De-crabbing at the flare to land without drift.

[A37575]

From Flight Test forum:

Hello All,

Another "where" question from Leo....

If you had to perform cross-wind testing in Europe in next few weeks (days...) on a mid-size biz-jet,

I have often wondered why Boeing in their 737 FCTM, publish three methods of performing cross-wind landings. They are, touch-down in a crab, the de-crab technique (with removal of crab in the flare), and the side-slip technique. Each is then described in detail.

From my first flight I was taught it was essential to touch down without drift and freely admit I needed much patient instruction before I could do this consistently well and with confidence. After graduation from Wings course my early years of flying Lincoln bombers (tail-wheel type for the younger generation :ok:) made me apprehensive of crosswind landings in these huge beasts as the squeal of tyres and bursts of power from outboard engines was the norm in strong crosswinds.

Touching down without drift has been taught as ideal with every aircraft I have ever flown from Tiger Moth to various fighters, bombers and transports. Apart from the Boeing 737 that is. In fact deliberately touching down without removing drift is a recipe for a ground loop in tail-wheel types.

Obviously, Boeing doesn't see any problem with landing their aircraft with drift which is why it is mentioned as one of the three choices. Is this because Boeing are aware that many pilots are unable to "see" drift and if so, not to worry, because the aircraft natural directional stability will prevent grief from happening?

Or is Boeing merely being pragmatic and aware that a significant number of pilots are incapable of consistently touching down without drift and therefore the manufacturer increases the structural strength of the landing gear to take this into account?

Surely, if properly trained in the simulator or the real aircraft, pilots should be able to track accurately on final and at the flare remove all drift by the time-honoured method of de-crabbing and touching down straight? Of course it takes practice and a modicum of finesse but this is not a new technique.

Despite Boeing assurance that it is safe to land with drift applied up to the landing cross-wind guideline speeds, this should not be carte-blanche for pilots to be lazy simply because they cannot be bothered to apply finesse. From observation of hundreds of crosswind landings in the simulator it is evident that many crews seem incapable of judging the decrab or touching down astride the centre-line with drift eliminated.

I was once told by an experienced but highly cynical Boeing test pilot, that all sorts of pilots will fly their aircraft and for this reason Boeing SOP's cater for the lowest grade. One can understand his sentiments from seeing how the QRH has been dumbed down almost to kindergarten standard with flow charts designed to cover the oaf to the ace pilot.

My point is why deliberately touch down in a crosswind with drift still present when it should not be beyond the average pilot to align the aircraft with the runway heading during de-crab and land without drift? Complacency, perhaps - or sheer laziness?

[rudderrudderrat]

Hi A37575,

I agree that being aligned with, and astride the centre line is ideal.
However, with a large cross wind there are a few problems:

1) As soon as the alignment process starts, the aircraft will start tracking towards the down wind side of the runway. If touch down is not accomplished quickly, then it gets expensive.
2) So bank into wind needs to be applied to prevent drifting. We are limited by bank angle in case we scrape an outboard engine pod.
3) The swept wing produces lots of roll with yaw, which has to be counteracted with aileron. If the cross wind is close to maximum limits, then the ailerons may be close to saturation with the very large yaw (to align perfectly).

The landing gear is stressed to happily accept up to about 5 degs of drift on touch down.

So with big crosswinds, the compromise is to remove some of the drift (to within 5 degs of runway track), and hence apply less bank into wind (prevents pod scrape), and have less aileron cross control applied thus leaving more control available (in case it gusts.)

[BOAC]

A37 - your last para is answered by your penultimate - and it is not 'cynicism'. When you make an aeroplane that will be flown all over the world by all sorts of pilots, what would YOU do? Produce an aircraft 'manual' in several editions, one for aces who have come up through tailwheelers down to edition 5 for those 3rd worlders who have their first swept jet in their company? Then 'control' distribution to avoid legal traps?

Surely the 'aces' will know enough to work out for themselves how to land a 737 in a crosswind without pictures? I cannot see the problem.

[A37575]

Landing without decrabing, I would on a wet runway.

