bueb0g
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Max Demonstrated Crosswind - True Meaning?

Tue Dec 25, 2012 12:22 pm

Max demonstrated crosswind seems to have a lot of interpretations, even from reputable people, so I've always been a little confused.

Some people say that it really means nothing, as it's just the max xwind component they found in the flight test schedule. I know that this is strictly true, but these types of people are likely to say that therefore, it's fine and dandy to land above this, as long as the pilot is happy with the conditions, and that the demonstrated limit really means next to nothing.

Then there are people who treat it almost as a legal limit; they won't land above the limit and would rather divert, even if they're potentially happy with the winds, because they say the aircraft isn't technically certified for those winds.

But I've also heard that the max demonstrated crosswind isn't an aerodynamic thing at all - it's just the maximum wind that an aircraft can land in, with no crosswind correction, without damaging the landing gear or tyres.

What is the correct way to think of it? Are all the above correct in some way? Until I know for sure, I'll continue treating it as a legal limit.
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Flyer732
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RE: Max Demonstrated Crosswind - True Meaning?

Tue Dec 25, 2012 12:47 pm

I was taught that it was the maximum crosswind component experienced during the flight testing. It is by no means the absolute limit of the aircraft. For example, the Cessna 172 has a "maximum demonstrated crosswind component" of 15 kts, however I've had to land one in 30kt direct crosswinds. Its all about the rudder authority, when you lose rudder authority, you've reached the maximum crosswind the aircraft can handle.

However, its all about the pilots abilities and comfort level.
 
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Starlionblue
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RE: Max Demonstrated Crosswind - True Meaning?

Tue Dec 25, 2012 1:10 pm

Quoting bueb0g (Thread starter):
Then there are people who treat it almost as a legal limit; they won't land above the limit and would rather divert, even if they're potentially happy with the winds, because they say the aircraft isn't technically certified for those winds.

Max demonstrated is not a legal limitation. As Flyer732 says, it is what has been demonstrated by the manufacturer. Having said that, if you land beyond it and ding the plane or yourself, your insurance company might take a dim view....
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo
 
PapaChuck
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RE: Max Demonstrated Crosswind - True Meaning?

Tue Dec 25, 2012 2:19 pm

I like to think of it as a CYA limitation provided by the manufacturer. A Skyhawk may indeed be able to handle a crosswind in excess of 15 knots if competently flown, however Cessna cannot be held liable for damage if you attempt to exceed that limit.

PC
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CosmicCruiser
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RE: Max Demonstrated Crosswind - True Meaning?

Tue Dec 25, 2012 3:11 pm

at my airline it IS a limit. you don't land in anything stronger. but at the max you will have your hands full. much stronger and you risk a wing tip.
 
jetlife2
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RE: Max Demonstrated Crosswind - True Meaning?

Tue Dec 25, 2012 3:59 pm

It is just what it says, the max crosswind demonstrated via test. It is not only the aircraft controllability. For turbofan engines, excessive crosswind will distort the inlet flow. Engines are tested to demonstrate their tolerance to this and this is part of the data that is used in setting the aircraft flight manual limits. Exceeding these limits risks stalling an engine, depending on the degree that the engine can tolerate beyond the certified limit, if any.
My views are not necessarily the views of the GE Company
 
SUPRAZACHAIR
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RE: Max Demonstrated Crosswind - True Meaning?

Tue Dec 25, 2012 8:30 pm

Most airlines have a crosswind limit in their AOM/FOM. For us in the dash it's 36kts, which is indeed a bit of a handful.
 
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zeke
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RE: Max Demonstrated Crosswind - True Meaning?

Wed Dec 26, 2012 6:20 am

Quoting jetlife2 (Reply 5):
For turbofan engines, excessive crosswind will distort the inlet flow. Engines are tested to demonstrate their tolerance to this and this is part of the data that is used in setting the aircraft flight manual limits. Exceeding these limits risks stalling an engine, depending on the degree that the engine can tolerate beyond the certified limit, if any.

The A380 was tested to over 50 kts, from a pilots perspective due to the aircraft inertia it was not a problem at all, the people who did have a problem with it is as you suggested, the engine manufacturers for the reason stated.
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tdscanuck
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RE: Max Demonstrated Crosswind - True Meaning?

Wed Dec 26, 2012 7:31 pm

Quoting bueb0g (Thread starter):
Some people say that it really means nothing, as it's just the max xwind component they found in the flight test schedule.

It is the maximum crosswind found during the *certification* flight testing. They may or may not have hit higher during non-certification. It doesn't mean nothing, it means that the regulators found that the airplane was safe to land up to that speed and they haven't checked any higher.

Quoting bueb0g (Thread starter):
I know that this is strictly true, but these types of people are likely to say that therefore, it's fine and dandy to land above this, as long as the pilot is happy with the conditions, and that the demonstrated limit really means next to nothing.

This is absolutely not true. There are multiple limits you may run into with higher crosswinds...landing gear load, wheel load, steering authority, nacelle clearance, etc. There is no basis to assume that the airplane will be fine at any speed significantly higher than the demonstrated maximum.

