Well, the magenta bug moves with the ground speed mini function. This means that the autothrust will always have to command the VAPP target and should not be 10/15 knots above it.
I must confess that it happens a lot with our A320's (IAE) where the speed is between 5 to 10 knots above the VAPP target.
FCOM Volume 3, chapter 4, section 10:
VREF : Reference speed used for normal final approach.
Equal to 1.23 × VS of configuration FULL.
Displayed on the MCDU APPR page if landing is planned in CONF FULL (VLS CONF FULL)
VAPP : Final approach speed.
Displayed on MCDU APPR page.
Calculated by the FMGCs.
Represents : VAPP = VLS + wind correction + 5 knots.
The flight crew may modify VAPP through the MCDU. During autoland or when A/THR is on or in case of ice accretion or gusty crosswing greater than 20 knots, VAPP must not be lower than VLS + 5 knots.
VAPP TARGET : Represented by a magenta triangle.
Calculated by the FMGCs
Gives efficient speed guidance in approach during various windy conditions.
Represents :
VAPP TARGET = GS mini + actual headwind (measured by ADIRS)
GS mini = VAPP - TOWER WIND (headwind component along runway axis calculated by FMGC from tower wind entered on MCDU).
FCOM Volume 1, chapter 22, section 30 :
SPEED MODE IN APPROACH PHASE
When the aircraft flies an approach in managed speed, the speed target displayed on the PFD in magenta, is variable during the approach.
This managed speed target, is computed in the FMGS using the "ground speed mini function".
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.
TWR WIND
It is the MAG WIND entered in the PERF approach page. It is the average wind as provided by the ATIS or the tower. Gusts must not be inserted, they are included in the ground speed mini computation.
TWR HEADWIND COMPONENT
The TWR HEADWIND COMPONENT is the component of the MAG WIND projected on the runway axis (landing runway entered in the flight plan). It is used to compute VAPP and GS mini.
CURRENT HEADWIND COMPONENT
The actual wind measured by ADIRS is projected
on the aircraft axis
to define the CURRENT HEADWIND COMPONENT (instantaneous headwind).
The CURRENT HEADWIND COMPONENT is used to compute the variable speed target during final (IAS target).
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 2 limits :
0 knots as the minimum value (no wind or tailwind)
+ 15 knots as the maximum value.
The crew can manually modify the VAPP and TWR wind values on the PERF APPR page.
SPEED TARGET COMPUTATION
The FMGS continuously computes a speed target (IAS target) that is the MCDU VAPP value plus an additional variable gust.
The gust is the instantaneous difference between the CURRENT HEADWIND COMPONENT and the TWR HEADWIND COMPONENT. It is always positive (or equal to zero for no wind or tailwind).
The IAS target is displayed on the PFD as a magenta triangle moving with the gust variation. VAPP is the IAS target minimum value.
GROUND SPEED MINI (GS mini) COMPUTATION
Ground speed mini concept has been defined to prevent the aircraft energy from dropping below a minimum level during final approach. The GS mini value it is not displayed to the crew.
Last edited by Pander216; 10th Aug 2006 at 22:34.