Constant Ground speeds....?

[mondriver]

I drive the 75!

I'm told on the Airbus....320/321 etc, the auto thrust maintains a constant Groundspeed......


Could someone explain to me, if this is indeed the case, why ?!!!!


I am not understanding !!!

[Basil]

Hmm, be a bit embarrassing if it had a steadily increasing tailwind to, say, 150kn :eek:

[idg]

Indeed the 320/1 autothrust does modulate speed in relation to groundspeed. The PFD displays a Vapp target during approach, so in fact either the autothrust or manual thrust can follow this variable target. It is called Ground Speed MINI(mum)

FCOM says:

GROUND SPEED MINI FUNCTION PRINCIPLE

The purpose of the ground speed mini function is to take advantage of the aircraft inertia, when the wind conditions vary during the approach. It does so by providing the crew with an adequate indicated speed target. When the aircraft flies this indicated speed target, the energy of the aircraft is maintained above a minimum level ensuring standard aerodynamic margins versus stall.

If the A/THR is active in SPEED mode, it will automatically follow the IAS target, ensuring an efficient thrust management during the approach.

The minimum energy level is the energy level the aircraft will have at touch down if it lands at VAPP speed with the tower reported wind as inserted in the PERF APPR page.

The minimum energy level is represented by the Ground Speed the aircraft will have at touchdown. This Ground Speed is called "GROUND SPD MINI".

During the approach, the FMGS continuously computes the speed target using the wind experienced by the aircraft in order to keep the ground speed at or above the "Ground Speed Mini".

The lowest speed target is limited to VAPP.

The speed target is displayed on the PFD speed scale in magenta, when approach phase and managed speed are active. It is independent of the AP/FD and/or ATHR engagements.

Wind is a key factor in the ground speed mini function.

As you can see a tailwind will not have any affect on the Vapp target. Normally it's affect is transparent to the crew but during strong wind conditions the speed changes can be quite dramatic!

[TopBunk]

The main reason for Ground Speed Mini is to guard against low airspeed in windshear / microburst / CB activity.

Take the microburst case, you fly the approach at say a constant 130kts IAS and into an increasing headwind component, so the Gspd decreases at the same rate. As you come out of the positive shear and into the negative shear, the headwind rapidly changes to a tailwind and suddenly your IAS falls rapidly and you have BIG problems (a la Delta (?) Tristar 15 years ago or so into DFW (?)).

With Groundspeed Mini, your initial 130kts IAS would increase by an amount equal to the increase in headwind component, giving a constant groundspeed, maybe to say 170kts IAS. On coming out of the positive shear your IAS would reduce rapidly but from a higher value, back towards the datum of 130kts. Result aircraft safe(r).

Some problem areas have been found to exist with the A319 current software and it can cause some flap limiting speed issues and be fun when asked for 160 to 4d when you resume 'managed' speed at 4 miles for the speed to increase to 175 or so!

Hope this helps

[Basil]

I believe Cathay use a min GS final approach technique on manual power too.
Personally I just add a 'suitable increment' and, if it turns to worms, it's a go-around :=