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B737 NG Crosswind Landings

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B737 NG Crosswind Landings

Old 27th Dec 2003, 05:39
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Post B737 NG Crosswind Landings

Boeing recommends no wing down in the flare above 13 kts of crosswind (for my variant) due to the possibility of pod or flap strike.

One of the recommended techniques is to fly a drift corrected heading all the way to touchdown (ie land sideways in up to 40 kts of crosswind). Apparently the undercarriage is designed for this. It works well in the simulator but how does the aircraft behave?

My prior experience is in larger aircraft where the norm was to use wing down to arrest the drift and rudder to align with the centreline in the flare.

Does anyone out there actually land the B737NG sideways in strong crosswinds and if so, is it a comfortable landing?
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Old 29th Dec 2003, 06:48
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I've been off the NG 800 about a year but I've never seen that 13 kt restriction in my limitations sections, and I've never heard that Boeing recommends landing in a crab above 13kts.

I've landed wing-low in 25kts of x-wind. My experience is that if you touch down in a crab you'll get one hell of a shimmy and shake and the airplane will be cocked off about 3 degrees into the wind on taxi in. Looks weird. You'll also scrub the tires up pretty good.
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Old 29th Dec 2003, 10:52
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dvt,

Thanks for the reply. It is not a limitation but a recommendation in the FCTM.

For the -800 it says:

"Sideslip only (zero crab) landings are not recommended with crosswinds in excess of 20 knots at operating empty weight, or 24 kts at maximum landing weight for flaps 30".

My first post was a little misleading; touching down with full drift applied is mentioned only for very slippery runways.

The basic recomendation for crosswinds, above the specified value, is to have the wings level at touchdown instead of using the wing down method that I have used for years in larger aircraft.

Two techniques are mentioned. One involves straightening the aircraft just prior to touchdown, whilst keeping the wings level. The other, for strong crosswinds, mentions a mixture of wing down and crab into the flare and then using rudder to straighten the aircraft as the first wheel touches.

I was wondering how more experienced operators achieve the basic recommendation, if at all.

Last edited by FlareArmed; 29th Dec 2003 at 11:03.
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Old 29th Dec 2003, 21:57
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The autoland crosswind limit is 15kts with ny operator. There is no rudder channel and it will be fully crabbed at touchdown. No mention of runway state.

Certainly in B757/767 the sideslip from very short finals was more comfortable.

The preferred teaching technique to new pilots on jets was, in the flare, to reduce the crab to zero with rudder, keep wings level with aileron, close the thrust levers, OH and don't forget to flare as well. 4 dimensional flying close to the ground. Good game for the old boys never mind the new ones. Closest thing to being a helicopter stick stirer you'll get.
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Old 29th Dec 2003, 23:21
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I think you have more control over the airplane in the wing-low method. On a slippery runway I'd still go with wing-low, because if you take the aileron out of the wind at any pint during the landing phase you're going to slide to the downwind side of the runway quickly. On runways where they've done a lousy job plowing and you only have +-25 feet from centerline this could cause quite a problem.

On slippery days with crosswind, the objective is to keep the airplane on centerline and under control and get it stopped ASAP. My experience is that wing-low gives you more control longer during rollout. With wing-low you've already got the right control surfaces defected for touchdown and rollout. Why confuse yourself? Keep that deflection working for you and steadily increase your surface deflection as your airspeed decreases and a slippery runway will pose little control problem. Landing wings-level invites control difficulties during rollout, not to mention a pretty crappy landing as well.

As I recall, the max demonstrated 800NG x-wind landing was something like 31kts. I understood this was demo'd the using wing-low. Like I've said, I've done wing-low with about 25 kts of x-wind with no problems. I felt like this method could safely handle another 10kts of x-wind, without a steep sideslip that would scrap a pod.

Use Autobrakes and Autospoilers and keep the aileron into the wind on landing and you'll have great results with this method. The autospoiler system won't let you go dancing around on one wheel for too long, especially if the x-wind is from the right. You'll get all your ground spoilers at once.

I flew the B-52 on active duty. That Boeing airplane had a x-wind crab landing system. Wing-low was not an option because of the wing span. With that system you actually touched down with significant crab. It was kind of a crazy way to land an airplane looking out the copilot's window to stay on centerline. If Boeing keeps putting bigger engines on old airframes maybe we'll see that system come back again.
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Old 30th Dec 2003, 00:13
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Having flown over 8000 hrs on the 737 and having benefitted from training by the chief test pilot himself back in the 70's..the 'crab -decrab' method of crosswind landing is the best and will always allow the airplane to be positioned straight down the middle of the runway in the worst of cross winds. This method also works on the 777.

Maintain the drift angle until flare, then begin feeding constant rudder and aileron into the wind to maintain wings level. The airplane will be perfectly positioned on the center of the runway and no drift down.
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Old 30th Dec 2003, 13:10
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777AV8R,

Thanks for the information.

Using the method you describe, is there any problem with drift without having the wing down?

How strong a crosswind have you used it in?
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Old 30th Dec 2003, 13:20
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I also use the crab method. I heard it said you hold the crab till the other guy starts to squirm.
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Old 30th Dec 2003, 14:10
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FlareArmed...absolutely no driftdown is encountered in the maneuver. The aircraft arrives perfectly on centerline with both maingear touching simultaneously.

I have used this technique with winds over 30 K with absolutely no problem. I found that it was best to keep some thrust 'on' until touchdown as the airplane can be a handful in strong crosswinds especially in gusty conditions. Changing thrust / acceleration vectors by reducing thrust seemed to complicate matters. So, a minimum of thrust was kept on and it minimized extra 'dancing' on the controls!
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Old 30th Dec 2003, 15:08
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Everyone,

Thanks for the quality information. I'll add the 'crab-decrab' method to my nav bag.

777AV8R: A friend of mine was particularly interested in your point about leaving a small amount of thrust on until touchdown. It sounds like a good technique.

RAT 5: I am amazed that Boeing turned out an aircraft with 2 axis auto-land! We are in the habit of squeezing a little bit of rudder on during an autoland and it seems to take some of the bite out of the crosswind.
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Old 30th Dec 2003, 18:32
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F.A.

I presume this is because there is no rudder control. Is it possiblr with 2 autopilots? I wonder if a fail passive system can have rudder control.

I agree about rudder control on touchdown. I found you have to very quick with rudder steering, (strong cross wind) once the mains are on the ground, to aim the nose wheel donw the centre line as it touches. The weather-vane effect plus any residual crab has you nearly going off-road in spectacular fashion. If it's raining and at night increase the blood pressure factor by XX.
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Old 30th Dec 2003, 20:53
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I feel and think to an airplane is not natural to fly with cross controls (eg. left aileron and right rudder ), so the latest the point at which you do so the best.

What I try to achieve is the crab-decrab method, but slightly mixed, at the very touchdown moment, with the wing low technique.

So I keep the crab as long as possible, and when I decrab, I decrab by 95%, and I keep 1 or 2 degrees nose into the wind, and drop the wing into the wind by 2 or 3 degrees....

This will really prevent any drift and provide an even smoother touchdown - cause one wheel touches slightly before the other.

Now, all the above is valid on dry runways.
The more slippery it is, the more you should touch without decrabbing, as suggested by Boeing.
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Old 30th Dec 2003, 21:57
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Smile

I am sure my question will create an interesting response....

Has anyone tried using asymetric thrust in the flare? By retarding the downwind Thrust Lever while decrabing has worked well for me during strong gusty crosswind landings resulting in a controlled and smooth touchdown. Been doing it for years from light to heavies. I think the real answer is... you will find whatever technique works for you best. I have found that some who run into trouble don't seem to like to use the rudder as much as maybe they should.

I have not seen this technique mentioned anywhere. However it works very well... for me.

Ps. Just to avoid any confusion.... both Thrust levers are at idle on touch down. The asymetric condition is just momentary while decrabbing. .... Just don't pull the wrong one first or the result could be less than desirable!
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Old 30th Dec 2003, 22:04
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Anyone have any figures, or good guess's, as to the bank angle req'd before one gets a pod strike in the NG.

I assume the oleo would be close to fully compressed before strike occurs, although I guess its possible to be trundling along briefly with one main on(de-crab tecnique), and partial oleo compression, for a brief moment before spoilers deploy and lift is gone.

Never the less, full compression would provide the min figure for bank before pod strike - any ideas??

Which is most critical? pod, or flap trailing edge at full setting?

Ps
Saw a 727 last week that had a wing tip/leading edge device scrape.

Not sure what crosswind component he had, but I was also in that same day and there was nothing really significant when I was around.

Not to say that he didn't encounter a strong one however.

Aircraft on the ground for a few days b4 departing.
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Old 30th Dec 2003, 23:31
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qnc3guy

<<Just don't pull the wrong one first or the result could be less than desirable!>>

No offence is intended by my following comments:

I've no doubt it may work for you but I don't believe such techniques are to be recommended for three reasons.
a) It is an unnecessary overcomplication which you may get wrong at some stage in the future with possibly dire consequences.
b) The person in the other seat will almost certainly have not been trained in such a technique and so will be unsure of what is a 'normal' aircraft response.
c) The manufacturer has not approved such a method. (To the best of my knowledge)


Regards
Exeng.

