Resuming VNAV

[Muchimi]

Hi -  I have a question:  once vertical navigation is interrupted (for any number of reasons, for example, armchair pilot operator error), how does one convince the avionics to resume the vnav profile?   VNAV is active, the vertical guidance (the purple VS indicator) is correct and showing, but the plane is not following the commanded VS to follow the VNAV.

It took me a while to figure out how to tell the plane to follow the FMS speed bug but I can't find the magic sauce to the same for the vertical navigation. 

Thanks for any tips!

[mmosko76]

In your PERF menu, check LSK 6L, it should say "Resume M.XX" or something along those lines. Activate that and it should switch back to VNAV.

 

[Muchimi]

Thank you, will try that!

[Graeme_77]

On the real thing, most pilots will only use VNAV for descent.

It's actually an Airplane Flight Manual Limitation that you must not use VNAV and FLC when in Mach FLC mode.

[I9311I]

9 hours ago, Graeme_77 said:

On the real thing, most pilots will only use VNAV for descent.

It's actually an Airplane Flight Manual Limitation that you must not use VNAV and FLC when in Mach FLC mode.

What about SID crossing restrictions when given a maintain altitude that is higher? Do they insert the crossing restriction altitudes one at a time, then change and climb on FLC? 

[Graeme_77]

Each “At” or “Below” altitude will be selected with the altitude dial as normal, rather than relying on VNav to stop at the intermediate altitudes. 

[L1ntzi]

On 1/18/2022 at 1:30 AM, Graeme_77 said:

It's actually an Airplane Flight Manual Limitation that you must not use VNAV and FLC when in Mach FLC mode.

Hi, could you please show the section of the AFM where this is mentioned?

Thanks

[Graeme_77]

It's listed in Limitations under FMS and Vertical Navigation:

VERTICAL NAVIGATION

Vertical guidance deviation information is an aid to assist pilots to comply with altitude restrictions and plan descent profiles. When using vertical guidance the pilot must use the primary barometric altimeter to ensure compliance with altitude restrictions, particularly during instrument approach operations.

Use of VNAV vertical guidance is prohibited when the barometric altitude is corrected to the landing field elevation (QFE).

Use of VNAV vertical guidance is prohibited with a single ADC source when the 3rd FMS is reverted in.

It is prohibited to select MCRZ from the FMS VNAV CRUISE page.

Use of VFLC (FLC with VNAV) is prohibited during climb when the airspeed reference is displayed as Mach

It is prohibited to select VNAV following a go-around during the missed approach procedure.

[maub]

This is really odd, I've being using VFLC for the climb without issues. What is the prefered method IRL?

[Graeme_77]

The model doesn’t have any odd behaviour with VFLC, and the reason for the limitation is unknown. IRL the VNAV is typically used only for descent anyway, as I mentioned above. You can use FLC in the climb, just not VFLC.

[richjb]

On 1/19/2022 at 6:31 PM, Graeme_77 said:

The model doesn’t have any odd behaviour with VFLC, and the reason for the limitation is unknown. IRL the VNAV is typically used only for descent anyway, as I mentioned above. You can use FLC in the climb, just not VFLC.

The same limitation is in the CL300/350 too, and to common to all Bombarider Collins FMS equipped aircraft.  The reason for the limitation is that if for some reason the FMS looses the speed values entered into the PERF VNAV CLIMB page, the climb Mach speed reverts to M.50, which would be very bad if you are climbing in VFLC and the FMS ACTIVE SPEED on the PERF page in a ACTIVE <mach> speed, i.e., the speed bug and VFLC speed reference is pink.  The airplane would pitch up and try to track M.50. At FL330, that would get exciting!
 

You can climb in VFLC with and IAS value in the PERF ACTIVE <speed> active, i.e., a pink speed bug set to IAS, but when it transitions to Mach, you need to change to FLC, or you can move the speed bug which will change the target speed from the FMS ACTIVE speed to a FCP speed, i.e., CYAN/blue bug.  Simply put, when in FLC or VFLC, you want the target speed bug showing cyan/blue.  
 

Rich

[richjb]

On 1/17/2022 at 8:56 AM, Muchimi said:

Hi -  I have a question:  once vertical navigation is interrupted (for any number of reasons, for example, armchair pilot operator error), how does one convince the avionics to resume the vnav profile?   VNAV is active, the vertical guidance (the purple VS indicator) is correct and showing, but the plane is not following the commanded VS to follow the VNAV.

It took me a while to figure out how to tell the plane to follow the FMS speed bug but I can't find the magic sauce to the same for the vertical navigation. 

Thanks for any tips!

There are a few ways you can do this. 
 

First, if you are going to a single altitude constraint, say an along track waypoint, destination/-30 at 10,000, then you can use the DIRECT TO button on the CDU, line select the altitude into the scratch pad, and then re-enter it for the along track waypoint. That will re-sync the VNAV and execute a vertical “direct to”.  If you’re in VNAV mode, e.g., VALT or VVS, the aircraft should recapture the VNAV path and change to VPATH mode.  You can do this with any altitude constraint and create a vertical “direct to”.  
 

The second method is used of you’re descending on a STAR with multiple constraints. You can use the same method above to execute a vertical direct to the next altitude constraint.  However, of that constraint is an “at or above”, then you have just changed that constraint into an “at” constraint. That will affect the downstream VNAV path angle after that constraint.  You will comply with the altitude constraints on the STAR, but you may fly them at a much shallower VNAV path angle.
 

Basically, the Collins VNAV uses the default VNAV path angle (3.0 but can be changed) and the lowest “AT” constraint, or the lowest “AT or BELOW” constraint, and then checks that 3.0 degree path back up through the remaining higher altitude constraints. If that path meets all coded/pilot entered altitude constraints, then the 3.0 path is used. If not, then the VNAV path is “smooth” between the constraints where is doesn’t work until it can again project a 3.0 degree path back to the cruise altitude for TOD.  

On a STAR with many altitude constraints, the best way to recapture the VNAV path from above when ATC interrupts the descent is to use either VVS or VFLC and descend at a rate equal to or faster than the target vertical speed shown on the vertical speed indicator (pink target bug) or as shown on the MFD Vertical Path information display.  The only catch is that if the target constraint is a window altitude constraint, one that is both “at or above” and “at or below” altitudes, you need to make sure you capture VPATH before you reach that waypoint. Otherwise, you risk not meeting the “at or below” constraint. This is my preferred real world method. 
 

Rich

Edited March 1, 2022 by richjb

[honeycool75]

On 3/2/2022 at 12:18 AM, richjb said:

There are a few ways you can do this. 
 

First, if you are going to a single altitude constraint, say an along track waypoint, destination/-30 at 10,000, then you can use the DIRECT TO button on the CDU, line select the altitude into the scratch pad, and then re-enter it for the along track waypoint. That will re-sync the VNAV and execute a vertical “direct to”.  If you’re in VNAV mode, e.g., VALT or VVS, the aircraft should recapture the VNAV path and change to VPATH mode.  You can do this with any altitude constraint and create a vertical “direct to”.  
 

The second method is used of you’re descending on a STAR with multiple constraints. You can use the same method above to execute a vertical direct to the next altitude constraint.  However, of that constraint is an “at or above”, then you have just changed that constraint into an “at” constraint. That will affect the downstream VNAV path angle after that constraint.  You will comply with the altitude constraints on the STAR, but you may fly them at a much shallower VNAV path angle.
 

Basically, the Collins VNAV uses the default VNAV path angle (3.0 but can be changed) and the lowest “AT” constraint, or the lowest “AT or BELOW” constraint, and then checks that 3.0 degree path back up through the remaining higher altitude constraints. If that path meets all coded/pilot entered altitude constraints, then the 3.0 path is used. If not, then the VNAV path is “smooth” between the constraints where is doesn’t work until it can again project a 3.0 degree path back to the cruise altitude for TOD.  

On a STAR with many altitude constraints, the best way to recapture the VNAV path from above when ATC interrupts the descent is to use either VVS or VFLC and descend at a rate equal to or faster than the target vertical speed shown on the vertical speed indicator (pink target bug) or as shown on the MFD Vertical Path information display.  The only catch is that if the target constraint is a window altitude constraint, one that is both “at or above” and “at or below” altitudes, you need to make sure you capture VPATH before you reach that waypoint. Otherwise, you risk not meeting the “at or below” constraint. This is my preferred real world method. 
 

Rich

I'm getting the msg "Invalid Direct-to" even after selecting the altitude into the scratch pad. What seems to be causing this?

[Pils]

1 hour ago, honeycool75 said:

I'm getting the msg "Invalid Direct-to" even after selecting the altitude into the scratch pad. What seems to be causing this?

It’ll do this if the required descent angle exceeds 6°, for one.