Vapp in gusty conditions

[Cecco]

Confirm, you add the headwind component and half the gust factor to Vref to get a increased Vapp in gusty conditions; e.g, RWY36, Wind 270/16, gusts up to 24 kts, Vref: 108 kts. 8 kts. (=headwind component) + 4 kts. (=halft the gust factor) are added. Vapp: 108kts.+8 kts.+4 kts.=120 kts. Vref remains unchanged.

Cheers
Cecco

[galaxy flyer]

Depends on type and AFM or FCOM; more accurately on it's certification standard. The two types I fly, GLEX and CL 605, the GLEX the addditive is only 1/2 the gust; the CL 605, it is 1/2 the prevailing wind and all the gust.

GF

[dan1165]

The CL 605, it is 1/2 the prevailing wind and all the gust
for a total of max 20 kt .

[Kiltie]

For the Boeing 737:

Default calm wind minimum Vapp = Vref+5kts.

...then adjusted for half the headwind component and the full gust component, aggregate total adjustment not to exceed Vref + 20kts.

Landing performance allows for the gust component to be carried on top of Vref to the point of touchdown if preferred, but the headwind component should have diminished by the 50' Vref point.

Vref is nominated but not adjusted.

These procedures come from both Boeing's FCTM and 2 different airlines, but check your own manuals for what applies for your type.

[Mohit_C]

A quick question whilst on this topic. Would you apply these corrections to the wind vector reported by tower whilst commencing approach or to the wind vector calculated by the onboard computer? In the latter, wouldn't it keep changing as you're approaching the ground so would need to constantly adjust your Vapp?

[ant1]

RWY36 wind 270/16, 8kt head wind component :confused: I think :=

Am I missing anything?

[zonnair]

Nope, your missing nada!! :ok:

[Kiltie]

Mohit C you raise a very valid point.

I'm not aware of anything published in this regard. Personally, and others may disagree, I use the TWR reported wind components. If the components increase during the approach, I might announce to my colleague that I am increasing the factor and ask him to re-bug the new Vapp (if I am flying manually). But, good CRM and practicality would dictate this is not something changed every few seconds during the approach. Once the factor is added on it is arguably better to stick with it if the wind decreases. Who's to say the wind won't increase again? Performance figures (B737) tolerate this with the caveat of never exceeding Vref+20kts.

[qwertyuiop]

RWY36, Wind 270/16, gusts up to 24 kts, Vref: 108 kts. 8 kts. (=headwind component) + 4 kts.


I think ant1 has a valid point.


Surely that is all x wind and no headwind.

[TruthHurts]

Do not see where the head wind is.

Rwy 36 and wind is 270????

Am I missing anything?

[Wireless]

No it's just a crosswind. I can't see where the headwind he got came from. I guess technically there would be a tiny headwind I suppose as you turn into it to allow for drift but not practically speaking

[A37575]

Boeing require the half the HW component is to be bled off on final so the aircraft crosses the threshold at Vref plus gust factor. Few operators actually do that in practice, which goes some way to explaining why the excess speed especially on a slippery runway, can lead to a long float and subsequent landing, and danger of running off the end.

[Capt Claret]

B717 FCOM, all gust, plus 1/2 steady state wind component above 20 kits, to a max 20 kg additive, with a min additive of 5 kits.

Eg, 30 kt wind gusting 45 kts would be 5 kts (30-20 = 10 & 1/2 of 10 = 5) plus 15 kts gust, for a total Vapp of +20.

[Geardownandlocked]

A quick question whilst on this topic. Would you apply these corrections to the wind vector reported by tower whilst commencing approach or to the wind vector calculated by the onboard computer? In the latter, wouldn't it keep changing as you're approaching the ground so would need to constantly adjust your Vapp?

I had a discussion with my captain about that a week ago. On our MD11's the wind indications on our respective ND's are slightly different due to IRS drift differences, and the closer you get to the ground the more inaccurate, even to the point of being totally useless (indicating eg. 3 kts head on the left side, 4 tail on mine). We are supposed to use the tower wind, but this isn't always accurate either, depending also on the airport where you're flying into. We just try to be on the safe side, that's all really.

BTW, our company policy is either the full wind above 20 kts, OR 1/2 gust factor, whichever is HIGHER. Minimum Vref+5, maximum Vref+20.

G.

[john_tullamarine]

Keep in mind that the protocol is a bit rubbery -

(a) the steady wind is presumed to reduce within the notional boundary layer so it probably will drop off approaching the runway - the half factor attempts to handle this in some sort of rational way

(b) the gust is presumed to be random so it might catch you out at any stage of the approach - better to allow for all of it through to the landing

(c) 20kt max tries to balance having too much against too little IAS approaching the runway

Nothing at all terribly scientific in this stuff - just an attempt to load the dice in the pilot's favour.

A few other threads on the subject have more description on the considerations for those interested in running a search.

[GAMEOVER]

http://i1131.photobucket.com/albums/m546/MACHO691/Diapositiva1.jpg?1289505059

I am sorry for the size...:E

[Cecco]

Can you post that again, it's all blurred.

Cheers

[FullWings]

I have to say I've never been too worried about getting these additives exactly right, seeing as on many types if you've got the autothrottle in, you don't apply them. I feel most pilots can do a better job with the thrust than the autothrottle can, anyway...

