Originally Posted by
hans brinker
GS mini is not for gusts. It does what the name says: ground speed minimal. Simplified: It takes your current headwind component, checks it against the head wind component for the runway based on the wind on the perf page, and adds the difference to Vapp. If the difference is negative, it doesn't subtract, to not go below Vapp. This will keep your ground speed and kinetic energy relatively constant.
Suppose the ground winds are calm, the headwind on the approach is 30kts until 200', Final approach speed is 140kts. In product B, you would be flying 140kts IAS, and have 110kts GS. Passing 200' you would still have 110 kts GS, and your airspeed would be decreasing towards 80kts IAS, with a huge power increase required. Not a good situation, so Airbus decided to introduce GS mini. Difference between headwinds aloft on the approach and on the ground is 30kts, Vapp is 140 kts, so bug at 170 kts (yeah, on the 320, can be pretty close to 177, I have seen that). As you pass through the change in wind, GS stays 140, the IAS changes from 170 to 140, no huge power increase required, and everything is great.
Is it perfect? No, but is a lot better than not having it. The higher approach speed means higher power, and as you enter the shift to less headwind that can lead to the speed increasing before the AT reacts, so probably less than 100% HC would be enough.......
Finally, someone gets it and concurs with what I posted earlier. It's about the difference in surface and aloft winds, for the reasons mentioned already.
nothing to do with transient gusts.