punkalouver There is a lot of confusion about what VNAV and LNAV actually are and what are they not.
The VNAV declared not available by ANSP is the result of the profile not taking you to the touchdown point. That invalidates the Instrument Approach Procedure, which for VNAV by definition must bring you with a constant slope to 50' over the threshold.
During VNAV+LNAV I.A.P you don't need(*) to check ALT-DIST as the sensor-instrument-autoflight-database combo is certified to be trusted.
Just like an ILS approach (with its own set of pre-conditions). On the day the VNAV would still work fine only bring them to the original threshold.
With LNAV you rely on lateral guidance from the FMC but the vertical profile becomes your task to solve, that's what the ALT-DIST table is for. Then you fly a target 3° profile along the charted values same as any other 2D IAP.
Think of LOC-only approach.
The ILS v.s. LOC analogy is helpful since, in a similar fashion,
- the 2 separate (VNAV+LNAV) and (LNAV) procedures are printed on the same sheet of paper
- the 2 separate (VNAV+LNAV) and (LNAV) procedures are completely identical laterally and vertically - trajectory wise
- the 2 separate (VNAV+LNAV) and (LNAV) procedures have different minima (DA/DDA/MDA+buffer/MDA)
- the 2 separate (VNAV+LNAV) and (LNAV) procedures require different authorisation (crew training, on-board installation, quality assurance of the database)
There's a small catch about the vertical guidance provided on the flight deck. For a LOC-only approach you would not the GP (most cases). Since the lateral and vertical profiles are both generated by the FMC onboard a VNAV+LNAV capable ship, when you fly LNAV-only, none of the displays, data and automation modes normally associated with VNAV is degraded in any way. Here the difference between
- flying the certified computer-generated profile and trusting it without cross-check (VNAV) and
- flying the ALT-DIST table with your own brainwork, using the FMC distance information to assure obstacle clearance (LNAV) - while the verical profile for VNAV remains visible and accurate
gets very very thin. But it is there! Yes, unless we talk low temperatures outside the IAP envelope, flying LNAV on a VNAV+LNAV capable aeroplane is pretty much only a state of mind.
This comes BEFORE we discuss which automation modes are available for each of the two options. Their overlap is brutal and the confusion is made worse by the fact that the most advanced vertical guidance mode of AP/FD is also called 'VNAV'.
(*)+(**) any explanation on this probably deserves a standalone thread.
- - - - - - - part 2- - - - - - - - - - - - - - - - - - - - - - - - - -
What wiggy explains about the no-modification rule for VNAV is a certification/authorisation requirement, breach of system integrity.
The easiest, most straightforward ways of flying an LNAV approach to a displaced threshold are to
a) Use the VNAV
autopilot mode to drive the aeroplane's vertical trajectory along the self-generated profile, while the brain is manually checking the ALT-DIST table (LNAV rules!!), as if nothing was amiss. Once visual, disconnect and manually deviate 80' above the profile, then fly the airplane towards the desired touchdown point.
b) Use the V/S autopilot and while manually checking the ALT-DIST table (LNAV rules) deliberately stay 80' above the checkpoint altitudes. This takes you down the perfect profile for the displaced landing.
Obviously, had there been a TEMPO LNAV procedure with officially published corrected altitudes, the 'A' above would be illegal.
