The takeoff phase of flight starts with the selection of takeoff thrust at the start of the takeoff roll.
PEGASUS FMC
On airplanes with the PEGASUS FMC installed, if GPS is not available the FMC position is updated when the autothrottle is engaged for takeoff.
The takeoff mode of the autoflight system provides flight director guidance until LNAV and VNAV are engaged after takeoff. Preparation for this phase starts in the preflight phase and includes entry of the TAKEOFF REF page data.
The takeoff phase changes to the climb phase when climb thrust and VNAV are engaged. The climb phase continues to the top of climb point, where the cruise phase starts.
When changes are made to the departure runway and SID, the DEPARTURES and TAKEOFF REF pages must be modified to agree. The modified data are entered the same as during preflight.
During takeoff, the autothrottle commands the selected thrust and the autoflight system provides pitch and roll commands through the flight director.
FMC roll commands are provided when LNAV is engaged.
PEGASUS FMC
When climb thrust and VNAV are engaged the FMC provides pitch commands to maintain the climb speed until the acceleration height is reached. At the acceleration height the FMC commands acceleration to 250 knots.
NON PEGASUS FMC
When climb thrust and VNAV are engaged the FMC provides pitch commands to accelerate.
The VNAV commanded speed is limited by the airplane configuration. At acceleration height, VNAV commands a speed 5 knots below the flap placard speed.
Passing 10,000 feet, VNAV commands an acceleration to the economy climb speed, which is maintained until reaching the cruise altitude unless the climb profile contains other constraints.
During the climb, VNAV complies with the LEGS page waypoint altitude and speed constraints. A temporary level-off for a crossing altitude restriction is accomplished at the commanded speed.
When the climb speed profile causes an anticipated violation of a waypoint altitude constraint, the FMC shows the CDU scratchpad message UNABLE NEXT ALT. A different speed profile that gives a steeper climb angle must be manually selected.