VNAV with multiple altitude constraint

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matpietsch
Posts: 268
Joined: Sat Dec 21, 2019 10:02 pm
VNAV with multiple altitude constraint

Post by matpietsch » Mon Feb 10, 2020 10:48 pm

Hi,

When flying with VNAV I have a hard time to fly to different altitude constraints.
Lets say I have two constraints, one at 10,000 ft and one at 5,000ft. VNAV will perfectly fly you to the 10,000ft one even when 5,000 ft is dialed in on the MCP. But after that nothing happens. On the PFD it only displays VALT and PATH is armed but the plane does not decend further.

Heronjr
Posts: 110
Joined: Tue Jan 20, 2015 1:04 am
Re: VNAV with multiple altitude constraint

Post by Heronjr » Tue Feb 11, 2020 2:59 am

With me, sometimes does descend, sometimes not. I don’t know if it depends on whatever conditions met along the future path. What I know is that HF2 brought nice advances on arrival procedures (holds and tears) but VNAV at times is still a mystery.

KenG
Posts: 157
Joined: Sat May 09, 2015 2:33 am
Re: VNAV with multiple altitude constraint

Post by KenG » Tue Feb 11, 2020 1:52 pm

When PATH is armed it is looking for the next descent point. You should see another TOD in front of the airplane when it will intercept the next 3-degree glide path.

That being said, I have noticed some inconsistencies as well. It has not been enough to point to a particular problem, but I have seen some missed descents or even climbs in PATH mode. Unfortunately, I would have to fly the same procedure multiple times to try and determine if it is a logic problem, database problem, or something else.

I also remind you that the King Air VNAV is not a set and forget system like other airplanes, you do have to manage it along the way.

If you want to force it to descend I recommend in VNAV selecting VS so you get VVS. This will take you off the PATH and allow you more control of the VNAV descent while still meeting all step-downs.

matpietsch
Posts: 268
Joined: Sat Dec 21, 2019 10:02 pm
Re: VNAV with multiple altitude constraint

Post by matpietsch » Tue Feb 11, 2020 3:27 pm

Referring to your top paragraph: I do not always see a new TOD.

And what is the difference between VS and VVS? Does VS level off at the selected altitude and VVS at the next constraint?

And I agree that it is not a set and forget system - I am not aware of a turboprop that has one - but at lest it should be consistent ;).
Or better: like the real thing. If the real one is crap make this one the exact same crap. If it is great then.. you get the point ;).

KenG
Posts: 157
Joined: Sat May 09, 2015 2:33 am
Re: VNAV with multiple altitude constraint

Post by KenG » Wed Feb 12, 2020 2:32 pm

I would need to see the specific flightplan and try it myself to see what was going on and if it is logically correct IAW what I would expect the system to provide. Flightplan and screenshots of your CDU, FGP, and PFD. VNAV is just too complicated.

VS is vertical speed and only operates based on the Altitude Selector.

VVS is VNAV Vertical Speed and operates through the FMS with the constrained waypoints. The aircraft will leveloff at either the FMS constrained waypoint or the Altitude Selector altitude, whatever is higher. You can tell because ALTS is the Selector and ALTV comes from the FMS.

When the flight plan descent altitude constraints are aligned so a smoothed path can be flown through the constraints, VNAV will not level at the intermediate altitudes. However, the PFD does show the next altitude constraint. The arm field shows that VNAV will do one of the three things that follow:
• Capture the FPTA when it is reached (ALTV armed).
• Capture the preselector altitude when it is reached (ALTS armed).
• Continue descent when aircraft crosses the constraint altitude (no ALTV or ALTS armed).
So why I would normally expect to see a TOD with the next descent and an armed PATH if the descent is smoothed then that might not happen. So maybe you are seeing a failure of PATH to continue correctly on a smoothed PATH. Other than a smoothed case this is what I would expect for a VPATH.
VPATH.png
VPATH.png (27.79 KiB) Viewed 1302 times
.

