Demonstrated crosswind and flaps

[FlyingForFun]

A question regarding "demonstrated crosswind", if I may?

I know that, in past discussions about demonstrated crosswind, answers as to exactly how it is proven have been pretty vague. E.g. there doesn't seem to be any requirement to use one particular method of landing (wing down vs crab).

But is there a requirement that full flap should be used, as per a "normal" landing?

The reason I ask is because of some comments on this thread, regarding landing a BE76 in 20kt crosswind.

The demonstrated crosswind for this type is 25kt, so the crosswind we are discussing is well within the demonstrated abilities of the aircraft. Yet several posters have suggested using partial, or no, flap.

Would the test pilot have used full flap in demonstrating the 25kt crosswind in the BE76?

I have, in the past, used less than full flap in strong crosswinds, but I'm talking about crosswinds well in excess of that "demonstrated" by the manufacturer (e.g 20kt+ in a C152, which has a demonstrated crosswind of 12kt, and starts getting a bit tricky above 17-18kt).

FFF
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[Piltdown Man]

I'd suggest that any conditions requiring non-standard operating procedures will be clearly highlighted in the AOM. If they ain't there, it's bar talk and total bollocks!

[westhawk]

Originally Posted by FlyingForFun

Would the test pilot have used full flap in demonstrating the 25kt crosswind in the BE76?

That's a good question FlyingForFun!

I had never thought of this, so I went looking to see if the FARs for certification of part 23 airplanes gave any hint. The only reference I found regarding max demonstrated crosswind capability is quoted below. This is from a section regarding operational information which must be included in the AFM. No mention I could find in the certification flight test guide either.

Originally Posted by FAR part 23

§ 23.1585 Operating procedures.
top
(a) For all airplanes, information concerning normal, abnormal (if applicable), and emergency procedures and other pertinent information necessary for safe operation and the achievement of the scheduled performance must be furnished, including—

(1) An explanation of significant or unusual flight or ground handling characteristics;

(2) The maximum demonstrated values of crosswind for takeoff and landing, and procedures and information pertinent to operations in crosswinds;

Drats! No clue as to whether any particular flap configuration is required when determining the max demonstrated crosswind. But since procedures and information pertinent to operations in crosswinds is required,it seems prudent to presume that it was determined using whatever procedures and techniques are recommended in the POH or AFM for crosswind ops.

I would be inclined to believe that your difficulty with the crosswind landing was, at least in part, attributable to the effect of having the rudder trimmed into the operating engine. This effect is excacerbated if the operating engine is on the upwind side. When you reduce the throttle, it's like stepping on upwind rudder. Not what you needed! You'll be better prepared now that you have experienced it and discussed it. Next time, you'll anticipate this effect and allow for it.

Much success to you in your training.

Best regards,

Westhawk

[EGBKFLYER]

I'm interested in this area of enquiry too.

I fly 6 different single engine types regularly, of which 4 are flapped. In none of the POHs does it suggest that less than full flap should be used when landing in crosswinds.

My question is why some teach the use of partial flap for strong crosswinds? What advantage is gained and what is the perceived risk if full flap is used?

[FlyingForFun]

Thanks for the comments. I will check tomorrow whether the POH says anything about use of flaps in a crosswind, although off the top of my head I don't think it does, which, it seems, would suggest that 25kts should be quite do-able with full flaps with both engines operating.

However, Westhawk's comments about the effect of the asymmetry on the landing are very valid. (Just to clarify, although it's not relevant - I am in no way connected to the original question in the other forum, and it's not me that's experiencing the difficulty - I was just reading the other thread and wanted to clarify some points re. the replies.)

EGBK - approaching flapless will universally require a higher approach speed, and the higher the speed, the less effect a crosswind will have. (Use your CRP1 to prove this, or simply draw vector triangles on a piece of paper if you prefer.) There might also be some extra control authority with less flap, although this will be type dependant (and I have also heard that the oposite might be true on some types, where at least partial flap will improve aileron authority).

FFF
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[High Wing Drifter]

FFF,

Like you I have often pondered why. I arrived at a pretty similar conclusion; the combination of extra drag and low speed mean you need to crab more, and more crab means more rudder authority is required for roundout. But even that seems a little weak as we're talking about one single degree for what would be a 20deg crab at 76kts with a 25kt component.

I suppose I can understand why you would want to take off some flap in gusty conditions, but not xwind.

[Genghis the Engineer]

All normal flight test and certification practice is that the maximum demonstrated crosswind was demonstrated using the technique recommended in the POH.

