Recently, I was descending into Kabul (OAKB) with a 777-200ER and my VNAV suddenly increased the descent rate to -3000fpm — too fast to slow down even with flaps and spoilers. This resulted in taking over manual control and a subsequent go-around in order to prevent a crash.
VNAV is not a human - it’s a coded program that gets you to the altitude you set at a specific waypoint. You probably activated VNAV too late, or there was a big altitude difference between 2 close waypoints, VNAV probably requires a steeper descent to get to the set altitude in time.
As an example:
Case 1 (Activating it late): If you activate VNAV too late (let’s say 50nm from airport), it will have to descend 30000ft in less time than usual, causing an extremely steep descent.
Case 2 (Waypoint altitude differences): If you have 2 waypoints 10nm apart from each other but there is a 10000 feet difference between them, immediately after reaching the first waypoint it will start an extremely steep descent in order to get to the assigned altitude at the next waypoint.
A steep descent may cause you to not be able to slow down, especially if it’s as steep as -3000fpm. If this does happen, make sure to arm flight spoilers, possibly flaps 1 (This will deploy just the slats which will cause the plane to slow down), and get the throttle to idle.
I engaged the VNAV roughly 30 minutes before my top of descent. I used a pre-programmed approach route, which did involve a tight turn to align with the runway. So perhaps the 14,000ft and 11,000ft (roughly) waypoints were too close when the VNAV proceeded onto the second altitude. Also, this happened immediately after I slowed down and put my flaps to 5°.