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V/S can be used to adjust the rate of descent. The use of low values of V/S, e.g. less than 1000 fpm, may be appropriate for small altitude changes as it makes the guidance smoother and needs less thrust variation. In areas of high traffic density, low values of vertical speed will reduce the possibility of nuisance TCAS warnings. As a guide, a value of -1000 ft/min is appropriate when in close proximity to other aircraft. The A/THR mode will automatically revert to SPEED and adjust thrust to maintain the target speed. In this configuration, the use of speedbrakes is not recommended to reduce speed, as it is inconsistent with the A/THR mode. When in V/S mode, pay particular attention to the speed trend, as the AP will attempt to maintain the selected V/S irrespective of the effect this has on the aircraft speed. If the pilot selects a very high V/S, the aircraft may be unable achieve both the selected V/S and target speed with idle thrust. In this case, the AP/FD will guide to the target V/S, and the speed will increase. When VMAX is reached the AP will pitch the aircraft up so as to fly at a V/S to maintain VMAX.
In OP DES or V/S mode, the level arrow . is displayed on the ND to indicate the interception point with the altitude set in the FCU.
Descent
FCTM REV 1 (6 JUN 05)
SPEED CONSIDERATIONS Managed
The managed descent speed computed by the FMGS defaults to ECON speed and provides the most economical descent profile as it takes into account weight, actual and predicted winds, ISA deviation and Cost Index (CI). If a speed is inserted into the PERF DES page for operational or policy reasons, then this becomes the managed speed for the descent. Once the descent phase is active, the managed descent speed cannot be modified. The managed descent speed also takes into account any speed constraints, which may be modified during the descent phase, e.g. the default speed limit, which is normally 250 kt below 10000 ft.
In turbulent conditions, adjust the speed or Mach target to allow adequate margin below VMO/MMO. In severe turbulence, select turbulence penetration speed. If the speed rapidly approaches the upper end of the managed speed band with the autopilot engaged, a transient increase above VMO is possible. Monitor the trend vector and, if an exceedance looks likely, either select a lower speed target on the FCU or select OP DES and a suitable speed below VMO. If the rate of exceedance is rapid, fly the aircraft manually. The autopilot will disengage if high speed protection is activated.
Selected
If necessary, the descent speed can be selected on the FCU. This may be required to comply with descent constraints, ATC clearances or during weather avoidance or penetration. In selected speed, there is no longer a target speed range.
DESCENT MONITORING
The PFD displays the magenta Vertical Deviation (VDEV) symbol which, within +/. 500 ft, indicates the aircraft's vertical displacement from the computed descent path. This information is only accurate if the aircraft is close to the lateral flight plan with the waypoints having sequenced correctly. The actual VDEV is also displayed numerically on the PROG page. VDEV information is available both in managed and selected descent.
At lower altitudes, when in HDG or TRK, the energy circle on the ND indicates the required distance to descend, decelerate and land from the present position.
The managed descent profile from high altitude is approximately 2 1/2.. As an estimation of the distance to touchdown is required to enable descent profile monitoring, it is important to ensure that the F-PLN plan page reflects the expected approach routeing. Gross errors in the descent profile are normally a result of either incorrect programming of the MCDU or non-sequencing of F-PLN waypoints, giving a false distance to touchdown.
FCTM Ch 10 refers.
5.30.6
A330/A340 Climb, Cruise And Descent
REV 1 (6 JUN 05)
FCTM Descent
DESCENT ADJUSTMENT
Weather avoidance or ATC vectoring may require descent profile adjustment.
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