Emirates A380 crosswind at Schiphol today -- question about technique

[sshanky]

Hi,

I suspect most have seen the video of the Emirates A380 crabbing at Schiphol today during the storm. It wasn't too different from many of the extreme crosswind videos I've seen, but it seemed he took quite some time to straighten up, even after touching down. I've always been intrigued about how that works -- when the wheels touch down when they're not aligned with the runway.

Firstly, I wanted to know what people think of the landing seen in the video. Was it done very well, or did the pilot wait too long to straighten the plane?

Second, is there anything to be worried about when you touch down at an angle? Doesn't it stress the undercarriage or the frame, and could it cause the plane to veer off to an angle?

Thanks!

[DaveReidUK]

Airline pilot doing what he's paid for ...

[RAT 5]

1. I've always been intrigued about how that works -- when the wheels touch down when they're not aligned with the runway.

You need to apply downwind rudder PDQ to straighten the fuselage & therefore the undercarriage with the runway axis.

2. Second, is there anything to be worried about when you touch down at an angle?

Worry is perhaps not the correct word. If the undercarriage has been designed and stressed to do this, then it is allowed. There are interim techniques to reduce the stress, e.g. impart a little side-slip in the latter stages, but each a/c has limitations on there degree of side slip.
What is important is to straighten the longitudinal axis of the a/c. The wheels are pointing into wind; the a/c, due to the rudder, wants to head off into the wind. If you do nothing, or are too late in removing the drift angle, you will go off-roading PDQ. Is that a worry? NO, it is an alert that you need to be sharp about it.

[Chris Scott]

Good morning in California, sshanky, and welcome to the PPRuNe madhouse of whingeing, armhair experts!

Is this the video you're referring to?
Emirates A380 lands at Schiphol Airport in Amsterdam during Storm Angus | Stuff.co.nz (http://www.stuff.co.nz/travel/travel-troubles/86720326/emirates-a380-lands-at-schiphol-airport-in-amsterdam-during-storm-angus)

If so, it was likely taken at the weekend, rather than yesterday, Easy for Dave Reid to talk, but in fact I entirely agree with him.

To be more specific, the handling pilot or autopilot (more likely the former, as autopilots traditionally have lower crosswind limits) has done a decent job in a fairly strong crosswind - as far as one can tell from the evidence of the low-positioned camera.

The runway could best be described as "very wet", which has a relevance to the landing technique in at least two respects:
1) Firm touchdown required to break through the water to the hard surface, ensuring main-wheel spin-up for subsequent directional control and braking;
2) Following from (1), no need to de-crab fully before main-wheel touchdown. The reaction of the tyres to the side load will be much less than on a dry runway.

In this case the pilot over-flares, and then seems to de-rotate slightly to settle the main-wheels on the runway.

You are right that, on a dry runway, landing without de-crabbing puts an unnecessary stress on the main gear assemblies and - coupled with the into-wind weather-cocking effect after touchdown - can potentially lead to the aeroplane veering upwind before nose-wheel touchdown. However, the great inertia of a big jet tends to mitigate that effect, and landing-gears are designed to withstand the drift angles involved.

Before someone pulls me up, must admit to no knowledge of the A380, but I notice that reverse is deployed on the inboard engines only. If all 4 engines are fitted with thrust-reversers, that may be a precaution in crosswinds? Judging from the resulting spray, which advances almost to the intakes, full reverse was used.

PS
I see RAT 5 has got in first while I got sidetracked. Yes, the other crosswind technique is to make the approach with the appropriate direction and amount of sideslip to enable the fuselage to be aligned with the runway centreline throughout. So the upwind wing is low, and the aircraft kept straight with enough downwind rudder. The upwind main gear lands first, after which the pilot allows the downwind wing to settle to wings level, keeping the fuselage straight with rudder. However, many big jets are limited in the bank angle at touchdown by outboard engine pods and/or trailing-edge-flap extremities. Also, any remaining coffee cups will slide off the side of the fold-down tables... :rolleyes:
Nevertheless, as RAT 5 says, the compromise is for the pilot to fly a crabbed approach, but to lower the upwind wing slightly during the flare and de-crab. That normally works well.

[Feathers McGraw]

A380 only has reverse on the inboard engines, IIRC it's because the outers are sufficiently close to the runway edge that you run the risk of blowing grass and earth everywhere so it wasn't deemed sensible to install 4 reversers.

[RAT 5]

Thanks Chris. I had not seen that video, and IMHO, considering the risk of PIO close to the ground, the guy did a very fine job. It seemed the downwind wheel might have touched first, but it was soon under control. The flare was just right and there was no rock & roll.
I've not flown these side-stick thingies, and I wonder if my ham-fisted agricultural technique would be suited. Give me 3000psi direct to the panels any day.
B757/767 I used to enjoy de-crab at 200' and slip it in onto one side in a straight line. It was so much easier than stirring the pudding and peddling all at the same time in the last 30'. The advantage of long legs; the beast not mine.

