EvilMonkey

Again speaking of the E-Jets, it's using GPS to give you a calculated glidepath which you can leave coupled down to LNAV/VNAV mins. And you're correct, it frees up your altitude bug for the Missed Approach Altitude. One major advantage is that it lets you fly the approach nearly identically as an ILS. The only major procedural difference is that you're in LNAV for lateral NAV and VNAV for, well, VNAV. In the E-Jets, the autopilot is also less aggressive in pink needles (GPS/RNAV) than it is in green needles (LOC/GS on an ILS.) Interesting side note, the E-Jets cannot track a VOR in NAV mode, so if you want to truly track a VOR in green needles you have to do it with the heading bug like an old DC-9. But I digress...

Another interesting thing about using a VGP approach versus an ILS...when you are tracking an ILS glideslope, the GS needle is representing a descent angle to a fixed point, i.e. the touchdown zone. So, let's say you're staying a dot high to avoid the wake turbulence of a heavy jet in front of you. Staying one dot high would represent say a 3.25 descent angle to the same fixed point as staying right on the glideslope and descending at 3.0 degrees. So, although your vertical speed for a given airspeed would be greater riding the GS a dot high, you would still theoretically touch down at the same point, at least ignoring flare/roundout.

This is not the case with VGP. As I said in my conversation with USMC, the glide path that VNAV is giving you is FIXED. It does not increase in sensitivity as you get closer to the ground. So, let's say you were riding the VGP a dot high. You would remain a fixed height, say for example 50 feet, above the intended glidepath for the approach. You would not be going to the same point on the runway, and your vertical speed for a given airspeed would be identical. Also, your descent angle would remain 3.0 degrees (provided the original glide path was 3.0 degrees.)

Just a quick disclaimer, I'm not sure what angle a dot high represents for an ILS, nor the height above glide path a dot high rerpesents for a VGP, just threw some numbers in there for an example.

Hope this answers a few more of your questions.

galaxy flyer

FlightTest

The vertical path used in VNAV, whether VNAV or VGP, is based on the two lateral waypoints and the barometric altitudes, hence "Baro-VNAV". At colder than ISA temps the path will be shallower than at ISA, conversely at higher than ISA temps. This is because the true altitude at the first waypoint will be, in cold temps, lower than standard by a greater deviation than the lower waypoint i.e. the runway. All paths based on Baro-VNAV will have temp limits which must be observed unless the FMS has temp compensation.

Raw GPS fixes are not presently using GPS heights unless augmented with WAAS creating a LPV glide slope.

GF

twin angel

Ladies and Gents, when flying an RNAV (GPS/GNSS) Approach with LNAV/VNAV minimums do you push the APPROACH MODE of your flight guidance system?

galaxy flyer

On Collins ProLine, all versions, Yes press APPR to ARM the approach mode and VGP. VGP requires APPR and VNAV.

GF

EMAW

[QUOTE=twin angel;2580957]Ladies and Gents, when flying an RNAV (GPS/GNSS) Approach with LNAV/VNAV minimums do you push the APPROACH MODE of your flight guidance system?[/QUOTE

The key is DA vs MDA. APPR/VNAV (VGP) for DA, NAV/VNAV (VPATH) for MDA if it’s authorized.

galaxy flyer

Not necessarily true. You can use APPR/VGP on a Baro VNAV to either a DA (H) or MDA on Collins equipment.

GF

Adlerdriver

For the 777, 767, 757, 737, MD-10, MD-11 and I'm pretty sure the A320, the answer is "no".

On those aircraft (as equipped when I flew them or fly them now), the approach mode is used exclusively when flying an ILS.

An RNAV approach uses FMS programmed routing which is tracked by using LNAV (or "NAV" mode in some aircraft). Vertical constraints at specific waypoints are also entered in the FMS and vertical control using those constraints during the approach is accomplished using VNAV (or "PROF" mode in some aircraft). So, "Approach" mode is never engaged and the entire RNAV is flown using LNAV/VNAV or NAV/PROF.

cougar

For the newer Boeing aircraft (737NG, 787, 747-8, and later 777) this is called Integrated Approach Navigation (IAN). The FCCs use the vertical path from the FMC database, and the FCCs treat it like a glide slope. These aircraft have Navigation Performance scales which change to IAN scales after capture of the lateral and vertical approach path. Currently, for our 747-8 fleet, the only method of flying a Non-ILS approach is using IAN, which requires the selection on the approach switch, just like an ILS. The FMAs for lateral would be BCRS, LOC, or FAC (FMC generated Final Approach Course), and the vertical is G/P. Missed approach is typically set at G/P capture just like an ILS. As mentioned, the default vertical RNP on final approach segment is set at 135' and is linear to touchdown. The IAN vertical G/P scale will turn amber at one half scale which is approximately 70 feet. This is just under the AC 90-105A tolerance for vertical deviation on final (+ or - 75'). And like a coupled ILS, the IAN approach, once coupled requires TOGA or A/P and A/Ts off to change modes.

EMAW

Will it do it either way, yes. Is it necessarily smart, no. APPR/VNAV (VGP) is used for DA because it won't level itself off at a selected altitude. If you select NAV/VNAV (VPATH) it will level at the first altitude reached (selector or VNAV alt). Which is, why it's better to use NAV/VNAV (VPATH) for MDA mins for continuity.

cougar

The method of flying the approach doesn't determine the minimums (DA or MDA). It is the approach type that determines either a DA or MDA. If the approach is an APV, Approach with Vertical Guidance, then a DA may be used, in fact, it is published. The only APV approaches I'm aware of are RNAV (GPS) to LNAV/VNAV minimums, LPV approaches, and RNAV (RNP) approaches. All other Non-xLS approaches are Non-Precision approaches with a horizontal ROC which requires an MDA. To use a DA in lieu of an MDA for these approaches requires OpSpec C073.
Further, MDA is generally no longer used when using CDFA method. Typically a Derived Decision Altitude is set 50' above MDA, for the decision point to land or go around.