Side Slip (wing down/cross control) Landing Technique on Airbus (A330)
 

Dear Learned Ones,

I learned that Airbus recommends the crab/decrab at flare method while Boeing does the wing down/cross control side slip method at about 400'.

During conversion from Boeing to Airbus, I was told once by an instructor that the Airbus aircraft will crash if the cross control method of Boeing is used on the Airbus. However he did not explain or elaborate further.

Can someone please give me a technical explanation on this? Your help is very much appreciated.

Many thanks.


ps: someone also told me that the A380 may be able to do the side slip method.

titaniumwings
In AB FBW when you move the stick to one side it is rate roll demand so computers will position flight controls to give you that. You are trying to fly straight with wing down so the computers will try to correct that by use of spoilers, aileron and rudder. It may lead to over use of rudder which can have serious consequences. When you are try to do something that is against the design concept the result cannot be predicted.

That's funny because I've landed the 330 with wing down into wind just like a 777 and it doesn't get confused or crash....:D

Indeed it's supposed to behave just like a real Aircraft.

A330 FCTM---


LATERAL AND DIRECTIONAL CONTROL FINAL APPROACH In crosswind conditions, a crabbed-approach wings-level should be flown with the aircraft (cockpit) positioned on the extended runway centerline until the flare. FLARE The objectives of the lateral and directional control of the aircraft during the flare are: • To land on the centerline, and • to minimize the lateral loads on the main landing gear. The recommended de-crab technique is to use all of the following: • The rudder to align the aircraft with the runway heading during the flare. • The roll control, if needed, to maintain the aircraft on the runway centerline. Any tendency to drift downwind should be counteracted by an appropriate lateral (roll) input on the sidestick. In the case of strong crosswind, in the de-crab phase, the PF should be prepared to add small bank angle into the wind in order to maintain the aircraft on the runway centerline. The aircraft may be landed with a partial de-crab (residual crab angle up to about 5 °) to prevent excessive bank. This technique prevents wingtip (or engine nacelle) strike caused by an excessive bank angle. As a consequence, this may result in touching down with some bank angle into the wind (hence with the upwind landing gear first).

So.

In the Flare---

Rudder to align the nose straight down the runway.
Aileron used as required to keep it over the centerline and not drifting downwind and if the xwind is strong enough land on the upwind wheels first. Yep a small gentle sideslip.

Basically like a 777.

You will find that unlike a 777 the Airbus won't require Aileron to keep the wings level while pushing in the rudder during a gentle crosswind, the FBW will do that for you. ( it's supposed to but........)

Thanks a lot for bringing up the points. It is what I had wanted to discuss.

As villas mentioned; the FBW flight computer may introduce an input which is contrary to that of a pilot.

As ACMS mentioned, the comparison I am trying to make is between B777 to A330. Also the part of which it can be flown like any other aircraft may need more thought. But you will be a great person to compare as I believe you have flown both the B777 and A330.

Consider a conventional turn, any aircraft (other than airbus) will require the pilot to hold in the input on the aileron to continue the turn. For airbus just demand a roll rate till the bank angle and release to neutral-this is not conventional.

I believe the B777 sets itself up for sideslip at 400', this can be observed on its autoland.

The A330 has normal lateral law till practically touchdown. Considering the way it coordinates for the turn, it therefore cannot do a wing down at height (400' or higher) like the Boeing.

During flare however, momentarily wing down before the coordinated lateral inputs by the flight computer will result in the wing down touchdown. This is just my interpretation of the normal lateral law.

Please correct my understanding accordingly.

ps: by this theory, with an all engine flameout and still under Normal Law and the engines do not light up, it is not possible to the side slip/ forward slip method to lose height while still maintaining track on finals

ACMS, can I just check where is this part about the wing will remain level even after you centre the heading with rudder is interpreted from.

It may be the same principle that I use to reason out the above.

Thank you very much.

If you are on a straight and level flight then you put in a rudder input, using the Lateral Normal Law as a guiding principle, what would the aircraft do?

