Recently I had a philosophical discussion on board with a crew about the use of VNAV without LNAV engaged. I was surprised to discover that they were unaware of any of the potential pitfalls associated, so I add a section to the P&T.
I should say right now that I’m not actually happy with what I’ve written yet – it needs some series simplification, not unusual for anything I write the first time.If you read this and have thoughts on it – please comment below.
Practices & Techniques : VNAV without LNAV, without … VNAV
LNAV/VNAV work best when engaged together. Assuming a correctly programmed FMC, LNAV takes care of the lateral track, accounting for all sorts of turn constraints, flyby and flyover waypoints, calculating turning radii, all while providing accurate distance/speed data for the calculation of fuel, time and descent path. Meanwhile VNAV – when following the programmed LNAV track and assuming reasonably accurate wind data – will calculate the most efficient descent path and keep the aircraft to it. Speed/Altitude constraints At, Above, Below or a combination thereof are complied with irrespective of the MCP altitude setting – when a constraint can’t be met the crew (for the most part) are informed in a timely fashion. LNAV and VNAV work great in harmony together. And then there’s what happens when you split them up …
LNAV without VNAV
LNAV without VNAV is not a train smash. For the most part LNAV works fine on its own – however it’s been observed that crew sometimes forget their vertical profile is no longer being guarded by the LEGS page constraints – particularly when the engaged LNAV-FLCH climb/descent happens to comply with the calculated VNAV path. If the MCP altitude selector is not set correctly, and compliance with vertical constraints not monitored, altitude exceedences and off path deviations can result.
VNAV without LNAV
VNAV without LNAV seldom makes sense – particularly when the aircraft is in VNAV PATH. In this case the calculated descent path is being maintained (VNAV PTH) based on a projected lateral path that the aircraft is no longer following. Inevitably either more or less track miles are involved in the calculated descent and VNAV PTH makes less and less sense. In addition the aircraft may level off unexpectedly for altitude constraints on a flight leg that bears no resemblance to the actual lateral tracking of the aircraft. Updates and changes to the LEGS page can further corrupt the VNAV PTH calculation. It is highly unusual for VNAV PTH to be the correct vertical mode when not following the associated LNAV track.
VNAV SPD without LNAV is somewhat better in that at least the aircraft is usually descending in IDLE (or a fixed thrust setting) and the implication is that the PF is monitoring vertical path. Speed Control (and the option of additional thrust when the auto throttle is in HOLD) is at the behest of the PF – in this case one remaining trap here is that the VNAV PTH can re-engage by itself with (or without) LEGS page changes and now you’re in VNAV PTH without being on the LNAV track.