Boeing Several operators have reported in-flight events where various Autopilot Flight Director System (AFDS) pitch or roll modes, such as LNAV, VNAV or HDG SEL, became un-selectable or ceased to function normally. Typically, these types of faults do not generate a failure annunciation to the flight crew and may be caused by an MCP hardware (switch) problem. This bulletin provides crews with guidance to temporarily resolve or work around such faults.
If an AFDS anomaly is observed where individual pilot-selected AFDS modes are not responding normally to MCP switch selections, attempt to correct the problem by disconnecting the autopilot and selecting both flight director switches to OFF. This will clear all engaged AFDS modes. When an autopilot is re-engaged or a flight director switch is selected ON, the AFDS default pitch and roll modes should engage. The desired AFDS pitch and roll modes may then be selectable. If the above action does not correct the fault condition, desired flight path can be maintained by selecting an alternate pitch or roll mode. Examples are included in the following table:
| Inoperable or Faulty Autopilot Mode | Suggested Alternate Autopilot Mode or Crew Technique |
|---|---|
| HDG SEL or HDG HOLD |
Set desired heading, disconnect AFDS and manually roll wings level on the desired heading, and re-engage the AFDS. The AFDS will hold the established heading. |
| LNAV | Use HDG SEL to maintain the airplane track on the magenta FMC course. |
| VNAV SPD or VNAV PTH (climb or descent) |
Use FLCH or V/S. Note: V/S should be selected for descent on final approach. |
| VNAV PTH (cruise) |
Use altitude hold. If altitude hold is not directly selectable, use FLCH to automatically transition to altitude hold. |
| LOC | Use LNAV and monitor and fly the approach referencing localizer raw data. |
| G/S | Use V/S or VNAV PTH to descend on an ILS approach, and monitor and fly the approach referencing glideslope raw data. |
Boeing Flight Operations Technical Bulletin 74, 15th November 2005