I get the opportunity to make crosswind landings in the 330 with anywhere from a 10 to 30 knot direct component fairly regularly so this is my 2 pesos. These are just observations gleaned from drinking myself into an obsessed self-analytical stupor after making a few LEGENDARY "crunch-on" crosswind "arrivals". They have since gotten a bit better. Read on for some ideas; take what you can use and chuck the rest.

In normal law, roll and yaw control remains the same until the main wheel touchdown, at which point lateral control blends down to a direct stick-to-deflection relationship.

Pitch, on the other hand, changes to a direct stick-to-elevator relationship around 100 ft RA. Autotrim also stops, and at 50 feet a slight nose-down moment is applied to get you to pull through in the flare. This all combines to basically make the airplane feel like a Cessna 172 IN PITCH during the flare and touchdown, assuming the autothrust isnt jumping around too much. In roll, if you use a "kick-out in the flare" kind of technique and have a gentle touch on the stick, it also will feel fairly intuitive if you just look out the window and fly it.

Autothrust, however, can really mess up the feel of the flare on a gusty day.

In Normal Law ABOVE 100 ft, the FBW maintains 1 "G" corrected for pitch, regardless of gusts or thrust changes. That does not mean it will necessarily hold the PITCH exactly stable, but the G LOAD; subsequently the pitch stays pretty close. So let's say as you motor down short final, you hit a big old sinker and lose a few knots below Vapp (assume we are below 400 ft so GS Mini has stopped "mini-ing", and above 100 ft so we are still in fully normal Normal). The airplane will hold CLOSE to the current pitch as the FBW applies up elevator to increase "G" against the "sinker". Simultaneously (actually a little after), the A/THR will blast in some power to bring up your speed. This would conventionally cause a non-FBW airplane to pitch UP due to "pitch-power couple", but of course the FBW counters this as well. So to you the airplane's path feels fairly steady, with no input needed on your part except for small little "guiding" touches when the path wanders off as the airplane works to maintain "G".

Now we get into "flare mode", below 100 feet. Remember our PITCH CONTROL ONLY has returned to a DIRECT relationship between stick and elevator, with no autotrim. A/THR, however, is STILL in "SPEED" mode. Let's say you hit that same sinker NOW, at 80 feet. The nose will drop with the "sinker", and you will correct for it. This feels natural and lets us have an intuitive feel for the airplane's energy state just above the ground, where it matters most. However, as the power surges in the "pitch-power couple" is also back with us, and the nose wanders up and down. In a Boeing, we have our hands on the throttles and feel them moving, giving us an instinctive grasp of what the power, and therefore the pitch, will do. On Airbus aircraft that is not the case. You really don't know what the hell the thrust is doing until a split second after it "does", because you are looking out the window and the engines are too far back to hear until they really crank up. So when the autothrust starts jockeying itself in gusty winds just above the flare, a lot of "stick-and-rudder" type pilots will feel just a touch "out of the loop".

So going back to the gusty crosswind landing, if your SOPs allow it, manual thrust is much more intuitive, assuming you have practiced using it. A good technique on the A330-200 is to set the power for around 1.6 ~ 1.7 EPR while all set up and riding down final. Use small power changes (.1 or less) to counter airspeed TRENDS, and allow the exact IAS to wander slightly in the gusts, preferably on the higher side. The autothrust is rough in gusts because it AGGRESSIVELY tries to maintain the EXACT speed at low altitude; maintaining ENERGY STATE is more the name of the game hand flying. It is also MUCH smoother on the passengers' ears (and the neighbors' around the airport).

Most important this puts the pitch-power couple COMPLETELY back in your hands, taking away the variable that seems to destabilize crosswind landings more than any other IMHO.

So we have pitch dealt with. Now the lateral. Roll stays in Normal (roll-rate demand) till the ground. So same as pitch, it will NOT lock in your bank angle, but your RATE. So zero stick deflection equals zero rate. It will try to correct back to zero if it is disturbed, and that correction puts it back more or less where it started, but not always. So there will be some "guiding" to do here as well BUT NOT MUCH. That does mean that as you squeeze out the crab angle with the rudder, it will try to hold zero roll rate. It does a good job of this right up to the point that a gust nails you in the de-crab. NOW, it will zero the roll rate but not always exactly on wings level. This is where we usually start "stirring the pot" and it gets really wonky (like the Lufthansa A320 near-accident a while back). Small, steady motions are the key. Big, steady motions if those don't do it, but above all steady and not jerky. Think Cessna 152 more than Boeing 747. Remember a given lateral stick deflection commands a given roll rate; the airplane may take a moment sorting out how to give that to you, so hold the command STEADY until it either starts moving in the right direction or you are ABSOLUTELY sure that it will not. Let it do the work for you; by getting your control inputs "out of phase" with what the airplane is trying to do for you, the famous "Airbus Hula" will begin and your landing is SHOT.

