VAPP calc w/ HW

[CaptainMongo]

VAPP calculation is:

VLS + 5 + 1/3 rd magnetic head wind component not to exceed VLS + 20.


On the PERF APPR page when I test this it doesn't work like that. Ie, I adjust the wind component (by inputting HW winds down runway at 10, 20, and 30 knots) but VAPP does not increase as I would expect. What VAPP appears to calculate is VLS + 5 + the 1/3 rd headwind component above 5.

What am I missing?

[vilas]

VAPP COMPUTATION




FCOM says it differently quoted below:


VAPP, automatically displayed on the MCDU PERF APPR page, is computed as follows:


‐ VAPP = VLS+1/3 of the TWR HEADWIND COMPONENT, or


‐ VAPP = VLS +5 kt, whichever is the highest.


"1/3 of the TWR HEADWIND COMPONENT" has two limits:


‐ 0 kt as the minimum value (no wind or tailwind)


‐ +15 kt as the maximum value.

[Amadis of Gaul]

VAPP calculation is:

VLS + 5 + 1/3 rd magnetic head wind component not to exceed VLS + 20.


On the PERF APPR page when I test this it doesn't work like that. Ie, I adjust the wind component (by inputting HW winds down runway at 10, 20, and 30 knots) but VAPP does not increase as I would expect. What VAPP appears to calculate is VLS + 5 + the 1/3 rd headwind component above 5.

What am I missing?

The +5+1/3 is probably a poor translation from the Frenglish. Either way, I don't think those extra 5kts are all that critical.

[CaptainMongo]

Wow.

Ok so the "Thales New FM Pilot Guide Release 1A" on page II.5 - 25 says:

VAPP [5L] This field is modifiable even if the field is dashed. Defaulted value is computed by the FMS, based on: - VLS + 5 KT + wind corr. (KT), - Wind correction equals 1/3 entered head wind component (KT), - Minimum VAPP is VLS + 5 and maximum VLS + 15 Value reverts to computed value when cleared, or when DEST RWY is changed.

Is the max VAPP adjustment 15 or 20 knots (our company manual has it as 20) Is there a newer Thales guide?

Vilas - could you please give me the page and effective date of the FCOM you referenced?

Amadis - my point is not the five knots, my objective is to ensure our manuals are correct.

[Amadis of Gaul]

I'd go with what your manual says, your company is the one paying your bills, not Airbus. And again, what's 5kts among friends?

[vilas]

CaptainMongo

The one I quoted was from Honeywell.
DSC-22_30-90 P 10/12
FCOM ← I → 15 FEB 13


It is also applicable to following MSNs


Ident.: DSC-22_30-90-D-00011975.0002001 / 17 AUG 10
11 Applicable to: MSN 0557, 0976-1818



However For following MSNs it calculates the way you stated.


Ident.: DSC-22_30-90-D-00011975.0001001 / 17 AUG 10
10 Applicable to: MSN 0138-0497, 0724-0943


VAPP COMPUTATION


VAPP, automatically displayed on the MCDU PERF APPR page, is computed as follows:


VAPP = VLS+5+1/3 of the TWR HEADWIND COMPONENT


"1/3 of the TWR HEADWIND COMPONENT" has two limits:


‐ 0 kt as the minimum value (no wind or tailwind)


‐ +15 kt as the maximum value.

[FlightDetent]

Seconded.

To a certain astonishment I recently learned too, that the (for me) standard calculation: Vref + highest of (5 kt ATHR or 1/3 HW) is actually a result of MOD from about 1997. Originally and on some airframes still, it is Vref + 5 kt ATHR + 1/3 HW.

Now, the tricky part is when manuals and FMS setting differ in the above respect, to find out which of them is wrong! :ouch:

CaptainMongo: PM me if you need the MOD number, I'd need to go digging for it.

[CaptainMongo]

Vilas,

I am flying MSN 871 today. The VAPP calculation is +5 or 1/3 up to 15, that isn't correct according to your listing, any ideas?

[vilas]

What I stated is from the manual, possibly as FD says your aircraft may have been modified.

[FlightDetent]

https://ctrlv.cz/shots/2016/10/18/LRb2.png
https://ctrlv.cz/shots/2016/10/18/5IVs.png

The DU ID is not really the same as FCOM reference, hence you'd go looking at FCOM DSC 22_30-90 I: SPEED MODE IN APPROACH PHASE -> Vapp COMPUTATION.

[safetypee]

Amandis,"what's 5kts among friends".
Quite a lot when viewed from the overrun area.

Has anyone knowledge of the underlying logic.
The +5 or 1/3 version has some correlation with previous practices, particularly if the wind addition was triggered by turbulence / gusts in stronger winds.
The alternative +5+1/3 appears to be an unnecessary buffer in steady winds, however if the the aircraft system is susceptible to wind gradient change then the double additive might be understandable.

Is there any automatic runway performance adjustment for these speeds?

[Amadis of Gaul]

Amandis,"what's 5kts among friends".
Quite a lot when viewed from the overrun area.



If your runway calculations hang on 5kts (assuming you're serious), then I dare say you're living somewhat dangerously.

[vilas]

safetypee
Actually the addition of 5kts is for ATHR otherwise not needed. Without ATHR it is 1/3 of headwind component. Airbus because of ground speed mini function doesn't need any addition for gust. Although later on airbus recommended the pilot to increase at least 5 knots in strong gusty winds.

[FlightDetent]

sftp: The performance data are the same, based on Vapp=Vref, without any speed additive. The corrective indexes table requires 90 m per 5 kt extra speed, and then it is up to the pilot to know how his Vapp is calculated.

With a quick calc, 5+ kt is +9,5% on kinetic energy. But YOU :E already knew that.

[safetypee]

Thanks for the replies.
FD, If I did know that (unlikely), then I have forgotten ;) . But then again I am not qualified on the Airbus.
Nearly 10% KE increase seems a large amount, what would this represent in stopping distance?
Furthermore this thread has identified some ambiguity for calculating the speed increase and thereafter perhaps some difficulty in recalling which adjustment applies and then considering a revised landing distance.
In practice the 5 kt discrepancy might not be a large effect, particularly with the introduction of FOLD, but on other types these 'small' amounts might seriously deplete the safety margins - margins which matter when landings are not as anticipated.

[FlightDetent]

Cheers, lost in translation. My bad, take two:

With about 64t landing weight +5 kt is +9% KE, add 5 kt tailwind and we are looking at 16% more KE. That does play a role - that's what I meant you did understand, but some others above may not appreciate fully.

Relevant e.g. here (https://www.google.co.uk/search?q=skiathos+airport&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjlyNWPquXPAhWFvhQKHZMHDhMQ_AUICSgC&biw=1421&bih=892).

5 kt adds approx 60-90 m on actual lading distance as per the Airbus 320 LD tables.

The Vapp is automatically calculated by the FMS on Airbus, from which you start calculating LD, so no ambiguity there. The gist of the thread being that suprisingly, there are two certified algorithms how to calculate Vapp for what are identical airframe-engine-brake configurations.