This doesn't mean that you can't use a different flap setting, different technique, or operate at greater crosswinds - only that you'll either be going against the advice (not instruction) or outside the testing done during certification.

G

[westhawk]

FlyingForFun:

My apologies for confusing you with the poster in the "other" thread!

The advantages and disadvantages of using full, reduced or no flaps for landing in crosswind conditions will certainly vary according to many factors. Specific aircraft type handling characteristics at various loadings, runway dimensions/surface conditions and obstacles may influence the decision as to which setting is most appropriate for that flight.

Where no guidance is provided by the AFM/POH, the decision of which setting would be best to use is left to the individual pilot based upon their own knowledge and experience both generally and with the specific type concerned. I did not find any recommendations regarding crosswind technique in my old BE-76 manual. My Cessna manuals are more descriptive, in that specific recommended techniques and considerations for crosswind ops are provided.

Generally speaking, in my personal experience, I find that tricycle gear equipped aircraft which must be slowed significantly from approach speed prior to allowing it to touch down in order to "protect" the nosewheel are more comfortably landed at a reduced flap setting in gusty and/or crosswind conditions. Half flaps has normally worked very well for me in the Duchess, Seneca and all trike gear singles I have landed while faced with any notable crosswind component. I have always found it very instructive to take students out on a good crosswind day and have them practice landings at different flap settings to experience it for themselves.

This is an excellent subject for discussion since there are a great many factors to consider and prioritize according to the specific aircraft, pilot and situation.

Best regards,

Westhawk

[Tankengine]

Less flap = faster approach speed = less % of xwind versus fwd speed [ie: less crab]

[EGBKFLYER]

Obviously still missing something here...

Westhawk - if you say that less flap = higher approach speed, surely that doesn't square with your argument that using less flap in gusty/ x-wind conditions helps with the difference between app speed and touchdown spd (i.e. reduces it, if I understand your post?)

Tankengine - your theory I understand except that for the aircraft I fly, the gain in approach speed is in the order of around 5kts, which makes next to no difference to crab angles in my view (about 3 degrees witha 25kt component between 65 and 75 kts).

I'm still concluding that other than increasing app spd by a margin to allow for gustiness, small singles can be flown with full flap as normal, whatever the wind conditions are.

[7FF]

One reason for using reduced flap on a 744 is not because of the cross wind, it has more to do with the wind gusts associated with strong winds.
Using full flap 30 in gusty conditions requires a Vref additive and as the 744 has a huge inertia wind gusts are seen on the PFD. This means that the operation can become very close to the flap load relief limit. Therefore, if the flap load relief operates the flap automatically reduces to flap 25. That's fine until the flare and as the IAS reduces the flap can reposition itself to 30 causing a balloon and touchdown further down the runway.

[EGBKFLYER]

Very good reason if you fly 747s then, which unfortunately I don't . The same explanation is clearly not valid for my little spam can, no matter how gusty it gets.

[westhawk]

Originally Posted by EGBKFLYER

Westhawk - if you say that less flap = higher approach speed, surely that doesn't square with your argument that using less flap in gusty/ x-wind conditions helps with the difference between app speed and touchdown spd (i.e. reduces it, if I understand your post?)

My point about the advantage of using less flaps when landing most light aircraft in a crosswind was related to the pitch attitude that goes with each flap setting at approach and touchdown speeds. Generally, the less flaps used, the higher the pitch attitude will be at both approach and touchdown speeds. Less change in airspeed during the flare and less change in pitch attitude is required as a normal consequence. The nose was allready nearer the touchdown attitude before flaring. This also allows the touchdown to occur with the nosewheel further above the runway while still retaining a reasonable stall margin to account for gusts. Once the mains are rolling, lowering the nose reduces AOA and lift, allowing weight to be transferred from the wings to the wheels. The chance of lifting back off are now reduced and the tires can now assist in controlling drift.

With full flaps, you must either land flatter or slower, often both. A larger pitch change is usually required in order to achieve an acceptable landing attitude when using full flaps, especially if approach speed additives were applied. A larger change in airspeed during the flare is required. Once on the ground, weight on wheels will be less with full flaps once the nosewheel is in contact with the runway if the landing was flatter because the nose will be closer to the runway at touchdown and the AOA cannot be reduced as much before the nose tire makes contact and prevents the nose from being further lowered without taking weight away from the mains. The wings are still producing plenty of lift with the flaps down and the plane is light on thr wheels. The flaps could be raised, but this not generally perceived as a good habit in complex aircraft due to a history of people inadvertantly raising the gear instead of the flaps!We've all seen both the flat landings with porposing, and the very slow landings with the associated drift and directional control problems. Not to say that everyone will have these problems, just that it is rather common.

