Reduced Flap setting in Gusty x-Wind Conditions

[jumbojohn]

I thought that I know why we landed with less than full flap in gusty x-wind conditions, then I read this months "Air Brained" in Pilot Magazine and now I'm not so sure? Any offers?

[essouira]

I don't have the article but I'm fascinated ! can you give us a brief idea of what it said ?

[bookworm]

An approach in turbulent air with a gusty crosswind may make the use of partial flaps preferable. The reasons for this are than with less than full flap the aircraft will require less of a change of pitch to establish the correct landing attitude.

I agree with the assertion that partial flaps can be preferable, but I don't think that's a good reason. I'm not even sure that it's true -- it looks to me as if in general it requires a smaller change in AOA to provide the same change in speed with full flap. It's all very well claiming, as it does elsewhere in the answer, that the touchdown should be made in a "level flight attitude", but if you touchdown while still at flying speed, it's going to get interesting in many aircraft.

My understanding was always that some aircraft had better aileron authority with only partial flap.

[Kermit 180]

Pitching the nose up requires greater elevator control input with flaps deployed, compare flaring with and without flap and see the differences for yourself.

I thought the reason for deploying partial flap as opposed to full flap was for stability, especially on a high wing aircraft where the full wing surface is exposed to the relative airflow (from the front and side in the case of a crosswind). With full flap the majority of the lift envelope is around the mid area of the wing, making the aircraft less stable than it would be if the envelope was evenly spread across the whole wing plan. I do not agree entirely with this concept for low wing aircraft, where the wing on the downwind side is sheltered by the fuselage. Also using less than full flap will aid penetration through gusting winds and wind shear by using a higher approach speed. Also I would not endorse landing flat. I have never had any problem with landing any aircraft in a crosswind with the nose wheel clear off the ground or tail down in a three point attitude.

Kermie

[FormationFlyer]

Hi folks,

Interesting this...

I use full flap for the most part...however...if conditions get *extreme* I will use a flapless approach and use a higher airspeed.

Why? Because higher airspeed means more control. In gusty conditions you add half the gust factor anyway...

As for ' if you touchdown while still at flying speed, it's going to get interesting in many aircraft. '.

Whats the reasoning behind this statement?

If you insist on stalling it in the flare YOU ARE ASKING FOR AN ACCIDENT. Why? Because you have nothing left to control the aircraft with - all the controls are fairly ineffective, use of aileron could lead to tip stall and digging a tip in. In X/Wind & Gusty conditions you dont want to hold a flare off....you want the wheels on the ground fairly pronto to give you better directional control (nosewheel a/c at least).

I will also add that the crab & kick straight technique is also a good way to cause undercarriage stresses during cross-winds - especially more extreme crosswinds...Personally I use a combination technique - thus landing wings level with no additional control movements to make when the controls are at their most ineffective.

Hope this helps,
FF

[bookworm]

As for ' if you touchdown while still at flying speed, it's going to get interesting in many aircraft. '.

FF asked

Whats the reasoning behind this statement? If you insist on stalling it in the flare YOU ARE ASKING FOR AN ACCIDENT.

A good point FF, and I wasn't suggesting stalling it in the flare. But the assertion that you should touch down in "level flight attitude" in the original text doesn't make a lot of sense. Level flight at what speed? If, as I suspect, it is proposing touching down on all three wheels at the same time, then that attitude may not correspond to a speed that is conducive to staying down.

Kermit 180

An interesting point about stability but wouldn't it work the other way round? On a high wing, the lateral stability comes from the interference of the airflow with the fuselage when the aircraft slips. If the lift is coming predominantly from close to the fuselage, the effect would be more pronounced rather than less. The reverse would be the case on a low wing.

[Numptoid]

A reduced flap setting is appropriate for crosswind or 'gusty' conditions for the following reasons:

1. Ensuring adequate control over the aircraft, sure. And a positive touchdown on a sensible part of the runway (ie, not at the other threshold) is appropriate. Techniques for tricycle / tailwheel / monowheel differ - always refer to FM/POH for guidance.

2. Stability. The lateral stability of the aircraft is REDUCED with flaps lowered due to the change in spanwise lift distribution (more lift near the CG) - hence less or no flap is a consideration.

3. The stalling angle of attack (critical angle) generally decreases for conventional (subsonic) aerofoils as flap is lowered (although coef. lift increases). Thus you have reduced protection from the dreaded 'vertical gust.'

