REASON FOR WIND ADDITIVE DURING LANDING
REASON FOR WIND ADDITIVE DURING LANDING
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Boeing recommend adding half the steady headwind component to the Vref. The steady stream airflow usually begins about 2000 feet above the surface. Below that height, the wind speed is gradually retarded due to surface friction. It follows therefore that unless a speed adjustment is made by use of say the throttle, there will be a gradual bleed off with IAS once the aircraft passes from being in free-stream flow at 2000 ft.
Boeing recommendation includes the statement that head-wind additives should be bled off approaching touch-down such that the IAS will be at Vref at the threshold. By this statement a 15 knot HW component additive on top of the published Vref, would need to be gradually bled off from 2000 ft down. In practice, few operators actually do this. Some assume you will lose the 15knot additive in the flare. Not so - unless it is gusty, and then it becomes another story altogether.
If the autothrottle is used to control the IAS, then I understand that Boeing state that additives are not needed any more, because the autothrottle system will counteract any sudden bleed off and recover to correct speed. This assumes that the pilot cannot think as fast as an autothrottle system and needs additives to allow for his slower reaction to speed bleed.
I believe that one of the causes of over-runs on wet runway windy landings is that wind additives have not been bled back and the aircraft crosses the runway with excess speed. Surprisingly few operators bleed off the steady headwind component to cross the threshold at the correct height at Vref.
The above comments are centred purely around steady headwind components and do not address gusts.
Boeing recommendation includes the statement that head-wind additives should be bled off approaching touch-down such that the IAS will be at Vref at the threshold. By this statement a 15 knot HW component additive on top of the published Vref, would need to be gradually bled off from 2000 ft down. In practice, few operators actually do this. Some assume you will lose the 15knot additive in the flare. Not so - unless it is gusty, and then it becomes another story altogether.
If the autothrottle is used to control the IAS, then I understand that Boeing state that additives are not needed any more, because the autothrottle system will counteract any sudden bleed off and recover to correct speed. This assumes that the pilot cannot think as fast as an autothrottle system and needs additives to allow for his slower reaction to speed bleed.
I believe that one of the causes of over-runs on wet runway windy landings is that wind additives have not been bled back and the aircraft crosses the runway with excess speed. Surprisingly few operators bleed off the steady headwind component to cross the threshold at the correct height at Vref.
The above comments are centred purely around steady headwind components and do not address gusts.
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The addition is to provide a safety margin from the stall/stickshaker during gusty conditions and to lessen the effect of the wind gradient during landing.
The ATR book for example, says 1 knot per knot HEADwind or gust value in full though max 15 knots.
The ATR book for example, says 1 knot per knot HEADwind or gust value in full though max 15 knots.
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I always thought that wind coorection are taken from the wind which is reported at 20-30 feet above R/w so 2000 feet logic does not make sence to me (correct me if i am wrong)and I agree that boeing does recommand wind correction but airbuss restrict you to a correction of max 20 knots and if you have configuration problem which requires you to put in 20 knots then they dont want you to put any correction.How can we ecplain that?
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bob3,
I am assuming you have transitioned from a Boeing to an Airbus, I will try and answer your question using A320 data.
After selecting the destination runway and you are programming the APPR page on the MCDU you enter in the QNH, temp, magnetic wind/direction and MDA/DA, and you select if you are landing conf 3 or conf full.
The box will have come up with a Vls based upon you weight and configuration. Vapp is a calculated number, this is explained in the QRH on page 2.25.
For a normal landing (including a planned conf 3 landing) and you are doing an auto land or using auto throttle Vapp=Vls+5kt+wind correction. If you are not using auto throttle Vapp=Vls + wind correction.
The wind correction is 1/3 headwind (excluding gust) to a maximum of 15 kts.
Based upon the ATIS wids you enter on the APPR page the aircraft calculates what is know as Ground Speed Mini (GS mini) which is equal to Vapp-ATIS Headwind component (max of 10 kts). The aircraft uses GS mini as the reference speed for the inbuilt automatic windshear protection during an approach.
As you are doing the approach with managed speed the aircraft is constantly monitoring your ground speed and constantly changing the managed approach speed = GS mini + actual H/W (or T/W) component, but it is not less Vapp and not greater than Vfe.
When using managed speed the aircraft will keep an IAS of GS mini + actual H/W (or T/W), minimum of Vapp, maximum of Vfe.
As the aircraft is monitoring the airspeed and ground speed, you do not need to add the wind and gust factors as on a Boeing.
For an abnormal/emergency landing you add the approach speed increment to Vref. The approach speed increment is listed in 2.26 of the QRH. The approach speed increment is referred to as delta Vref.
Calculating what wind component to add for a abnormal/emergency landing is the where most people have trouble.
If your delta Vref is greater than or equal to 20 kts Vapp=Vref + delta Vref. For example if you have a green+blue hydraulic failure, delta Vref from 2.26 is 25 kts, and say for 68000 kg landing Vref = 138 kts, so Vapp=138+25=163.