For most landings, you have a good power-to-weight ratio (<MLW) and, *if you're stable*, can regain airspeed pretty quickly if necessary. The spread of speeds available is generally only 15kts, from Vref+5 to Vref+20. When Vref is 150kts+, I don't feel the need to add much more, especially if the runway is approaching limiting length. Overruns due to excess speed on T/D, prolonged flares, wet runways, tailwinds, etc. are right up there on the list of accidents that keep repeating themselves...

If it's a gusty day, keep on top of it! :ok:

[bloodshed]

Hi full wings,

I generally agree. However, the computed Vappr values to me are max values.
Repeat: MAX!!!!

I've seen so many mates who do a great job in telling me what their Vappr would be; only to then add 5 for grandma, another five to "play it safe" and sometimes more for their inaptness. And they wonder why I would not let them fly when things get interesting and/or RWYlength is restricted...

Aside: yes, flying manually with autothrust engaged is calling for trouble.
To me it is a no-no.

Else: on many airports you have a windsock close to the touch down zone. To me this is providing the only true information- opposed to 10 min old TWR wind or IRS-derived erroneous winds. Use it! And your brain...

[Escape Path]

Aside: yes, flying manually with autothrust engaged is calling for trouble.

Hi Bloodshed, may I ask why? I'm not familiar with A/T operation. Is it because it lags responding to the pilot pitch inputs?

Thanks in advance

Escape Path

[Capt Claret]

Aside: yes, flying manually with autothrust engaged is calling for trouble.

The Douglas/Boeing 717 FCOM recommends the use of Auto-throttle when landing, manual flight or autoland.

[galaxy flyer]

Yes, thread drift, but what is the reasoning behind manual flying/manual throttles. It is a limitation, not the way we train or fly on the GLEX. The A/T move, so you know exactly what the throttles are doing, if you need to takeover, one click and you have control instantly. The Navy even flies in that configuration for shipboard traps.

GF

[Mansfield]

GF,

I believe that Boeing, at least, has a concern about manually responding to autothrust induced pitch changes in the case of under wing mounted engines. This may be why the 717 FCOM allows autothrottle use when in manual flight; on the other hand, it may be an artifact from Douglas that has not changed.

Boeing is pretty clear about their preference for manual throttle during manual flight on the 767/757. That said, none of the operators that I have flown the 767/757 for actually adhere to that; typically, the autothrottle remains engaged until quite a bit closer to the runway than the autopilot. The concept that I was trained with was simply that if your throttle hand moves forward, then your flying hand moves forward, and vice versa.

The Boeing explanation of eliminating speed additives during autothrottle use is that the autothrottle system is very aggressive when increasing thrust in response to a speed decay, while it is rather lazy about reducing thrust in response to a speed increase. Therefore, it will operate conservatively in dynamic situations, resulting in an average speed somewhat on the high side of bug.

[galaxy flyer]

I only rode the jump on EAL B757s and they used the auto throttles most of the way down final, even at KDCA visual to 18 down the Potomac.

GF

[stilton]

Certainly, when performing an autoland with autothrottles engaged of course
no speed additives are necessary on the 75/67



However the autothrottles are not approved for use to touchdown while flying manually. Unlike an Autoland they will not go to idle in the flare, they will stay in speed mode attempting to maintain the MCP speed.



During a manually flown approach, even with autothrottles engaged until close in, the usual, half the steady and all the gust additive must be added to target speed.

[decurion]

Although not required by regulations, from a safety point of view it is wise to make corrections to the approach speed in case of strong gusty wind conditions to avoid reductions in stall margins. The speed correction given by 1/2 headwind + gusts with a maximum of 20 kt. is often advised by aircraft manufactures such as Boeing, Fokker, and McDonald Douglas. However, it should be realised that it is not an official requirement. It is not part of the aircraft certification (unlike stated in one thread). It is left to the operator to use this or not. Also there are another corrections used. For instance Airbus advises the following wind corrections depending on the aircraft model:

· A320/A330/A340 series: 1/3 of the headwind component (excluding the gust), limited to 15 kt;
· A310/A300-600: 1/3 of the tower average wind or the gust increment, whichever is higher, limited to 15 kt;

Another method is the one recommended by Bombardier and Cessna which only adds half of the reported gust to the approach speed.

[john_tullamarine]

These various techniques/recommended practices have, as their intent, assessment of three problems -

(a) the steady wind is expected to reduce as the aircraft gets nearer to the ground. The typical presumed profile is the one-seventh power law flight test/certification boundary layer equation

speed(1)/speed(2) = (height(1)/height(2))^(1/7)

or some other empirically derived relationship. Typically, the atmospheric boundary layer is considered to be appropriate below, say, 2000ft and provides a useful means of estimating speeds at heights different to the measuring station

(b) gusts are random and may bite you at any stage during the approach and landing

(c) come over the fence increasingly too fast for the runway and conditions and you progressively increase the risk (probability) of an overrun

What's the answer ? Good question ..

The recommended practices are intended to provide some guidance on how to manage the three considerations. None is precise, none is intrinsically more generally "correct" than another .. but all are usefully helpful in the real world.

[Mansfield]

Stilton is correct...I should have clarified that in my post.

For those who have not already seen them, NLR has done some pretty good work on this topic, and I recommend reading their crosswind papers:

http://www.nlr.nl/id~4382/l~en.pdf

http://www.nlr.nl/id~5114/lang~en.pdf