VNAV Descent with wrong altitudes

[curtoavida]

Hello... i want to use this topic for ask about VNAV descent because when i select my Star and APP in FMS, my route map shows up my TOD in pink color but when my star have some altitude restriction the VNAV doesn’t obey altitudes restrictions i come to high every time and my TOD not correct to obey the altitudes for example yesterday i was flying PAFA >> PAVD and in the PAVD have app by VOR (JOH) have to pass in this VOR about 7.800FT but when i descent By VNAV i pass about 20.000FT in this VOR(i was flying FL-300) in the FMS altitude set correctly for pass in this VOR’s but VNAV doesn’t obey the altitudes restrictions i was descent with GS-300 and VS - 1.500Ft/min... so what i’m doing wrong??...

[KenG]

Thank you as VNAV really deserves its own post.

First, let's look at the VNAV modes of the FGS and what they mean:

VNAV Modes.png (120.19 KiB) Viewed 705 times

So setting up for the STAR we need to do a few things with the FGS for it to be able to properly follow VNAV.

First thing is to make certain that the FMS contains the correct constraints for the STAR. On the right side of the waypoints, we should see altitudes, they may be just an altitude (crossing at a specific altitude,) and altitude with an A (cross at or above the altitude,) and altitude with a B (cross at or below the altitude,) or a set of altitudes one with an A and one with a B (cross at or between the two altitudes.)

The altitudes in the FMS will only be the altitudes that are restrictions on the arrival. It will not include the minimum altitudes.

Once we confirm the altitudes are correct in the FMS then we need to set up the FGS.

First, we need our clearance by ATC and this may come in different forms. Usually, turboprops get an altitude to descend to and we won't need VNAV for a direct descend. We may also get a waypoint and altitude to be at (VTO.) Very rarely we may be given the descend via the arrival in which we will meet all of the speed and vertical constraints on the arrival.

For the King Air, you are in charge of the speed restrictions. To put the FGS in VNAV then we need to select the correct altitude in the preselector. VNAV still abides by the preselector even in VNAV. So if ATC has given me a higher altitude then set that in the preselector, otherwise if given the descent via just set the lowest altitude in the arrival. Second, you must arm VNAV, select VNAV on the FGP. You should then see PATH in white and the system will start descending at the computed TOD and you will also see the Flight Plan Target Altitude (FPTA) show in magenta above the altitude ladder and the Vertical Speed Advisory Pointer (VSAP) appear in the vertical scale as a magenta circle.

NOTE: The vertical speed advisory pointer on the VSI shows the climb and descent rates computed by the FMS using the flight plan altitude
constraints. Display of the vertical speed advisory pointer DOES NOT imply that the aircraft is capable of meeting the required climb or descent rate.

WARNING: Manually initiating a descent to follow the vertical speed advisory pointer prior to reaching the TOD will result in aircraft flying below the VNAV computed path and should not be done unless cleared by the ATC and the obstacle clearance is assured.

The vertical speed advisory pointer shows the following:
• The minimum average vertical speed to climb from present position to the next climb constraint if the rate is at least 100 fpm.
• The initial vertical speed, if a descent were initiated at the present position, to the next descent altitude constraint. The pointer shows
only if the required descent rate is at least 100 fpm, and the TOD at current altitude is within 50 NM or vertical deviation is displayed. The
pointer shows in all VNAV modes. The vertical speed needed for descent shows in the VNAV MFD window as a number.

Vertical speed needed to altitudes with AT OR BELOW or BETWEEN constraint is not shown because there is no minimum value.

Armed Modes:
When coupled VNAV is active, the vertical armed fields on the PFD show the armed modes, if any. There are three armed fields. One field
shows the armed holding mode (ALTS for the preselector or ALTV for the VNAV target altitude). A second field shows the armed VNAV vertical mode (Flight Level Change (FLC), VS, PTCH, or PATH). The third field shows the armed glide mode (GS for an ILS glideslope or GP for a VNAV glidepath). ALTS and Glideslope (GS) modes are controlled by the FCS even when VNAV is engaged. VNAV can arm any of the modes that follow:
• ALTV, PTCH (climb or descent).
• FLC (climb only).
• VS, PATH, GP (descent only).

The general principles for arming the modes are as follows:
• In the climb phase, VNAV arms for FLC if the aircraft is holding at a VNAV FPTA, the preselector is above the current aircraft altitude, and there is a down-track climb altitude constraint.
• In the cruise or descent phase, VNAV arms for PATH if there is a down-track descent altitude constraint, the preselector is below the current aircraft altitude, and all other conditions are normal.
• GP is a special case of PATH. VNAV arms for GP instead of PATH when APPR is selected on the Flight Control Panel (FCP) inside the terminal area.

One of the benefits of the FMS is that it will look at the entire arrival and where it can it will smooth the descent trying to keep a 3-degree descent (or whatever is set int he PERF Descent section.) Thus you may see the FMS adjust the PATH to meet all of the constraints, but not necessarily cross at the specific altitude. This is when more than one constraint is in the FMS and the angle is more than 1 degree between multiple points.

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.

VPAs on the second and subsequent descent segments are modified to provide the path continuity. The computed VPA shows on the LEGS page. A descent path is always computed to the last descent altitude constraint in a flight plan or before a discontinuity, regardless of the altitude type. If the last descent altitude is BETWEEN, the path defaults to the upper altitude constraint.

The descent path is computed through AT OR ABOVE, AT OR BELOW, or BETWEEN descent constraints that are not last in the flight plan or before a discontinuity. This allows the FMS to maintain the selected VPA and perform path smoothing whenever possible to create a continuous descent path through these constraints. AT OR ABOVE, AT OR BELOW, and BETWEEN constraints are always honored but they do not restrict the path. In the approach phase of flight, VNAV computes a descent path that flies to the first altitude constraint in the approach transition even if it is specified as AT OR ABOVE in the flight plan.

For a smooth descent path, the FMS cannot select a VPA greater than the maximum allowable VPA for the aircraft. If adjacent altitude constraints require a steeper path than the limit for the aircraft, the lower altitude constraint shows in yellow on the LEGS page. The message UNABLE FPLN ALT shows on the message line to alert the pilot to the need to resolve the vertical path ambiguity.

In the above I have just touched on VNAV, there is a lot more! But, hopefully, it gives you something to look at and anticipate what the airplane should be doing.

That being said, I am not a fan (personal preference) of the PATH mode. IMHO it can cause more trouble then it is worth and I have seen wonky stuff happen. I prefer to control the VNAV by selecting VS (thus VVS.) It gives you a smart VS mode that will meet all constraints in the FMS while keeping the pilot in control of the FGS. Again, this is my personal preference and may be hampered by the FFS not being as good as the actual airplane even though they run Rockwell Collins software.

Good luck with VNAV and let us know if you have more questions. Please understand that the testers and developers are way ahead on builds so what you are seeing may have already been fixed as we work to get SP1 out the public. But, still please report inconsistencies.

[curtoavida]

Thanks a lot Ken for your answer this will help e too much to understand the complexity of VNAV... have a good flights!!..