Sign in to follow this  
Driver170

Touchdown with crab on slippery runways

Recommended Posts

 

Touchdown with crab:

 

It is recommended to use this method when landing on slippery runways as it reduces drift on touchdown and allows for rapid deployment of the spoilers and autobrake as all main gear have touched down simultaneously. However, rudder and aileron input to de-crab after touchdown must be applied in order to maintain proper directional control.

 

This method is not recommended on dry runways with strong crosswind conditions as on landing the aircraft will tend to track upwind until the correct de-crab technique is accomplished. This lack of initial directional control is undesirable. Fly the nosewheel onto the runway after the aircraft is tacking the runway centerline.

 

I just can't figure out why its recommended on slippery runways and not dry runways?

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

Touchdown with crab:

 

It is recommended to use this method when landing on slippery runways as it reduces drift on touchdown and allows for rapid deployment of the spoilers and autobrake as all main gear have touched down simultaneously. However, rudder and aileron input to de-crab after touchdown must be applied in order to maintain proper directional control.

 

This method is not recommended on dry runways with strong crosswind conditions as on landing the aircraft will tend to track upwind until the correct de-crab technique is accomplished. This lack of initial directional control is undesirable. Fly the nosewheel onto the runway after the aircraft is tacking the runway centerline.

 

I just can't figure out why its recommended on slippery runways and not dry runways?

 

I have seen in done on dry runways with all different types of airliners. Touchdown, on mains and then kick the nose over with rudder. 

Share this post


Link to post
Share on other sites

 

 


I just can't figure out why its recommended on slippery runways and not dry runways?

 

It tells you why:

 

 

 


It is recommended to use this method when landing on slippery runways as it reduces drift on touchdown and allows for rapid deployment of the spoilers and autobrake as all main gear have touched down simultaneously.

 

 

 


This method is not recommended on dry runways with strong crosswind conditions as on landing the aircraft will tend to track upwind until the correct de-crab technique is accomplished. This lack of initial directional control is undesirable.

 

(My highlights).

 

If you think about it -- on a slippery runway the reduced friction between the tyres and the runway will allow the aircraft to continue "drifting" sideways for a period after touchdown. On a dry, grippy runway, however, when you touch down in the crab the tyres will "grab" and the aircraft will want to start going where the nose is pointing (i.e. off the side of the runway).

 

That said, as Bob points out, in reality you will see all manner of crosswind techniques, some more questionable than others...

Share this post


Link to post
Share on other sites

Right cheers guys. Think i'm getting a grasp of it!

 

Simon - If you land in the crab on a slippery runway, will the aircraft not keep on sliding sideways off the runway? Will it not be easier to control on a dry than a slippery?

Share this post


Link to post
Share on other sites

 

 


If you land in the crab on a slippery runway, will the aircraft not keep on sliding sideways off the runway? Will it not be easier to control on a dry than a slippery?

 

When you're in the crab, the nose is pointing upwind, but the aircraft is travelling in the direction of the centreline (because of the drift). When you touch down, the tyres will slide on the slippery surface and the aircraft will continue to travel along the centreline (obviously as the aircraft slows and the tyres start to grip you will eventually need to point the nose in the right direction, as the FCOM says).

 

On a dry runway, as I say, because you have more grip if you touch down with the nose pointing off-centreline, rather than skidding the tyres will grip immediately and want to start taking you wherever the nose is pointed.

 

Bob's link provides a good summary of the techniques.

 

Just as a small point -- Bob used the term "kick off the drift", which is a is commonly used but IMO slightly misleading phrase as it implies an aggressive application of rudder. FS aircraft are a little variable in the way in which they model yaw-induced roll, and I'm not sure what the NGX is like in this regard, but if you "kick" the rudder in a real jet you are likely to get a very alarming roll (in the "wrong" direction in the case of a crosswind landing) -- the rudder is a big, powerful control surface.

 

I personally would argue that you never "kick" or "boot" the rudder - you apply rudder pressure. To that end, I prefer the term "squeeze" off the drift, which I feel is a better description of what you're actually doing  :wink:.

  • Upvote 1

Share this post


Link to post
Share on other sites

 

 


I personally would argue that you never "kick" or "boot" the rudder - you apply rudder pressure. To that end, I prefer the term "squeeze" off the drift, which I feel is a better description of what you're actually doing

 

Agree, even in a light twin there's no kicking or rapid rudder deflection...that's just not intuitive when you're in the cockpit. A gradual application of correction just looks so much prettier LOL

Share this post


Link to post
Share on other sites

Agree, even in a light twin there's no kicking or rapid rudder deflection...that's just not intuitive when you're in the cockpit. A gradual application of correction just looks so much prettier LOL

 

Here's a BA 747 landing in quite a marked crosswind. The pilot reduces the crab just before touchdown with a lot of rudder into wind so the sideways strain on the landing gear is minimised. Pretty good I'd say.

 

 

Share this post


Link to post
Share on other sites

 

 


The pilot reduces the crab just before touchdown with a lot of rudder into wind so the sideways strain on the landing gear is minimised. Pretty good I'd say.

