Add 5 Kts for X-Wind Landing??

[Ace on Base]

I dont know if this has been done before, but I am curious as to where this practice came from, why, and the aerodynamic benefit from additional speed on Vref.

I am led to believe it was a standard practice for a major (now kaput) Australian Airline, if so, why? Also, what other airlines use this practice?

Was or is there an advantage for adding 5 kts for a Jet, If so, is there an advantage for adopting this practice in prop driven twins (turbine or other)? Is this practice adopted by crew of turboprop aircraft over 5700kg (ie metro, saab, braz etc)?

Does this practice have anything to do with wind shear possibility close to Vref, spool-up times, blanketing effect of the wing on the lee side of the fusalage, or to cater for wind gusts? Additionally, what was the x-wind components that would warrent an increase in Vref ie wind speed and angle to direction of flight path?

I am really interested in the reasons and theory behind this practice, I do recall reading somewhere (possibly in "handling the big jets") about this practice. I dont have a copy handy, so if someone could advise as to the concept behind - this would be appreciated.

Thanks,

Ace:confused:

[Capt Fathom]

Wasn't around when I trained and never heard of it since! :confused:

[Ace on Base]

Capt. Fathom,

Yes, well, it is not sitting right with all those years, hours, and instructional experience that have pocketed. :(

I have basically tried to "cut-up" all the aerodynamics theories to resolve this one, and still find that it cant or isnt fitting - I am not saying that I am right (before I get pounced on by the knowledge police :ouch: ), but the argument that was poised to me is that the higher the speed, the higher the drag (yes), so the faster the deceleration and less effect the X-Wind has on the Aircraft - And I say "THATS UDDER :mad: CR@P!!"

I think that the increase in Vref was for Wind Gusts - 1/2 the diff between wind and gust strength should be added to Vref, but I cant be certain - ATPL theory is but a distant memory! :{

Anyone else wanna throw their hat in the ring??

Cheers

Ace :)

[404 Titan]

Ace on Base

I only add knots to Vref or should I say Vapp (an airbus thingy) for windshear. Unfortunately it is quite a regular occurrence in Hong Kong when the wind is coming in from the South over Lantau Island at more than 10 kts.

[Ace on Base]

404 Titan,

I only add knots to Vref or should I say Vapp (an airbus thingy) for windshear. Unfortunately it is quite a regular occurrence in Hong Kong when the wind is coming in from the South over Lantau Island at more than 10 kts.

Is this a Company SOP Thingy, or
an Airbus Flight Manual thingy, or
a pilot thingy?

What are the circumstances, ie what wind strength and X-Wind component induces a decision to increase Vref (Vapp)?

What is the formula you use to calculate the increase, or is it a SWAG (Scientific Wild @rse Guess)?

Cheers

Ace

[MAX]

I fly Boeing aircraft and Mr boeing has us doing the following:

5 knots is added to Vref always and becomes the bug speed. A target speed if you will that can be bled off in the touchdown.

When its a little more windy half the constant head wind is added and then all the gust, if needed, as explained previously. Only up to +20 mind.

Actual Vref (full flaps) does not change.

I cant be bothered reading the manuals but when the aircraft has a 45knot crosswind and its gusting there can be quite a bit of veering/backing and the target speed gives you some buffer as that airspeed WILL be changing.

All the above is SOP in my company but is preached by Boeing (for the type).

Hope this helps?

MAX:cool:

[Ace on Base]

MAX,

Thank you for your reply, yes now that you mention it - half the headwind component and all the gust sounds farmiliar. But I think this is in the event of a reduction in headwind or to counteract wind shear when you have a reasonably heavy a/c that will take a fair power change to get moving again (also to keep a safety margin over spool up times?)- is this a fair assumption?

Has your company (or Boeing) have a SOP for an increase in Vref speed (flaps full) for the soul reason of X-Wind Component?

I am trying to get my head around why an airline (or anyone for that matter) would carry an additional Ykts to overcome Xwind component?:ugh:

Perhaps there is a reason for it and I am trying to establish why!

Please advise.

Ace:hmm:

[MAX]

Ace,

Yes a fair assumption. Some companies may fly a constant groundspeed but this will prob come down to there SOP's?

After a quick brouse I can see nowhere that Boeing suggests increasing bug speed for a crosswind. In fact for a constant 90 degree wind the add on is correctly zero.

There is the standard 5 knots always added. The rest is added headwind component and gust. If for example the wind is 45 degree the headwind from that is added accordingly.

Vref full flaps does not change.