It is important to appreciate that a crosswind and 'gusts' (or windshear) are separate issues although often occur together to make life interesting. When using reduced or no flap it is important that we can still comply with FM/POH data for LDR. This may not be possible if excessive increments to Vref/Vat are applied indiscriminately 'for mother.'

Hope this helps.

[FormationFlyer]

Bookworm.

Indeed. I personally discourage a 'level flight attitude' (or 3 point nosewheel landing)!. When I say wings level I mean precisely that...touch down on two main wheels at the same time. I naturally assumed everyone would protect the nosewheel and would therefore be in the 'normal' landing attitude - (sorry if I confused anyone) - as opposed to a wing down one wheel first landing which must be used when the airplane is no longer capable of slipping enough wings level to maintain straight flight down the centreline.

Oh and a word on wing down technique...it doesnt encessarily mean the wing is 'down'. As I just said unless the X-Wind becomes extreme the a/c actually remains wings level - even though you have aileron applied.

This has been a *really* interesting thread. Cheers folks!

Hope this helps,
FF

[bookworm]

Numptoid

3. The stalling angle of attack (critical angle) generally decreases for conventional (subsonic) aerofoils as flap is lowered (although coef. lift increases). Thus you have reduced protection from the dreaded 'vertical gust.'

Not sure I follow this. At a particular lift coefficient, what would seem to matter is the difference between the AOA required in that configuration and the stalling AOA. From the plots I've found in Abbott and von Doenhoff, it looks as if there's slightly more margin with flap than without. Certainly the stalling AOA decreases, but the AOA for approach speed is lower still.

FF

Oh and a word on wing down technique...it doesnt encessarily mean the wing is 'down'. As I just said unless the X-Wind becomes extreme the a/c actually remains wings level - even though you have aileron applied.

Now you are confusing me. How can you have aileron (and presumably opposite rudder to stop it turning) applied with the wings level?

[Cardinal]

Something that can be taken from the airline world is the possibility of a tail strike. Without flaps the nose will be higher to generate the same lift coefficient. Obviously a normal approach in most trainers doesnt create a problem in this regard, but if it's botched it gets interesting. The wind was howling yesterday, and I recommended a no flap landing to my student. Used to teaching landings with a normal flap setting, when my student flared a little high I didn't react because I'd "rode out" arrivals like this one before. The airplane came down hard on the mains and whacked the taliskid hard as well. I didn't make the connection, so we went up and tried it again, with the same loud result. I know, stupid move on my part, but something to consider.

[Ivan Ivanovich]

Another reason we use less flap for a gusty approach is for the reduced drag. Should a drop in wind occur the aircraft will lose lift. Power will be required to limit the sink. If max flap - and therefore max drag - is used then it will take longer for the aircraft to respond and recover. Just what you don't want on short finals.

[STATLER]

Alot of people seem to advocate the reduced flap settings in gusty conditions but i am not convinced.

1)correct me if i am wrong but arn't the landing performance graphs in the pilots operating hand book devised on the whole using full flap.

2) what if the strip is field length limiting for the aircraft type,to get in you must use full flap.

[FormationFlyer]

Statler: 1. Yes. 2. your call.

You must remember that there are many circumstances where it is not wise to make an approach. For instance a *short* but soft ground strip - two opposing techinques are required, the short field app and the soft field app - the two cannot really be combined and therefore if you find yourself field length limited in this situation then you gotta make the call...but avoiding the situation would be a good move.

If you go into a short strip you use a short field technique...this technique involves you coming in on the approach with LESS margin above the stall. A severe gust could *stall* your aircraft. Not good. Therefore you then apply margins (0.5*gust) - remember that this increase in airspeed is also NOT covered in the performance charts - therefore you must allow suitable factors. I think somewhere there is a pink AIC that details sensible ones - something like a 10% increase in speed requires a 10% (or is 20%?) increase in landing distance. However, by doing so you may find yourself without enough runway - so just dont go. There is *no* situation that requires a planned landing at a runway that is too short.

So...golden rule is take ALL conditions into account. If you insist in perform a short field landing to get into your short runway in gusty conditions you could well be endangering your aircraft, your life and the lives of others near your approach path...not worth the risk.

Hope this helps,
FF

[Tinstaafl]

Related topic here

[bluskis]

Tinstaafl I followed your link.

Beardy had an explanation which confirms my findings that an approach in gusty conditions is much more comfortable with reduced or no flap.

My thinking was that the effect of a gust was unlikely to be equal on both wings, hence the unequal increase in lift is more pronounced, causing a stronger roll, with flap than without.

The caution against making a flapless approach into too shorter a strip is of paramount importance.

