Tailwind limit - why?

Aircraft Tires have a quite limited maximum speed. Could that be the reason?

Udo nailed it…

exceeding tire speed limits

The speed rating on my tires is 160mhp / 140kn, i do not think i will try to operate with a tail wind that could make me reach those during TO or LDG ;-)

Try taking off in a DA42 on a 960m tarmac runway with a tail wind of more than 10kt. You will then see why most aircraft have tailwind limits.🙂

So you think you don’t need more runway length to take off with a tailwind?

There’s also minimum climb gradients to consider, probably with airliners – single engine minimum climb gradients.

So you think you don’t need more runway length to take off with a tailwind?

Of course it does, for the (two) obvious reasons. But Peter’s point, I think, is that commercial operators impose a (10 kt?) limit regardless of the usable runway length.

If so, my guess is that the second of those reasons – the reverse wind gradient, tending to increase airspeed as the aeroplane approaches the surface – is something that ILS and PAPI glideslopes can’t adjust for.

This wouldn’t affect many light GA pilots, who are trained and used to fly without such “landing aids”.

The POH of many aircraft give a tailwind limit.
I can’t remember what the DA42 was but others on here can look it up.
I have always considered the tailwind limit in a similar way to the demonstrated crosswind limit.
If you go through the graphs of many aircraft and look at the distances required with a tail wind you would be surprised at how much even a few knots of tailwind adds to take off and landing distances.
In creating the POH info for any aircraft the test pilots have to rely on the weather conditions of the day and runway lengths in order to get an accurate picture. Those figures then have to be remodelled mathematically to take into account issues such as take off weight and age of engines etc.
So a test pilot flies a 757, probably well short of maximum weight and finds with say a 8kt tailwind he just manages to take off after 2km of runway on a 3km runway. (The figures will also be taken during landing with the same tailwind.)The mathematicians then get to work on their computers, add safety margins which will cover both take off and landing. Operators will then transfer these figures to their SOPs. There are several books and articles from test pilots Richard Farley (who I believe was also a test pilot on the Harrier) for one who describe their tests and what happens in detail and how actual conditions are impossible to achieve over the whole range of the flight envelope.
And before someone mentions that the crosswind figure is a maximum “demonstrated” which it is. This is also due to the conditions available during testing. However, depending on your piloting skills and the width of the runway it is possible to safely land or take off in much greater crosswind. The same goes for tailwind limits, although they are much riskier to go beyond unless you have a very very long runway.
And if you do leave the runway when going above these limits, how do you think your insurance company or your airline bosses are going to feel about it.
@Jacko although there is a reverse wind gradient the wind also changes direction by up to 30° as you get nearer the gound so in calculations a 10kt tailwind (for standard calculation purposes) becomes a 5kt crosswind and a 5kt tailwind which is much more difficult to deal with as aircraft naturally attempt to turn into wind. (TBH 5kts isn’t such a great problem but it can become one at greater wind speeds).
But take the case when you are below 2000ft and the tailwind itself is 10knt, and you try to land your DA40. Your landing configuration approach speed is say 70kts IAS, you are now approaching at 80kts. Doesn’t sound a lot but at 70 kts you would probably touch down and stop from 50ft in somewhere between 300m and 400m. At 80kts you will find yourself floating, wondering when the wheels are going to positively touch the surface and remain there and as soon as they do, possibly stamping on the brakes at around 750metres and wishing you had worn your oily flight suit rather than your light coloured trousers.
These are things you can and perhaps should try for yourself if you can find a long enough runway without too much traffic to worry about you landing contre QFU.
Gliders often land contre QFU in competition so that using their airbrakes they can land at the far end of the runway without having a long tow back for the beginning of the runway for the next launch. But even they don’t do it if the tailwind is too great.

Jacko wrote:

If so, my guess is that the second of those reasons – the reverse wind gradient, tending to increase airspeed as the aeroplane approaches the surface – is something that ILS and PAPI glideslopes can’t adjust for.

This wouldn’t affect many light GA pilots, who are trained and used to fly without such “landing aids”.

It’s the wind gradient for sure. Or at least that is the main problem for small airplanes. This is the same effect as in dynamic gliding/soaring. With a head wind, you will of course take off with lower ground speed, but when climbing, you head into stronger head wind, and therefore climb with a considerable higher VSI. When landing, the opposite happens, and you lose airspeed, resulting in a short run.

With a tailwind, this is turned inside out. Taking off, and you could in fact end up in a situation where you cannot climb with any effective climb rate at all, fighting the gradient. When landing, you will increase airspeed due to the gradient, and will float forever, and do it at a high ground speed.

I don’t know about the airline limit of 10 knots. I would guess it is a design constraint. The aircraft must be able to take off and land safely (with no adverse effects) with a 10 knot tail wind.