VGP: have you used it?
 

• Have you used Vertical Glide Path (VGP) on GPS approaches?
• How does it work?
• Why not just use the LPV and VNAV for vertical guidance?
• Is it useful?
• When can you use it?
• What airframe or gps install did you use it on?


Thanks

-FT

My understanding is it comes on certain Honeywell FMS units and is used to get glidepath indications for visual approaches from the VDP on down. Yes? No? It has to be already be a visual descent the way I understand it, which would happen at the VDP in most cases.

Speaking Collins now, VGP is an approach mode for Baro VNAV that replaces VPTH. So, you enter an approach with a charted Baro VNAV descent gradient; arm APPR and VNAV, and intercept the course and path; the FD mode changes to VGP and the FD/ AP now behave like it is an ILS. Particularly the ALT Selector which can now be set to the missed approach altitude. The FD will NOT have a level-off altitude as in a non-precision approach with a MDA, the pilot will see a GP to the runway.

It does make compatible approaches more like an ILS; is not a substitute for LPV (it was developed prior to LPV) and is on all ProLine 4 and 21 installations, at least all Bombardier ones.

GF

The E-170/175/190/195 has the capability to do VGP approaches. On the E-Jets, it works much like an ILS only the needles are magenta instead of green. The sensitivity remains constant, unlike an ILS, so the course and glideslope don't get squirrelly close to the ground. We (S5) can shoot these approaches using LNAV/VNAV minimums on any compatible RNAV approach.

EvilMonkey -

If I understand it right - there is nothing to get squirrelly. It is a math problem whereas an ILS is an electronic signal in space most likely bounced off the ground and susceptible to all sorts of irregularities - - correct?

USMCFLYR

You got it. The "needle" sensitivity remains at a mathematical fixed sensitivity, 0.3 RNP, both for the lateral guidance and glidepath.

I know what you mean by squirrelly, though one I flew recently seemed to be doing dutch rolls just inside 10 miles on the localizer. Not so sure if that was the signal or my own poor autopilot, but it was noticeable!

USMCFLYR

Interesting. I've seen the 170 autopilot do some goofy things when someone drives a plane through the ILS critical area...kind of like a pronounced phugoid cycle with the pitch. What aircraft do you fly?

King Air 300.
The autopilot is not the best and sometimes you can't really tell up front if it is the AP or the signals that are bad/degraded. Some signals actually have bends or steps in them and that is just the way they are engineered.

USMCFLYR

Winning response, thanks. Various pieces of gear are hard to understand if you have never used them. I have not used baro-VNAV so am guessing you can't couple to most baro-VNAV approaches although you can possibly still get the FD indications.

The way I understand VGP is, it gives you a calculated glidepath based on whatever it uses (blended RNAV, Baro-VNAV, GPS?) which allows you to fly a coupled approach all the way down. There apparently is also some advantage to this besides being coupled, such as freeing the alt bug for other purposes which suggests it would need to be stuck at minimums otherwise.

Getting warm?