The secrets of the guys I've seen who do really good crosswind landings mostly involve:

Use manual thrust. Since you are controlling thrust directly now, idle the throttles slowly like you would in a Boeing, to reduce pitching tendencies. Squeeze out the crab as late as possible DURING the flare and not before, to minimize lateral stick corrections. Small, STEADY pressures on the stick; use STEADY, CONSTANT PRESSURES in pitch below 100 ft, with small, STEADY, intermittent pressures in roll that ALWAYS RETURN TO NEUTRAL.

And even then sometime the damn thing just drops like a brick, and lands itself on the wrong wheel while everyone else in the cockpit laughs at you.

Good luck!

villas,
This Airbus produced document doesn't agree with you.

http://www.skybrary.aero/bookshelf/books/179.pdf

"With higher crosswind (typically above 15 kt to 20 kt crosswind component), a safe crosswind landing requires:
•
− A crabbed-approach, and
− A partial decrab prior to touchdown, using a combination of bank angle and crab angle (achieved by applying cross-controls).
On most Airbus models, this requires touching down with:
− Maximum 5 degrees of crab angle, and
− Maximum 5 degrees of bank angle."

hikoushi,

Great writeup. :ok:
Thank you very much for sharing your experience and the enlightenment. Really appreciate it.


Regards.


ps: If you are on a straight and level flight then you put in a rudder input, using the Lateral Normal Law as a guiding principle, what would the aircraft do? TQ.

Some really interesting lines written by Hikoushi...:)

Goldenrivett
If there was no problem with side slip technique then all that write up was unnecessary, then why not fly wing down cross control like any other aircraft? There is a problem as I mentioned. If you carefully read what you quoted it is applicable in your own words prior to touch down. It does not mean you fly side slip approach and there is a bit of jugglery involved. You fly the crab all the way to flare and when you align the nose down the runway if you start drifting down wind you put on max 5 degrees bank to stop that and if the crosswind is too strong you land with partial drift. I Don't know about 777 but I have flown a lot the 747 and A320 is not flown like that. And I have to disagree with hikoushi about manual thrust in gusty wind. It is exactly opposite of what Airbus recommends. You don't knock out GS Mini and convert AB FBW to Boeing it is not a safe way to fly. It is safer to change your company and fly a Boeing instead.

ACM, goldenrivette
Your post 3 and 4 are contradictory. In post 4 FCTM extract does not tell you to use side slip technique. You only use roll to stop the drift after flair and in that you may keep some crab and some bank in very strong crosswind. It is done prior to touch down and not in approach.

titaniumwings
Your post 6 is correct except all engine flame out part. I don't know what you mean by that. Any way with all engine flame out you do not remain in normal law. With both AC busses and G+Y knocked out you are in alternate law.

vilas,

It should be in alternate law. I suppose that means that side slipping to lose height is then possible?

Thanks for checking n cheers.

titaniumwings
It is not that you cannot side slip the Airbus but you should not. And I see that you are obsessed with side slip for loosing height. This is very dangerous as it may cause brake up of the tail plain. Use of too much and rapid rudder input caused fatal accident of A300 in New York. It is not to be done.