If your delta Vref is less than 20 kts Vapp=Vref + delta Vref + wind correction, up to a maximum of 20 kts. For example landing at 68000 kg, Vref =138 kts (QRH 2.25) with a SEC 1+2+3 fault the approach speed increment from 2.26 is 10 kts. Say your landing on runway 36, and the wind is 060/12, wind correction = headwind comp is 6 kts, then Vapp=138+10+6=154kts. However if the wind was say 060/30, the wind correction = headwind comp = 15 kts, but the wind correction (15) plus the delta Vref (10) > 20 kts so Vapp=138+20 = 158 kts.
Edit : moved the wind from 030 to 060, thanks
[This message has been edited by Zeke (edited 18 May 2001).]
I am assuming you have transitioned from a Boeing to an Airbus, I will try and answer your question using A320 data.
After selecting the destination runway and you are programming the APPR page on the MCDU you enter in the QNH, temp, magnetic wind/direction and MDA/DA, and you select if you are landing conf 3 or conf full.
The box will have come up with a Vls based upon you weight and configuration. Vapp is a calculated number, this is explained in the QRH on page 2.25.
For a normal landing (including a planned conf 3 landing) and you are doing an auto land or using auto throttle Vapp=Vls+5kt+wind correction. If you are not using auto throttle Vapp=Vls + wind correction.
The wind correction is 1/3 headwind (excluding gust) to a maximum of 15 kts.
Based upon the ATIS wids you enter on the APPR page the aircraft calculates what is know as Ground Speed Mini (GS mini) which is equal to Vapp-ATIS Headwind component (max of 10 kts). The aircraft uses GS mini as the reference speed for the inbuilt automatic windshear protection during an approach.
As you are doing the approach with managed speed the aircraft is constantly monitoring your ground speed and constantly changing the managed approach speed = GS mini + actual H/W (or T/W) component, but it is not less Vapp and not greater than Vfe.
When using managed speed the aircraft will keep an IAS of GS mini + actual H/W (or T/W), minimum of Vapp, maximum of Vfe.
As the aircraft is monitoring the airspeed and ground speed, you do not need to add the wind and gust factors as on a Boeing.
For an abnormal/emergency landing you add the approach speed increment to Vref. The approach speed increment is listed in 2.26 of the QRH. The approach speed increment is referred to as delta Vref.
Calculating what wind component to add for a abnormal/emergency landing is the where most people have trouble.
If your delta Vref is greater than or equal to 20 kts Vapp=Vref + delta Vref. For example if you have a green+blue hydraulic failure, delta Vref from 2.26 is 25 kts, and say for 68000 kg landing Vref = 138 kts, so Vapp=138+25=163.
If your delta Vref is less than 20 kts Vapp=Vref + delta Vref + wind correction, up to a maximum of 20 kts. For example landing at 68000 kg, Vref =138 kts (QRH 2.25) with a SEC 1+2+3 fault the approach speed increment from 2.26 is 10 kts. Say your landing on runway 36, and the wind is 060/12, wind correction = headwind comp is 6 kts, then Vapp=138+10+6=154kts. However if the wind was say 060/30, the wind correction = headwind comp = 15 kts, but the wind correction (15) plus the delta Vref (10) > 20 kts so Vapp=138+20 = 158 kts.
Edit : moved the wind from 030 to 060, thanks
[This message has been edited by Zeke (edited 18 May 2001).]
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Very large aircraft, or aircraft that don't have the high thrust to weight ratio of most modern big twins accelerate relatively slowly, around three knots per second. If you were flying into a 30knot head wind at a Vref of say 140knots then of course your groundspeed would be only 110knots, your inertial speed. If as you descend on your approach the wind shears away to L&V then you faced with an IAS of only 110knots, 30 knots below your Vref. Very close to, if not at Vstall with some obvious consequences. Some operators use a wind additive i.e fly faster than Vref to counter this possibility, others fly what they call a 'reference groungspeed technique'. This technique involves calculating the minimum groundspeed you should fly in the prevailing conditions to arrive over the threshhold, after having passed through any shear, at or close to your planned Vref. Then you won't be faced with having to accelerate a heavy aircraft from a very low inertial speed to recover lost airspeed. That's a very basic version of it, if you want any more details e-mail me.
Why is it always easier to see the typo's after you've posted??
[This message has been edited by Bullethead (edited 09 April 2001).]
Why is it always easier to see the typo's after you've posted??
[This message has been edited by Bullethead (edited 09 April 2001).]
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Quite interresting. To go back to the original question; If I fly exactly Vref over the threshold, as determend in the AFM, any windcomponent should not affect my stallspeed margin. Wind only changes my groundspeed over the runway. If I do not ad an extra couple of knots to my Vref, my groundspeed would be less (with an headwind component), so decreasing my landingroll. Now I can understand adding a HWC in gusts for safety margins on my Vref, but to always add a HWC to my Vref still keeps me questioning this procedure (although I have to use it as well). Please, can anyone comment on this one?
Regards, E.
Regards, E.