Is there an outstanding operational reason for NOT removing drift before touch-down whether wet or dry runway? Surely the side strain on the landing gear is not desirable. In any case, if landing on a slippery runway with crosswind and not de-crabbing, the immediate result of full reverse thrust is the danger of going sideways due to the reverse thrust sideways vector. This is explained in great detail in the FCTM. By de-crabbing before touch down this danger is removed.

[A37575]

Surely the 'aces' will know enough to work out for themselves how to land a 737 in a crosswind without pictures?

BOAC. If it were my aircraft it is not the `aces` I would be worrying about. I call it the pursuit of excellence:ok:

[Mikehotel152]

I always enjoy sitting at the hold at LGW watching the incoming aircraft landing in the usual moderate and slightly turbulent southerly crosswinds we often experience there.

It always strikes me how serene and in control they look as they float gently by, sometimes dipping the upwind wheel as they flare, feeling for the tarmac, and at other times simultaneously firmly planting both bogies on terra ferma in a great puff of blue smoke.

This serenity contrasts with my own experiences of the cockpit landing a B738 on the same runway in the same conditions. The word frantic precedes the touchdown and something called relief follows quickly therafter as the centreline is regained. I'm sure many will admit to the same emotions.

When I started out flying the B738 - my first medium jet - I tried the de-crab in the flare method because, in theory, it seemed more natural. On my limited experience I have to say I found it very tricky. My stick and rudder technique was simply not good enough and the only way of improving is doing.

As with all things aeronautical practice makes perfect, but in my experience it's only when the crosswind component reaches and exceeds 15 knots that a proper technique is required. I would suggest that at typical B738 landing speeds, the more commonly found 10 knots of crosswind hardly requires much of a technique because the crab is very limited and drift minimal.

On the rare occasion I encounter a good 15-20 knots across the runway, with my relatively low hours on type, I therefore find the hardest part of the technique is judging the correct amount of rudder and aileron to correct the crab and keep the wings level while also flaring the aircraft enough to provide a reasonably firm touchdown without risking a float.

As we airline pilots only touch the rudder on take off to maintain the centreline and in engine-out scenarios in the sim, getting the feel for a relatively aggressive de-crabbing and flaring maneouvre at low speeds and so close to the ground is tricky. I'm sure the experienced pilots on here will cite this as yet another example of the loss of handling skills in pilots, but I'm just being brutally honest. In the meantime, I hand fly as much as I can to remedy this shortfall.

I've watched a number of far more experienced pilots landing the B738 in moderate crosswinds and reckon the most commonly used technique is the sideslip as described in the FCTM. If I see the de-crab in flare technique on the line it invariably involves a late flare and very firm touchdown, especially on a wet runway.

For the moment, I find 'stretching-out' the de-crab and flare using the sideslip technique and accepting the slight drift is better for a pilot of my experience level. If this relegates me to the kindergarten of flying, so be it. As I get more experience of the aircraft and moderate crosswinds I expect to contract the manoeuvre until it's more akin the 'ace' de-crab in the flare technique beloved of my esteemed colleagues.

[hetfield]

Just get the f*** down!

Kick in what ever it takes, whenever you want....,

BUT DON't SCRATCH IT!

[Tmbstory]

A37575:

I was always taught and in actual flying, I would "de crab" during the flare and if necessary, slightly lower the into wind wing to stop any drift and land.

[cosmo kramer]

I've watched a number of far more experienced pilots landing the B738 in moderate crosswinds and reckon the most commonly used technique is the sideslip as described in the FCTM. If I see the de-crab in flare technique on the line it invariably involves a late flare and very firm touchdown, especially on a wet runway.
Whaat? Where do you fly?
The smoothest possible landings on the 737 is when the runway is a bit wet. I presume it's because it delays the wheel spin up a bit + allow for a slight drift to go unnoticed.

To achieve a smooth touchdown in 737NG in general, you have to de-crab during flare. I can't tell you why, but the NG will punish any sideways movement with the familiar rattling of anyone/thing onboard - no matter how low your rate of descend is when touching. This also includes a 5 knots crosswind. De-crab during flare technique can also be used even if the wind is calm to correct a slight error in the alignment. If you just put in rudder to align the nose and don't lower the wing a bit simultaneously, you create your own drift.

Landing straight is the key element to making smooth landings in the 737NG, which is a bit tricky aircraft when it comes to making greasers. Any sideways movement and the landing is ruined.

