An aircraft's autopilot is coupled to the flight management system (FMS) in LNAV mode during an RNAV (GPS) approach. Compared to HDG or VOR NAV mode, what does LNAV coupling primarily provide?
- ALateral guidance that follows the programmed GPS flight plan track, including curved path legs, rather than a single VOR radial or magnetic heading
- BDirect automatic control of engine power settings to maintain a constant descent gradient
- CAutomatic autopilot disconnect whenever GPS signal quality degrades below RAIM limits, without any annunciation
- DVertical guidance based solely on barometric altitude that replaces the need for glidepath information
Show answer & explanationAnswer & explanation
Correct answer: A. Lateral guidance that follows the programmed GPS flight plan track, including curved path legs, rather than a single VOR radial or magnetic heading
LNAV mode provides lateral navigation guidance by following the GPS-derived flight plan, including curved or complex path legs defined in the FMS/GPS database, unlike VOR NAV mode (a single radial) or HDG mode (a fixed magnetic heading). Vertical guidance and engine power are handled by separate systems (VNAV, autothrottle), not LNAV itself.
Why the other options are wrong
- B. Engine power control is handled by an autothrottle system, not LNAV.
- C. RAIM degradation triggers an annunciation and mode reversion, not a silent disconnect.
- D. LNAV is a lateral mode; vertical guidance is handled separately (e.g., VNAV or LPV glidepath).
LNAV Mode (Autopilot/FMS Coupling)
LNAV is a lateral autopilot mode that follows the GPS/FMS programmed flight plan track, capable of flying curved legs, unlike a single VOR radial or fixed magnetic heading.
- Lateral guidance only; vertical handled by VNAV/LPV separately
- Can fly curved path legs unlike VOR NAV mode
- RAIM loss triggers annunciated mode degradation, not silent disconnect
Memory trick: LNAV follows the GPS breadcrumb trail, curves and all.