Final answer:
VNAV descent modes, part of an aircraft's flight management system, include VNAV PTH (Path), which targets a glide path angle for waypoints, and VNAV SPD (Speed), which focuses on maintaining programmed speed during descent.
Step-by-step explanation:
VNAV descent modes are part of an aircraft's flight management system (FMS) that help pilots to manage the descent phase of a flight efficiently.
VNAV, which stands for Vertical Navigation, calculates the optimal descent profile according to performance data, weather conditions, and air traffic requirements. The two primary VNAV descent modes are:
- VNAV PTH (Path): This mode targets a specific glide path angle to reach a predetermined waypoint at a certain altitude. During VNAV PTH descent, the aircraft's autothrottle system maintains speed by adjusting engine power.
- VNAV SPD (Speed): In this mode, the airplane descents to maintain a programmed speed. If the aircraft's speed deviates from the set speed, the autothrottle adjusts the power or the flight path is altered to correct the speed.
These modes assist in ensuring a smooth and fuel-efficient descent, while also adhering to air traffic control restrictions and the aircraft's performance limits.