Boeing Several crew reports have been received of erroneous flight director pitch commands consisting of differences between the captains and first officers pitch indications during initial climb after takeoff. Flight director pitch command differences of 5 to 7 degrees have been experienced after takeoff rotation and prior to reaching 1000 feet above the ground.
On one occasion, an operator reported that either the Captain's or First Officer's flight director pitch bar has commanded 21-23 degrees with the offside flight director commanding 15-18 degrees. The pitch command bar differences remained until another autopilot pitch mode was selected VNAV, FLCH, etc.
The flight director anomaly was also observed during a Boeing 757 flight test in which both the captains and first officers flight directors commanded to V2 instead of V2 + 15 knots when taking off with all engines operative. a review of the flight test data confirmed that the “dial up” (increasing the value displayed in the speed window by rotating the IAS/mach selector) logic in the flight control computer (FCC) control law was set whereby the FCC commanded the MCP selected speed to V2 instead of V2 + 15 knots. The scenario to cause this condition was determined to be as follows:
Flight director turned ON in the takeoff mode, airplane on the ground, followed by V2 being set (increasing speed direction only) with a very rapid spinning turn of the IAS/ MACH selector (more than 15 knots in approximately one tenth of a second).
If the conditions in the above scenario occurs, the FCC(s) (depending upon FCC processing time differences) may command one or both flight directors to use V2 rather than V2 + 15 knots as a reference after takeoff. The V2 speed reference will remain until both flight director switches are cycled or a different pitch mode is engaged. If both flight director switches are in the OFF position when V2 speed is set, or if the flight director switches are cycled to OFF (both must be off at the same time) and back to ON after setting V2, then the rotation rate used to set the V2 speed has no effect on the FCC logic and the flight director commanded speed will be V2 + 15.
Dialling the MCP speed selection in the decreasing direction has no effect on the FCC logic regardless of how fast the IAS/mach selector is rotated.
Flight crew personnel should be advised to not make rapid inputs when setting V2 for takeoff. A smooth steady rotation rate should be used when turning the IAS/ MACH selector to a higher speed value to prevent the anomaly from occurring. Regardless of the occurrence of this anomaly, safe operation of the airplane is not affected as the flight director will not command speed below V2.
Boeing Flight Operations Technical Bulletin 38, 24th October 1991