Question: why exactly does high wind slow (not stop) operations?
 

Question from curious SLF:

Apparently there were delays at LHR today due to high winds slowing operations - now obviously I can understand high winds stopping operations altogether, but what exactly is happening when operations are simply slowed by the wind?

What is happening is that aircraft are slowed by the wind. Imagine an aircraft flying in still air at 200 knots. If it then encounters a headwind of 40 knots, its speed over the ground reduces to 160 knots hence everything slows down.

At LHR arriving IFR flights are still separated horizontally by distance and not time (...yet). High winds mean a strong headwind component on approach which means a slower groundspeed therefore it takes longer to fly the separation distance. The result is a reduction in movement rate from around 40 per hour to nearer 30 per hour with consequent delays. Each aircraft would be nearer 2 minutes apart instead of around 1 min 40 secs.

If arriving flights could be separated by time (as departures are) then the flow rate and runway utilisation rate could be maximised to suit runway conditions. It would mean getting a following aircraft to (as an example) "fly 1 minute 40 seconds behind the (type) ahead".

(Edit: Sorry HD, slow keyboard skills....!)

Depending also on the wind direction ,as aircraft have a `crosswind` limitation for take-off and landing,dependent on type and Company/crew limitations.That means that the aircraft may have to divert to an airfield with more favourable runway directions.If so,that means aircraft and SLF are not where they want to be,which means that there will be delays until the weather abates....Same as when it snows.....

Thanks all. Seems obvious in hindsight! :ok:

So as the question has been asked of delays. How fast would the wind have to be going (assuming 180d against direction of takeoff / landing) before the airport gets closed. And presumably a much lower speed as a cross wind?

Nice to have additional non-parallel runways if you can!

You'd be running into significant problems long before headwind killed operations due to zero flow rate.

As has been said crosswinds much above 35-45 knots are going to be above most aircraft's crosswind limits. Even with the wind blowing along the runway some aircraft have a maximum wind speed for ground operation (50-70 knots), and then again many aircraft have a maximum wind speed for door opening/closing ( again typically up around 50 -70 knots).

I'm sure there will limits for the ground folk as well - rumour has it the new tower at LHR has a wind limit, above which it cannot be occupied - any truth in that?

JABIRD.... Major airports are not closed due to wind. It's up to the crews to decide whether they can accept the wind, not the airport.

Another consideration might be a limiting wind speed for the radar....I have to switch ours off at 70 knots(as far as I can recall) to prevent damage to the turning gear. Lack of an airfield radar could slow traffic rates.

Groundpeed mini
 

I am surprised that no mention of "Groundspeed Mini" function fitted to all Airbus has not been mentioned, as this automatically increases the Vref approach airspeed so as to maintain a constant groundspeed, regardless of approach headwind.
The only problems when using this function are the large power reductions which can happen close to the ground, as headwind reduces, and the danger of a flap overspeed, if speed momentarily increases in high wind turbulence.
The problem with selecting this function "off" using "selected" Vref is that automatic speed targets in the event of a go-around are lost.
Unfortunately the benefits of GS Mini are useless unless all aircraft are using it at the same time, so for Heathrow, selected Vref for most of the approach is the norm.
Hope this is some help.

Emoclew,
Slight correction re. GS mini.
Not all Airbuses are fitted. There are still quite a few A300/310s around without this.

dixi188
My apologies.
Emoclew

Another factor is time on the runway - If the aircraft touches down at 120kts groundspeed in no wind it will reach its runway exit sooner than if there's a headwind of 20 kts (td speed 100kts) - this can be a big factor if the runway exits are widely spaced, adding as much as 20 seconds or more to the rollout.

Unless of course the higher landing speed with no headwind results in you missing the first exit and having to roll an extra 800m to the next one!

In which case the developer should have put the exit further down the runway...:D

In all seriousness, if you miss the first exit at the higher landing speed and keep your speed up to vacate at the next one, there's very little difference - it's time on the runway that counts:ok: