Blackbird Sims / Milviz Forum Archive

Hi Guys,

when flying e.g. the RNP Z Approach into LOWS (click HERE for chart), the following happens:

After passing ETROK at 10000ft with APPR FMS and VNAV engaged, the plane does descent on the correct path (VPATH as active vertical mode, altitude selector is set to the OCA of 1800ft).

However, I don't manage to avoid level-offs at the altitude constraints coded into the FMS approach procedure (which are also visible written in magenta in the PFD, above the vertical speed tape). So at WS806 (altitude constraint 7400ft or above), VALTS will engage as the active vertical mode, and the aircraft will level off at 7400ft.

When I then activate VVS mode and descent with a good rate (e.g. 2000fpm), the plane does capture the VPATH again from above - but at the next fix (in this case WS805), it will do the same as before. The only way to avoid the level-offs in any way is to disable VNAV completely and descent in VS mode, without any vertical guidance.

When passing the FAP (WS804), the VGP mode becomes active, and the rest of the approach works as expected.

Is there a way to cancel altitude constraints coded into FMS approach procedures, other then deleting them from the LEGS page within the FMS? I would guess that's what the ALT CANCEL push-function of the altitude selector knob is for, but this one seems to do nothing at all.

I didn't find anything in the Proline 21 Operations Manual, but I guess I'm just missing a small thing here.

Thank you,
regards

Chris

Hi,

Once you are in VALTV, VNAV is protecting from leaving that altitude. You need to be in VALT, which means the current VNAV altitude constraint must be different from aircraft´s current altitude. Just switch VNAV off, descend in VS/PTCh/FLC and then reengage VNAV when appropriate.
And ALTS/VALTS case, you can't engage any other vertical mode, be in VNAV or not, until you move the Preselector altitude to a different from aircraft's current.

Tomas

Hi Tomas,

thanks for your response!

The Altitude Preselector was set down to the OCA of 1800ft during the whole approach, at least until VGP became active after passing the FAP, when the Missed Approach Altitude was set.

Does that mean, at every fix with an altitude constraint, I need to disable VNAV before passing, and re-engage it shortly after? This doesn't sound really practicable...

Thanks for your support!

Regards,
Chris

Hi Chris,

it is supposed that VNAV altitude constraints are to be respected.
For example, you have this sequence of wpts and altitude constraints:

WPT1 - 5500A
WPT2 - 2500
WPT3 -1600

VNAV possibly set a path to cross WPT1 above 5500 and level off at 2500, which is mandatory.
Suppose that you are in VPATH mode and a/c levels off at 2500. AP mode changes to VALTV, and you can't force a descent unless you deselect VNAV.
Now, as soon as you cross WPT2 VNAV restriction changes to 1600, and your AP mode changes to VALT. In this situation, these are the possibilities:
- VNAV armed a new PATH to WPT3, which the aircraft captured after crossing WPT2; AP mode changes to VPATH and descent starts.
- Vpa between WPT2 and WPT3 is too tight so PATH can't be armed. Aircraft will remain in VALT and you can then use any manual descent mode (VVS,VPTH,VFL) to go down.
- Vpa between WPT2 and WPT3 is too shallow, hence aircraft will remain in VALTV until new TOD is reached; if vpa is above 1.0 deg then it might start a descent in VPATH mode, or it might switch to VALT, now letting you use any manual descent mode to force down.

Tomas

Hi Tomas,

thanks for the in-depth explanation!
It took me a few days to really think about it and do some test flights, therefore sorry for the late response

I guess, the problem here is that the descent angle is always calculated in such a way, that the level-off is initiated always very short before the respective waypoint (even if it's "or above"). So, as soon as it's possible to re-engage further descent after passing this waypoint, the aircraft is always way above the desired descent path.

So technically, at those approaches I tried, it's almost always the 2nd of the three possibilities you described. I need to select a manual descent mode for a dive back onto the VPATH. Those "roller-coaster" approaches can't be good for the comfort of the vPax in the back...


What I'm doing now is to manually delete the next altitude constraint on the FMS LEGS page, as soon as the "1000 feet-to-go" sound rings. But this can't be the correct way to fly those approaches I guess?

Chris

Hi Chris,

VNAV constraints on approaches contain several variations, with different characteristics.
In some it is possible to reach the FAF with the whole ride in VPATH mode, in others it happens what you have found, aircraft levels off one waypoint before FAF and this makes difficult to capture the path and comply with FAF altitude. I've found this is more usual in ILS approaches, where it seems the path is built to make an aircraft level off before the FAF as to capture the GS already leveled (so VNAV to FAF is ignored).
I think any method you use that suits your requerimets should be fine. Our KA instructor once told us that he use to fly many approach paths out of VNAV mode, manually commanding the descent, in part because of these kind of issues.

Tomas