Common crosswind landing errors such as wheelbarrowing, heavy side loading or skidding and porposing are seen more commonly when full flaps are used than when reduced flaps are used. The most common error exhibited in crosswind landings is the use of excessive airspeed, no matter what flap setting is used! I simply say that in my personal experience, using an intermediate flap setting increases the margin for error somewhat, and generally better results are achieved by most pilots of light aircraft in my observation.

Also keep in mind that some aircraft AFMs do contain recommended crosswind landing techniques which include the suggested use of reduced flap settings. The reasons for this probably include what I've stated above, as well as control effectiveness considerations and perhaps some other factors I have omitted.

Let me reiterate that different aircraft, different situations and different pilots may justify or even demand differing techniques. However, in general, I find that reduced flap settings provide for more consistent results for most pilots under most circumstances when performing crosswind landings in most light GA/trainer airplanes. There are bound to be exceptions! I encourage practicing at all flap settings in all aircraft flown, as much as the situation allows.

I do hope this clears up any confusion my earlier post may have induced, even though I can see by reading it myself that this may not be the case. This is difficult stuff for me to convey on the internet! I'll keep working on that.

Best regards,

Westhawk

[A37575]

In light singles or twins the crosswind technique should be the same regardless of less or more flap. More flap (full flap, that is) gives you the advantage of less float and that is most desirable. Less flap will naturally cause a prolonged float which is precisely what you don't want. Not only does it eat up the landing length from over the fence but it exposes you to more chance of getting a gust and you will have to continually increase aileron and rudder movement as the aircraft slows up in the air while floating until worse case you run out of both controls just before touch down. Also increased chance of wheel-barrowing if you have to force the aircraft on the ground due long float with less flaps.

[EGBKFLYER]

Westhawk - thanks for that - now much clearer and I agree that it's horses for courses. At least I can explain a little more about the thinking behind the reduced flap approach technique now...

A37575 - I don't agree that less flap will definitely cause more float. It will if you come over the threshold at a higher than required speed, which most folks seem to do. I think yours is a good description of most average light aircraft pilots' attempts at flapless landings, regardless of what the wind is doing!

[barit1]

Originally Posted by Tankengine

Less flap = faster approach speed = less % of xwind versus fwd speed [ie: less crab]

As long as runway length is not limiting, this is good news.

Regarding "float" w/no flaps: The flapless attitude should be "nose high" compared to what a pilot may be used to. However some pilots may fly attitude more than IAS, and adopt the more familiar nose low attitude, thus coming over the numbers faster than intended - with consequent float.

[westhawk]

Originally Posted by A37575

In light singles or twins the crosswind technique should be the same regardless of less or more flap.

Perhaps my choice of the term technique to describe the tactic of using less flaps during strong or gusty crosswind operations should be reconsidered. Done!

Simply stated, minimum touchdown pitch attitude must be achieved prior to touchdown. This attitude can be reached with less pitch change in the flare and at a higher airspeed without float when less flaps are used. Rate of deceleration is reduced and, as a consequence, so is the difference between approach speed and touchdown speed as long as appropriate airspeed control is exersized on approach.

Very low airspeed at touchdown, while often desirable in many circumstances, is certainly far less so during crosswind and/or gusty conditions. The slower the touchdown speed, the less control authority is available from the flight controls, so the greater the displacement required to maintain drift and alignment control. It is more likely that full flight control displacement will be reached under these conditions than if the aircraft is touched down at the higher airspeed allowed by the lesser flap setting.

Generally speaking, reduced flaps will allow the aircraft to remain controllable with higher crosswind components than is the case when using full flaps. The actual maximum crosswind component at which the airplane can be controlled acceptably is so largely dependent upon the skill with which the individual pilot employs their chosen technique and tactics that the phrase Horses for courses as used by EGBKFLYER does seem apropos!

I would never presume to say that my viewpoint is the only right one. Instead, I simply present my observations, and the reader is free to use them as they might. It is my hope, as always, that discussion of such matters inspires further thought and understanding for all of us.

Best regards,

Westhawk

[barit1]

Well put, Westhawk.

With a taildragger, it can be good to try to steer for the downwind side of the runway, so that when weathercocking occurs you have some room to manuver.