Demonstrated cross wind.........

[Esperanza]

I've spent the last two years discussing with a colleague the relevance of the manufacturers demonstrated cross wind. The figure given in the aircraft's POH.
I've always treated this figure as a limit. Especially with regards to student pilots. My colleague doesn't consider this figure to be a limit. He states that the only limiting factor is the the skill/ experience of the pilot at the controls of the aircraft. I can see that he probably has a valid point. However, I'm just wondering how both the CAA and the insurance companies view the importance of this figure. I would be interested to hear the thoughts of others on this matter. Has anyone out there fallen foul of either the CAA or insurance companies as a result of flying with what could be viewed as an excessive cross wind?

[FlyingForFun]

Oh dear, not this one again.

If the POH does not say it's a limit, then it's not a limit. The Cessna 152 and (older) 172 manuals explicitly state that it is not a limit.

What is limiting, though, if we're talking about a school, is the school's FOB. If the FOB says the POH number is to be treated as a limit (or gives any other limit), then it becomes a limit.

FFF
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[Human Factor]

FFF is correct. However, I would suggest that the prudent pilot thinks very carefully before trying to "demonstrate" a greater crosswind. IMHO, the best way is to think of the "demonstrated crosswind " as a pointed hint.;)

[FormationFlyer]

I was discussing this with the chap who owns 3 a/c at our club and apart from being an airline captain is also a licenced aero engineer.

He told me that the demonstrated component has nothing to do with aerodynamics or pilot abilities. The testing that results in this figure is based on landing in a crosswind using a crab approach where the pilot DOES NOT align the a/c with the runway before landing - i.e. they land with the full crab still on.

The published figure is the figure at which the undercarriage will structurally fail with that full crab still on.

This is why it is easy to land an a/c with a 'demonstrated' of 17kts in a wind with a component of 25kts in a wings level condition. Yes there comes a point where dynamics come into play, however, technique covers much of it.

As FFF says - if it doesnt say limit in the POH/FM it isnt one.

However if you fly under the rules within a club FOB and that specifies X-wind LIMITS...then they are legal limits and must be complied with.

I know one FTO where the demonstrated is 17kts but the FOB restricts instructors to 25kts and all other licence holders to 17kts.

[hugh flung_dung]

FF:
The Flight Manual phrase is "Max Demonstrated" and I have always read and been taught that it meant what it said on the tin - the max that was demonstrated during certification testing. There must obviously be a structural limit (that varies with mass) but this is not directly related to max demo'd xwind.

BTW, I believe that FAA certification before the 60s/70's didn't require xwind capability to be demo'd but that there was a requirement that handling was "safe" down to 20% of Vs.

HFD

[cavortingcheetah]

:hmm:

In the case of airline flying the sort of modis operandi rule was not to taxi if the gust factor, straight or cross, was greater than 50kts.

In the case of flying instructors, the 'limit', especially as it relates to training aircraft, appears in the handbook in order to afford instructors a convenient out when they feel that the pupil might be somewhat meteorologically challenged or when they, themselves, do not feel quite up to the sorties in the booking sheet?:cool:

[QNH 1013]

We obviously have a difficulty in that different pilots believe the "max demonstrated cross wind component" figure has been obtained in different ways. Without the manufacturer telling us how the tests were carried out we can only speculate.
My personal view is that for large transport aircraft, the figure is obtained as suggested by FF above, i.e. the aircraft is landed with crab and no attempt is made to align the u/c prior to touchdown because this will give reproducible results. I don't agree, however, that the u/c will fail at the max demonstrated figure, there will be a safety margin, but the lateral forces might well be at the design limit for the u/c. This can be easily confirmed by strain guages on the u/c during certification.
The lateral strength of the u/c is only one possible limiting factor. An aircraft with stronger undercarriage could lift a wheel (or bogie) or otherwise lose directional control beyond a certain cross-wind component.
So unless the manufacturer tells us how the figure was arrived at, we can only speculate. However, unless the figure is quoted as a limit, it isn't a limit, and some aircraft don't have any figure quoted at all.
Don't get me started on inaccurate reporting of surface wind at some airfields !

[FlyingForFun]

I don't believe FF's theory about the demonstrated x-wind relating to a landing with no correction.

I've heard this theory many times, but never seen anything written down. However, consider the Piper Seneca. I believe that most variants have a demonstrated x-wind of 17kt (please correct me if I'm wrong, though - I've only flown two variants, and not seen the manual for the others).