The classic on the other hand will allow a fair bit of drift to go unnoticed.

[stilton]

Just as an aside. Watch some of the videos of Boeing doing crosswind testing (all models)



No attempt is made to establish a sideslip or straighten out at all prior to touchdown.

[Tee Emm]

It is a known effect of thrust reversers, you are debating about decrabing or not, you are not debating about using reversers.

The way I read the post by A37 is he makes the point quite validly that if you touch down in a crosswind with crab still applied on a slippery runway the immediate application of reverse thrust will cause the crab to remain in place and possibly get worse as the net sideways reverse thrust vector exacerbates the slide.

By de-crabbing at the flare and therefore eliminating all cross wind drift, immediate reverse can be used without risk of going sideways. Seems logical.

[JazzyKex]

Different techniques are best practiced for different reasons.

I began my jet flying in a 146 where pod scraping was of no consideration when straightening up but I preferred to de-crab in the flare. I took that technique to the 757 where with a crosswind of 40kts some positive controlling was required to stop any drift if you mis timed the flare! The ops manuals for each offers both techniques as perfectly legitimate.

The straightening in the flare technique is great until you move to something like the 777, where on a normal day it works absolutely fine. However the machine is capable of autolanding in a 40kt crosswind and although rare, there are occasions when you disconnect the autopilot after the aircraft has started its own crosswind technique (sideslipping) below 500'. In that scenario you better be well practiced at landing with that technique too unless you intend to let the aircraft weathercock and reapply your "straighten in the flare technique" all in the last couple of hundred feet! The pitch attitudes and amount of aileron use are considerable, significantly different and very uncomfortable for those who have not encountered and practiced it before that dark and stormy night!

It boils down to knowing your aircraft and taking the opportunities to practice the possible methods (sim or aircraft - I don't think MS Flightsim counts looking at some of the answers here!!!).

As has been mentioned above look at the crosswind testing of the 777, doesn't seem to bother Mr Santori much to have a LOT of drift on on touchdown, and what he doesn't know about Boeings really isn't worth knowing!!! Also as for the reversers argument????? Watch the video and think about what happens when a couple of hundred tons of metal touches down with drift on....the drift does not stay on long!!! It straightens up pretty pronto comfortably or not...definitely at lot faster than the reversers start to affect your directional control. Nice to have your theories...now go look at a areal aeroplane! :ok:

[Checkboard]

Crosswind testing is to prove that the machine won't be broken with ham-fisted handling - not to demonstrate how to land in a crosswind! :suspect:

[ImbracableCrunk]

Crosswind testing is to prove that the machine won't be broken with ham-fisted handling - not to demonstrate how to land in a crosswind! http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/cwm13.gif

Exactly. Maybe if I have an RTO, I should tell the firefighters to wait until the 5 minutes are up before they can put out the brake fires.

[STBYRUD]

Back to topic: Am I the only one here who questions the idea of landing in a crab on a wet runway? Why set everything up for a sideways skid right away? There is plenty of chance to do that during the rollout :}

[Desert185]

I've only flown four types of heavy aircraft. B-727, L-382, DC-8 and B-747 (in that order). I was a check captain/instructor on the last three.

The Herc and the 747 didn't like landing in a crab due to the mainwheel/landing gear configuration, but a slight crab, especially on a wet or cluttered runway, was doable.

The DC-8 was the worst gusty crosswind airplane of the bunch, particularly with the CFM engines. If the conditions were near the XW limit and gusty, I would land the DC-8 in a crab. With the robust gear design, there were no issues and the company(s) actually encouraged the technique, rather than suffer a possible pod strike.

The Herc and the 747 are good XW airplanes, so kicking them straight @ 10'-20' in the flare with a bit of bank remaining during higher crosswinds wasn't an issue. The EGPWS altitude callouts in the flare were a pretty good assist in judging height and sink rate prior to touchdown.

The 727 had wingtip clearance issues, so one had to be careful with too much bank during the flare. A slight crab during touchdown, if necessary was OK.

In little airplanes, I teach and practice flying final in a crab and going to wing low for the touchdown sometime during the flare. Start the kickout early until you get a feel for the technique and then progress to a later correction as you get comfortable. I have never been a proponent of slipping from a long final in any airplane for various negative reasons.