 

I am surprised that he kept the nose gear up for a couple extra seconds. I was taught to get all the rubber on the road expeditiously in a stiff crosswind; however, considering he already has lots of rubber on the pavement with the mains I guess the nose wheel isn't adding that much resistance to lateral movement. Interesting.

 

My favorite crosswind arrivals are in the B52, which cannot drop a wing to cross control coming out of the crab. It has steerable main gears, set by the pilot during approach to allow the aircraft to roll forward with the fuselage at an angle to direction of travel. That is amazing to see.

 

My problem landing is when there is no crosswind LOL... here in Corpus Christi TX a normal day has at least 15 kt crosswind component coming at ya and I've seen 43 kts off the nose 30 deg, (22 kt crosswind component)  one gets so used to it that when it's calm the touchdown tends to be ...., shall I say aggressive.

Share this post


Link to post
Share on other sites

Agree, even in a light twin there's no kicking or rapid rudder deflection...that's just not intuitive when you're in the cockpit. A gradual application of correction just looks so much prettier LOL

 

I mean't kicking out the crab, not stomping on the rudder like the Airbus Pilot did climbing out of JFK and tearing the plane apart..  Cheez.......

Share this post


Link to post
Share on other sites

Like always great explanation simon :) makes sense and thankyou all for helping me out.

Also, it eats up tires if you land on a dry runway with drift.

 

Very true

Share this post


Link to post
Share on other sites

I mean't kicking out the crab, not stomping on the rudder like the Airbus Pilot did climbing out of JFK and tearing the plane apart..  Cheez.......

Zakly when was that bob? Can't find any references ....

 

Chas

Share this post


Link to post
Share on other sites

Zakly when was that bob? Can't find any references ....

 

Chas

 

Was over a decade ago. It was an Airbus A310 or something like that.. Got caught in windshear on takeoff and tried to use the rudder to get out of it, but it put too much stress on the aircraft, and the tail ripped apart in flight. At least that's what I think what happened.. What he was doing wasn't exactly wrong, it was just wrong for that type of aircraft. I believe he flew C130's before, and that was actual the correct method of exiting windshear on that type of aircraft. But, the C130 has a much, lets say, "stronger" tail and rudder section, that could handle that kind of stress.

Share this post


Link to post
Share on other sites

Was over a decade ago. It was an Airbus A310 or something like that.. Got caught in windshear on takeoff and tried to use the rudder to get out of it, but it put too much stress on the aircraft, and the tail ripped apart in flight. At least that's what I think what happened.. What he was doing wasn't exactly wrong, it was just wrong for that type of aircraft. I believe he flew C130's before, and that was actual the correct method of exiting windshear on that type of aircraft. But, the C130 has a much, lets say, "stronger" tail and rudder section, that could handle that kind of stress.

 

Wasn't wind shear. It was some wake turbulence from a 747 that had just taken off and was about 5 miles away which is normal spacing. They said that if the PF hadn't man handled the rudder as he did, the aircraft would have easily flown out of the wake turbulence. The PF exacerbated the situation by using full a rapid rudder inputs, which caused the rudder to fail, and separate from the aircraft. At that point, the aircraft basically broke apart with the engines separating from the wings, and it did a flat spin into the ground. Airbus had cautioned pilots not to use the rudder this way prior to this crash, but apparently the airline did not cover this in their pilot training. It was an A300-600

  • Upvote 1

Share this post


Link to post
Share on other sites

Among other things, as I recall, the engines functioned like gyroscopes, since they were at high rotational speed just after takeoff. They kept going on the previous course when the aircraft didn't.

 

Mike

Share this post


Link to post
Share on other sites

It was also a design/maintenance flaw in the Airbus. The rudder attachment points were designed with carbon fibre brackets through which steel plates and fixings were inserted to attach to the tail surface. Rudders shouldn't fly off at initial climb speed, although the co-pilot was making aggressive inputs. There have been quite a few rudder failures associated with Airbuses. I don't know if they've fixed them yet but they are also notoriously over-sensitive to manual input at anything over 200 knots. 

Share this post


Link to post
Share on other sites

They found out after this as well that Va (Maneuvering Speed) does not guarantee you more than one complete abrupt control movement without exceeding limits. The rudder can exceed over 4 times normal limits by giving full left and right deflections.  This was one element they highlighted during a recent advanced upset recovery training that I recently went to. 

Share this post


Link to post
Share on other sites

Wasn't wind shear. It was some wake turbulence from a 747 that had just taken off and was about 5 miles away which is normal spacing. They said that if the PF hadn't man handled the rudder as he did, the aircraft would have easily flown out of the wake turbulence. The PF exacerbated the situation by using full a rapid rudder inputs, which caused the rudder to fail, and separate from the aircraft. At that point, the aircraft basically broke apart with the engines separating from the wings, and it did a flat spin into the ground. Airbus had cautioned pilots not to use the rudder this way prior to this crash, but apparently the airline did not cover this in their pilot training. It was an A300-600

Yeah, I wasn't too sure what it exactly was, and what type of old-school wide-body airbus it was. But I remember now.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this