correction for tail wind on approach

[aquarium1]

My Co. says add 1/3 head wind to approac speed or 1/2 gust, limited to max 15 kts.

My question is what to do in case of tailwind or gusting tailwind?
:ugh:

[airman13]

circle to land or opposite runway.....

[InSoMnIaC]

Or if you do land with a steady tailwind and it is within the aircraft limit you don't add any wind correction same for crosswind. The reason you add the correction due to the headwind is in case the headwind component suddenly drops during the approach. if it does drop by half then your IAS will momentarily drop to your vref speed.. ie still safe.

If that happens with the tailwind component it will only serve to increase your IAS which might destabilise your approach but at least you won't run the risk of stalling on short final.

[stubby1]

What if there is gust in the tail wind.

Do you add 1/2 the gust factor (as the reason for doing so in headwind gust will dictate so ?)

[blind pew]

If the wind is gusting significantly then the mean wind will probably be over max tailwind component!

[Max Angle]

My question is what to do in case of tailwind or gusting tailwind?
Please tell me you are not operating a commercial aircraft with such a poor grasp of the basics. This is flight school stuff.

[JABBARA]

All commercial types advises to increase the approcah speed by a certain factor depending on headwind (e.g 1/3 f HW component).

"Mistakenly" it is considered a safety factor to overcome a sudden decrease in HW, as protecting the normal app speed (KIAS).

No, this is not correct, it may be only a side effect.

The main reason: As increasing the App speed depending on HW, the cockpit view (cut off angle) remains same for a fixed path approach. For example in an ILs approach with 3 degrees, if the same app speed is maintained regardless of a strong headwind, body attitude (but not AOA) would be higer than normal, as reducing the cockpit view through windshield.

[flyburg]

JABARRA,

I've been trying to get my head around this! Have to admit, never heard of this. Do you have any reference to this?

Thanks

[rudderrudderrat]

Hi flyburg,
if the same app speed is maintained regardless of a strong headwind, body attitude (but not AOA) would be higer than normal, as reducing the cockpit view through windshield.
Don't loose any sleep trying to get your head around that one - it doesn't make any sense to me either.

[FatFlyer]

On a 3 degree slope, if you have a strong HW but use the same airspeed, the groundspeed will be lower, therefore required vertical speed lower (approx 3 time G/S?) so there will be a slightly higher nose attitude. If you are a bus driver you can watch the "ground speed mini" function add lots of airspeed in a strong headwind and see the lower nose attitude. However i do not understand how this would affect the cut off angle being talked about significantly as a one or two degree pitch change would not obscure the view of the runway. eg when landing with flap 3 instead of full on an A320, the pitch attitude is about 5 degrees instead of 2.5 but does not affect the view of the runway threshold?
Also no knowledge of what to do with a gusting tailwind other than land the other way.

[flyburg]

Hi rudderrudderrat, thanks, but too late, haven't closed an eye :( guess I feel pretty stupid but even with above explanation can't figure it out. I know you need more power in a strong HW. Am I correct in stating that the added HW introduces a vector which actually changes your path in space but not when related to the GP? That is the only thing that makes sense.

Have to admit, never gave too much thought to it, just did it but interesting nonetheless!

Actually fatflyer, one or two degrees will make a difference on the ground segment you would be able to see. Groundsegment being the distance on the ground limited by your forward visibilty and the cutoff angle. One of the reasons when visibility is low we usually use flaps 30 instead of 25 when landing. Pfft, I did know that one ;)

[rudderrudderrat]

Hi flyburg,

If you had a huge head wind during your ILS App, so that your ground speed was zero, then you'd apply more power and have to fly level to stay on the ILS glide path. Your pitch attitude would be 3 degs higher than normal, so the view out of the window would show the runway apparently 3 degs lower than normal. But by increasing the approach speed, you could lower the nose and still fly level, and help restore the normal picture.

[flyburg]

Well, if my groundspeed was zero, I'd have a hard time following the glide path, I could position myself over the runway though and hover down :ok:

[RAT 5]

To expand the question into the performance and legal envelope: I have tried for many years to get an official answer from my company on the legal question of gusty tailwind limits. No success yet. You have a 10 or 15kt tailwind legal limit and no option for the other rwy. The ATC wind is 13G25kts and it is a -15kt approved rwy. Can you land legally? Simple question; at least I thought so. No problem with the company answering the X-wind question, but this one has been filed in a black-hole. The performance dept says they would use all the gust for landing distance calculations, but that was not the question. Is it legal to land? Even the manufacturer says it is up to the airline, except the airline is keeping it secret. Can anyone help?

[JABBARA]

Hi flyburg,

Sorry for this confusion.

I cannot give any written reference but, on a delivery flight of a B 737-800 in Seattle, one of the Boeing Engineer who is in charge of documentation, he explained one of our question regarding the speed selection difference between B 737-400 and 800 by this way. Frankly speaking, even I forgot what was our question then so he explained this way (long time I am flying with Airbus 330, so the concept is completely different).