Doing crosswind landings well is part of being a pilot. Obviously, proficiency comes with experience, practice and mistakes, but achieving quality, end-results is the goal to becoming a competent pilot. At least thats the way it used to be...

[chksix]

No-one has mentioned the shimmy dampers of the main gear.
The Boeing 737 Technical Site (http://www.b737.org.uk/landinggear.htm#General)

[pauli767]

No Sir, Boeing has not dumbed down the checklist.

Unless, you are a Sim Instructor, TRI/TRE, the checklist are not a familiar read but for once , or twice a year. They have tried to make the reading of the checklist as simple as possible. Considering the errors I see, in the simulator, on a regular basis, it is understandable, Boeing has tried to make it simple.

Considering you background ( Auzzie), I understand your pious attitude.

Nuff said.

Walnuts:=

[Intruder]

The 747 will tend to align itself with the runway immediately after touchdown as long as you have no drift. This will negate the problem with the reverse thrust vector in most cases. It doesn't really matter which method you use to stop the drift, as long as you do it.

If you land in a slight slip, the aileron input used for the slip will also tend to keep the wings level once the wheels are all on the runway.

[Jo90]

Surely the prime objective in a XWind landing is to have the beast tracking parallel with the centreline at touchdown. If this is not achieved there is a sporting chance that momentum will overcome tyre grip leading to the need to call out the greenkeeper (and the guys to replace some R/W edge lights etc etc). To actually be on the centreline is a bonus.

Having operated four types, alternating between de-crabbing and side slip, I would say de-crabbing is easier to learn but more likely to result in not achieving that prime objective when you get caught by a gust at just the wrong moment.

My preferred technique is to ease in the rudder around 300ft enough to remove about two thirds of drift. Reducing Xwind component as you get nearer the ground usually results in being pretty much aligned by the time you get to the flare, and if circumstances permit you can finesse the rudder at that time.

Reduced sideslip on short finals means more control authority in reserve from ailerons/spoilers.

[NARVAL]

You have said it all I think! To further think on the fact that a good wave-off is better than a botched decrabbing (wind becoming unmanageable...) may I recall a beautiful and terrifying video on the subject?
Dailymotion - A320 atterrissage vent travers hambourg - une vidéo Webcam & Vlogs (http://www.dailymotion.com/video/x59ogg_a320-atterrissage-vent-travers-hamb_webcam)
I remember, long ago, when I passed a test to become an instructor in the french air force, the officer testing me offered his cap with pieces of paper. I should choose one, on which was the lesson for the test. I took one: "cross-wind landings". And first at the blackboard, I explained the technique...then for fifty minutes in the little jet (FougMagister) I taught him. Weather cavok. Wind calm. The most difficult lesson I ever taught, since he was stubborn enough to ask me to go through with it.

[john_tullamarine]

That video is why most of us, in a strong gusty crosswind, will prefer to set up a slipping final approach coming up to or over the boundary fence rather than in the flare - makes the flare cognitive feedback loop much easier by removing a bunch of inputs from consideration.

If the pilot had fed in some slip (the other way) during the yaw it would have been a fine landing ... a bit of cognitive overload one thinks ?

[rh200]

may I recall a beautiful and terrifying video on the subject?



Holy cr@p, there would have to have been some underwear changes after that one, after extracting the seat cushion that is.

[Desert185]

NARVAL:

That video reminds me of the video one sees of aircraft landing at the completion of the old Kai Tak, IGS13 approach. Some demonstrate effortless proficiency and others demonstrate a distinct lack of knowledge and capability, regardless of the aircraft flown...although mostly done with the easiest of XW airplanes I have ever flown...the B-747.

I would like to know the rest of the story and see previous and succeeding aircraft make that attempt before making a judgment call on that singular film clip.

[cosmo kramer]

I'm not an Airbus man. But as far as understood from that incident, the cause was an "software feature" in the airbus that limits the trow of the ailerons after touchdown.

Unfortunately they had a gust and touched down with the downwind main gear, where after it was impossible to bring the wing down with the limited trow.

I believe Airbus reprogrammed the software after that incident. Correct me if I am wrong.

