consequences of exceeding the X-wind limit

[Pilot16]

Hi,

Just wondering, if the aeroplane , say in one particular leg of the flight or during the take, exceeds or very near to the crosswind limit, what should the pilot expect?

Thanks.

P16

[Maarten]

If you land it without bending it: nothing.
If you do bend it the insurance will not pay up.

[Aussie Andy]

Expect to run out of rudder authority - especially when landing crosswind.

[Dufwer]

P16, could you explain what you meant by
in one particular leg of the flight
Crosswind limits apply to take-off and landing. If you exceed them, or get close to them while not current in crosswind technique, you could lose control of the aircraft. :ooh:

D


Dude~, you'll have to go for short post like mine if you want to beat FFF on the draw ;-)

[FlyingForFun]

What do you mean by "crosswind limit"?

If you mean the "maximum demonstrated crosswind" that you'll find in the POH, then, quite possibly, nothing will happen - depending on aircraft type. Many aircraft can be quite safely landed in crosswinds well above the demonstrated limit. The reason for the limit is often simply that this was the strongest wind the test-pilot could find to fly it. On the other hand, in some aircraft the max demonstrated crosswind really is a limit. The only way to know for sure is to speak to other pilots of that type of aircraft, or try it yourself if you're feeling brave enough.

However, if you do try this, and you bend the aeroplane, don't expect much sympathy from the insurance company.

Assuming you're approaching wing-low, you will simply run out of rudder authority at some point during the approach. When this happens, you add power, go around, and find somewhere else to land. So trying it is unlikely to cause you any problems, as long as you have an appropriate diversion lined up. Bear in mind, though, that as you slow down, the controls become less and less effective, so you will need progressively more rudder... if you're right on the stop as you start to flare, then you will run out of rudder during the flare, and that's going to be bad!

If you're de-crabbing, I'm not really sure what would happen, but I suspect that the aircraft will start to drift sideways by an unacceptable amount before it straightens up, which could well end up in a bent undercarriage. I don't particularlyl want to test this theory out.

FFF
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[Dude~]

Flying for fun must have posted as I wrote my waffle. He/she said exactly what I wanted to, though I can't expree myself nearly so clearly!


say in one particular leg of the flight

In flight it doesn't matter what the crosswind is doing. However, if it is very very strong, a light aircraft may not be able to make headway over the ground.

As far as on the runway is concerned, the crosswind limit is the point up to which a particular type can safely maintaine the runway centreline. If the crosswind is slightly above limits, say 18/19 kts, providing the ailerons are held fully into wind to prevent the upwind wing lifting, at best, the plane would deviate from the centreline due to insufficient authority from the rudder.

I'm sure there's much more to it though, ie, fuesalage cross section, aircraft lenghth, (moments around the main gear) and of course runway surface and tyre pressure and strength. ANd that old twin engine chesnut - pilot competancy!

I suppose if there was say a 30kt crosswind component, on the approach, a warrior or similar could track the extended centreline, albeit with horrendous crab. But it would be imposible to land wing down with opposite rudder and zero drift and then maintain the centreline.

Going off at a tangent, I once landed a warrior at Le Touquet at its limit, (i think it was 30/40 degrees from the right and 19kts gusting 27) Now, I said to myself, I'll approach, and if at anytime I dont like it I'll divert. However, as I flared and was pretty much committed, I began to feed in increasing ammounts of right aileron and left rudder. It surprised me when I reached the rudder stops, but what surprised me more was that my damm knee borad prevented me from getting full aileron deflection so as I touched down in 27kt gusts I had to remove my kneee board. Now of course, its just habbit pre landing to chuck all the maps and crap in the back

[incubus]

I rotate my kneeboard (A5 size) to the side of my right leg for takeoff and landing - I find it is the best way to get full & free movement of the controls in the 152/172

[Tinstaafl]

In flight: Not a thing. As far as the a/c is concerned there isn't a crosswind - it all arrives from the front. Presuming no sideslip, of course.

On the ground: No great drama as long as the control surfaces are held appropriately and depending on just how much over the limit (1kt? 10kts? 100kts?). There will be a tendency for the a/c to weathercock into the wind regardless. At some point taxiing becomes impossible to the strength of wind overcoming steering authority &/or ability to keep the a/c from becoming airborne or getting flipped. That applies no matter where the wind is from.

