With an Stec 60-2 one can force the autopilot to track the GS by pressing the ALT button. The normal sequence is to intercept from below and to accomplish this with the aircraft in heading mode and altitude hold mode flying the vector to final, the pilot presses HDG and Nav simultaneously to fly the final vector heading heading until the intercept to capture the localizer or lateral course (NAV, HDG, and ALT should be annunciated). On intercept of the course, HDG will extinguish and APR will annunciate. , ALT will remain on until all the correct signals are present for a period of time and then GS will also annunciate to indicate it is armed. When the intercept point is reached, ALT will extinguish. If the conditions are not met for the GS to arm, and the pilot presses ALT (already engaged), this extinguishes ALT and turn on GS, the result being a sharp move towards the GS from above or below.
Autopilots other than the CIII that have GS tracking capability can also intercept from above or below the GS. If APR is engaged and the aircraft is directed thru the GS zero indication (from above to below or the more normal case from below to above), GS tracking will take over just past the cross over.
Timothy,
Any airplane I have flown with A/P has had either a VOR/LOC button and an APP button, or has had a VOR/LOC and a GS button. In the first instance, pressing APP the aircraft will NOT climb to capture the GS. In the second case, it’s a bit harder to say because I can’t remember the A/P model and it was a very very weird and defective airplane, however pressing GS in it would make it climb to intercept. I stress again that this was a very quirky airplane with lots of problems so can’t really suggest that this is normal.
Flying an ILS we use APP mode which as stated above will only hold the aircraft level until it intercepts.
flying an LNAV/VNAV approach we use LNAV and VNAV modes (naturally). Pressing VNAV when above the ‘glideslope’ will make the aircraft either descend, or fly level until the point at which it can descend to the FAF at idle thrust, at which it will start the descent as long as we have the platform altitude in. If we are already are below the ‘glideslope’ pressing VNAV will command level flight until intercept.
I can not (part from the quirky aircraft mentioned above) think of any approach or any normal autopilot approach modes which would cause a climb to intercept a glide path.
Many thanks for the detailed explanation, jwoolard and NCYankee 
I have to admit, this is damn complicated…
… +v glideslopes on LNAV only approaches? IMO this is a killer feature.
I agree 100% – LPV is very slow coming to Europe in an operationally significant way (airports with Customs but no ILS, etc). I have written to Avidyne asking the question.
The G1000 supports Baro VNAV, but not for the intermediate and final approach segments. The only exception to this is the Garmin King Air retrofit STC which does support Baro VNAV for approaches. The WAAS GPS units are also certified in the US to fly the LNAV/VNAV minimum line on an RNAV approach using the WAAS for vertical guidance in lieu of a Baro VNAV system. There are some approaches outside of the WAAS service area, for example in Puerto Rico, that WAAS is not permitted to fly the vertical for the LNAV/VNAV and this is noted on the approach charts. Flying the LNAV/VNAV with Baro VNAV does not require WAAS for the lateral guidance and uses the Baro VNAV for the vertical, but has temperature limitations. If the approach is within the WAAS service limits, a WAAS GPS may fly the LNAV/VNAV without any temperature limitation since the vertical guidance is not affected by temperature. The vertical path flown by the WAAS is not identical to the one flown by a Baro VNAV, but the vertical full scale deviations are limited on the WAAS path so that the two paths are close enough. The Baro VNAV path is a curved path in space whereas the WAAS VNAV path is a straight line in space.
Which light GA avionics support BARO-VNAV?
None – not even the G1000 can do this on an approach.
The same glideslope can however be flown by a waas gps, if the approach is approved for this.
The UK and Sweden (at least) seem to be lagging behind approving VNAV approaches for GPS. Germany certainly supports them; I flew one at EDDG on Saturday.
Do the non-garmin boxes give +v glideslopes on LNAV only approaches? IMO this is a killer feature.
Actually interesting to know if the new Avidyne boxes do an advisory glideslope.
Not quite: LNAV/VNAV approaches can be flown either by baro-vnav or by waas gps. This tracks the VNAV glideslope as surveyed and intended by the procedure designer. Some VNAV approaches are not permitted for GPS operations – in this case you will only get LNAV guidance: you won’t get a +V glideslope.
For (hypothetical) approaches with all three of LNAV, LNAV/VNAV, and LPV minima the system will choose the best available. The LPV glideslope may well be coincident with the VNAV glideslope, but the presentation would be different as the LPV glideslope has a much smaller distance to full scale deflection (it’s more like an ILS).
LNAV+V is a garmin provided pseudo glideslope linking the FAF to the MAP. If the gps degrades from VNAV or LPV to LNAV only then the +V glideslope will be inhibited.
Do the non-garmin boxes give +v glideslopes on LNAV only approaches? IMO this is a killer feature.
So a WAAS GPS can fly an approach published for BARO-VNAV, with vertical guidance?
Which vertical profile is it tracking?
The +V advisory glideslope, or the LPV glideslope?
Which light GA avionics support BARO-VNAV?
Isn’t LNAV/VNAV only for BARO-VNAV aircraft i.e. not light GA?
No, not at all. It was originally used for aircraft with FMS systems and used BARO-VNAV. Now can be flown with WAAS GPS.
which is exactly what it does in Sidney NY.
Presumably only if you go below the MDA, which is fair enough… The Sidney is a well discussed case but it is a non-precision IAP so…
Obstacle clearance below the MDA is not assured if the L+V trajectory is continued, except on ILS or GPS/LPV.
Isn’t LNAV/VNAV only for BARO-VNAV aircraft i.e. not light GA? I am not sure.
