Automatic Flight
Autoflight systems can enhance operational capability, improve safety, and reduce workload. Automatic approach and landing, Category III operations, and fuel-efficient flight profiles are examples of some of the enhanced operational capabilities provided by autoflight systems. Maximum and minimum speed protection are among the features that can improve safety while LNAV, VNAV, and instrument approaches using VNAV are some of the reduced workload features. Varied levels of automation are available. The pilot decides what level of automation to use to achieve these goals by selecting the level that provides the best increase in safety and reduced workload.
Note:  When the autopilot is in use, the PF makes AFDS mode selections. The PM may select new altitudes, but must ensure the PF is aware of any changes. Both pilots must monitor AFDS mode annunciations and the current FMC flight plan.
Automatic systems give excellent results in the vast majority of situations. Deviations from expected performance are normally due to an incomplete understanding of their operations by the flight crew. When the automatic systems do not perform as expected, the pilot should reduce the level of automation until proper control of path and performance is achieved. For example, if the pilot failed to select the exit holding feature when cleared for the approach, the airplane will turn outbound in the holding pattern instead of initiating the approach. At this point, the pilot may select HEADING SELECT and continue the approach while using other automated features. A second example, if the airplane levels off unexpectedly during climb or descent with VNAV engaged, LVL CHG may be selected to continue the climb or descent until the FMC can be programmed.

737 Flight Crew Training  Manual
Early intervention prevents unsatisfactory airplane performance or a degraded flight path. Reducing the level of automation as far as manual flight may be necessary to ensure proper control of the airplane is maintained. The pilot should attempt to restore higher levels of automation only after airplane control is assured. For example, if an immediate level-off in climb or descent is required, it may not be possible to comply quickly enough using the AFDS. The PF should disconnect the autopilot and level off the airplane manually at the desired altitude. After level off, set the desired altitude in the MCP, select an appropriate pitch mode and re-engage the autopilot.
Recommended Pitch and Roll Modes
If the LEGS page and map display reflect the proper sequence and altitudes,
LNAV and VNAV are recommended. If LNAV is not used, use an appropriate roll
mode. When VNAV is not used, the following modes are recommended:

LVL CHG is the preferred mode for altitude changes of 1,000 feet or more. V/S is

preferred if the altitude change is less than 1,000 feet.
If unplanned speed or altitude restrictions are imposed during the arrival, the
continued use of VNAV may induce an excessive workload. If this occurs, use
LVL CHG or V/S as appropriate.

AFDS Mode Control Panel (MCP) Faults
In-flight events have occurred where various 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. These faults may be caused by an MCP hardware (switch) problem.
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 clears 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 this action does not correct the fault condition, the desired flight path can be maintained by selecting an alternate pitch or roll mode. Examples are included in the following table:
Copyright . The Boeing Company. See title page for details.
1.52 FCT 737 (TM) October 31, 2006
737 Flight Crew Training  Manual
Inoperative or Faulty Autopilot Mode  Suggested Alternate Autopilot Mode or Crew Technique 
HDG SEL  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 LVL CHG or V/S. V/S should be selected for descent on final approach. 
VNAV PTH (cruise)  Use altitude hold. If altitude hold is not directly selectable, use LVL CHG to automatically transition to altitude hold. 
　
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