KATL STAR/VNAV problem

I was descending into KATL and the vnav/star was descending me then it had me descend so fast that my autopilot disengaged, does anyone know what the problem was?

Was the V/S flashing yellow? I know that the limit of VNAV is -3000fpm, so it sounds like you exceeded the VNAV’s ability, and the attitude of your plane was too much for A/P to handle, so it automatically disconnected. I’d recommend taking a gander at how far apart your waypoints with altitudes are, and if the spacing relative to the altitude needed to lose is too short, then slow down earlier so you don’t push the VNAV to its limits.

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it was on the star.

Many STARs have very tight spacing between fixes, and with the tight spacing, there’s occasionally big jumps in altitude difference. I recommend you take a look at the distance between fixes with altitudes, and if it looks like you need to lose a lot of altitude in a short distance, then slow down in advance so you don’t have VNAV nosedive your plane.

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I was at around FL140 going 230kts IAS

Again, there are some STARs where 230IAS can still be too quick to maintain a reasonable V/S when descending. My suggestion for you is to just keep an eye on your fixes, and slow down further as required. Or, if you want to throw realism out the window, eliminate the altitude for that fix, so VNAV skips that step.

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ah I shouldve done that.

Lessons learned for next time! :)

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To be honest most altitude restriction are at or above on STARs. If available you can have a look at a chart to clarify, but please don’t take every altitude restriction as the recommended or set altitude at that waypoint.

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Oh for sure, but most people (myself included), it’s extra work to stray from the already set ones lol

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I do understand that, but it’s not realistic nor reasonable to fly 6000feet for 40+NM only because it’s the minimum. But in the end it’s personal taste as you said.

Which arrival were you doing? @juan_tavares

Adding on to Danny and Julian:

I experience this from time to time. As you may know, STARs have a crossing restriction for certain waypoints. There are also instances where the STAR has crossing ranges. For example, on the ROBUC3 for Boston, waypoint RUIZE has a range of FL230-FL210. That’s not all that drastic but there are instances where the range can be 17,000ft-11,000ft. I believe IF would choose the lower altitude in these instances. As a result, It’s a good practice to find the STAR of choice on the web and look at these things.

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I forgot this happened 1 month ago.

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