The exception is the first variant, which has a demonstrated x-wind of just 12kt. The difference between this and the other variants is that, in the early version, the ailerons and rudder were connected to each other, making it more difficult to use the crossed controls necessary to carry out a x-wind landing. This would seem to be counter to FF's theory.

(I don't know if the rules are different for larger aircraft, though, as 1013 suggests.)

FFF
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[VFE]

I'm with you FFF.

One is no longer naive enough about flying to dismiss a theory straight away without careful consideration but having been told during my CPL training days that the x-wind limit was the speed at which maximum aileron deflection would not be sufficient to stop the windward wing lifting I'll continue using that explanation for my students. If a limit is a limit then the point surely must be not to exceed it.... so it's all acedemic in that sense.

But coming onto max demonstrated (as is being debated)...... my understanding is that this is purley what the test pilot flew in the time allowed for testing and therefore may possibly be exceeded. I have never heard about the landing gear aspect regarding max-demonstrated and maybe I'm wrong and FF is right but, what was that age-old aviation saying about there being no old bold pilots? ;)

Perhaps someone could ask the likes of John Farley over on the Flight Testing Forum to take a peek at this thread and it's recent cousin over on the Private Flying forum as I'm sure we could all benefit from his valued insight into such a matter?

VFE.

[six_degrees]

I agree, there are a lot of different ideas about this subject.

Where I learnt to fly, we were taught that the maximum demonstrated crosswind was a figure obtained during testing, where the pilot landed at the prescribed flight manual speeds using normal crosswind technique and directional control was still maintained.

This figure - so I was told - is not limiting as an increase in approach and touchdown speed (a common technique in a crosswind landing) will provide greater control and thus directional control may still be maintained.

As mentioned previously, connected rudder and aileron controls limit the amount of 'crossed-controls' available and will thus limit the demonstrated crosswind figure.

Additionally, the authority maintained by the rudder in combination with the friction provided by the tyres on tarmac will contribute to this figure (landing on a grass strip in a crosswind tends to be ugly on the ensuing ground roll as the aircraft decelerates).

Does anyone else find this credible, or was my instructor making things up??

[hugh flung_dung]

FFF:
The Seneca 1 has a 13kt max demo'd xwind and at this XWC the last part of the approach (rudder to align, wing-down for zero drift) requires huge amounts of rudder and aileron input (getting near to full deflection). Obviously power on the upwind engine helps if the runway isn't limiting, as does a little crab.
You're correct that there's an aileron-rudder interconnect (because of the low roll response at low speed and the adverse aileron yaw) but I assume this uses a spring; it certainly doesn't limit the ability to use the aileron/rudder independantly.


VFE:
A limit based on wing lifting due to a xwind is a little difficult to swallow. Admittedly the secondary effects of sideslip in a zero-crab approach lead to a downwind roll (which needs to be countered with aileron) but the aileron response depends totally on forward airspeed. On the ground a pure crosswind will only cause wing lifting once a wing starts to lift and the ailerons will not have any effect because of the low or zero airflow over the wing.
Can you explain your theory a little more?

HFD

[VFE]

Can you explain your theory a little more?


I just spent about 40 minutes thinking about this and writing a bloomin' essay which on proof reading was complete twaddle, ha! Aerodynamics - gotta love it!

Your point about forward airspeed stands to reason in my mind so now I'm a little shaky on whether what I was originally taught is actually correct afterall! Putting my aerodynamacists hat on for 1 moment instead of 40 - I'd say this theory is actually based on the max aileron authority required to counter the applied rudder when the aircraft is being flown at vAT in a crosswind.

This kinda ties in with your posting and following all my fruitless cerebral activity I'm now going to take a nap!

Cheers, don't ask me anymore questions - anyone would think I'm an instructor...! Errr... http://www.thestranglerssite.co.uk/forums/images/smiles/100.gif

VFE.

[P.Pilcher]

As I've said before, in 1971 I had a holiday in Canada and decided to get a Canadian tourist pilot permit on the basis of my U.K. PPL. I had about 60 hours total experience then.
I checked out on a Cessna 150 and there was agentle X-wing of about 10 knots blowing. From that day to this I have never forgotton my Canadian instructor's debrief: Guess you were taught to fly by an ex-RAF instructor huh? O>K you put it down quite nicely by pushing off the drift with rudder just before touch down. Over here, we drop the wing into wind and line the nose up on the runway with rudder. You can put a Cessna 150 down with 40 kts across that way, no sweat!"
I have never tried to put a C150 down with 40 kts straight across, but those words have enabled me to land all sorts of aircraft safely in all sorts of crosswinds since. I am also assured that the technique works perfectly on B707's 737's and 777's to name but a few.

