How is windsheer handled

[phnuff]

On saturday, I was holding at A1 in a TB10 at EGGW watching a 737 on final: clearly, they were not having the smoothest approach and this resulted in a 'firm' landing . As they reached the far end of the runway, they reported windsheer which meant they had lost 15kts on short final. If speed drops on short final in a piston, it is pretty straght forward to open the throttle because power is pretty much instant, however I understand that with jet's, there is a lag of 5-10 seconds before power is available and on short final, there may not be 5-10 seconds.
So the question, How do you cope with windsheer in a jet? Is it a matter of carrying more speed on final or does a jet produce sufficent thrust during the lag period to counteract say a 15kt reduction?

[kiwi1]

For starters we use approach idle (ie. a higher idle than ground) so that if TO/GA pwr is reqd (TakeOff/Go Around) the engines only take 2-3 secs to accel. Also on the A320 series there is "managed speed" which compares the atis wind entered into the FMGC and the instantaneous wind coming from the IRS's and increases/decreases the "managed speed" as approp. Also later versions of the airbus have "Predictive Windshear" which use the a/c's wx radar to scan ahead below 2500 Radio Alt, on the takeoff/approach looking for possible windshear signs (precip. in certain patterns/areas, I think ?) Have just moved onto the 777 and it has the same sort of functions, ie. below 2500RA it will say (and display) "Monitor Radar Display/Windshear Ahead" and show where it is predicting shear on the navigation display.

The old style low bypass turbojet/fans did take a long while to accelerate, and you had to be careful of stall/surging them, but todays modern engines are all/mostly protected by good FADEC's (Full Authority Digital Engine Controls) and you can push the thrust levers all the way fwd as fast as you like, and the FADEC's manage the accel. safely.

So the answer to yr q. as I see it, yes they do take a bit longer to get to full pwr vs a piston, but we have lots of nice "toys" to help us with this fact.

Cheers, Kiwi1 :ok:

[qwertyuiop]

Vref is constant for a given weight. It then has an addition for the wind (1/2 the constant wind plus the gust speed). The approach is therefore flown at a higher speed.
PS. I think airbuses do it slightly differently.

[AirRabbit]

I would hope that a flight crew who encountered some sort of anomoly regarding airspeed on final approach would provide more information than merely reporting "windshear on final." Some specifics are almost always needed to help following flight crews in making decisions.

Losing any airspeed on final is technically a shearing of the relative wind - although not the classic "windshear." How much airspeed was lost or gained, over what time period or altitude change, where was it on final, and what kind of airplane was it?

I would think it would be more than a little difficult to explain why you dinged the wing, got the tail, or just plain crashed when the guy in front of you reported "windshear on final." If that's all the information available , I think I'd wait for more information.

[Peregrine falcon]

Go-Around is the first thing that comes to my mind when encountering a significant instantaneous loss of airspeed. If the added wind margins do not prove to be sufficient perhaps one shouldn't be landing there and then. ;)

Never too proud to go-around.

:ok:

[Capt Claret]

phnuff

On both jet types I've flown (BAe146 & B717) at landing flap a signifficant amount of power is required to maintain V-APP (usually V-REF +5), so the engines are already spooled up and respond quite quickly to thrust lever advancement.

[Dan Winterland]

The Airbus managed speed protection system is known as 'GS Mini'. It adjusts your bugged speed to achieve a minimum groundspeed. The autothrust (if engaged) foolows the command. However, the Airbus doesn't always cope (esp. a heavy A321) and situations like the one you describe happening to the 737 aren't uncommon in the bus.

The windshear function is a part of the GPWS and will tell you that you are encountering it - i.e you have to have it to get the warning. The Predictive Windshear System does use the Wx Radar, but you have to have moisture in the air for the radar to detect it. This is a common misconception and I often hear Pilots I fly with talk about their reaction to a PWS event during the deaprture brief when there isn't a cloud in the sky!

[Piltdown Man]

And do we think that pilots report "Windshear" when turbulence might be a more appropriate description? I'm thinking that the (mechanical) turbulence generated by strong winds generates as many plusses as minusses and excluding the wind gradient close to the ground, is effectivley speed neutral. That is not to say the "Windshear" does not exist. I'm just suggesting that we too often call "Windshear" instead of "Turbulence". The basic reason we carry a few extra knots is to allow a few to get "robbed".

[Don Coyote]

I agree Piltdown often aircraft will report windshear when it is straightforward blustery conditions.

Before someone mentions it, I know that just because no one else reported it does not mean that the next aircraft will not suffer from windshear, however it is more often than not just blustery conditions.