P.S. I use the 'crab' method on the 737 and find it works perfectly well within the limitations of my own handling skills. If there is any tendency to float (probably as a result of a slightly misjudged flare) then I drop the 'into wind' wing slightly. Dropping the wing stops the float and keeps you on the C/L.

As instructedI tried the sideslip (zero crab) approach and landings on the 777 but never felt very comfortable with the technique.
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Old 31st Dec 2003, 01:53
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EXENG

In all honesty.... I agree with you whole heartedly.

To my surprise.... most have not even noticed when I have done this.

And as I suspected the response to a nonstandard technique is as expected.


Respectfully
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Old 31st Dec 2003, 02:04
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Light x-winds....it really doesn't matter which technique you use. Yes you can even land in a crab. Won't be pretty.

However, in high/gusty x-winds if you simply "kick out the crab in the flare" with no ailerons or leveled ailerons you will drift downwind. This technique is dependent upon you making contact with the ground ASAP, if you're to keep it on centerline. If you float in the flare, you will drift off centerline. Aero 101. Once you've drifted from centerline you can't get it back that close to the ground. You must accept an off centerline landing and correct back on rollout or go-around.

I've tried this "decrab" technique and have found it to consistently result in a lot of dancing on the rudder peddals, drifting, and poor landings with the shimmy and shakes. You can go ahead and try this technique, but those who advocate it haven't tried it in high x-winds/gusts. In these conditions, you simply MUST go wing-low, whether you've done so consciously or not if you're to keep it on centerline.

Master the basics! And the basics are wing-low and landing on the upwind wheel in high gusty x-winds. Do it the same way everytime.
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Old 31st Dec 2003, 06:19
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DVT

<<In these conditions, you simply MUST go wing-low>>

I don't agree. In these conditions you fly the A/C in accordance with the manufacturers approved technique. That may or may not give you the option of 'wing down' as opposed to 'crab'.

'Kicking out the crab' leads one to imagine abrubt control inputs to the uninitiated. 'Pushing off the drift' might be a better description; that combined with into wind aileron whilst entering the flare should lead to a successful landing on the C/L.

Dancing on the rudder pedals is most certainly not a recommended technique on any swept wing jet I've come across. A measured amount of rudder to push off the drift is recommended.

<<those who advocate it haven't tried it in high x-winds/gusts.>>
Actually I believe a lot of us around here have indeed tried it in x-winds with gusts up to the x-wind limit. Certainly in the northern hemisphere winter one gets to try it rather a lot.

And finally you say: <<You must accept an off centerline landing>> I don't accept such a landing, and nor I believe do most of my colleagues.


Regards
Exeng
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Old 31st Dec 2003, 06:33
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Strong Crosswinds

A Boeing recommended technique for strong crosswinds (no value given) is to crab into the flare with some wing down and then straighten with rudder as the first wheel touches. I haven't tried that yet.

wandrinabout: The angle of bank for a pod strike, on the NG series, varies with pitch angle. At a normal touchdown pitch of +5 degrees, the flap track fairing or nacelle will strike the ground at about 13 degrees of bank (source: B737 FCTM).

The worst case is at a pitch of -3 degrees (that would be some touchdown!) where the flap track fairing will strike at about 8.5 degrees angle of bank and the nacelle at 9 degrees.

The numbers differ fractionally for a -300/400/500.

With the wing-down method, once the attitude is stable, it can be held waiting for the touchdown. With the crab-decrab method, a prolonged flare would probably not be ideal because of the inevitable drift.

I am going to try it, particularly with the help of the HUD (Nice work if you can get it!). The HUD counts down to the nearest foot and the wheels touch precisely at the 'zero' readout everytime. Also the Flight Path Vector symbol gives a useful indication of tracking.
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Old 31st Dec 2003, 07:55
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The boeing advertised technique of decrabbing during flare works fine with me. As I'm taught this principle from early on and works fine from gliders, cesnna's all the way up to the bigger metal things. A few notes I observed for the 737 series i fly now;
1. A note is made about keeping the power on earlier in this threat. I'm strongly against that as the NG series with flaps 30 is a long floater already if you misjudge (especially the -800 series).
2. Do not be afraid to land it with a little crab, as the landing gear is designed to take a lot of crab (re form boeing after our pilot community started questions about it).
3. Control inputs are much smoother with this technique. People have much more diffilculty in finding a stable decrab control setting early on than during a dynamic manouvre as the flare. And you don't have to get it perfectly right during flare, as long as the aircraft's nose is on it's way to runway heading is enough and will already provide a smooth touchdown. Time is too short to develop drift I noticed (especially true for more heavy epuipment)
4. Notice how much aileron you need during a 35 knot X wind take-off. Thats the amount you need during landing.
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