintain course, you should not be crossing ailerons with rudder unless you want to lose altitude, not maintain course in crosswind.

The reason you were never able to get much of an answer is that aerodynamically there's no difference between a forward and side slip once you're in it. In both you end up with opposite rudder and aileron, and the yaw string pointed off to the rudder side*. The difference is the way you enter it. Basically in a forward slip you have the same course (ground track) after entry, but your heading (where your nose is pointed) is changed. A side slip leaves you with the same heading, but with a ground track bent off to one side. It's boils down to how you manage the lateral velocity and acceleration caused by the bank during the transition from coordinated flight to the slip.

IOW, a side slip is just a turn followed by a forward slip down your new ground track.

In practical terms, a sideslip is what you use to counter a crosswind during a landing, a forward slip if you're too high and need to lose some altitude. Aerodynamically they’re identical, just the direction you intend to go is different (with the crosswind your intended direction is a diagonal relative to the runway, with excess altitude you're intended direction remains straight ahead).

The distinction is one of those things in aviation, that, IMO, are artificial ideas that should have been dropped decades ago, since having it just confuses the issue (for example, let’s say you’re too high on final *and* have a cross wind, now what?). Like the whole to/from VOR idiocy (and trust me, you don’t want to get me started on that).


*What? You don't have a yaw string? OK, OK, the ball off in the other direction. Power pilots, *sheesh*  Wink
 
BoeingOnFinal
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 4:40 pm

Quoting Rwessel (Reply 15):
A side slip leaves you with the same heading, but with a ground track bent off to one side.

What? A side slip changes your heading from the crab angle to alignment of the centerline, that is the whole point of the side slip. To reduce strain on the landing gears. And the ground track is the same, for most approaches, from turning final to touching down.
norwegianpilot.blogspot.com
 
KLM772ER
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 4:45 pm

Quoting Bio15 (Reply 8):
30° is excessive, 17°-plus is too.

You are right here! Sorry If I phrased my self incorrectly..
I didn't wanted to tell you that a crosswind-landing angle is normal above the 17 degrees you mentioned above...
Thats to much! By the way your calculation is definitely right  bigthumbsup 

Quoting Bjoernvenghaus (Reply 1):
It actually don't have to turn 90 degree, just maybe 30 degrees in this case...

I never really thought of the angle before (didn't had too..) and just had a look at the picture and as you said, the perspective fooled me

Quoting Bio15 (Reply 6):
There is NO chance to land at a 30° angle from the centerline. A landing gear couldn't pull that off unaffected.

Say you had a 40 knot crosswind component and land at 130 knots, a crabbing approach would point your nose around 17° away from the centerline. That being considering the extreme



Quoting Bjoernvenghaus (Reply 7):
http://www.airliners.net/open.file/0223248/L/

this one has at least 30 degrees.. though it is not a crosswind landing..

The photo for sure is the other extreme and a landing like this will in most cases suffer some damage
I just wanted to say that there is still a bit reserve in the stability of the landing gear to take a bit more than 17 degrees if necessary... That's why I said: though it is not a crosswind landing..
It is the same as with the wing of the 777 which can withstand up to 150% greater force than ever realistically would be exerted on it
 
rwessel
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RE: How Does Crosswind Landing Works?

Fri Mar 16, 2007 4:54 pm

Quoting BoeingOnFinal (Reply 16):
What? A side slip changes your heading from the crab angle to alignment of the centerline, that is the whole point of the side slip. To reduce strain on the landing gears. And the ground track is the same, for most approaches, from turning final to touching down.

You're right, that was seriously unclear. The slip always happens in the air mass, and a slide slip leaves you with a lateral motion within the air mass, which would leave you with a diagonal ground track assuming a still air mass. But the usual reason to do a side slip is so the motion of the air mass and the motion from your side slip cancel out and you remain on track to hit the runway.

Still the point I was making was that aerodynamically a slip is a slip.
 
bio15
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RE: How Does Crosswind Landing Works?

Sat Mar 17, 2007 12:14 am

Quoting Bjoernvenghaus (Reply 17):
I just wanted to say that there is still a bit reserve in the stability of the landing gear to take a bit more than 17 degrees if necessary...

I would bet on that my friend. Design teams usually take the highest load attainable in flight and multiply that by a factor which usually depends on the industry. Furthermore, this is multiplied by the safety factor which comes from the manufacturer. The final figure is what the aircraft part should withstand, which is considerably higher than any load actually felt during regular flight.

With the exception of a tire burst or collapsed landing gear, main landing gear damage should not prevent the aircraft from taxiing to the gate and then to maintenance, so yes it might have happened and gone unnoticed. Such events include landings above Maximum Landing Weight which compromise the aircraft structure as well.


cheers
Alfredo

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