DFC90/DFC100 Digital Autopilot

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DFC100 and VNAV

Printed From: Avidyne
Category: Avidyne General
Forum Name: DFC90/DFC100 Digital Autopilot
Forum Description: Topics on the Avidyne DFC Digital Autopilots
URL: http://forums.avidyne.com/forum_posts.asp?TID=115
Printed Date: 24 Nov 2024 at 7:55pm
Software Version: Web Wiz Forums 12.01 - http://www.webwizforums.com

Topic: DFC100 and VNAV

Posted By: AviJake
Subject: DFC100 and VNAV
Date Posted: 07 Nov 2010 at 2:04pm

Now that we are closing in on DFC100 certification, a lot of questions are popping up about VNAV. What is it exactly? How does it work in R9-DFC100? and a lot of specific scenario questions.

First a general statement - as soon as we are through the final FAA TIA flight test, we will post the DFC100 Pilot Guide for all to read. There is always some risk that something might have to change as a result of the final FAA flight test and we don't want to publish a pre-cert version that may have to change a week later.

That being said, this forum thread can be used to answer any specific questions that may come up.

Let me start with a VNAV question that came up in a recent DFC100 webinar. The question was along the lines of "Are step down fixes available in VNAV?" The short answer is yes. As way of more information, think of the type of approach being conducted when VNAV is engaged. In both cases, it is assumed the desired approach is part of the active FMS flight plan.

For a precision approach (LPV or ILS), if the autopilot was in VNAV mode prior to starting the approach, any intermediate step down fixes between the IAF and FAF will be achieved and departing the FAF, the autopilot will automatically toggle out of VNAV mode and into ALT hold mode with GS armed. If the vertical position of the aircraft was such that glide slope can be immediately picked up, the autopilot will immediately toggle to GS mode and this may be so fast that ALT may not have been noticed.

For a non-precision approach (LNAV, LOC, etc), if the autopilot was in VNAV mode prior to starting the approach, any intermediate step down fixes between the IAF and FAF AND inside the FAF will be achieved. The important point to understand in this case is that when the active FMS flight plan leg terminates with the MAP, the autopilot will automatically toggle out of VNAV and into ALT hold mode. Some approaches have multiple step down fixes between the FAF and MAP and all but the last one will be followed by VNAV mode.

I hope this addressed the specific question.

Please use this thread for any more VNAV questions for a R9-DFC100 equipped system.

Steve Jacobson
VP Product Management
Avidyne Corporation

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Steve Jacobson
sjacobson@avidyne.com

Replies:

Posted By: Turrisi
Date Posted: 07 Nov 2010 at 2:53pm

How about an LNAV+V approach? Currently the R9 system makes selection of capturing a GPS derived GS or flying the step down fixes pilot optional. The way it is done with the S Tec 55X is to switch from NAV GPSS mode to NAV/APR. How will the DFC 100 mange this? I assume VNAV could be used for the straight VNAV approach without the GS but what if you want the GS?

Secondly, I understand from the webinar that only altitude restraints that are built into the FMS waypoints will be able to be followed by the VNAV function. Other altitude sets will be done by the ALT/VS mode. So in flying a flight from start to finish (ending in an approach), what would be the most efficient way to cycle the altitude modes? Take off and climb with an ALT/VS setting? Unless there is an intermediate enroute altitude restricted crossing fix, would there be any reason to use VNAV enroute?
Thanks,

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Brian

Posted By: AviJake
Date Posted: 07 Nov 2010 at 4:06pm

If you want to do step downs on an LNAV+V, just leave the AP in VNAV. If you want to do the +V pseudo-glideslope, then put the AP in GS.

The most compelling reason to use VNAV enroute is that the TOD will be visible on the map and the FMS will start the descent for you upon reaching that point. As for mode management, I would recommend the following sequence. During preflight, either select an approach at the destination or enter an altitude constraint on the destination leg (I suggest using the 1000AGL default). On climbout, use VS/ALT or IAS/ALT to reach the cruise altitude. Then, engage VNAV. Manage enroute altitude changes with VS/ALT, but then always go back to VNAV after leveling off. Because of the defined downpath constraint, VNAV will compute the TOD and start the descent at the correct time. If you start out with a constraint on the destination and then select an approach as you get close, VNAV will automatically adjust TOD for the first constraint in the approach.

