Originally Posted by
vilas
As per the FCOM I quoted, in approach with flaps full the GSmini limit is VFE177-5=172kts a margin of 5kts from VFE and in approach with flaps three the limit is VFE NEXT which is 177kts while the VFE flap3 is 185 i.e. margin of 8kts from VFE Conf3. So the extra protection from VFE offered in CONF3 is a mere 3kts as compared flaps full. If you consider the much less drag of flaps 3 it is easier to exceed VFE in 3 than in full. The comfort factor is only imaginary. There is no dispute about using FLAP3 in expected windsheer conditions or where better climb gradient is required.
citation 2 I used the word gusty conditions what I meant was turbulent conditions.
Last thing, instead using home made pickles why no one in all these years asked the manufacturer what his idea is about the matter?
Thanks for clarifying that,
vilas.
One minor observation, if I may: I think it's important not to confuse GS-mini - i.e., the minimum g
round-speed calculated for an approach by the GS-mini system - with the "managed" IAS it calculates to comply with that minimum GS (and therefore, by definition, the Vapp). One way of avoiding possible confusion may be to refer to the latter datum as "GS-mini IAS", and the former as "GS-mini ground-speed" or "GS-mini GS".
Once the wind and flap setting have been entered into the PERF page, the GS-mini GS will remain fixed. But the GS-mini (managed) IAS will fluctuate according to the headwind component.
I agree that Flaps 3 is unlikely to be much help in avoiding a temporary VFE exceedance in the common situation that
Unhooked describes, although the figures suggest - as you say - it gives an improved margin of 3 kt.
For beginners, can we put some numbers to a typical, gusty approach scenario and compare the two configurations?
Whatever wind has been entered into the PERF page, the Vapp for Flaps 3 will always be 5 kt higher than for Flaps Full.
Accordingly, the fixed GS-mini ground-speed will be 5 kt higher with Flaps 3. The GS-mini IAS will also be 5 kt higher with Flaps 3, unless VFE considerations come into play.
Flaps 3 landing
Let's take a simple, sea-level/ISA example, perhaps at night, where the runway HWC (head-wind component) is 10 kt, and the FMGC has calculated a Flaps 3 Vapp of 140.
Therefore, GS-mini GS is 130.
At 4-miles final, with a selected IAS of 170 (previous ATC requirement), the flight crew select Flaps 3 and then push the speed knob to command managed IAS for final approach.
At that moment, the HWC is 50 kt.
The system therefore calculates a GS-mini IAS of 180, but limits it to 177 (the VFE Flaps Full).
As the IAS reaches 177, the HWC increases by 10 kt in a gust, causing an instantaneous rise of IAS to 187.
So the VFE Flaps 3 is momentarily exceeded by 2 kt.
Flaps Full landing
Now let's use the same wind conditions as above, so the FMGC has calculated a Flaps Full Vapp of 135.
Therefore, GS-mini GS is 125.
At 4-miles final, with a selected IAS of 170 (previous ATC requirement), the flight crew select Flaps 3 and then Flaps Full, and then push the speed knob to command managed IAS for final approach.
At that moment, the HWC is 50 kt.
The system therefore calculates a GS-mini IAS of 175, but limits it to 172 (VFE - 5).
As the IAS reaches 172, the HWC increases by 10 kt in a gust, causing an instantaneous rise of IAS to 182.
So the VFE Flaps Full is momentarily exceeded by 5 kt.
IMO, we can conclude that the extra margin from VFE of using Flaps 3 is indeed 3kt, but the aeroplane is slightly more "slippery" when the pilot wants to slow down.