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Old 4th Oct 2008, 01:00
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Chris Scott
 
Join Date: Jan 2008
Location: Blighty (Nth. Downs)
Age: 77
Posts: 2,107
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SRS ends, strictly speaking, at the acceleration altitude, not the thrust-reduction altitude. These are normally the same (by default), but can be different if the crew chooses.

GS-MINI is, as the name implies, based on a minimum ground speed. before the approach, once the crew can predict with reasonable confidence what the surface-wind is likely to be at the landing threshold, they enter the figure into the approach Performance page. The FMS works out the correct Vapp, which is a threshold IAS based on weight and headwind component.

Now: let's take the sea-level ISA case, where IAS=TAS; a Vapp of 130kts; and a predicted headwind of 10kts entered into the Performance page. On a conventional aeroplane stabilised at Vapp (130kts) at a height of 500ft, if the headwind is 30 kts the GS will be 100kts. But when it comes over the threshold, where the headwind is only 10kts, it will need to have accelerated to a GS of 120kts to maintain the required Vapp of 130. This will require a lot of extra energy (from the engines), which may cause problems, particularly if the loss of headwind happens suddenly (like at night).

It makes sense, therefore, to ensure that the GS remains at or above 120kts throughout the approach, even though this initially results in a higher IAS (150kts at 500ft in this case). The "managed" speed (IAS) target on the ASI (used by the pilot and the autothrottle) goes up and down with the headwind, but never below Vapp. Reaching the threshold, provided the actual headwind equals the predicted figure, the speed target will be Vapp. If the wind is higher, the speed target will be above Vapp. This should not be a problem for stopping in the runway length, because the GS will be no higher than originally planned.

In practice? Works very well, particularly using manual throttle, provided the pilot uses the "managed" speed as a target speed; not a minimum speed. The power changes required in windsheer are far less than on a conventional aeroplane, because the GS (kinetic energy) is steady.

CHARTS can be clipped to the pilot's table, but this is normally stowed for take-off and landing. There's another clip by the DV window, under the chart light.

COMMS are conventional, varying with airline SOPs.
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