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IMPORTANT: 744v2 SP1 FLAP Logic

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    Posted: Oct-30-2015 at 9:00am
Hi All,
 
Here is information on another 744 SP1 system logic refinement:
 
FLAP Speed Logic

 

The PFD command airspeed bug (CAB) follows commands by the FMC when in VNAV PTH.  In other words, it follows what is programmed in the FMC.
 
In the descent and while above the speed transition point (240/10000) the CAB will usually point to ECON descent speed. The speed below 10000ft is 240KTS and, as the descent continues, the CAB will move to the next speed constraint or a speed/alt restriction at a waypoint as required by an approach procedure or entered by the flight crew.
 
If FLAPS are inadvertently selected when the airspeed is above the FLAP placard speed, the CAB will move to the placard speed less 5KTS, if this speed is less than the next speed constraint.
So if the aircraft is descending at 240KTS with the CAB on 240 and FLAP20 is selected, the CAB will go from 240 to 225 (5KTS less than the flap 20 limit of 230KTS). This is assuming that no other deceleration is immediately necessary to meet the next constraint.
 
If the CAB is driven below minimum maneuver speed (top of the PFD Speed Tape Amber Band) the A/T will increase thrust to maintain speed above the amber band. However, the speed bug will not move from where it was below the amber band.  This speed protection is not dependent on where the CAB is pointing and the CAB will not move.
 
So why is this important?  Let’s say the aircraft is past the approach speed restriction point with FLAPS up.  The real world is modelled and the CAB will drop to, for example an approach speed of 179KT, only a few knots above the Red Bricks.  The airspeed will now decay from 240kts, flight crew entered DES speed restriction or the waypoint speed restriction.  However, A/T airspeed protection logic will keep the airspeed 5KTS above the top of the Amber Band (Min Maneuver Speed).  As FLAPS are selected the Amber Band is driven down until airspeed reaches the CAB, in this example 179KTS, at which point the airspeed holds.  It’s also important to recognize what is happening if FLAPS are inadvertently applied, during cruise for example 
Best,
Jim




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