[DaveReidUK]

Nice video of an aircraft that can land, and even taxi, with up to 20° of drift without any problem. :O

https://www.youtube.com/watch?v=TCUHQ_-l6Qg

[Chris Scott]

Thanks for that, Feathers McGraw. Bit like a B707 at Blantyre-Chileka (30-metres width), which is one of the reasons Air Malawi used a VC10 for a few years in the 1970s before Lilongwe opened.

Hi RAT 5,
The FBW Airbuses are not allowed to be flown with crossed controls. Can't speak for the A380, but Airbus used to claim that the A320 would hold its wings level during de-crab if the sidestick remained neutral (i.e., no roll demand). In practice, a momentary upwind roll-input during de-crab had the desired effect of enabling the upwind gear to touch down first.

I gather the off-set gear angle on the B52 has to be preset before landing, Dave? Of course the B737 main gear has a degree of castoring ability, as you can often see if you follow one along a straight taxiway.

[andytug]

Not sure how true this is, but remember reading that the 747 landing gear is designed to stand touchdown at up to a 45 degree angle to runway direction , which is pretty amazing considering the weight/speed involved. Anyone know if the A380 is similar?

[DaveReidUK]

I gather the off-set gear angle on the B52 has to be preset before landing, Dave?

I believe so. After carefully noting which side the crosswind is blowing from. http://cdn.pprune.org/images/smilies/embarass.gif

Not sure how true this is, but remember reading that the 747 landing gear is designed to stand touchdown at up to a 45 degree angle to runway direction , which is pretty amazing considering the weight/speed involved.

If that's true, it's amazing. And somewhat pointless, if you think about how strong a crosswind would be required to produce a 45° drift angle.

I'd be fascinated to know your source.

[andytug]

Couldn't remember where I saw it again, though this suggests limit is 25kt crosswind rather than an angle as such. First video shows a fair old angle...

Question for pilots: max crab angle for a jet ? - Pelican Parts Technical BBS (http://forums.pelicanparts.com/off-topic-discussions/580708-question-pilots-max-crab-angle-jet.html)

[Chris Scott]

Hi andytug,

AFAIK you are right. In my experience on a wide variety of types, the crosswind "limit" (often referred to as "the maximum demonstrated [in certification]") was always a wind component. So, for example, a 35-knot maximum would allow (say) a 50-knot wind up to about 44 degrees off the runway bearing; a 30-knot maximum would allow (say) a 40-knot wind up to about 50 degrees off. One complication is any stipulation relating to gusts.

When using the crab technique, the angle required to track the runway centreline at a given crosswind component will increase as the aircraft's TAS (true airspeed) decreases. On a given type, and if everything else remains equal, the TAS reduces with the all-up weight (mass). So an empty aeroplane will fly at a larger crab-angle than one at maximum landing-weight.

On big jets, a crab-angle of over 20 degrees when crossing the threshold is not uncommon. On smaller, slower types - such as feeder-liners - it may be considerably higher. That's when the side-slip technique (see my previous post) really comes into its own on types that permit it.

[wiggy]

andytug

I'd really caution that there appears to be, putting it politely, a bit of confusion and incorrect info contained in that link you posted, and I'd hate to see it perpetuated here.

Just to be clear last time I looked the 744 certainly has not got the B-52 main gear system where you can "skew" the main gear prior to touch down to account for expected drift (and FWIW you can't do that on the 777 either).

I think some of the guys on the pelican thread are getting confused/misunderstanding the function of the body gear steering (747)/rear axle steering (777) - that's only available at slowish speed whilst taxying.

As for the comment in that place that there is no dry crosswind limit "per se" I'm hoping that's a typo. There's most definitely a crosswind limit for dry runways on most if not all aircraft. It's been a while since I flew it but dry cross wind limit for the 744 on a dry runway is well above 25 knots, it may be operator dependant but it's more like 35 knots, if not more.

[DaveReidUK]

So, correct me if I'm wrong (I threw away my Dalton Computor many years ago :O) - to produce a crab angle of 45° would require a crosswind component roughly equal to 70% of TAS.

That I'd like to see ...

[Chris Scott]

Quote:
"...to produce a crab angle of 45° would require a crosswind component roughly equal to 70% of TAS."

Yes, Dave, that's what I make it too (W/C = TAS x sin 45).

(In the simplest case, where the W/V is straight across the R/W, the ground speed would equal the wind strength.) Could Boeing be making allowance for the Hurricane season? :rolleyes:

[TwentyThree4]

Because the Airbus A380 FCTM (sic) SOP stipulates that the pilot sits above the centerline in a crosswind landing, the main gear is displaced a considerable distance downwind in a strong crosswind.
It is counter intuitive for the pilot to remove drift before touchdown, in a situation where the main gear is displaced (so) significantly downwind.
That is the fundamental reason that videos of A380s in actual strong crosswinds do not illustrate, the de-crabbing maneuver prior to touchdown.
The first requirement, sit above the centerline precludes the second, to decrab before touchdown.
QED

[WindSheer]

Fantastic landing.

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