Lateral Navigation (LNAV)
LNAV uses the flight management system (FMS) to follow the entered flight plan laterally (left and right). The LNAV mode can be armed on the ground before takeoff or in the air.
The LNAV mode can engage when the route path is intercepted or when you are close to the route (within about 2NM either side).
Vertical Navigation (VNAV)
VNAV is a combination of vertical steering and automatic airspeed adjustments. It factors in the altitude and speed constraints on the route as well as the aircraft performance and economical factors. VNAV can be armed on the ground before takeoff or in the air.
VNAV engages directly during the climb, it then commands a climb airspeed and levels off at altitude constraints.
When the cruise altitude is reached (VNAV armed) VNAV PATH engages. Before the top of descent (TOD) you can set a lower altitude, then the autopilot will descent automatically at the TOD.
During the descent VNAV PATH engages when you intercept the planned vertical profile of the route.
After takeoff the vertical navigation target airspeed (VNAV SPD) is kept below the maximum allowed flap speed with a 5 knot margin.
- When above flap retraction speed retract the flaps by one setting to allow a higher VNAV SPD target
- Repeat until flaps are retracted
During approach phase VNAV tries to slow down to approach airspeed. The airspeed is kept above the minimum speed for the next lower flap setting.
- Extend flaps by one setting to allow airspeed to drop to the next lower airspeed
- Set landing flaps when you are getting below about 2000ft RA
When you capture the glide slope VNAV disengages and you have to manually adjust airspeed again.