Approach Timing

(Michael) wrote

This method is used by the flight control software of the Airbus A-320
series of aircraft. I **** you not. Straight from the lips of an
A-320 captain. The autothrottles adjust to a given groundspeed on
approach. Groundspeed, not airspeed. No, I don't know why either -
but he insists that it's true.

Well...I have a pretty good idea why without ever having been
in an Airbus. MicroBurst. On an ILS using a "normal" autopilot
and autothrottles, when approaching a microburst, the autothrottles
(or human pilot) will retard the throttles to maintain Glideslope
and IAS and then shortly after passing the microburst, one finds
himself with reduced power and rapidly reducing airspeed. Been
there--Done that!
By flying a groundspeed that is proportional to the strength of the
microburst on the front side of the microburst, one just might survive
the backside. Without the Groundspeed input, one just adds some
arbitrarily chosen number of knots to the IAS, wanting at the same
time to keep it as low a number as possible because you had to get
rid of any excess prior to touchdown.
In really bad conditions, indicated by a larger than normal difference
between IAS and GS, I always kept the GS up.

Bob Moore
ATP B-727 B-707
PanAm (retired)