Quoting bueb0g (Thread starter):
Then there are people who treat it almost as a legal limit; they won't land above the limit and would rather divert, even if they're potentially happy with the winds, because they say the aircraft isn't technically certified for those winds.

Crosswind isn't a certified limit. So it's technically true that the aircraft isn't certified for those winds, but it's not technically certified for *any* crosswinds. It's just a demonstrated performance. However, all airlines should have a crosswind limit in their OpsSpec and *that* should be the legal limit.

Quoting bueb0g (Thread starter):
But I've also heard that the max demonstrated crosswind isn't an aerodynamic thing at all - it's just the maximum wind that an aircraft can land in, with no crosswind correction, without damaging the landing gear or tyres.

There are multiple things. For most modern designs, the gear can take considerably more than any realistic crosswind will throw at it (I know one aircraft where the gear structural limit is more than twice as high as the max demonstrated crosswind)...in a modern twin, you're more likely to have a nacelle clearance problem than a gear structure problem.

Quoting bueb0g (Thread starter):
What is the correct way to think of it? Are all the above correct in some way? Until I know for sure, I'll continue treating it as a legal limit.

What's in your OpsSpec is your legal limit for operation. The max demonstrated number means that the regulators were satisfied, and demonstrated, performance up to that speed. There is no guarantee for any type of performance beyond that limit...you may have 30 knots of margin, you may have 2, there is no way for an operator to know.

Quoting Flyer732 (Reply 1):
Its all about the rudder authority, when you lose rudder authority, you've reached the maximum crosswind the aircraft can handle.

At typical landing speeds (and all engines operating) rudder authority is not very likely to be your limiting factor (you're well above Vmca and Vmcg at touchdown). Once you get the wheels down your nosewheel steering authority is going to be more important.

Tom.
 
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larshjort
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RE: Max Demonstrated Crosswind - True Meaning?

Wed Dec 26, 2012 10:17 pm

This is what can happen if you land with too much crosswind. The two arms of the fork holdng the nosewheel snapped off, they are ~1"x3" of aluminum.

The aircraft got a new nosegear, engine and prop and was flown unpressurized to Odense, Denmark ODE from Nizhny Novgorod GOJ. About 4 m2 of skin needed to be replaced together with much of the wheel well skin.


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CosmicCruiser
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RE: Max Demonstrated Crosswind - True Meaning?

Wed Dec 26, 2012 11:48 pm

"a man's got to know his limitations" Inspector Callahan

I've done 30kts in the sim and you feel like you don't have much rudder left.
 
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tb727
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RE: Max Demonstrated Crosswind - True Meaning?

Thu Dec 27, 2012 1:36 am

I did one the other day at or maybe a little over the max in the pouring rain in PVD, it was shifting and gusting as we came in. I don't think I want to do that again any time soon. I've done the max before in just about everything I have flown but in nicer weather and it's not too bad. When you start throwing in precip or a wet runway and things can get hairy quickly.
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WingedMigrator
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RE: Max Demonstrated Crosswind - True Meaning?

Thu Dec 27, 2012 5:15 am

Quoting CosmicCruiser (Reply 10):
I've done 30kts in the sim and you feel like you don't have much rudder left.

The sim isn't likely to be accurate in that regime...
 
Max Q
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RE: Max Demonstrated Crosswind - True Meaning?

Thu Dec 27, 2012 6:02 am

Quoting tb727 (Reply 11):


I did one the other day at or maybe a little over the max in the pouring rain in PVD, it was shifting and gusting as we came in. I don't think I want to do that again any time soon. I've done the max before in just about everything I have flown but in nicer weather and it's not too bad. When you start throwing in precip or a wet runway and things can get hairy quickly.

I don't think there's a better aeroplane to be in than a B727 in those conditions.
The best contribution to safety is a competent Pilot.
 
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zeke
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RE: Max Demonstrated Crosswind - True Meaning?

Thu Dec 27, 2012 9:06 am

Quoting Max Q (Reply 13):
I don't think there's a better aeroplane to be in than a B727 in those conditions.

Any aircraft with more inertia will keep tend going where it was already going,
We are addicted to our thoughts. We cannot change anything if we cannot change our thinking – Santosh Kalwar
 
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tb727
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RE: Max Demonstrated Crosswind - True Meaning?

Thu Dec 27, 2012 6:41 pm

Quoting Max Q (Reply 13):
I don't think there's a better aeroplane to be in than a B727 in those conditions.

Yeah, she took it like a champ. I added about 5000 extra pounds of fuel on since we were empty, I think it helped.
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musang
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RE: Max Demonstrated Crosswind - True Meaning?

Fri Dec 28, 2012 1:57 pm

Quoting PapaChuck (Reply 3):
I like to think of it as a CYA limitation provided by the manufacturer

I've always seen it as that also, especially in the private flying arena where manufacturers face lawsuits by people doing rediculous things in light aircraft and suing because the manual didn't specifically prohibit it. A Cessna 172 can easily deal with more than 15kts, as mentioned earlier, but Cessna won't commit to it. CYA.

