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x-wind landings on big jets

Old 02-01-2009, 08:10 PM
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Default x-wind landings on big jets

Almost always I have seen big jets like the 757, 747, 737 using the crab method on x-wind landings. Why not use the down wing method instead? Is the crab method more preferable than the down wing method? Also, at the last minute when the rudder is being kicked to align the nose with the centerline, I have noticed that sometimes the nose isn't exactly aligned with the runway. I see some side loading. Are landing gears on big jets designed to be "twisted" like this without causing damage? On a 172 you really feel the side load and it is no bueno as you know.
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Old 02-01-2009, 08:39 PM
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Originally Posted by PearlPilot View Post
Almost always I have seen big jets like the 757, 747, 737 using the crab method on x-wind landings. Why not use the down wing method instead? Is the crab method more preferable than the down wing method? Also, at the last minute when the rudder is being kicked to align the nose with the centerline, I have noticed that sometimes the nose isn't exactly aligned with the runway. I see some side loading. Are landing gears on big jets designed to be "twisted" like this without causing damage? On a 172 you really feel the side load and it is no bueno as you know.
Q. Why not use the down wing method?
A. One big reason is for ground clearance: the wings are long on big jets and the engines are slung low, so you run the risk of having something scrape the ground. The other big reason is aerodynamic: in the "wing down" scenario, the airplane is in a sideslip, which risks having the air flowing around the fuselage "blank out" the tail, causing you to lose elevator effectiveness, which is not good at low speed. In a crab, the airplane is not side-slipping.

Q. What about side loading on the gear?
A. While it's not ideal to land with side load, the main gear on large jets can handle some side loading. In fact, some large aircraft like B-52's, have steering mechanisms that can be steered to land in a crab.
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Old 02-01-2009, 09:04 PM
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Default Crab vs. wing-low

Pearlpilot,

I was just a line pilot, not an instructor, but FWIW here are some thoughts on the matter:

1. On an instrument approach, you don't align the airplane until you see the runway, which could be at a very low altitude, so why not do it that way all the time?

2. Varying winds down final would require constant rudder inputs with a wing-low procedure. This is uncomfortable for those riding near the tail.

3. Perfect alignment at touchdown is rare no matter which technique is used. The gear is quite strong, and landing in a small crab is preferable to being off centerline.
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Old 02-01-2009, 09:18 PM
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For an approach with a crosswind, it is preferable to fly crabbed into the wind. A crabbed approach requires less power and less fuel due to the decrease in drag and body forces. If you apply a forward slip (crosswind controls (low wing high rudder)) early, you increase the drag resulting in a higher power setting, pilot fatgue (holding the rudder for long periods), and is uncomfortable for the passengers.

I think what you are witnessing are heavies commencing their approaches in a crabbed condition, but, as they start their flare, they will transition to a forward slip (wing low, high rudder). Only the pros will wait until the last second to do this. Most folks I fly with will start at 200 feet or so with a slow rudder input to align the nose and a corresponding wing low to kill the rolling moment.

Aircraft are designed to withstand certain loads, check out FAR Part 25.485: Side load conditions -- FAA FARS, 14 CFR.

While on active duty, I was once tasked to find the max crosswind components for various aircraft. Boeing gave the max demonstrated and max theoretical crosswind components for their aircraft. Additionally, they also sent us the max crabbed landing components. These numbers were much lower than the other numbers.

The takeaway ... yes, they are designed for some loading, but not alot. If you don't kill the drift or land in too much crab, this happens ...

http://www.ntsb.gov/Events/2005/FedE..._animation.wmv

-Fatty

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Old 02-18-2009, 11:26 AM
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As my Chief Pilot has said, "Whether it's a Baron or a Boeing, use the crab method all the way to the flare, then align the logitudinal axis with the runway."
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Old 03-15-2009, 04:49 PM
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777 Flight manual recommends using the wing low method up to 30 kts cross wind and to use a blended method over 30 kts.

I use a wing low method whenever possible. As do most of the guys I fly with.
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Old 03-15-2009, 07:36 PM
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That is interesting. I wonder if that's because the 777 can't hold runway heading using a forward slip (wing low) above 30 knots?
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Old 03-15-2009, 07:42 PM
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Default Getting a little "crabby"

Originally Posted by KC10 FATboy View Post
That is interesting. I wonder if that's because the 777 can't hold runway heading using a forward slip (wing low) above 30 knots?
Or maybe those monster engines would get dangerously close to the ground with that much "wing low" attitude.
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Old 03-15-2009, 07:58 PM
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That's a very good point Tomgoodman.
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Old 03-16-2009, 12:00 AM
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Originally Posted by PearlPilot View Post
Also, at the last minute when the rudder is being kicked to align the nose with the centerline, I have noticed that sometimes the nose isn't exactly aligned with the runway. .
I used to use the kick method on the 72 all the time. Great airplane and responded to inputs rather quickly. On the 74, NO WAY am I gonna do the kick method. To much mass, slow to respond and a much longer fuselage arm. I tried it once or twice and it wasn't pretty.

Wing down on a 72 = drags the outboard slat
Wing down on a 74 = drags the pod
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