VNAV seems to take precedence over AP ALT setting

[rwilsonlj 3]

After correcting several silly user errors, I flew a KLAX to KSFO flight that utilized the RNAV approach to RW10L at KSFO. I noticed that even after setting the AP altitude to 3500 ft for the NORMM waypoint (the IF for the approach) the plane kept descending. It repeated this after I set the AP altitude to 1800 ft for the XATTU waypoint (the FAF for the approach). I noticed that the PFD info at the left top was VALT PATH in green. Was the AP trying to follow a GP (I don't think this approach has vertical guidance) that caused it to continue descent? Probably user error, but it seemed incorrect/

[JRBarrett 675]

10 hours ago, rwilsonlj said:

After correcting several silly user errors, I flew a KLAX to KSFO flight that utilized the RNAV approach to RW10L at KSFO. I noticed that even after setting the AP altitude to 3500 ft for the NORMM waypoint (the IF for the approach) the plane kept descending. It repeated this after I set the AP altitude to 1800 ft for the XATTU waypoint (the FAF for the approach). I noticed that the PFD info at the left top was VALT PATH in green. Was the AP trying to follow a GP (I don't think this approach has vertical guidance) that caused it to continue descent? Probably user error, but it seemed incorrect/

This particular approach has only LNAV minimums, so it cannot be flown with VNAV active. The vertical path has to be flown manually in VS mode.

I don’t know if the FMS “knows” that VNAV is not authorized, so it may indeed have been trying to fly a calculated vertical path. The CRJs I work on as an avionics tech do not have the coupled VNAV option, so I am not knowledgeable as to what it will or will not do in a case like this. Perhaps one of the r/w CRJ pilots on the forum can chime in.

 

I assume that before you arrive at the IF NORMM, you would need to get out of VNAV mode by selecting vertical speed, and select the VS to whatever descent rate already existed at the time you made the switch. The aircraft should then level off at 3500, and you would then conduct the rest of the descent to XATTU and MDA in VS mode, while monitoring altitude constraints as in a standard non-VNAV RNAV approach.

[rwilsonlj 3]

Thanks for the info. I suspected the VNAV thought he was still in charge as the indications never changed on the PFD and the two waypoints did have altitude restrictions shown. It either was trying to maintain the calculated path or trying to meet the restrictions. It appears the CRJ attempts to do a pseudo LPV or some such even on non vertical guidance approaches. I will try again and turn VNAV off before arriving at the IF. Aerosoft really needs to tidy up the avionics glitches. 

[Anthony Pariselli 0]

+1 on this. Its happened twice since the last update. VNAV just busts through wtvr setting you have in ALT or any restrictions in the FMC. 

[JRBarrett 675]

On 5/31/2021 at 11:42 AM, rwilsonlj said:

Thanks for the info. I suspected the VNAV thought he was still in charge as the indications never changed on the PFD and the two waypoints did have altitude restrictions shown. It either was trying to maintain the calculated path or trying to meet the restrictions. It appears the CRJ attempts to do a pseudo LPV or some such even on non vertical guidance approaches. I will try again and turn VNAV off before arriving at the IF. Aerosoft really needs to tidy up the avionics glitches. 

This may indeed be an avionics glitch, but I cannot say for sure it is. If a particular RNAV printed approach plate shows only LNAV minimums but not LNAV/VNAV, then it would normally be the pilots’ responsibility to insure that VNAV is de-activated before arriving at the IF. I do not think the nav data contains information as to what kind of minimums are authorized for a particular RNAV approach that would deactivate VNAV automatically (if it is not permitted), but I could be wrong about that.

[rwilsonlj 3]

3 hours ago, JRBarrett said:

This may indeed be an avionics glitch, but I cannot say for sure it is. If a particular RNAV printed approach plate shows only LNAV minimums but not LNAV/VNAV, then it would normally be the pilots’ responsibility to insure that VNAV is de-activated before arriving at the IF. I do not think the nav data contains information as to what kind of minimums are authorized for a particular RNAV approach that would deactivate VNAV automatically (if it is not permitted), but I could be wrong about that.

I don't disagree with you that the VNAV should be deactivated. However, in other SIMS, the aircraft will not bust the AP altitude setting on a LNAV only approach. The AP knows what type of approach is being flown and LNAV only requires the pilot to descend to the appropriate altitude for each waypoint on the approach. If a RW CRJ5/7 pilot is reading, let us know.