Very Strong Gusty Crosswind Landings In A Quad Vs A Twin

[Kilda Ste Hilda]

Recently a junior colleague posed this question to me and I was stumped...I couldn't give her a definitive answer.

According to her, based on personal observations and you tube videos, quads seem to handle better, stable and land reasonably well in strong crosswinds.

Twins tend to rock and roll all over the place. I gave her the answer a former instructor in the Far East told me years ago when I posed a similar question. He opined that the quads like the B747 and A380 have very good " wing loading " on account of the outboard engines, and hence are stable in gusty crosswind conditions. He elaborated on further with other reasons but I only remeber the " wing loading " part as I thought it was BS, good for a laugh. Well, it sounded like wing loading because he explained about the geometry, moments of force and engine weight effects on the wing. Well, I didn't buy it then but I found myself regurgitating the same to my colleague! It's embarrassing!

Anyone with a better explanation?:)

[Sidestick_n_Rudder]

Never flown a quad, but perhaps it has to with inertia, ie 2 engines on the wing give a a higher moment of inertia, thus better resisting rolling moments induced by turbulence?

[John Farley]

The initial (NB initial) response of any aircraft to a lateral gust will be determined by two factors:

1 The rolling moment due to sideslip (or if you prefer it the amount of ‘dihedral effect’ present in the design).

2 The rolling inertia of the aircraft.

It is likely that the roll inertia of a quad is greater than a twin so for a given similar dihedral effect in both categories the quad will be less likely to bank quickly compared to the twin.

I have stressed initial response because very rapidly other factors come into play. The main ones being any control input following the gust arriving and the aerodynamic roll damping term present in the wing design.

(BTW if anybody wonders why I write about ‘dihedral effect’ rather than dihedral angle it is because several aspects of aircraft design introduce the same effect as dihedral angle)

[John Farley]

Sorry S n R you made the point better than me!

[Owain Glyndwr]

It would be interesting to hear the views of pilots who have flown both the A330 and A340!

[fantom]

It does not matter if you are in a magnificent Airbus.

Nothing matters.

[Owain Glyndwr]

I was hoping for something more interesting!

[John Farley]

Owain

Both the A330 and A340 are FBW. This means control inputs will be made by the FBW system as soon as the aircraft starts to roll and so if we assume both FBW systems are equally competent the two aircraft will both subsequently respond in the same way to the same gust regardless of any roll inertia differences. That is what FBW is all about.

[divinehover]

I've flown B727, B747, A320, A340-200/300/600 and A330. There is no difference. The only significant point with the big Airbus's is that over active sidestick handling will get you into the poo very quickly. Other than that, fly them like a C172.

[Sidestick_n_Rudder]

Hi John,

It was good to read a more technical explanation, than mine :)

Could you elaborate a bit on the factors affecting aerodynamic damping?

Regarding the Airbus FBW - my personal opinion is that its handling in gusty crosswinds is pretty appalling. Lots of rolling, partly due to slow response of the FBW and partly due to overcontol (one can argue that this is due to poor piloting technique, but the system is IMHO quite conductive to it).

[Owain Glyndwr]

Thanks John.
Point taken, but I just didn't know if AI had tuned the gains for each machine or not. If not theremight have been a difference

[John Farley]

S n R and Owain

I have no knowledge of Airbus aircraft but have an old CTP mate at Dunsfold who very much does. He says the trick is not to try and correct the aircraft as soon as it rolls due to turbulence but let the FBW do what it designed to do. Non intuitive and maybe easier said than done for many pilots.

Re roll damping as we all know the upgoing wing loses AoA and the down going wing has an increase. This produces asymmetric lift in the sense needed to reduce the roll rate. This effect is enhanced by increasing the aspect ratio. It is also greater the steeper the lift curve slope of the aerofoil concerned.

[glendalegoon]

maybe its because the "quads" go senior and the senior guys know what they are doing?

;)

[zerozero]

I've flown both the 747 and 767.

No big difference as far as I'm concerned.

The sight picture is different due to cockpit height and length of fueslage (i.e. the 747 cockpit is higher and farther from the centerline--closer to the runway edge--during big crabs).

But the physical handling of the aircraft is pretty much the same.

...for whatever my opinion is worth...
:8

[JammedStab]

According to her, based on personal observations and you tube videos, quads seem to handle better, stable and land reasonably well in strong crosswinds.



About 20 years ago I was on a vacation in LA on a very windy November day. It was a direct 30 knot crosswind. I decided to watch some aircraft land in LAX. There were many go-arounds but all were narrowbody and mostly 737. Some mentioned windshear. Not one of the 747's went around and there were quite a few.

The most interesting aircraft that made it in while I was there and I kid you not....A Beech 18.

[John Farley]

I have stressed initial response because very rapidly other factors come into play. The main ones being any control input following the gust arriving and the aerodynamic roll damping term present in the wing design.:hmm:

As I said the behaviour after the initial response will be modified by any control input.

Don't forget autostabs as well as FBW will move the controls.

So I reckon the original notion that quads would be steadier than twins is probably most likely to be true if you have manual controls and no stabs or dampers! Those were the days.

[No Fly Zone]

In no particular order, the Big Quads have 2x-3x or more mass and thus are more stable, especially when already on a ~stable course. One responder notes that the Big Quads are usually driven by older, more experienced pilots. Those older, more (often FAR MORE) experienced drivers 'tend' to fly with a much lighter hand and, where appropriate, allow the embedded FBW systems to do their auto-correction before the human interface can react. Most recognize that current FBW systems react faster - and with a lighter hand than does the human.
That convoluted mouthful said, in a difficult, gusty x-wind approach, I'd much prefer to be in a heavy quad. Why? That older, more mature, more experienced driver is far more likely to call "Go Around and Divert," and simply get the heck out of Dodge City that is the light-weight driver. He did not become 'old' or accumulate 20K+ hours because he took risks just to keep the schedule intact. I'll always prefer to ride behind the light-handed, experienced 'old farts,' in a difficult situation. Ahem - just my two cents' worth. (I too am an old fart. I drive, but only little stuff. I've NEVER found it necessary to land under questionable conditions and yes, I've spent more than one night away from home, waiting for the weather to improve. The decisions to divert must have been good ones, because I'm still here to talk about it. :ok:

[bubbers44]

"The most interesting aircraft that made it in while I was there and I kid you not....A Beech 18."

Now why doesn't that surprise me? It would be fun to watch most of these new age pilots land a Beech 18 in a 30 knot crosswind. I didn't get to fly one of these new generation planes with the tail wheel in the front until prepping for my PPL. They said I needed a plane with fancy instruments, radio and a battery for the check ride.

I still find learning in a tail dragger made all these fancy Boeing planes quite easy to land in 30 knot gusty crosswinds. The Beech 18 and Boeing 767 both land quite well using the same technique.

[con-pilot]

I still find learning in a tail dragger made all these fancy Boeing planes quite easy to land in 30 knot gusty crosswinds. The Beech 18 and Boeing 767 both land quite well using the same technique.

I landed a DC-3 in a 30kt crosswind one day, which was not planned trust me, and had more of a problem taxing in than I did landing.

[PGA]

I have flown 319 / 320 / 330-200 / 340-300 / 340-500 / 380-800 and I find in a crosswind they all handle almost exactly the same, must be the FBW. The 319 was easily over controlled, once again due to the FBW sometimes being a bit slow, but in essence there is no difference as far as I'm concerned.