Sign in to follow this  
JDWalley

G1000/GMC710 Approach question

Recommended Posts

First off, even though I'm writing about my experience of the Carenado C208 Grand Caravan EX, I'm posting this here because I suspect it applies to all (payware?) aircraft using this form of the glass cockpit, not just Carenado's offering.  (Feel free to move it if you think it only applies to the Carenado aircraft, or if it better belongs in some other forum.)

 

I have my first payware GA aircraft with an implementation of the G1000, in this case combined with the GMC710 autopilot.  In flying from one location to another, I'm able to put myself on a pre-created flightplan using NAV on the autopilot using GPS mode.  I'm also able to use the PROC key to set up and activate an approach to the destination airport (in this case, a GPS approach, since the runway I'm using doesn't have ILS).  The aircraft is flying that approach laterally just fine.

 

Do I need to also invoke APR mode on the autopilot?  If so, what is supposed to happen?

 

I ask this because, in my experience, I get "LOC" annunciated in white on the PFD, but it never turns green (not surprising, since -- as I already pointed out -- there is no ILS on this runway).  But the aircraft will generally, a couple of miles out on what I seem to have established as a glideslope based on the PAPI lights, suddenly start climbing steeply (no, ALT HOLD mode isn't on; it got switched off when APR was invoked, and, besides, I dialed the altitude down to field level as a precaution), occasionally stalling due to airspeed drop in the process, and I have no choice but to switch off the autopilot and wrestle the aircraft down to the ground in what has suddenly become a short-field landing more appropriate to bush flying.

 

Does this mean that, when I'm doing a GPS (i.e. non-ILS) approach with the G1000/GMC710, I shouldn't invoke APR mode, and that the latter should only be done with ILS approaches?  If that's the case, what is the proper procedure for carrying out a GPS approach with the G1000 combo?  If, on the other hand, I should invoke APR mode on the GMC710, what is supposed to happen?  Unfortunately, the Carenado documentation is basically just a "guide to all the buttons and menus" that doesn't given the big picture, and everything else I've searched on the web seems to leave out that information.

Share this post


Link to post
Share on other sites
Help AVSIM continue to serve you!
Please donate today!

What approach are you flying at what airport?

 

Are you following the charted altitudes on the approach plate?

 

 

The autopilot might be trying to follow a vertical profile for that approach and you may be too low.

Share this post


Link to post
Share on other sites

O.K., more details (after trying this out several times):

 

I'm flying the GPS 34L approach to KPAE (field altitude 606) out of KSEA.  The approach chart has three waypoints, RARYO (altitude 3000), USDAW (altitude 2200), and ZEREN (altitude 1160).  Unfortunately, the database supplied with this aircraft appears to lack the final waypoint -- the approach only has RARYO, USDAW, and RWY34L.

 

I approach RARYO at 3000.  Upon passing RARYO, I cut throttle, descend to 2200, extend flaps and configure the aircraft so that it's going at around Vref+5 (78 MPH) when it arrives at USDAW.  Upon crossing USDAW, I invoke APR on the autopilot.  The PFD shows LOC annunciated in white (not surprising since, as I said before, there's no ILS approach for this runway). 

 

What I find happening is that the aircraft, left to its own devices, will hold level at 2200 until, maybe, about half-a-mile from threshold, when it suddenly shifts into a steep descent, visually aiming at what appears to be two-thirds of the way along 34L.  I can generally get it down after switching off the AP, but don't want to know what the final vertical descent rate was -- I'm sure my insurance company wouldn't be happy! :wink:

 

In retrospect, what appears to have been happening beforehand is that the autopilot wants the aircraft more than 1500 AGL until just before the runway, and was forcing it to climb if I was lower.  O.K., but that seems like a pretty extreme way to land, if you ask me...or is the problem that, with a GPS approach, I shouldn't even be invoking APR mode on the AP at all, but only do that on an ILS approach?

Share this post


Link to post
Share on other sites

In retrospect, what appears to have been happening beforehand is that the autopilot wants the aircraft more than 1500 AGL until just before the runway, and was forcing it to climb if I was lower.  O.K., but that seems like a pretty extreme way to land, if you ask me...or is the problem that, with a GPS approach, I shouldn't even be invoking APR mode on the AP at all, but only do that on an ILS approach?

 

According to the charts, you should have intercepted the glideslope from the FAF, USDAW at 2200 (MSL). In order to do GPS approaches you need to have a GPS system that is WAAS capable. I do not believe the Carenado units are. As far as I know, RXP GNS and Flight 1 GTN are the only units that provide support for WAAS approaches. You also need to check minimums on charts to make sure LPV (localizer performance with vertical guidance) is listed in the minimums. KPAE 34L has an LPV, but not all GPS approaches do. APR mode is only activated when there is vertical guidance of some kind - from an ILS, or otherwise from GPS approaches with LPV and unit that can support them. In instances where there is no LPV, you maintain the altitude profile in the charts down to MDA, and either land once you have a visual or go missed approach if you don't.

 

I'm not sure how to explain the behavior of the airplane after you engaged APR. When you engage an autopilot mode for which there is no data available the flight director doesn't have anything to go on, it's like flying without the autopilot on at all. The airplane's behavior might have been completely random. You would still have lateral guidance if NAV mode is enabled, but once APR is enabled without any glideslope data your vertical guidance is effectively gone. It's also a bad idea to dial down the ALT to field level. Keep it at the FAF altitude or DA at the lowest.

 

These links might help to clarify GPS approaches and WAAS/LPV:

 

http://www8.garmin.com/aboutGPS/waas.html

 

http://www.duncanaviation.aero/videos/understanding_waaslpv/what_is_lpv.php

Share this post


Link to post
Share on other sites

Well, if you need WAAS to do an LPV, and Carenado doesn't support it, I can see why I wasn't getting anywhere. I'm almost getting the sense that what I've been experiencing has been, when I invoke APR on 34L, the Carenado is actually grabbing the ILS on the reciprocal 16R (even though I don't have NAV1 properly tuned), and is trying to fly a 16R ILS approach 180 degrees reversed, putting the intended touchdown point at the north end of the runway. As weird as that would be, it would seem to fit what's been happening. In any event, if LPVs aren't possible with this aircraft, I won't bother with APR mode, but just let NAV mode set me up laterally while controlling the descent manually.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this