Crosswinds dont seem to be a prob.

MAX:cool:

[DirectAnywhere]

I hate to be anal (but I'm going to do it anyway)!!

In any crosswind - even at 90 degrees - there will always be a headwind component as a result of the fact the aircraft is cocked into wind.

Someone with an FMS or Jepp computer could work out how much it is, but at 35 kts. with a Vref of 130 kts I reckon it'd be 6 kts, give or take, as headwind.

Have a look at your GS relative to your TAS next time you're flying at right angles to a jetstream.

[Capt Fathom]

Effective TAS = TAS multiplied by the (COS of the Drift Angle)
Eg. Wind at 90 degrees abeam at 100kts, TAS 450, = 12deg drift.
450*(Cos12) = 439kts effective TAS.
Rivetting stuff I know!

[A37575]

There is no Boeing requirement to have additives purely because there is a crosswind. In light winds it is recommended that 5 knots be added to the calculated Vref speed and this additive is bled back approaching touch-down. Boeing do not define the term "approaching touch down." Practically speaking this means crossing the threshold on Vref.which is perfectly safe as Vref has safety factors built in. Crossing the threshold at Vref was normal Boeing procedure when the 737 and 727 were certified, but over the years there has been a tendency for pilots to hang on to the headwind additives until the flare. This habit extends the landing distance which is OK on a dry runway but not good technique for a short wet runway.

[haughtney1]

I cant honestly remember when there was a time when I had an exact 90 X/wind...hence its the old rule of thumb 1/2 the head wind component...and all the gust up to 20 knots.
Orrrrr a quick check of progress 2 in the scan at about 200' radio.



:)

[MAX]

I took the 90 degree quote directly from the Boeing manual. Their pilots fly it better then me so I aint gonna be anal.

MAX:cool:

[Sunfish]

From a light aircraft perspective:

The Whitts flying web page alleges that the FAA practical test standard for a normal landing is Vref +- 5kts.

Shortfield and soft field approaches 1.3 Vso +10 -5 kts plus half the gust factor.

Whitts again alleges that in strong crosswinds you should add some speed so that your control surfaces (mainly rudder) have more authority. The maximum crosswind (as opposed to the demonstrated) the aircraft can handle is reached when the controls do not have sufficient authority to hold the aircraft straight.

[belowMDA]

Ace, just to further clarify what MAX has said the 5kt increment has little to do with spool up times. At all times on final approach ideally you would have the engines spooled up above 50%N1 (give or take) This is especially the case when getting closer to the ground. The engines should be spooled up regardless of what speed Vref+(increment) is.
This may not be the best explanation but it is how I see it at this time of the morning:}

[DirectAnywhere]

I agree the Boeing books say you don't have to correct for it and the only time I've seen it is in the sim (training capts must have their little bit of fun)!!:)

[Ace on Base]

Thanks all for replying, seems that there are very valid reasons to add X Kts to VRef due to headwind and gust factor - but still no valid reason to add X kts to Vref due Xwind!!

I still cant see that there is any advantage in adding wind in a Xwind situation, furthermore, all I see is that if speed is added for Xwind, deceleration will take longer, hence more time exposure to such a wind - particularly once in ground effect!

I am certain of this aerodynamic fact - seems to have been the effect on every A/C I have flown up to 5700 kg, my guess is that it would be the same effect right through to a fully loaded B 747-400 or A380!!

It just boils down to physics!!

Thanks again,

Cheers

Ace

[piontyendforward]

It all depends on which version of the B737 FCTM you have.

With the change of wing from the Basic 737 i.e. the 100 and 200 non AVD to the B737-200ADV the old Vref40 = V2 F15+15=V2 F1 no longer worked, (to safely manoeuvre in the go around). By adding five knots to the Vref F40 or F30 then you were fully manoeuvrable again in the go around. Hence all landings with the 732,3,4,5 (not sure of the NG's) had a minimum of Vref + 5

Early FCTM versions had words like "landing with a tail wind the Vref + 5 increment may be bleed off in the flare to touchdown" Later versions don't mention this at all but do say "do not apply wind corrections to Vref 40 (30)+ 5 for landing with tail winds"

The QRH still has statements
"Maintain Vref 15+5 knots minimum on final approach. Apply normal wind corrections but do not go below Vref 15 + 5 to assure adequate speed for the go-around"

From the Boeing wind additive chart for landing without autothrottle (FCTM revision3) a 90 degree crosswind at 15 knots has a wind additive of 0 and an approach speed of Vref + 5.