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Steve Jacobson
sjacobson@avidyne.com

Posted By: Turrisi
Date Posted: 07 Nov 2010 at 4:25pm

So on an LNAV +V when you want to fly the glideslope, just select GS and I presume you would do that when the next waypoint is the FAF?

On any approach when a missed approach is flown, other APs require disconnecting the AP and re-engage when on the miss. Is that needed with the DFC 100 or can you just hit the "activate missed approach" button and let the VNAV do the rest?

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Brian

Posted By: Turrisi
Date Posted: 08 Nov 2010 at 2:25pm

One other VNAV question: If the last waypoint in the FMS is an airport, I understand the VNAV will plan 10 get to 1000 feet above the airport at a range of 5 miles. Does that vertical constraint (1000 feet) have to be manually entered into teh FMS or will the VNAV function automatically assume it?

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Brian

Posted By: AviJake
Date Posted: 10 Nov 2010 at 9:33am

Let me tackle the three questions from the last two posts all at once:

For the question, “So on an LNAV +V when you want to fly the glideslope, just select GS and I presume you would do that when the next waypoint is the FAF?” the answer is GS should arm even for LNAV+V approaches.

For the question, “On any approach when a missed approach is flown, other APs require disconnecting the AP and re-engage when on the miss. Is that needed with the DFC 100 or can you just hit the "activate missed approach" button and let the VNAV do the rest?”, let me try to answer this by saying that on any approach other than a non precision an auto transition out of VNAV mode happens when the next leg terminates with the MAP so you always have to press something for the missed. Because of the timing issues with reaching minimums vs the MAOP at the threshold the pilot should disconnect, establish climb and hit NAV, VNAV after the MAP.

For the question, “If the last waypoint in the FMS is an airport, I understand the VNAV will plan 10 get to 1000 feet above the airport at a range of 5 miles. Does that vertical constraint (1000 feet) have to be manually entered into the FMS or will the VNAV function automatically assume it?”, the answer is that yes, the airport constraint has to be entered. When the FMS cursor is put over the alt const field, 5nm/1000’AGL will be the default value but pilot has to select it (by pushing in on the FMS knob) or change to a different desired value.

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Steve Jacobson
sjacobson@avidyne.com

Posted By: jfourrn52
Date Posted: 20 Mar 2011 at 1:55pm

Does VNAV consider or otherwise honor the selected altitude as set by the altitude bug?

For example imagine you are descending into an sea level airport from 16,000'. You have the setup the system to have you 1000' AGL 5nm from the airport and have intercepted and captured the VNAV profile (the autopilot is in VNAV). However you have only been given a descent clearance to 5000'. In other systems if you were to set the altitude selector to 5000' you would level there regardless of the VNAV profile.

Posted By: Turrisi
Date Posted: 20 Mar 2011 at 8:19pm

I tried this today and the VNAV honors the intermediate constraint altitude. So when you dial in 5000 feet, the VNAV will stop you at 500 feet. Push again after level off and it goes down to 1000 or what ever the next waypoint is set to go to.

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Brian

Posted By: AviJake
Date Posted: 20 Mar 2011 at 8:59pm

Brian is correct. The Alt Bug is honored in almost all of the vertical axis modes. (It is not honored during ILS glide slope operations)

Here is a snippet from the Pilot Guide:

"Altitude captures can be performed using IAS, VS, VNAV or PITCH."

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Steve Jacobson
sjacobson@avidyne.com

Posted By: jfourrn52
Date Posted: 21 Mar 2011 at 9:14am

Thank you for the answer.

I did look for a statement like that before I posted, honest! :-)

Perhaps in the next revision of the Pilots Guide you might consider a graphic showing a flight from showing how various modes interact with the VNAV flight plan and ALT and GS. Some people just love pictures!