P.P.

[bogbeagle]

Well, there must be a limit to the crosswind capacity of each aircraft. We are just arguing about how that limit is determined and specified.

If I were trying to sell an aircraft which I had manufactured, I would be keen to specify as high a figure as I could. So, I would guess that when Piper quote a demonstrated 17kt, or whatever, they are stating the highest figure which they reasonably can. I would also guess that they have half an eye on the law and possible actions which may be taken against them. The court would presumably look at what it is reasonable for the manufacturer to claim. Doesn't help much, does it?

I treat the Demonstrated figure as a limit...you've gotta have some tangible figure to work with.

I haven't seen a POH that specifies a speed increase when encountering a crosswind; .....I don't say that one doesn't exist, mind you.

[FlyingForFun]

HFD,

The Seneca 1 has a 13kt max demo'd xwind...
Ah, thanks for the correction.
...and at this XWC the last part of the approach (rudder to align, wing-down for zero drift) requires huge amounts of rudder and aileron input (getting near to full deflection)
So I gather, from a colleague of mine who warned me about this when I had to fly one of these for a couple of hours. But in those couple of hours, I never experienced cross-winds anywhere close to this.
You're correct that there's an aileron-rudder interconnect (because of the low roll response at low speed and the adverse aileron yaw) but I assume this uses a spring; it certainly doesn't limit the ability to use the aileron/rudder independantly
In which case, do you know what the difference is between this and the later Senecas with the higher (17kt) demonstrated x-wind? I know from personal experience that the Seneca 5 can be landed very comfortably in a 20kt x-wind, and from that experience I would guess it could handle quite a bit more. I'm not aware of any other difference besides the rudder/aileron interconnection - but I don't know the type intimitately enough to be certain of this.



Bogbeagle,

As I said earlier on this thread, Cessna have explicitly stated in their POH that their demonstrated x-wind is not limiting. What you say sounds plausible, but isn't correct.

FFF
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[VFE]

So hands up who'd like to volunteer landing a C150 in a 40kt x-wind.

Me? Nah, I'd sooner jab a fork in my nuts.

VFE.

[FormationFlyer]

Er...yeah I chicken out at 25kts Im afraid....40kts in the cessna...with a Vat of say 60kts....I think I might be in trouble.

My comments were a merely passing on what an engineer told me. I thought it was to do with ailerons previously as well. But if its based on aileron authority at a maximum then it doesnt explain why I can land a C152 at 20kts xwind component completely wings level when the demonstrated is a poor 12kts. By placing the 'wing down' after that I am happy up to about 25. Still the aileron has the required authority - so this explaination doesnt quite make sense. Incidentally isnt it interesting that soo many different a/c come up with 17kts.. C172, PA28, Seneca II...why 17kts?

[bookworm]

We obviously have a difficulty in that different pilots believe the "max demonstrated cross wind component" figure has been obtained in different ways. Without the manufacturer telling us how the tests were carried out we can only speculate.
The following is how the FAA tells the manufacturer to carry out the tests:
(I previously posted this in another thread on Private Aviation.)
AC23-8B FLIGHT TEST GUIDE FOR CERTIFICATION OF PART 23 AIRPLANES (http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/0/469cd77d24955f4e86256da60060c156/$FILE/Final-Part1.pdf) is worth a look.
107. SECTION 23.233 DIRECTIONAL STABILITY AND CONTROL.
a. Explanation.
(1) Crosswind. This regulation establishes the minimum value of crosswind that must be demonstrated. Since the minimum required value may be far less than the actual capability of the airplane, higher values may be tested at the option of the applicant. The highest 90-degree crosswind component tested satisfactorily should be put in the AFM as performance information. If a demonstrated crosswind is found limiting, it has to be introduced in Section 2 of the AFM.
...
b. Procedures.
(1) Crosswind.
(a) The airplane should be operated throughout its approved loading envelope at gradually increasing values of crosswind component until a crosswind equivalent to 0.2 VSO is reached. All approved takeoff and landing configurations should be evaluated. Higher crosswind values may be evaluated at the discretion of the test pilot for AFM inclusion.