I trained in Canada where they taught the wing down technique. When I got back to the UK I was made to use the crab technique by RAF QFIs. I continued with the latter until I got to ETPS and subsequently onto B Sqn at A&AEE. During my time there I did a considerable amount of work on landing techniques where I was given carte blanche to try what I liked. My findings were conclusive - wing down is far superior. OK, so you land on one wheel, but you are still, in fact, still partially wing bourne at this time. Advantages are as follows: the fine judgement on when to kick off drift is very difficult to get right and, if you don't, the ac will land with crab on which can significantly stress some ac. In strong, gusty X winds, with wing down, you land with the control deflections which are exactly what you want for the roll out which prevents the arm and leg blurr that usually follows a crab touch down. It is easy to land without drift because the instant of touchdown is not critical and so is a much smoother. I have used this technique with ac ranging fom Slingsby T67s through to VC10s taking Comet and Lancaster en route. The only ac not suitable for this treatment are things like 747s A340 and B52. Incidentally, I have fair experience on both A320 and B737. There is no mystery - they both handle like normal ac. In fact I found the A320 a little nicer to handle manually than its rival. (Retire to WWll bunker at the end of my garden). As you may appreciate this is a big hobby-horse for me, but I do wish the RAF had been broad minded enough in times past to get away from sacred cows and try different techniques.

p.s. I never ground looped a Harvard and so have disproved the old saying "There are those who have ground looped a Harvard and those who havn't yet"

Vilas.

The A300 accident was caused by more than simple unidirectional rudder movement. There were at least three rudder pulses, each one reinforcing the previous one. The ac didn't have a chance.

pontifex
I am aware of the reasons for that crash and after that Airbus has made a video presentation on use of rudder in transport aeroplanes. The point I am making is in Passenger jets you do not use manoeuvres like sever side slip to loose height.

titanium wings
In Normal Law side stick neutral the a/c will try to maintain zero roll rate. If you apply the rudder slowly the bank angle will not change much; if you boot the rudder the FBW cannot cope and the aircraft will roll initially then stabilise provided you maintain constant rudder deflection.

You can fly a side slip in an airbus the same way as any other aircraft... Rudder in the opposite direction to the bank angle, and fly the required attitude.

:ugh: That's a 'good' one!!!

Another thing which I was told; Airbus does not build its landing gears to withstand the lateral loads of landing without decrabbing. Such a 'technique' usually results in significant damage.

Just to share.


vilas; i am going through hell-day contingencies being new to the bus. One of the scenario given is all eng flameout. Am thinking what to do when you are on finals and high, not high enough for an orbit but high enough to have the speed being excessive. Better be prepared with my plans and options rather than thinking about it when the sxxx hits the fan.

TyroPicard,

Thanks for your reply. As you said because there is no input in the side stick there should be no bank. There is pitch and lateral law but there isn't any yaw law as I thought it is to supplement the roll (lateral). I think the yaw is used by the computer to coordinate the turn. Hence when there is no turn the computer will neutralize the rudder. But when the pilot makes a rudder input, will the computer try to counter this with the primary objective as 0 roll rate?

I am not trying to split hairs or be an engineer but I really hope to know and understand what the aircraft does especially maneuvers which is close to ground where I don't have much room to play or experiment with.

titaniumwings
If you are new to the Bus then FCOM and FCTM should be your bible. If you didn't understand something from that you can ask some one to explain in PPRUNE. Do not learn flying here. Lot of things some incorrect and some downright dangerous are suggested in these columns. The most dangerous thing to do is start making your own procedures in Airbus FBW. Airbus is flown only one way and that is airbus way. There is no place for experimentation listening to some one no matter how experienced he is. Do not compare it with previous aircraft. One very senior airbus industry instructor told me if you have taken a new girl friend do not compare her with your old one the result will not be pleasant. Also I can feel you are reading procedures as if they are horror stories. In air if you misjudged something there is limit only within which you can recover. If everything was recoverable there would be no accidents. You don't side slip big aircraft and first try to fully understand laws in normal flying and not aerobatics.

Once again I thank you those who give me advice and recommendations especially on techniques.

I have developed an approach to my operations (but always learning and in the process of refinement). As a pilot it is important to know the books. Attention to limitations are essential. Those are usually non-negotiable. When reading the books attention to the wording is important. Most of them are written with much thoughts. 'Must' and 'should' are used appropriately.

Beyond knowing the books understanding the underlying reasons helps to mature one's operations. This is especially true for recommended techniques. There are many instances of instructors debating over different techniques but the management usually takes the macro view of pilots exercising their judgement and determining what is the best course of actions. This is to the exception where certain techniques which may be prohibited due certain restrictions. These limitations must be adhered to.