[Turbine]

Load Factor with flap!

Decreased flap is only ever required when the crosswind (or any wind for that matter!) is accompanied by an associated turbulence for the following three main reasons.

1. The maximum load factor for most aircraft decreases with flap…

For example, a PA38 has a maximum load factor that decreases from 4.4 to 2.0g with the application of full flap. If you are on a turbulent approach, regardless of the crosswind component, common sense suggests that you should fly the aircraft in a configuration in a way that allows the maximum load to be safely applied upon it.

Prolonged strong gusts on the aircraft with full flap can possibly cause structural damage – even if that damage is only contained entirely to the flap system itself.

2. Reduced Angle of Attack

The reduced AoA with reduced flap in anticipation of a relative gust of wind from under the wing causing a stall is certainly a consideration, albeit not a major one. The main consideration once again is a preventative measure against overstressing the aircraft. (Increased AoA as a consequence of change in RAF = Increased Load Factor… and when flap is down on the aircraft, it will tolerate a lesser LF).

3. Increased Controllability

Additional speed on the approach is simply to assist with the control of the aeroplane in turbulence. With reduced flap the approach speed is normally increased as a product of 1.3V(new)s.

You could perhaps argue that by coming in faster you are subjecting yourself to the crosswind for a longer period of time? The advantages far outweigh this disadvantage assuming the correct technique is applied.


Note that the deceased flap is only ever a possible consideration in turbulence. A strong calm wind in itself is absolutely no justification for reduced flap unless it’s a general consideration (i.e. collision, damage, rough strip, flap failure etc…)

As for stalling onto the runway in a crosswind! You’re altering the flying characteristics of the aeroplane fairly dramatically in awkward conditions and will result in a degree of uncontrollability. If you do this out of ground affect it can make for a very unpleasant ‘arrival’ indeed.

I worked at one school that stated flap was always required, and when available, on a landing “to ensure high drag and high RoD in the flare so as to minimise the effect of the crosswind”. I always debated this philosophy since I was a firm believer of applying a technique that fit the conditions – and using the Ops manual as an eduacted guide.

The above technique suggests that by coming in fast you subject yourself to the crosswind for a longer period of time – NOT that this is a problem for most aircraft, BUT will one-day cause concern if you use the technique indiscriminately.

I know of one situation in Central Australia where a company actually had an STC approved for the operation of an aircraft into a gravel strip without flap even though the POH implicitly implied that under the circumstances it wasn’t permitted. After much red tape, revised TOLD, laising with the manufacturer and flight testing it was approved.


There was a recent article written on crosswind technique in an Aussie Aviation mag as well, jumbojohn. The author is a newly christened Commercial Pilot and I personally think that she doesn’t have the instructional skill, technical knowledge, commercial exposure or experience to write on any operational subject with any authority.

The author was writing about a well-known and very well regarded Sydney flying school and their teaching techniques. I don’t think she came close to articulating the Instructor’s message. Although the article was in the interest of safety I would argue that it could have quite possibly had the opposite effect. This mag has some serious issues to address if they want a serious readership and need to revaluate their expertise and maybe even their duty of care before they continue with such widely read dribble.


Fly in compliance with your Flight Manual, Approved Pilot’s Operating Handbook (POH) or company SOPs (or Part B). Don’t apply a technique that hasn’t been tried, tested and approved by an approved mechanism.

[Send Clowns]

If you put flap down then stability with respect to a crosswind gust is reduced for one additional reason not yet mentioned. A gust from an angle to one side will lift one wing because it will hit the whole of that wing, but the other wing will be shielded by the fuselage an only encounter the gust at the tip section. If flap is used it increases the lift on the inboard section of te wing only. Therefore the imbalance in lift will be greater, as this is the area not producing additional lift on the down-going wing.

Turbine is correct in that in a steady crossswind full flap is a better technique, for thereason he gave. The critical period is the time between kicking off drift and touching down. At this time you will start to drift sideways, so you want to minimise the time spent in this condition.

[slim_slag]

The critical period is the time between kicking off drift and touching down. At this time you will start to drift sideways, so you want to minimise the time spent in this condition.

Isn't this avoided by side slipping instead of crabbing?

[Turbine]

Slipping v's Crabbing

To those that made mention of it (including the author of the article I mentioned above )…

If you establish yourself nice and early on in the approach in the slipped condition, then you won’t have to worry with applying any crosswind technique on the landing. Can’t say that I entirely agree.