Take-off: Ditto no great drama depending on just how strong the x/wind component is. Hold the ailerons to prevent the upwind wind from lifting, rotate briskly to minimise the transition period from tyre borne to wing borne so that the tyres don't get dragged sideways as much. The a/c will tend to yaw into wind as soon as airborne. As rough rule of thumb whatever it yaws to will be approx. the drift allowance needed to maintain RWY track if your climb speed is similar to the airborne speed.

Landing: Becomes more of a problem, again depending on just how much over the demo. limit the x/wind component is. As others have said it's rudder input that becomes the limiting factor.

Using a wing-down/sideslip method then as speed reduces more rudder input will be required to maintain HDG. This is compounded because as speed reduces the drift increases so even more wing down will be needed to maintain position over the centreline. That in itself requires additional rudder input. Eventually you'll run out of rudder & the a/c will start to turn in the direction of the low wing.

Keeping position over the c/line isn't really a problem (big wing compared to rudder so it can generate a HUGE amount of sideways force to counter the x/wind). Use more wing down as speed reduces/xwind component increases.

Once the wheels are on the ground then the addition of nosewheel steering + differential braking assists control.

You can partially reduce the problem by limiting flap amount. The extra speed throughout the flare/hold off/touchdown reduces the drift for a given wind strength & makes the controls more effective.

Another option is to use the crabbed technique: A moment before the wheels are about to touchdown on the runway apply sufficient rudder to yaw the a/c (a flat turn so opposite aileron will be needed) so that the a/c points down the runway just as the wheels contact the surface. The disadvantage is that there's little margin to correct a misjudgement, unlike the wing down method.

The problem is that the rudder can only apply enough force to yaw the a/c at a certain maximum rate. The rate reduces as speed reduces due to reducing rudder effectiveness. The stronger the xwind then the greater the crab angle needed to track along the runway. That means more time is needed to yaw the a/c.

As soon as the yaw is started then the a/c will begin to drift. At some point the time needed to yaw to align the a/c vs. the amount of drift that develops throughout the process will limit how much xwind is feasable. If you like, you can think of it as running out of rudder too ie not enough rudder to yaw the a/c fast enough.

I tend to combine methods. I leave the yaw until shortly before wheels touch and also apply wing down. Sometimes I've also had to differentially close the throttles to increase the yaw rate.

[Aussie Andy]

I love threads like this ;) A couple of comments caught my eye:However, if it is very very strong, a light aircraft may not be able to make headway over the ground. Jeez! I've never come across an 80kt headwind, which is what it'd take for your average GA aircraft to have a groundspeed of zero or less... or were you thinking of microlights? But this does reminds of DHC-4 Caribous (see here if interested (http://oc-kahuna.com/RAAF_7.html) of 38SQN when I used to work at RAAF Richmond in NSW: when on final with full flap in a goodly headwind, they seemed to have a GS of nearly zero! Don't know what the fully flapped stall speed on these was, but pretty low I think (guess 35 or 40kts?)... tend to combine methods. I leave the yaw until shortly before wheels touch and also apply wing down. Sometimes I've also had to differentially close the throttles to increase the yaw rate. Funnily enough, we were talking about this the other weekend when our trip to Germany involved a number of gusty x-wind landings... when discussing our respective techniques (wing-low versus crab in versus combination), I realised I am not always conscious of which method I am using... just kind of do it without thinking about it! Following this conversation though, now that I am thinking about it, I seem to suffer paroxysms of indecision on every approach! I was better off just getting on with it!

Andy :ok:

[Vizsla]

In a no second choice situation I managed to peel off the two mains on a AA5B in 29kt 90 degree when limit was 15. Subsequent braking on the rims was interesting...

[Kingy]

On my Cub the max crosswind is supposed to be an entirely arbitrary and stupidly low 10mph/9kts. I exceed this figure very often. The aircraft, if flown in the right manner, will handle far more the this as the controls are quite powerful. I've done up to 20kts quite happily in the past using the wing (very) down method.

As the others have said, often it is quite possible for aircraft to handle more cross-wind than the max demonstrated. It is very dependent on the currency and ability of the pilot. A gusty wind is much more of a challenge than a constant wind also.

Why not grab a good instructor on a windy day and fly some crosswind circuits. I did this when training and it helped me become comfortable holding crossed controls on approach - a necessity for this kind of thing!

The ability to handle your machine upto, and perhaps, slightly beyond it's 'limits' certainly makes you more confident even if you are not always 'pushing the envelope'!

Kingy

PS Some crosswind limits are not as far off as the Cub's - don't do anything silly...;)

[Flying Farmer]

Gusting 45 kts in an Arrow, not going to say how far off the nose though!!!!!!!!
certainly an interesting arriaval, bu££er was we had to fly back to EG?? again interesting.
FF

[bookworm]

Take-off: Ditto no great drama depending on just how strong the x/wind component is. Hold the ailerons to prevent the upwind wind from lifting, rotate briskly to minimise the transition period from tyre borne to wing borne so that the tyres don't get dragged sideways as much.

I think this differs greatly between aircraft. Much depends on how much lift is generated before rotation, and how smooth the surface is. Combine a low-set wing, a nose-up resting-angle with a bumpy surface, and there's not much normal force there to allow the tyres to grip laterally. As a consequence, I find crosswind take-offs in some types harder than crosswind landings.

[FlyingForFun]

Exactly, Kingy. It's been a while since I flew the Super Cub, I think the max demonstrated crosswind was 12mph? Whatever it was, it can handle pretty much double that quite safely. On the other hand, the numbers for the Europa (I was originally quoted 12kts, but I have since seen 15kts on their website) are pretty close to the realistic limits. It does vary from one type to the next, and the safest way of finding out is to ask someone who knows the type well. (I discussed this with my Europa instructor, who also happens to be a Europa test-pilot - I figured he probably knows the type about as well as anyone else.)

Andy, you were talking about the cruise speed of light aircraft? My old instructor owned (still does own, I believe) a share in a single-seater Jodel. He used to race trucks down the M4 in it. He reckoned if he had a very strong tail-wind he could sometimes beat them, too! :D

FFF
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[BEagle]

Took a Warrior into that patch of mud and gravel known as Oxford Airport one windy day. Wind was about 25 knots straight down RW27.....but when we got there we found that it wasn't available as it was waterlogged. They wouldn't approve RW29. so the landing on RW19 was quite interesting - but the 'crab' technique worked fine and we touched down with no dramas. In fact I was more concerned with stopping the right wing lifting during the roll-out.

They wouldn't approve RW29 for departure; the take-off was probably more difficult than the landing. During the return to Cartoontown International, the turbulence was so rough that Pooley's made it from the front passenger seat to the back via the ceiling! I had a GS of about 45 kts on the way back at 1500ft....

[Genghis the Engineer]

Maximum demonstrated does mean just that - it's the greatest crosswind seen during certification and found acceptable.

It may be the actual safe limit, or the safe limit may be double that but they didn't get there in flight test. The only real absolute about that limit is that nobody should plan to exceed it unless they know the type extremely well.

And I for one have flown stationary or backwards several times in FW aeroplanes (and no, it wasn't a Harrier). Quite entertaining, but bad for the soul too close to the ground and can make navigation fiddly.

G

[Evo]

Maximum demonstrated does mean just that - it's the greatest crosswind seen during certification and found acceptable.


The CFI at my club is ex-airlines, and he told me that one of the aeroplanes that he flew had different max demo'd limits for crosswinds from the left and right due to the weather during certification. :rolleyes:

I believe that it is possible to get the max demo'd crosswind increased. I know of one company that was trying to do it, and last I heard it seemed likely to succeed. The original limits were rather low though.

[Aussie Andy]

FFF: :p re- the Jodel...

BEagle: at least you knew what the RWY's were... I once diverted into Oxford due weather and was quite stressed... they told me I was cleared to approach RWY "xx" but my brain heard "yy" (don't know what I said for the readback) and it was the nice man in the TWR gently enquiring why I was turning left instead of right that got me back on track! That was a bad day... haven't had one like it since!

Ghengis:And I for one have flown stationary or backwards several times in FW aeroplanes (and no, it wasn't a Harrier). Quite entertaining, but bad for the soul too close to the ground and can make navigation fiddly sounds fascinating: do tell! :uhoh:

Andy

[stiknruda]

BEagle wrote:

"but the 'crab' technique worked fine"

well of course it does old chap, just like they said it would at Stalag Cranditz!:D :D

Stik

[Genghis the Engineer]

Well, if you earn your living in flight testing you need to evaluate these limits - which means getting airborne close to them. On occasion it goes beyond what you were hoping for - unsurprising since you are deliberately getting airborne in nasty weather conditions.

Once had a TP (he was a Harrier pilot) in a light FW type at Boscombe, he had the circuit to himself since at 25kn total wind we were outside ejection seat limits so all the FJ had been put to bed. Wind got up to 35 kn which was beyond safe landing limits - no problem he had 4 hours fuel on board and plenty of daylight. So, he amused himself by flying go-around circuits then at the end of downwind on one, he stopped and flew backwards the wrong way down the downwind leg. SATCO took some time to get over that one.