[busTRE]

I'm not an Airbus man. But as far as understood from that incident, the cause was an "software feature" in the airbus that limits the trow of the ailerons after touchdown.

Unfortunately they had a gust and touched down with the downwind main gear, where after it was impossible to bring the wing down with the limited trow.

I believe Airbus reprogrammed the software after that incident. Correct me if I am wrong.

You're wrong!

The cause was part overcontrol by the HP and then a dual inputs (both ways) to correct which caused excessive roll one way then the other causing the contact of the wingtips. Airbus did change the roll authority of ELACs around that time but it was to dampen roll in Flap 3.

[Mr. Lowkey]

Here is a great video of Boeings in crosswinds.
‪Crosswind Landing Testing‬‏ - YouTube

[low n' slow]

I know this discussion deals with jet aircraft, but this video of the F50 i think shows a nice view of a typical crosswind landing and subsequent rollout. Notice how the sideslip increases at the end of the rollout.

LnS

‪Fokker 50 landing in east Iceland‬‏ - YouTube

[Denti]

@busTRE, he wasn't all that much wrong. Read the safety recommendations of the BFU derived from that incident. Two deal with landing mode and reduced aileron trow after a single main gear touch down and a wrong description about that in the standard issue airbus FCOM, probably since corrected. Airbus changed the description of the recommended crosswind landing technique on its own even before the report was published though.


The following systematic causes led to this serious incident:
• The terminology maximum crosswind demonstrated for landing was not defined in the Operating Manual (OM/A) and in the Flight Crew Operating Manual (FCOM), Vol. 3, and the description given was misleading.
• The recommended crosswind landing technique was not clearly described in the aircraft standard documentation.
• The limited effect of lateral control was unknown.

[capt. solipsist]

Ah, the Fokker 50. Landed one unaware that x-wind has reached 42kts (uncontrolled airport). Some rudder trim here, some aileron trim there - didn't even notice the wind until landing roll, when our fin was fluttering like crazy. Sorry if off-topic, that video just had me reminiscing :cool:

[STBYRUD]

Concerning that Lufthansa A320: Grab the report here:
http://www.bfu-web.de/cln_030/nn_226462/EN/Publications/Investigation_20Report/2008/Report__08__5X003__A320__Hamburg-Crosswindlanding,templateId=raw,property=publicationFile.pdf/Report_08_5X003_A320_Hamburg-Crosswindlanding.pdf

As Denti said the whole thing was completely unnecessary since the commander insisted on using the ILS on runway 23 with full crosswind (up to 47 knots as reported by the tower, beyond all limitations) instead of landing on runway 30 and shooting a LOC.

Handlingwise it seems like the thing caught a gust just as the PF straightened the airplane (maybe with not enough opposite aileron)... Nasty nasty nasty.

[rudderrudderrat]

Hi STBYRUD,

maybe with not enough opposite aileron They both had full opposite aileron - but even that wasn't enough because:

From page 46:
The aircraft touched down shortly after the 1,000 ft marker in the touchdown zone, about 2 m left of the runway centreline lights with the left wing 4° down and the fuselage pointed 2° to the right of runway allignment, whereupon the rudder pedals were returned to the neutral position.
The aircraft yawed towards the left, thereby increasing the lift from the right wing and decreasing that from the left wing. In spite of the co-pilot's right sidestick correction, this resulted in unintended contact between the downwind main landing gear and the runway. The objective had been to land with wings level. This was confirmed by the correct application of right sidestick prior to touchdown, which was intuitively supported by the Captain's sidestick (dual input).
After touchdown the aircraft yawed a further 5° to the left. The left main landing gear lost contact with the runway. At no time did the right main landing gear make contact with the runway. Lift dumpers (partial spoiler extension) remained inactive, because the necessary prerequisites were absent.
During the next few seconds the aircraft rolled to a 23° left wing down attitude in spite of the full right deflection of both sidesticks and application of right rudder. The switch to Ground Law limited the effect of roll control corrections. The left main landing gear again made contact with the runway. At about the same instant, the left wingtip made contact with the runway.

[Al Murdoch]

Ground Law? Crikey, I'm not an Airbus hater, but at least I know whats happening to the surfaces on my Boeing... I don't think my brain's big enough to cope with all that stuff.