I listen to the range of views and opinions especially those with accompanying reasons. This develops my understanding and judgement in determining the best course of action. I will comb the books and web for answers and when not satisfactory I may consult as much resources as I could. After getting the range of opinions I have to evaluate critically and determine the best course of actions.

Nonetheless I appreciate any effort to share. Again, much appreciation for the spirit of sharing guys. :ok: Also I will decrab for landing :)


Ps: Airbus told me that this aircraft can be flown like any other aircraft. But I feel that it is essential to know their FBW laws and characteristics. I also read their FCB: Yaw Disturbances During Takeoff Roll which pay special attention to the improper use of rudder. An accident that I have visited was AA 587 which was a B737 nonetheless. I try to learn from all the incidents and accidents, don't think I live long enough to make all the mistakes especially the ones I learned.

Cadet pilots are no more taught down wing technique in France and learn realistic cross-wind in airliners with passengers:{

Thanks for the info. Just for interest sake, can I verify that it is also such for Boeing, especially B777? Cheers

Can't say for the big twin, but for the 737 the sideslip (wing low) method is actually one of the boeing approved methods, up to a crosswind component of 17 kts at flaps 15, 20 kts at flaps 30 and 23kts at flaps 40.

Two other techniques are described, one being touchdown in full crab, which is approved up to the 40kts crosswind limit, but not recommended on dry runways, however is recommended on wet or slippery runways. The other one is the overall recommended technique of de-crab during flare.

The fail operational autoland uses a mix of wing low (up to 7° of bank if memory serves right) and crab.

decrab in flare on both 737 and a320. Have never seen a 737 come down with wing-low method on approach. If you do this you are a minority.

As for all engine out method, just stick the nose down and pitch for the runway threshold at FULL flap. You won't be at optimal glidespeed so you loose altitude faster.
Unless you picked the shortest runway in town the wheelbrakes will stop you, just don't try to achieve a smooth touchdown.

Usual hysteria about 'sideslip in airliners'


If it is that dangerous then why is it built in to every autoland system on modern Boeing aircraft ?


Try watching the different stages of autoland to see this wing down correction introduced by the autopilot, very smoothly while limiting the bank angle at touchdown.


In manual flight it nearly always produces a better, more predictable result than the 'push it straight at the last second' technique.

stilton
By slide slip I do not mean wing down approach. titaniumwings wants to slide slip to loose height. I would like to know which commercial aircraft manufacturer recommends this.

Thanks for sharing the different techniques on different types guys.

Denti, B777 seems to share similar techniques to the B737. The gears can take the no de-crab. But most pilots would.

I was asking for recommendations. Thanks to 737Jock for his recommendation

vilas;
I would like to reiterate my position: I am trying to investigate and reaffirm my knowledge of Airbus FBW laws and characteristics. In the process I also thought about contingency plans for all eng out forced landing piloting techniques especially to losing height at finals in case of being caught high and fast. I am not about to give up in that situation and have learned of a few techniques which did include side slip to lose height in my early days. I have not seen an outright limitation on this nor am I recommending it. Neither could I find a recommended technique for this situation in FCTM. I am open to suggestions and would be enlightened if I could be pointed to the reason(s) as to why not for this technique, be it references or even technical explanation. I have in mind more than 1 technique and had evaluated the plus and minus of each but I always like to learn from others if they are kind enough to offer.
Thanks in advance to those who do. Cheers guys.

Vilas----gidday again mate, in the flare during a strong crosswind you will need to apply ( with a crosswind for the right ) LEFT rudder and then RIGHT Aileron into wind and probably have to hold those inputs in to control the Aircraft smoothly onto the ground on the upwind wheels.

So isn't that a small sideslip?

Unless you are very quick and push the rudder quickly to align the nose just before touchdown and don't need any wing down you MUST have to apply opposite control input at the same time....

Sideslip.....

Anyway mate it works for me.

I assume that's what you do as well?

