Why does VNAV want to descent so slowly from so far way?

The title says it all. My VNAV always wants to descend like 170-180nm away from the airfield at something around -1700FPM, with the thrust at like 50-60%. Why is this? When I calculated my descent for an idle descent, I get 105nm from FL350. The method I’ve always used is the same method thry use in the user guide, or they get the same result anyway.
Thanks!

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I’m pretty sure the VNAV is based off of the real thing. Real life aircraft descend pretty slowly (as in, around 1800 or so). Either way, it’s meant to provide a descent to the altitude at a specific waypoint with precision.

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-1700fpm is quite a good descend for average commercial aircraft. Nothing to complain about this.

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Alaska hit the nail on the head with his explanation.

If you are descending from 105nm, you’re likely over -2,000 VS, which is quite steep. VNAV is accurate descent into a location at a controlled speed.You shouldn’t be doing an idle descent to begin with (if were going to be realistic here).

The VNAV will bring you from point to point with the altitude assigned and at a safe descent level.

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Ok, I always thought an idle CDA was the most realistic option. Where are we getting the “steep” from? Just curious.

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I do aggre with you @InfiniteFlightDeck as an idle decent is really not possible from my experience with VNAV. You don’t need VNAV from TOD all the way down to follow the altitudes on the STAR though, which nonetheless makes it a great tool.

It is the most efficient option, and the one pilots often thrive for IRL, yes.

Steep comes from factoring in a cruise that it likely above FL290 (which would be less steep than a cruise of FL410) at the range you are starting your descent from.

A good descent is between -1500 and -1800 (depending on where you are flying in based on terrain).

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Idle descents are rare but VNAV does do descents shallow as the current iteration is rudementary. The cureent VNAV doesnt consider if the Alt selected is a ceiling, floor, mandatory, or a range of acceptable altitudes. This results in STARs that have your altitudes as floors being calculated as a mandatory altitude so VNAV descends farther away. For instance a STAR i had today had a point 75nm out that had a floor of 10000 at that waypoint. Instead of allowing altitude to be at say 13000-15000ft VNAV in IF had it set automatically at 10000ft. To adjust for that VNAV had a TOD farther out and shallower than it should have. Right now I recommend changing your ALT selected for the waypoint to a higher one if you dant want to descend so far out and so shallow.

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I agree with you… I’ve just finished a flight to OJAI and descended via GENE6A STAR… This STAR has a ceiling at FL260 and 150 and 7000… VNAV wanted to reach those altitudes specifically without taking into consideration that you have a margin (waypoints with underlined altitudes on charts which means at or above)… Instead of doing a smoother descent; VNAV got crazy with -3000 ft/min although i did decelerate in an organized manner as i used to do because it’s my home base and I’m highly familiar with this airport with all its procedures… This behavior has got me deviated from the descent profile which eventually forced me to disconnect VNAV and shallowing the descent in order to decelerate… Moreover I noticed that the aircraft (B777-200ER) doesn’t follow the pink line correctly which represents the actual flight plan. On approach; the aircraft turned into final earlier than it should have and this rendered the approach a bit unstable when i pressed APPR (swerving like crazy)…

The descent angle for VNAV is 2 degrees by default, unless set otherwise. The descent angle for an idle descent is usually greater. The usual formulas mentour and captain joe give on their vids are for 3 degree descents.
I think VNAV is most useful for continuous descent operations, or when there are a lot of altitude restrictions and it is easy to miss some, and not really for descending all the way from TOD. I usually descent at a shallower angle initially, then continue to descend at 3 degrees later.

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There is nothing wrong with descending at a slow rate from a distance above terrain, the problem you encounter is the terrain. Thats why there is set alttitudes on STARS.

If you would like a -3 degree descent you can use my descent formula based off Captain Joes video. Remember that 3nm = 1000ft (for calculating parts of descent) e.g descent down to the transition of a STAR.

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More importantly, seperation from other traffic

Yep but that wasn’t part of the question. I was just explaining that descent above a certain FL doesn’t matter as there is typically no terrain encounters outside of a STAR.

Perhaps your speed, the faster you go the sooner it must descend, the slower the later. I have never asked what the intended descent rate is based on. @moderators can you reach out to a pg member and ask?

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The slower and faster you go doesn’t affect your TOD it only affects your descent rate.

But it does affect you descent rate to hit you waypoint and altitude.

It’s a 2 degree slope (IRL it’s closer to 3)

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for more realism

I will admit the calculated descent is a bit shallow. They set it at 2 degrees when most IRL will descend at 3 but in a way, that can be beneficial because some aircraft in IF are difficult to slow down at high descent rates. This is true irl as well but we usually descend earlier than the calculated path and then slow down or the aircraft VNAV will take into account the extra distance to level off and slow down.

P.S. you can always change the altitude targets to be steeper if you’d like it to be closer to idle descent or 3 degrees. The VNAV has been biasing towards the bottoms of any altitude blocks or whatever the “at” number is in an “at or below/above” so I usually change it to be the higher altitude or whatever would result in a 3 degree glide in between the block altitudes

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The real question is: why were you even above -2000ft/min on your descents?

I have NEVER been over -1500ft/min. I was actually scared VNAV was going a little to fast at -1700ft/min