The Weird and Wonderful Logic

A318/A319/A320/A321

Professional

Vol

8-03-8

10 July 2018

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 airplane 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 autot hrottle) 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 Thrust Lever, provided the pilot uses the "managed"

speed as a target speed; not a minimum speed. The power changes required in windshear are far less than

on a conventional aeroplane, because the GS (kinetic energy) is steady.”

Using MINI GS is not without drawbacks. You might see that your managed speed (and thus ground speed)

fluctuates and in a busy environment this could cause problem in the traffic flows.

SPEED REFERENCE SYSTEM

The Speed Reference System (SRS) will guide the aircraft along the vertical path during the take-off phase

(and go around stage). It prevents the aircraft from climbing too steep and loosing airspeed.

It is armed when V2 in entered on the PERF page of the MCDU and flaps/slats not fully retracted.

It is triggered by the thrust levers placed in the take-off position and will control pitch after rotation and it

will maintain an airspeed between V2+10 knots and V2+15 knots and at minimum a 120 FPM climb. When

vertical speed drops below 120 FPM the system allows the speed to decrease to V2.

FLAPS 1

Another fine example of Airbus logic. If you move the flaps lever from 0 to 1 airborne, the flaps will not

move! Only the slat (at the front of the wing) will deploy. Of course that would be too easy so on the

ground flaps 1 WILL extend the flaps and when you retract them from 2 to 1 they will stay out. Don’t even

try to get your head around this, accept that the engineers thought about this and that it is best for the

aircraft.