STAR waypoint starts too high to follow descent path

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Kracken130
Posts: 24
Joined: Fri Mar 10, 2023 11:31 pm

STAR waypoint starts too high to follow descent path

Post by Kracken130 »

Hello,
I set up an approach in the PMDG 737 800 to KPHX 8 via the HYDRR Star selecting HYDRR as the starting waypoint. I have the exported snapshot below.
When I initiate the flight, I find myself immediatley about 4000 feet above the VNAV descent path, despite being careful to enter the correct cruise altitude of 19000 feet as suggested by FSI panel when choosing that fix, and getting VNAV path to display as expected. I thought I was doing something wrong and reloaded several times and finally flew it through and discovered that I was arriving about 3000-4000 feet over the airport despite having VNAV PATH engaged. Interestingly despite being so far above the descent path, VNAV PATH accurately descends the aircraft accurately through each constraint along the path, and correctly controls speed etc, only it arrives about 4000 feet to high for the ILS. Looking later at the FMC one has to descend about 5000fpm through the descent to arrive at correct altitude for the approach. That is probably unachievable, and the autpilot seems to ignore that high descent rate all the way down. All very interesting.

What I discovered looking at the NAVIGRAPH chart is that there is a restriction to be between 19,000 and 15,000 feet at that intersection, and FSIPanel reads the higher of the two as the recommended altitude to start at. Logically if I had revised that to say 15,000 feet, 4000 feet lower and started there I am guessing the VNAV PATH would have set me up fine for the landing at KPHX.

Besides reviewing the charts carefully when using FSI's recommended altitude along a STAR I would ask what FSIpanel reads from the NAVIGRAPH database to set the recommended altitude, and if it might be possible due to situations like this to recommend the lowest altitude?

Regrettably when I try to attach the snapshot file I created, it says invalid file extension, so I cannot attach it. It is an .FSI file.

Thanks
Greg
jpgarraio
Site Admin
Posts: 1234
Joined: Fri Sep 15, 2017 10:30 am

Re: STAR waypoint starts too high to follow descent path

Post by jpgarraio »

Hi,

i will add .FSI to allowed file extension on the forum.

FSiPanel reads the safest altitude if published by Navigraph to have the best terrain clearance.
The idea is that if you select that altitude, you will be safe, you are however free to change it like you did from 19000 to 15000.

Regards,

J-P
Jean-Pierre Garraio
Developer
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Kracken130
Posts: 24
Joined: Fri Mar 10, 2023 11:31 pm

Re: STAR waypoint starts too high to follow descent path

Post by Kracken130 »

Thanks, now that I understand the logic I know what to look out for. And by bad luck while learning the program, I randomly chose what is probably one of the only STAR waypoints with a spread of 4000 feet on the altitude constraints.

Really enjoying this program by the way very helpful.

Greg
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