Windy weather 17/01 - groundspeed on approach?

[treadigraph]

Group of us in the office are looking at the Heathrow approach out of the window (now that the sky has cleared a little!), watching a BA 747 appearing to c-r-a-w-l down the glide slope - assuming the forecast windspeeds of gusts to 70mph (or Kts?) are accurate, we are wondering just how slow the groundspeed would be today...?

Treadders

[HEATHROW DIRECTOR]

A/c on final approach to Heathrow usually fly around 160kts airspeed. Groundspeed will be considerably slower if there is a headwind although the effect on the aircraft will vary depending on the direction of the wind.

As I write the wind speeds on the approach to Heathrow are over 60 kts so aircraft may be flying the approach at not much more than 100 kts, or around 90mph.... with most of the M4 traffic overtaking them!!!!

[GANNET FAN]

Looking out of the 8th floor window in the City just now, watching the line of 3 aircraft heading west, it almost looks as if they are hanging in the air, their forward progress is clearly slower with these winds.

[treadigraph]

Thanks HD, the difference in speed is really quite marked compared to less windy days - which shows that I spend too much time looking out of the window on clear days rather than at my PC! It's more or less overcast again now, so back to work...

[Georgeablelovehowindia]

Latest LHR windcheck is 270 at 40 gusting 57. OK straight down the runway now - it was 230 earlier, of course - but still quite challenging due to the windshear on final. Still some go-arounds.

100 knots groundspeed equates to 115 mph and if somebody is doing that down the M4, they're (a) running the risk of getting airborne themselves and (b) grabbing the attention of the folks in the blue and yellow checked Volvo!

I wonder what it's like in the new control tower?

[Groundloop]

Originally Posted by HEATHROW DIRECTOR

As I write the wind speeds on the approach to Heathrow are over 60 kts so aircraft may be flying the approach at not much more than 100 kts, or around 90mph.... with most of the M4 traffic overtaking them!!!!

Conversion the wrong way round? 100 kts more like 115 mph.


Just beat me to it, G-ALHI!!

[slingsby]

I would have loved to speed checked the pigeon who past my office window earlier, I would not like to have been on the receiving end of a departure from controlled flight,,,,, yeuk

[HEATHROW DIRECTOR]

Sorry for getting speed conversion wrong - put it down to age!!

[PPRuNe Pop]

Personally Bren, I wouldn't have mentioned it!

[runawayedge]

Pardon me for butting in......if the wind on the ground is at 60kts, chances are it will be considerably higher aloft.....on finals that could be anything up to 100kts......meaning groundspeed of about 66mph........chances are flashing blue lights in rear mirror are not for you!

[Geffen]

Around noon 4 thousand foot wind at LL 260/90

[Flightman]

Originally Posted by HEATHROW DIRECTOR

A/c on final approach to Heathrow usually fly around 160kts airspeed. Groundspeed will be considerably slower if there is a headwind although the effect on the aircraft will vary depending on the direction of the wind.
As I write the wind speeds on the approach to Heathrow are over 60 kts so aircraft may be flying the approach at not much more than 100 kts, or around 90mph.... with most of the M4 traffic overtaking them!!!!

Real basic question here, so apologies, but we were discussing it in our office today, and no-one had the answer.

Using HD's numbers above. If the aircraft is doing 100kts, and the 60kts headwind makes it up to 160kt, what happens if the wind just stops? Most people said the a/c would just fall out of the sky! I argued that wasn't safe, couldnt be the procedure and the aircraft must be flying " in a 160kt config"
( for want of a better phrase ) and if the wind just stopped, it would still be doing 160kt, but also 160kts over the ground. Am I right, wrong, or mad?

[Right Way Up]

Flightman, what you describe is called windshear. A loss of 60 kts would indeed lead to "falling out of the sky". Unlikely though to be anywhere near that magnitude.

[point5]

The KLm F50s inbound to LL today showed a groundspeed of 55kts! Naturally a B744 was shuvved right up its ar e!

[Noah Zark.]

There was a nice clip on the Beeb news of a 146 taking off from somewhere with a significant crosswind. Fine bit of filming, and flying!

[NWSRG]

Take an approach speed of 160k airspeed. Into a headwind of 60k this equates to a ground speed of 100k. And at touchdown, a ground speed of say 80k for an airspeed of 140k...

Now, given that touchdown is therefore at about 60k less than normal, is there any procedure that allows a reduced flap approach in high winds...ie. less flap, higher touchdown airspeed, but offset by the wind. Say an airspeed at touchdown of 180k, equal to ground speed of 120k due to the wind...still a lot less than a 140k normal calm touchdown.

I remember a windy approach into BFS on an Easy 319 where the pilot seemed to adopt this approach...and the lesser flap seemed to allow a more stable approach as well...

[Rainboe]

In February 1990, there were 4 separate 100mph (aloft) gales sweeping through the south of England in the 4 weeks, all with westerly winds. I remember looking at the Omega groundspeed on approach into Gatwick several times to see just over 60 kts groundspeed. It made for very long duration approaches!

[1DC]

Departed Glasgow in a Shorts 360 in a full gale one night and when airborne we seemed to be flying over the runway at walking pace. We then flew to Teeside with the wind astern and the pilot said we made the journey 15 minutes quicker than he had ever done it before.

[NudgingSteel]

Originally Posted by Flightman

Real basic question here, so apologies, but we were discussing it in our office today, and no-one had the answer.
Using HD's numbers above. If the aircraft is doing 100kts, and the 60kts headwind makes it up to 160kt, what happens if the wind just stops? Most people said the a/c would just fall out of the sky! I argued that wasn't safe, couldnt be the procedure and the aircraft must be flying " in a 160kt config"
( for want of a better phrase ) and if the wind just stopped, it would still be doing 160kt, but also 160kts over the ground. Am I right, wrong, or mad?

As RightWayUp says, that effect is called windshear. It's defined as a sudden change of wind speed or direction, that occurs significantly faster than the aircraft can accelerate or decelerate. As you say, if an aircraft was at 160kts airspeed into a steady 60kts headwind, it would be moving forwards at 100kts groundpeed. If the wind suddenly stopped, the aircraft would still be moving over the ground at 100kts (because it's a big lump of metal that has a lot of kinetic energy which remains the same for the next few moments) but the airflow over the wings is suddenly only 100kts...which is bad news until the aircraft can increase thrust and accelerate back to required airspeed. For an airliner, you're right, that would almost certainly lead to a stall. Fortunately the wind doesn't ever just suddenly and completely stop!
In the UK, windshear is rarely of such a large magnitude. (It's often encountered at an inversion layer). I had traffic report a loss of 40kts in turbulence on short final a couple of weeks ago as a squall was passing through, but that's the largest I've ever heard of here. Usually it's a variation of about 20kts maximum, up or down. If windshear is reported or predicted, crews will add an amount to their final approach speed to allow for the effect described above. It can go the other way; positive windshear suddenly increases airspeed and can cause the aircraft to 'balloon' or climb.
It's a bigger problem in places like the USA with the huge thunderstorms that can occur there, and microbursts can cause severe windshear effects many, many miles away from the storm.

[Dufo]

~40kts GS on short final to EGNS 21, done with a L-410.
And 25kts GS just before stalling over IOM VOR on a proficiency check a few minutes before, with the same a/c.