On another occasion - actually a few months ago, had to fly some climb performance and stalling tests out of a GA airfield "somewhere in England". It was a windy gusty day so we climbed up to about FL80 in the overhead. We had a GPS installed and for half an hour whilst we were consistently pointing into the prevailing wind it was reading within about 2nm of the same spot, a G/S of between 5kn and 20kn, and a constantly variable heading.

And once flying entirely for pleasure in a microlight with Mrs Genghis in the back for a trip out for lunch the forecast 20kn wind turned into 40 and I did a near-vertical walking-pace arrival at a Gliding club. As I'm sure others will agree, it reminded me that close to the ground strong winds are seldom steady and flying any kind of circuit becomes very hard because you can't help trying to use visual references and they do refuse to move in the direction and at the sort of speed instinct normally tells you to expect.

G

[ianhogg]

Once flew over killington lake (by the services on m6 in cumbria)
3 times without turning round in a first generation trike.
once at 1500 , once at 1500 blown backwards then once very determindly with the bar between my teeth at about 400 where the wind wasnt as strong but was more "sporting".God the enthusiasm of youth!!!.
Pip Pip.

[IO540-C4D5D]

My information (which is worth exactly what you have paid for it :O ) is that the xwind limit is largely based on rudder authority at the POH final approach speed.

So, if a large runway is available, and you really have to get down, you can fly it 5-10kt faster and "force" the plane down onto the runway, and should get away with it. Not that I would ever try it myself!

Obviously with a 50m wide runway it's easier anyway because you have lots of sideways drift leeway...

There are two schools of thought on how to land; the traditional is to always do a "minimum energy" landing, i.e. you virtually stall at the end of the flare. The other is to fly a bit faster and you have at least the two main wheels down before the stall warner horn goes off. It seems obvious to me that the former technique, while reducing the damage if you hit something (because you are landing slower) will have a much lower xwind limit because you have no elevator authority left and not much rudder authority either.

[Genghis the Engineer]

Not really, although rudder power is clearly a player. Here are a few real world examples - they all exist but for the purpose I'll elect not to name them.

#1 - large span, low wing, moderate lateral and directional stability and moderate rudder power. Crosswind limit is basically set by the need to avoid touching the wingtip on the ground, so the main players are lateral stability and geometry.

#2 - high wing, high rudder power, high directional stability, high lateral stability. We set the limit at about 15kn on that aircraft, although the controls made a 25kn X-wind flyable BUT the sideforces in sideslip were so high that we felt that no normal pilot could reasonably be expected to function in that environment. So, the big player there was directional stability.

#3 - high wing, roll control design that gave very little adverse yaw, moderate lateral and directional stability, very little rudder power (because in normal flight it wasn't needed). So in that case the crosswind limit, which was fairly low, was clearly determined by rudder power.

#4 - biplane, linked roll and yaw controls so no independent rudder control. The aircraft was deliberately landed crabbed right onto the deck (an interesting experience) but the undercarriage was specifically strengthened to take the loads as the aircraft touched down skewed then was pulled straight by it's inertia. So in that case the crosswind limit was determined by maingear strength.

G

N.B. If you've really got a 50m wide runway, land on the diagonal and thus reduce the crosswind component.

[FlyingForFun]

Have to admit that I think there's some truth in IO540-C4D5D's theory. That's why I always wheel-landed the Super Cub in moderate crosswinds - it all becomes a bit too uncontrollable at very low speeds just before touchdown if you 3-point it.

Genghis knows far more about these things than me, though. And I'd definitely agree with him that it depends on the type. But regardless of type, you will have more control authority at higher speeds, even if the control authority isn't the limiting factor.

But be careful of that nose-wheel (if you have one) when you land a little faster than usual :ouch:

FFF
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[Aussie Andy]

Good on ya Genghis: that was very interesting, and I'm glad to say I think I've learned something today... Cheers!

Andy :D

[john_tullamarine]

Super Cub glider towing ops routinely see the pair take off at whatever the wind might be .... with the tug then landing across the strip .. the more adventurous pilots have been seen to land with the wingtip not very far above the grass ....

Sometimes the fibro gable markers have been seen to take a beating ... I am told that it makes quite a "bang" for the occupant(s) ....

I do think that it is a pity to see the generally poor crosswind standard in training these days .....

As an aside, some of the US aircraft with linked controls can be a handful.... I was involved in upping the limit on a Rockwell Commander many years ago .... as I recall the US figure was 15 kt ... we chickened out at a little less than 20 kts as I recall ...