In order to prepare the reader for the discussion to follow, the Boeing 777 flight management system will be described in this section. In pilot terms this is called the mode control panel (MCP) and is located in the cockpit on the leading blunt edge of the glare shield. Notice the high level of complexity in such flight management systems in the discussion to follow.
Airspeed Cluster
The airspeed cluster of the flight management system shown in Figure 1 consists of the airspeed window, the airspeed dial selector, the engine thrust selector, the autothrottle selectors (A/T), LNAV, VNAV, and Flight Level Change (FLCH). During approach operations, intermediate and final target airspeed are selected by the flight crew based on two competing criteria. One is the assigned airspeed requested by Air Traffic Control (ATC) and the second and overriding airspeed is the performance limiting airspeed—if exceeded, the aircraft will enter a condition of dynamic instability.
Airspeed and airplane flight mode are selected as LNAV and VNAV relate to the active route of flight depicted on the navigation display; otherwise, Flight Level Change (FLCH) is used to change altitude. It is important to understand that the flight crew of Flight #214 was attempting to use FLCH during the final approach segment, which led to much confusion in part since this mode is not suited for such precision.
LNAV, if armed, engages if the airplane is within 2.5 NM of the active leg. During the final approach segment into SFO, if the crew had line selected the instrument landing system (ILS) approach to RWY 28L, they could have activated LNAV and used this feature to help provide runway alignment.
If engaged for a VNAV approach (which should have been utilized at the time, but wasn’t), the autothrottle operates in speed mode. When using FLCH for vertical trajectory control during approach, the autothrottle will not operate correctly.
The Heading and Track Cluster
The heading and track cluster of the flight management system shown in Figure 2 consists of the heading window, the heading selector, bank angle selector, heading-track selector, and the heading hold selector. Both heading and track will be necessary to be operational during the approach into San Francisco RWY 28L. If LNAV is engaged and the track line is intercepted, the window will display the track depicting the final approach course.