In my airline, its in the "Limitations" chapter and used to be described as "Max demonstrated" but now its just "Max crosswind" (with different figures for land, t/off, dry, wet, asymmetric, contaminated). Since its in Limitations we face a choice. Pull off a landing in more than the stated limit, and no-one will ever know you're a hero. Bend something, and you knowingly exceeded a limit and were irresponsible. So as CC mentioned above, regardless of how brilliant we think we are, we treat it as a limit.

Quoting tdscanuck (Reply 8):
This is absolutely not true. There are multiple limits you may run into with higher crosswinds...landing gear load, wheel load, steering authority, nacelle clearance, etc. There is no basis to assume that the airplane will be fine at any speed significantly higher than the demonstrated maximum.
Quoting tdscanuck (Reply 8):
you're more likely to have a nacelle clearance problem than a gear structure problem.
Quoting tdscanuck (Reply 8):
At typical landing speeds (and all engines operating) rudder authority is not very likely to be your limiting factor (you're well above Vmca and Vmcg at touchdown). Once you get the wheels down your nosewheel steering authority is going to be more important.

Surely all this depends on technique. If an aircraft is landed wings level and severely crabbed, there will indeed be sideloads on the gear and wheels but no nacelle clearance problem (unless its allowed to roll downwind on contact). If its crabbed down the approach but "ruddered straight" in the flare, there will be:

(a) minimal gear/wheel sideload at touchdown because the wheels are now aligned with the runway (although starting to drift downwind),

(b) no nacelle clearance problem if its done properly as the wings remain level, and

(c) no rudder authority problem IF there was enough rudder available to yaw it straight in the first place. In this case the nosewheel would touchdown aligned with the runway and approximately aligned with the track (the drift downwind is just starting) and one wouldn't be using the nosewheel steering until down to 50 knots or so anyway, when rudder authority runs out.

There is a school of thought which supports the notion that yawing it straight before touchdown is preferable as it gives an indication of:

(i) whether there's enough rudder authority available to straighten it out, and

(ii) how much rudder you'll be using after touchdown, i.e. you get a feel for it.

The crabbed landing method means you don't know how much rudder will be needed on the runway until you try it!

Regards - musang
 
jetlife2
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RE: Max Demonstrated Crosswind - True Meaning?

Fri Dec 28, 2012 6:53 pm

Knowing that there are many readers here with all kinds of different backgrounds: private pilots, airline captains, engineers, hobbyists: My recommendation to all, for the avoidance of any doubt, is to observe all published limitations. Much thought, testing and energy goes into the selection of these limits. It is misunderstanding the process if you perceive these as "CYA" for the manufacturer. They are protecting the operation of the aircraft based on the information and testing they have. Maybe in some flight conditions there is capability beyond the limit. Maybe in others there is not. Maybe additional testing or analysis could expand that particular limit: but today that has not been done, and it is what it is.

I am not debating any one specific limit here, just making the general point.
My views are not necessarily the views of the GE Company
 
CosmicCruiser
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RE: Max Demonstrated Crosswind - True Meaning?

Fri Dec 28, 2012 7:29 pm

Quoting WingedMigrator (Reply 12):
The sim isn't likely to be accurate in that regime...

I would say ours are more forgiving than real life.

Quoting musang (Reply 16):
no nacelle clearance problem if its done properly as the wings remain level, and

For the -11 there is to be no drift angle (or crab). Unfortunately if you "rudder out of the crab without using some wing down aileron then two things happen, you're heading downwind therefore side load on gear and you're no longer on centerline something that our trg dept takes very seriously. therefore we had no option but the slip to ldg in a xwind unlike the 777 guys who don't worry about it. It has been covered before but usually from the jets I've flown the wing tip is usually the issue not the nacelle. the graph shows as pitch increases on a swept wing jet it takes less and less bank to drag the wing tip therefore in a xwind you don't hold it off hoping for a beautiful ldg. I remember a case a few yrs ago where one of our 727s caught a tip and the wind was less than 10kts!! They just held it off trying to that greaser and the wind wasn't bad. It got the dump nozzle, wing tip and outboard aileron. and no it wasn't me  
 
musang
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RE: Max Demonstrated Crosswind - True Meaning?

Fri Dec 28, 2012 8:41 pm

Quoting CosmicCruiser (Reply 18):
you're heading downwind therefore side load on gear and you're no longer on centerline

Indeed, which makes it so important to complete the "de-crabbing" ideally, just before touchdown. The longer its held off, the more it drifts downwind. And the faster its de-crabbed, the harder it is to synch the aileron input to keep the wings level.

In a big x-wind, give me an ATR or 146 any time!

Regards - musang
 
CosmicCruiser
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RE: Max Demonstrated Crosswind - True Meaning?

Sat Dec 29, 2012 1:37 am

Quoting musang (Reply 19):
which makes it so important to complete the "de-crabbing" ideally,

There's the problem...."ideally" never happens "ideally" except maybe 05% of the time and I guess that's why we aren't allowed nor taught that.

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