View Full Version : Unreliable speed and the FPV

14th Mar 2004, 11:49
Can anyone out there explain which inputs to the Airbus (A320) Flight Path Vector are the the reason for the following statement (to be found just after the immediate actions for unreliable speed indications):

"If the altitude information is affected, the FPV is unreliable"

15th Mar 2004, 13:07
As I understand it the altitude is one of the inputs required for the calculation of the FPV.

The FPV is an indication of what the a/c is doing over the ground whereas the flight director is (mostly) related to what is happening with reference to the air mass. The IR knows the groundspeed and drift but also needs an altitude input to work out what FPA the a/c is achieving.

One of the failures that can occur is a partial ADR failure. This is (according to Airbus) quite possible and will result in failure of either the altitude or the speed outputs. If the alt on the #1 PFD fails the FPV will also go off. There is nothing (of course) about this in the FCOMs!

Incidentally the wind readout on the ND gives absolute nonsense readings if the a/c is unbalanced (beta target/lower trapezoid not centered). Therefore I must assume that the FPV in similar circumstances would be useless too...must try it tomorrow!

As always edited for spelling!

16th Mar 2004, 12:14
idg - thank you for the response. You're quite right that the FCOMs are often inadequate. Input from ADRs makes sense, but do you know any references for this? Would a partial failure of, say, ADR1 only affect the FPV on PFD1, thereby giving differing FPV indications on each side?

Interesting about the balance theory. Be careful what you do with the rudder when trying it out!



19th Mar 2004, 02:01
Sure wll be careful!
As I recall the FPV will dissppear frm boh PFDs whn there is problem with adr #1, but I will check an let you know. There never appears to be a dfferent indcation left to rt.