Hi Check Airman,
The BEA report of AF 447 accident
http://www.bea.aero/docspa/2009/f-cp...p090601.en.pdf on page 190 says:
"2.3.2 Operation of the flight directors
When an unreliable airspeed event occurs, the automatic control features (autopilot and autothrust) disconnected automatically. The crew could only then re-engage them by pressing on a dedicated push-button on the FCU. The flight directors behaved differently, insofar as the cross bars disappeared from the PFD, even though the flight directors were still engaged.
....
The credibility of the cross bars is strengthened by their disappearance followed
by their re-appearance: if they appear, it implies that the indications that they
display are valid;
ˆ Since they attract the crew’s attention (green colour and presentation in the
centre of the PFD), the presence of the cross bars could have influenced the
actions of the PF, notably in respect to his reaction to the stall warning;
ˆ It is only possible to be aware of the changes in active modes (when the cross
bars reappear) by reading the FMA, which is probably difficult to do in a high
workload situation induced by piloting or failure management tasks."
When the FDs reappeared automatically, their modes would be HDG and VS. If the aircraft had a rate of climb when the FDs re-engaged, then they would constantly request a pitch up command whilst the aircraft was descending in a stall.