Titaniumwings-------Also remember the Transat A330 dead stick approach after running out of fuel over the Atlantic, the machine glides so well they needed to use a lot of side slipping after getting too high on their approach. Worked well for them on the day and they managed to land and stop with blown wheels.....ok:

Not that I would sideslip in any other situation in a commercial Jet but in an Emergency like they had all bets are off!! You do what you need to..

ACM
You hold the crab till flare. After flare you straighten the nose with rudder or hold 5 degrees crab if too strong a wind and use stick as required to maintain direction or the centre line in that you may use 5 degrees of bank before touch down. That is the recommended technique for all not only for you. The dispute is can you do it from 100ft or 200ft? Airbus does not recommend it and the reason is stick out of neutral is rate of roll demand. If you want to do it ask the airbus. Don't develop procedures on your own or drag Boeing procedures in AB FBW.

OK, I'll review it for myself.

When talking about if you are able to land the Airbus FBW (I only know 320 family) in crosswind using the "wing down/cross control side slip method", I'd like to make different considerations.

- Airbus clearly recommends the crab technique. Why? They didn't tell us! But it is a recommendation. They didn't say it's a must or any other technique is prohibited or any other technique is not possible or discouraged because the flight controls. In the other hand if the manufacturer is recommending you something....better to follow it! But still a recommendation.
- There is nothing in the flight controls system that prevents you to do so. You make a roll input in the sidestick to produce a roll rate until you have the target bank and then go to neutral to order roll rate zero and the desired bank will be maintain (it doesn't matter if the target bank is zero or 5 or any other until 33 or if you are changing your heading or keeping it). Of course in real life you have to work on it the same way you have to do it in every landing, with the only difference that the target bank won't be zero. Simultaneously you use the rudder to align the aircraft with the runway centerline. You will flight straight and the computers will be happy because they are doing what they are supposed to do and nothing else. Just the same way you'd fly any other aircraft.
- Is it difficult? I don't think so, but as any other technique, has to be trained, and the more you do it the easier it become. Will you train it? I don't think so because it is not the recommended technique.
- ACMS and hikoushi: wise words, thank you!
- Titaniumwings: don't worry! You won't crash so easily! Just an aircraft! Enjoy it! :ok:

ACMS: How can anyone forget Air Transat 236, an Airbus 330. You put it in perspective.

Gryphon: I have considered your approach, interpretation and technique analysis. Totally agree with you philosophy and perspective.

Wise words by wise aviators!! Thanks for the learning and sharing session. :D

Vilas-----mate mate mate

Please re read my post particularly the underlined bold part where I stated with emphasis I was speaking of during the flare manoeuvre. No where did I say to start the side slip earlier, a technique I have seen on conventional aircraft but not on Airbus FBW. I don't use that technique myself either.

Eaelier I posted the A330 FCTM reference to crosswind technique which I do follow quite well even in gusty typhoons as per Airbus and company policy/training, so I'm not using my old 777 737 747 experience. ( although they are indeed the same )
Cheers

Villas,
How much side slip do you think you have with a badly trimmed aircraft during an engine failure on take off? Even with a big side slip, the bank can be held constant with neutral side stick. The tail doesn't fall off either.

At some time or another, most pilots will have flown the Airbus sim with lots of side slip and not realised until spotting the Beta target / slip indicator. You don't need constant aileron to control the slip induced roll. The computers are doing it for you.

What do you think the FBW computers are doing differently when side slipping at 2,000 ft with all engines at symmetrical power and flying with the slip indicator not centred during EFTO?

Goldenrivett
It is my well considered opinion that pilot should stick to Airbus recommendations in FBW aircraft. Because line pilot does not have access to the software, hardware or wind tunnel or has the training or opportunity to test fly. Also the manufacturer is in touch with all customers across the globe and is aware of all incidents. Therefore they are in a better position to approve or disapprove certain way of doing things. If you develop your own procedures in isolation there may not be a crash but in different circumstances and situations it may lead to undesirable state. Can you side slip an airliner to lose height like a small trainer? If your answer is yes I have nothing more to say but good luck. Till the A300 crashed in New York they didn't know that overuse of rudder can cause the tail to brake off did they?

ACMS
If you read what I have been saying you will realize that we are saying the same thing so where is the argument.