It’s probably similar to Boeing in general: even if not in LNAV, such as when in HDG SEL, the aircraft still calculates the path if it’s still in VNAV(also other modes), using the aircraft’s current position to the first active waypoint.
As for why the rate of descent is high when starting descent — you need to look at the details of what you did at that moment. For example:
• If you’re in FMC speed mode and set the lower altitude late, the aircraft will begin descending after the normal TOD. But if it’s not too far past TOD, the aircraft is still in VNAV PATH even though in reality it’s above the path, so it pushes the nose down to increase the ROD and rejoin the path. That’s why the ROD is high. It’s not related to HDG SEL.
• But if you set the lower altitude early before TOD, you won’t encounter this issue.If you observe the CDU, you’ll see that the aircraft will begin its process of descent a few miles before the TOD.
Or if you used MCP SPD that was higher than the cruise speed, this could also have happened. More details would be needed to know exactly what occurred.It depends on what you did that day.
If you’re not flying in LNAV, in holding, then use another mode. Or if you want to fly VNAV, then fly VNAV SPD instead of VNAV PATH, such as using MCP SPD or adjusting thrust manually
Last edited by Noknoipobin; 4th October 2025 at 04:36.