French investigators are recommending changes to flight-director logic after finding that the pilot of ill-fated Air France flight AF447 may have overly trusted the instrument as he struggled to regain control of the Airbus A330.
Disconnection of the flight director is normally part of the "unreliable airspeed" procedure. But after the A330 lost its airspeed information, and the autopilot and autothrust automatically disconnected, the crew left the flight directors engaged.
The flight director crossbars disappeared and re-appeared intermittently as the crisis unfolded, and French investigation agency BEA says that the "credibility" of the crossbars could have been "strengthened" by their re-appearance.
"If they appear, it implies that the indications that they display are valid," it adds.
In its attempt to explain the crew's failure to recognise the A330's fatal stall - including the apparent disregard of the stall warning - the BEA has queried whether the pilot might have been "tempted" to trust the flight directors "without validating the information presented".
"The concurrence of the information from the [flight director] with the stall warning may have undermined the credibility of the actions to take in response to the warning," it adds.
It says that the flight director "seems likely" to have "exerted an influence" on the pilot's cognitive processes, noting that its design - a green crossbar, in the centre of the pilot's primary display - attracts attention.
The "charged emotional" situation, combined with the workload, may have led the pilot to trust the flight director "independently of any other parameter", says the BEA, and that he may have viewed the flight director crossbars as a means of maintaining cruise altitude.
Even if there is uncertainty about whether the crew followed the flight director's indications while the stall warning was active, it adds, the crossbars' orders were "in contradiction" with the appropriate inputs and "may have troubled" the pilots.
The BEA is recommending a review of flight director display logic, to avoid conflict if the stall warning is triggered.
It adds that this review should also examine whether the flight directors should be consistent with the autopilot and autothrust, which require specific crew action to re-engage them if automatically disconnected.
In its final report into the AF447 accident the BEA also recommends that the position of flight director crossbars be among the parameters stored by flight-data recorders.