The above image shows ALTS constraints and not ATLV constraints. The same thing could be done with ALTV and either the PATH would be smoothed or there would be a TOD between each altitude where the FMS determines the distance between waypoints would cause a less than 1-degree angle and thus computes the 3-degree intercept. There are also a number of cases that can cause VNAV to fail, all of which should throw a message to the FMS.
Listed below are abnormal conditions that can cause the PATH or GP arm to become invalid and the associated CDU message that shows:
• Cross track distance exceeds the limits for the current environment (en route, terminal, or approach) (NO VPATH – XTK).
• Track angle error exceeds the limits for the current environment (en route, terminal, or approach) (NO VPATH – TAE).
• Another autopilot mode is manually selected (NO VPATH – PILOT CMD).
• A down-track heading leg (VECT) interrupts the vertical path (NO VPATH – VECTORS).
• A holding pattern is active (NO VPATH THIS LEG).
• The aircraft is on the FROM side of a waypoint for which sequencing is inhibited.
• Other conditions as described in the VNAV DESCENTS section exist (NO VPATH CONDITION).
Additional information is added below:
PATH
The descent path is defined by an altitude at a waypoint and a Vertical Path Angle (VPA). The VPA is the angle that defines a straight-line path through the target altitude at the target waypoint. The target altitude and target waypoint comprise the next descent altitude constraint in the flight plan ahead of the present position of the aircraft. The VPA will be one of the items that follow:
• The angle that appears on the ACT PERF INIT page.
• The angle entered by the pilot on the ACT LEGS page.
• The angle computed by the FMS to establish a smooth descent path.
• The angle required for executing a vertical Direct-To.
The LEGS page shows the VPA for the descent path directly above the target altitude for each descent segment. Preselector altitude must be lowered below the current aircraft altitude for VNAV to capture and follow the planned descent path in VPATH mode. Preselector altitude is ignored when the aircraft descends in VGP mode.
PATH SMOOTHING
When there are two or more altitude constraints in a descent path, the FMS calculates and flies a smooth descent path without intermediate altitude hold segments, as long as each of the descent segments has a VPA greater than the minimum allowable VPA for the aircraft of one degree. An altitude hold is inserted before the descent when two altitude constraints are so far apart that a straight-line path connecting the two constraints is shallower than the minimum allowable VPA or the VPA for that segment.

Raptor
Posts: 411
Joined: Wed Feb 26, 2014 3:07 am
Re: VNAV with multiple altitude constraint

Post by Raptor » Wed Feb 12, 2020 2:48 pm

WOW! Where is the "Like" button???

Thanks Ken!

matpietsch
Posts: 268
Joined: Sat Dec 21, 2019 10:02 pm
Re: VNAV with multiple altitude constraint

Post by matpietsch » Wed Feb 12, 2020 4:33 pm

Thanks lot Ken.

Flightplan is easy: use the RNAV 13R into KPSP. There is only one :).

KenG
Posts: 157
Joined: Sat May 09, 2015 2:33 am
Re: VNAV with multiple altitude constraint

Post by KenG » Wed Feb 12, 2020 5:18 pm

The only approach I see into KPSP 13R is the RNAV (RNP) Z RWY 13R.

Technically the airplane is NOT certified for RNP AR (Authorization Required) approaches.

Thus you are flying a procedure the airplane was not designed to fly in the real world. I am not even going to attempt to comment on a procedure that cannot be flow in real life.

The only certified approach for the King Air into KPSP is the VOR or GPS-B

matpietsch
Posts: 268
Joined: Sat Dec 21, 2019 10:02 pm
Re: VNAV with multiple altitude constraint

Post by matpietsch » Wed Feb 12, 2020 5:42 pm

Ok then I have to find another approach that is allowed.
These are nitty gritty things that a non type rated person cannot know ;). And RNAV is so different here in Europe all together.

KenG
Posts: 157
Joined: Sat May 09, 2015 2:33 am
Re: VNAV with multiple altitude constraint

Post by KenG » Wed Feb 12, 2020 7:12 pm

In the real world approaches the airplane cannot fly are filtered out so they cannot be selected.


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