First. Why on Earth would you sideslip the aircraft on the approach and then ‘into’ the flare rather than instead crabbing the aircraft and applying the proper crosswind condition?

Crabbing minimises the drag on the aircraft on the approach. What happens if you get an engine failure and find yourself with the controls all crossed up? What happens, given the turbulent and gusty conditions, if you require additional power but you are already using extra thrust to compensate for the additional drag of a slip? Applying crossed controls to the aircraft in turbulent conditions can also apply a significant stress on the aircraft since it essentially twists the aircraft like a tin can. Light aircraft should be flown (and landed) in a way that minimises the stress on the aircraft.

Some aircraft, like most of the Cessna series, don’t like prolonged slips as well. If you set yourself up in a slip on a 172 on very early final you could well starve the engine of fuel in a slipped condition!


Many believe that if they slip down the approach then you won’t have to apply a crosswind technique. This is incorrect. As you flare and the speed of the aircraft decreases, the aircraft will drift (and yaw) with wind as the amount of control you applied becomes less than what is required to keep straight.

The only advantage of slipping is you don’t have to think about what aileron and what rudder is required. This concern can be entirely eliminated with proper and competent instruction. It’s hardly an ‘advantage’.


Aileron arrests unwanted drift and rudder keeps the aircraft straight. Simple.

[slim_slag]

Turbine

First. Why on Earth would you sideslip the aircraft on the approach and then ‘into’ the flare rather than instead crabbing the aircraft and applying the proper crosswind condition?

We were not taking about the approach, but seeing as you asked.....
I would crab down final and start slipping on short final. Better for the passengers and better technique. I believe in teaching students to slip all the way, so they get to see the effects of aileron and rudder.

What happens if you get an engine failure and find yourself with the controls all crossed up?
Best glide, landing spot, memorised checklist. Controls would be 'uncrossed' rather early in that process, probably automatically.

What happens, given the turbulent and gusty conditions, if you require additional power but you are already using extra thrust to compensate for the additional drag of a slip?

I have plenty of extra power available in the pattern. I don't get that far from the runway, usually I am power idle from abeam the numbers. You may do differently of course, and standard teaching is to power the plane in. Even if you go power to 1800 abeam the numbers and keep that all the way - not that I would - you still have plenty of power. Besides, it takes zero time to go from uncoordinated flight to coordinated flight, it's not an issue.

Even so, it doesn't matter, you are on the same glideslope whether crabbing or slipping, and in both cases if your engine quits you will either land on the runway or not.

Applying crossed controls to the aircraft in turbulent conditions can also apply a significant stress on the aircraft since it essentially twists the aircraft like a tin can. Light aircraft should be flown (and landed) in a way that minimises the stress on the aircraft.

Airframes are incredibly overengineered and certified to handle stresses far higher than you will encounter in a side slip.

I agree with landing stresses being minimised, the gear should be looked after especially well, that's the weakest point.

Some aircraft, like most of the Cessna series, don’t like prolonged slips as well. If you set yourself up in a slip on a 172 on very early final you could well starve the engine of fuel in a slipped condition!.

If you arrive at your destination with that little fuel you are negligent. Probably an old wives tail anyway, and I said you should crab on early final. Slipping is for landing.

Many believe that if they slip down the approach then you won’t have to apply a crosswind technique. This is incorrect.

The slip is the crosswind technique.

As you flare and the speed of the aircraft decreases, the aircraft will drift (and yaw) with wind as the amount of control you applied becomes less than what is required to keep straight.

Indeed, you dynamically change your control input according to the conditions, its called flying. In practice, by rollout, you will have full aileron control input towards the wind - well you know what I mean

The only advantage of slipping is you don’t have to think about what aileron and what rudder is required.

Do you mean "The only disadvantage of slipping is you have to think about what aileron and what rudder is required." ????

This concern can be entirely eliminated with proper and competent instruction.

Yep.


Aileron arrests unwanted drift and rudder keeps the aircraft straight. Simple.

Indeed it is. That's why it's the proper configuration to be in during the flare. You gotta think about your landing gear, they tend to collapse if you apply nasty side forces to them, or more likely your tyres might just part company. You don't need either. If the tyres stay attached, they are going to be pushing you off the runway towards the upwind side, which is also the direction you are pointing, and you are at risk of ending up in the dirt. It's a very very bad idea to crab when landing, crosswind or not.

Gusts just need a bit more aquired skill. More fun too!!!

Anyway, to the thread, nothing wrong with using less flap when its gusty. The reasons you gave seem sound, and it seems to work better in practice too I'd say.