It also mentions that when using autothrottle position the command airspeed bug to Vref + 5. Sufficent wind and gust protection is available with the autothrottle engaged.

Later in the paragraph it also mentions that the steady head wind component can be bleed off as the aeroplane approaches touchdown while maintaing the gust component.

In summary the Vref + 5 is for manouver protection for go-around not wind correction.

[seandavis333]

hey guys, i am trying to find someone that downloaded the vid file for the brazilian 777, 747sp crosswind landing. the link that hosted it has taken it down and i had not yet saved it to my computer. am a CFI and want to use it for training.
anyone that has it saved please forward it to me at [email protected]!!

[Cloud Cutter]

I think the reasoning behind it is quite simple - just the same as landing with a reduced flap setting in a crosswind - if you are going faster you are subject to less drift - ie. the aircraft heading is closer runway heading. Also this provides for a more drawn out flare which gives more time to apply the required control inputs.

I don't think this has any benifits appart from teaching a new student in a lighty. Although I think Air Nelson are able to do reduced flap landings in the Saab if required, I don't really think it's necessary. You can't do it in a B190 (apart from zero flap) because they are not certified for partial flap landings.

In a jet I can't think of any reason (aside from normal gust factors) - they're surposed to be aligned with the wind pretty much right untill touchdown anyway.

[757manipulator]

Only time I can think of for a reduced flap setting for landing is for a S/E approach and G/A etc, having said that, the 757 I fly is certificated for flap25 and flap30 landings as part of our Ops manual:O

[dogcharlietree]

Ah, back in the good old Diesel9 days it was add half the wind and all the gust, up to a max of 15kts.

[Centaurus]

Piontyend. If you are correct in the +5 knots being solely because of GA performance, I would be surprised. If that was so, then Boeing would publish a caution on go-arounds from a bounced landing because of degradation of performance in the GA manoeuvre - and they don't. The +5 knots to Vref in light wind conditions is there (as far as I am aware anyway) to allow for the reduction in IAS due to wind gradient from the free stream flow that starts to diminish below 2000 ft above the ground. While it is common to see crews flaring at Vref plus half the HW component, this is not what is published in the various FCTM and as A37575 stated in an earlier reply, it can cause a unneeded float. And long floats can result in the danger of a tail strike.

Cloud Cutter. The object of the crosswind landing (as with most landings) is to touchdown with minimum float. Certainly in a Boeing there is no requirement to reduce flap setting simply because of a crosswind.

The additional speed with any reduced flap in an airliner or a C150 will make stuff-all reduced drift angle change and extra float only means more control juggling at a critical time as the speed drops off in the float.

At the max crosswind limit in Cessna singles and most twins, full flap should be acceptable as it involves less float and thus less exposure to sudden gusts. There is no shortage of accident reports where pilots of light aircraft have been adding knots for mum and the kids along with reduced flap settings and have finally over-run the available runway.

[Cloud Cutter]

I agree completely. I was just outlining a common perception. In fact it was standard proceedure a the school where I trained to do crosswind landings with reduced flap settings. I disagree with this practice for the reasons you've given.

The only time I've used reduced flap is when introducing a student to crosswind circuits - a flapless aproach provides more time (due to increased float) for a student to sort out the control inputs required. It is purely for demonstration purposes, and I normally only throw in one or two of these part way through the lesson. Otherwise it's full flap.

I certainly wasn't suggesting that pilots of large aircraft would consider this technique.

[Coander]

Might be a little late on this post but....

Just got back from sitting my OZ ATPL and in the Systems/Aerody exam casa seemed to think that you add half the the headwind to you Vref up to a max of Vref + 15 and then on top of that half your gust to a max of Vref + 20. eg.
W 100/20G30 = 10 + 5 = Vref + 15

But thats only CASA.....

[flyingkiwi]

I am definitely no expert in flying jets as im still doing upgrade training but what I have learnt so far is that in swept wings lift= speed, and in a xwind condition where there is a blanketing of the wing, a little extra speed can be a good thing.

If you end up even slightly below Vapp (yes im an Airbus driver too) this can have a significant effect on the flare, or more specifically the lack of arresting the descent rate in the flare. The swept wings are that critical in their speed.

I think every one whom has landed in Taipei will agree, if due to shear you end up at Vapp-5 you will have a very firm landing, so to bug up 5kts will give a form of protection. In my case my trainer laughed all the way to the gate, apparently he guessed correctly some miles out that I was going to bury it due to not having adjusted my Vapp.