But my humble knowledge of Aerodynamic and Flight Mechanic confirms what the engineer said, as simply explained by Fatflyer. If we look the things in the reverse way, this also explains why there is so small tailwind limit (usually 10 K) regardless of the length of runway where we land (even so long): Because in the HW case you can compansate body attitude by increasing speed, but in TW case you do not have this option as reducing speed below Vref. Result, at very strong tailwind you may hit the nose gear before main gear.


The Airbus Document "Getting the Grips with Cat II and III" explains how the visual segmet is effected by body attitude. As you are aware, 1 or 2 degrees changes of attitude by speed may seem small, but its impact on visual segment (particularly at high cockpit airplanes) will be more pronounced.

Maybe some other collegues can explain better.

Regards

[stubby1]

So to recap,

1. No correction for steady tail wind.

2. Gusty tail winds .. no mention in any training manual !

Hope someone can shed more on point 2.

[Notso Fantastic]

No correction whatsoever for tailwinds, gusting or otherwise. Just maintain normal Vref+5 without carrying excess. Period.

[poina]

Finally an answer that makes sense!

[Green Guard]

Not so ... actually it as a Fantastic answer.

as for difference between "ground" GS° and "air" GS°, maybe this one can add some light.
Air GS° = GS° x ( 1 + tailWind / TAS )
Air GS° = GS° x ( 1 - headWind / TAS )

[JABBARA]

From the equation given above, an example:
ILS GS=3 deg, TAS=140 and HW= 30K,
Then Air GS =2.35 deg,

Any idea about the required pitch change to reduce Air GS from 3 deg to 2.35 deg (equal to change of the flight path 0.65 deg)?

It may not be necessarily equal to 0.65 deg pitch change, could be greater...

[Capot]

A small whisper from the sidelines among you professionals...

What about the need to recalculate the landing distance required from where you eventually get the wheels down after reacting to a 30Kt gust up the chuff over the threshold, especially when the ground speed is now going to be faster by 2X the gust speed (ie 60 Kts faster) than it would have been landing the other way.

Was this a serious question? Are there any circumstances in which a commercial aircraft in normal operations would accept a runway with a gusting tailwind? "Gusting" implies - to me - more than a few knots. Is there a definition, if the gust speed is not provided?

[RAT 5]

"Are there any circumstances in which a commercial aircraft in normal operations would accept a runway with a gusting tailwind?"

Indeed there are. I was at an airfield with ILS and only circle. The circling MDA was high. This had been attempted and at MDA was still IMC. The ILS wind was tail 13G25kts: very long rwy with 15kts allowance. Can you legally land? It seemed a simple question to my Flt Ops, but after many years still no reply, hence my search for opinions or facts.

[john_tullamarine]

A nasty situation and, I guess, a problem at more than a few aerodromes around the traps.

The general philosophy involves -

(a) probably no consideration regarding body angles - consider that we have no great difficulty with different landing flap settings which entail different body angles. The reference to low vis considerations is a red herring, I suggest.

(b) flight test and AFM data is based on steady wind - one avoids gusty conditions like the plague due to the inability to obtain repeatable data.

(c) for a steady wind over a flat area, there is a ground layer (a bit like the boundary layer around an aeroplane in flight) which sees the freestream wind speed at a reasonable height decrease as the measurement is taken progressively nearer to the ground. Generally, this large scale wind shear is approximated by a standard equation routinely used in flight test work and usually presented in AFMs.

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

This has been discussed at length in previous threads.

Obviously, this will be subject to all sorts of perturbations where there are obstructions. However, for most larger airports, the flat area analogy is a reasonable starting point.

It follows that this boundary shear will see the W/V reduce during the later stages of the approach and landing and that a modest margin for approaches into headwinds (where the headwind is expected to reduce) is appropriate but not for tailwinds (where the tailwind is expected to reduce).

This is all fairly predictable and repeatable. It is anticipated that the additive will bleed off as the aircraft approaches the runway and landing.

(d) for a gusty wind, on the other hand, the wind variation is unpredictable and random. Therefore there is a sound argument for carrying a measure of the mean gust through until the flare to guard against unpleasant surprises.

(e) the planned landing for heavies generally is factored by 1.67 on the flight test performance landing demonstrations in steady wind conditions. It follows that this factor ought not to be squandered lest one be bitten severely by the Fickle Finger of Fate. Thus there will be a maximum additive. Typically, this will be up to 20kt and this sort of value dates back to BCAR days as I recall.

(f) consider that, to a first approximation, variations in landing distance will be related to variations in speed squared. If, for instance, we are looking at a typical Vref of, say, 130kt, add 20kt with no wind losses to the ground speed on a bad day, and you might be looking at an increase in distance in the vicinity of around 35 percent - that 67 percent fat can disappear VERY rapidly and a routine landing becomes a major risk consideration.

(g) my view is that landing on a critical length runway in significantly gusting tailwinds is the stuff of emergency situations only and very sweaty hand business.

[framer]

(a) probably no consideration regarding body angles - consider that we have no great difficulty with different landing flap settings which entail different body angles. The reference to low vis considerations is a red herring, I suggest.


Exactly. The whole body angle thing with no reference , just word of mouth from a Boeing Engineer, is tosh if you ask me.