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Tailwind

Old 13th Aug 2012, 09:52
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Tailwind

I had an interesting the other day, tailwind limit for landing is 10kts and the wind at destination was variable and all over the place.


RWY 16/34
Wind reported at 1000' was 340/20
ATIS broadcasting a average tailwind component of 6 kts at the 34 threshold.

At 500ft nice and stable with a 3 knot headwind according to the ND wind vector, passing 100' wind on ND was indicating 160/12kts, this coincided with a TWR report of Calm to 3 kts tail at threshold (confirmed with windsock). We were nice and stable so continued.

My question is, at what point do you go-around for a tailwind?
Does the ND vector override the TWR report and visual check?

I only ask as the FO queried this later on and I stated that my opinion had always been that as long as the wind at touchdown was good (as reported by the tower) and you were stable throughout the approach then you were entitled to carry out the landing even if the ND wind fluctuates over your limit. He wanted a reference for this and I couldn't find one either way. Would be interested to hear everyones opinions.

Obviously this is only really applicable in situations where you have the wind varying all over the place and there is not an 'obvious' runway to use.
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Old 13th Aug 2012, 10:01
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Concerning airbus, there is a time delay (don't know how long) of the wind reading on ND.
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Old 13th Aug 2012, 10:05
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Yep, my previous airline (big carrier) explicitly stated in the Ops Manual that the TWR wind could override the ND wind due to the delays in computation and the difference that can be present between ND 1 and ND 2. That is why I still apply that philosophy, FO wasn't convinced though.
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Old 13th Aug 2012, 11:11
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Had something similar recently: The wind given by tower differed to the airplanes ND, and I (PNF) constantly checked the GS indication against Vref. As far as I know, the GS is computed differently from the wind, thus more exact.

How would you handle that? Go around for high GS or rely on the wind to change to the tower wind?
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Old 13th Aug 2012, 12:00
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We had a bit of a discussion about this in our company recently and the position of the tech. management was that there were various possible errors in the wind computation, especially at low level or when crossed controls were being used. Because of this, wind readouts were deemed "advisory", so a G/A was not mandatory if they were out-of-limits but the tower winds were OK.

That said, I think it'd be unwise to ignore this information - personally I'd give serious consideration to throwing it away if the wind component was significantly outside book figures, as I might be experiencing something that was shortly to be reported.
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Old 13th Aug 2012, 12:26
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I would think the answer to this is (as it is too often) "It depends".

Our manuals refer to "wind", but do not say which one is to be considered. Is it the FMS/IRS calculated wind with its known lag and general inaccuracy, or is it the ATC report that only is averaged over 2 minutes? Both of them are inaccurate and only give a rough idea of what one might find when landing and taking off. On top of this, ATC at certain fields have been known to "tweak" the reported wind to suit the supposed needs of traffic - I have been told of calm wind and dry runway by ATC in a torrential downpour and more than 10 knots of tailwind already.

So with the ATC reported wind within the required envelope, if the landing at hand is close to performance limits, one will easily be way more G/A minded in case of an arising oddity like FMS showing an out of limitations tailwind than when landing on a non-limiting, long runway without terrain around and with a low landing mass.

Performance-wise, there is a little safety margin in the calculation, if I remember correctly: a tailwind is triply and a headwind is half considered in the runway analysis tables. While this is of course in no way an excuse for a sloppy performance calculation, it will usually cover for some minor wind fluctuations between the time the calculation is performed and the actual takeoff/landing.
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Old 13th Aug 2012, 15:35
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He wanted a reference for this and I couldn't find one either way.
The reference is in the limitations section of your manuals.

Wording would probably be "landing (wheels touching the runway) max tailwind component xx kts"

Not "flying" at 500 feet or 200 or 50.

Since we are not fortune tellers, the only way to know what the wind would be at landing (an event not yet taken place), is to rely on the reported wind from tower

Only after the landing has taken place are we able to check if it was actually within limits (how were we supposed to know before hand). To me it's like continuing beyond the outer marker if the visibility drops below minimum, we can continue, but it's not certain we can land. Same with the wind, if tower reports tailwind exceeding the limit, you have to go-around - otherwise you can continue and have a "look" if landing is possible (use of common sense/airmanship).

...and of course you checked your landing performance before hand if limited to check your margin.

I guess you already see it the same way, the above was written as argumentation to use with your F/O. If he is stubborn, ask for a reference where it's written that the ND wind should be used while still flying above the runway.

Last edited by cosmo kramer; 13th Aug 2012 at 15:37.
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Old 13th Aug 2012, 15:41
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Use most reliable sources. Tower wind is not always the most reliable source for many reasons.

As always it must be a judgement call.

Last year approaching a North Eastern Nigerian airport using the so called instrument approach tower gave the wind as calm. Steady 20 to 30 kt tailwind as indicated on the ND from about 1500 ft until about 150 ft. I went around with my last indicated wind as 27kt tailwind. An easy call to make but some are not so easy. Just judgement depending on such things as runway condition etc.

One thing is for sure if for some reason you left the paved surface and the the data recorder showed you had information on the ND that the tailwind was outside of limits then regardless of what the tower was saying that information would be used as yet another reason to hang you.


Regards
Exeng
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Old 13th Aug 2012, 18:08
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You can always find examples that are no-brainers. Even if indeed the wind was calm, if you have 27 knots tailwind at 150 feet, the reason to go around would be the 27 windsheer.

I am not going around though, because the ND/FMC says 15 knots tailwind at 150 feet (for a 10 knots limited aircraft), assuming tower tailwind was reported as within limit.

Myself:
...and of course you checked your landing performance before hand if limited to check your margin.
But I would have bumped my landing speed 5 knots for performance calculations if the runway length was limited.

That way I would not go over the end.
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Old 14th Aug 2012, 00:50
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I think comparing the TAS and GS on the ND would give you a much more accurate reading of tailwind/headwind component than the wind vector.
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Old 14th Aug 2012, 01:06
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Prog 4/4 on Boeing for Headwind/tailwind.

Winds at touchdown is what matters.

Big tailwind(Y) at X feet is grounds for a go-around. That's what the fourth stripe is for. Windshear, unstablized, etc, etc.

It's been my experience that the Boeing wind readout is more accurate than the Airbus wind readout. Granted Airbus were 1990's models but Boeing winds were accurate back then while the Airbus wind readout wasn't nearly as reliable.

Last edited by misd-agin; 14th Aug 2012 at 01:08.
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