So Uplinker,
You're on a Runway where perf figures suggest a 500m margin for takeoff.
Figures predicated on 5 kt headwind. Vr is 140.
Passing 110kts your eagle eyed PM takes his eyes off the engine display and notices a 7 kt tailwind on the wind readout. Neither of you had noticed anything untoward on the speed trend vector or tape.
What would you (or your captain expect you to) do?
Ensure a very positive rotation and/or select TOGA.
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How much time would you want to spend watching the speed tape and trend to assimilate it while taking off?
I can't believe I'm answering this. You have a split second to look at the size of the arrow and the movement of the tape before back outside or to the engine parameters. You don't need to process a value or compute anything. Just long enough to get a feel as to how long until the next speed (100 or v1).
If, as you claim, you have such little time to process a parameter, then by your own logic does it not make sense to look at a readout that has already been measured and processed by a computer and therefore only needs a glance rather than a longer look to assess its trend?
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You only need to glance at the arrow.
Unless your ND arrow increases in size with the wind direction you need to read the speed too. If youve seen the arrow going all over the place when the outside wind is vrb at 1-2 kts at altitude you'll understand. How do you know if that sudden tailwind is 1 kt or 10?
If the wind arrow is in the general direction you are expecting, then there's no need to look at it again. If you have allowed for 10 kts tailwind, you just need to glance at the windspeed readout to confirm it is no more than 10kts.
To look at the wind vector (which is of dubious reliability anyway)...
Why have folk got such a hard on about the 'reliability' of the wind readout? Do you trust the autobrake to keep you straight on landing?, because it does, and it uses processed data from the ADIRS to modulate the brakes to do just that.
... gives you no extra information, takes your eyes away from other more important items, and distracts with the cognitive awareness for other decisions.
Well it does give you extra information: the head or tailwind. Is that not an important parameter to check on the takeoff roll?
The second of time that the PM spends trying to work out if the wind has backed or veered, and what the subsequent headwind component has done would be much better spent looking at the engine parameters.
No working out is needed, that's the point - the wind arrow and speed readout tells you in the fraction of a second it takes to glance at it ! As for looking at the engine parameters, PM has just watched the speed tape on the PFD to announce 100kts, then while moving their eyes back to the engine instruments, glances at the wind readout on the way. Easy!
As for the size of the wind readout; the arrow is thicker than the V/S arrow, and bigger than the beta target indicator; surely you can see those? The numerical element of the wind readout is a similar size to ALT CRZ on the FMA - you can see that surely?
Windsocks. Hmmmm. So you are complaining about glancing at a processed readout which tells you the actual value of the wind, yet you advocate looking outside the cockpit, finding the windsock (at night - is it lit?), and assessing a head or tail wind from that. Is it more than 10 kts? And won't the windsock be behind you by the time you are into the take off roll?
And in what state are the windsocks in some second/third world airports?
I think I would rather rely on the on-board multi-thousand dollar measuring and processing equipment and computation power fitted to the plane.
(Remember, I am talking about the wind not being what it was after the tower passed it to you and you lined up, or the tower passing an incorrect wind. This actually happened to me and getting a heavy A330 airborne with an unannounced tailwind, was a moment of extreme concentration, I can tell you! - for a second or two I thought one or both elevators had failed. I now always glance at the wind readout on take-off)