Dreamflight767 wrote on Dec 31^st, 2018 at 3:23pm:

Hello: VNAV "run away" .

Funny you mention this. I have seen similar behavior in my real world flying. Guys were afraid to let the jet VNAV through 10,000, so they use the technique of 1,300 at 13,000 to slow. The jet would do this dive at the transition or at TOD.

The problem is that systems have a priority when it comes to speed and altitudes. Altitude is the first priority and speed is the second. The system will work to give you both. If both can't be maintained, It focuses on the altitude. This is what was happening when the nose would dump. It ignores the speed to guarantee altitude. Problem is, you have no speed protection in this scenario. Only once over speeding the jet or devices, it will adjust.

The solution to the scenario is to monitor the jet as you should be doing. Now days there are alot of complex arrivals and approaches with points close together. The FMC has to calculate descent at each point. This creates a sort of cone of silence for 2 to 3 seconds. When points are close together, the system starts getting behind with calculations. Before long, it dives to meet the altitude. If the descent profile is too aggressive/steep, you run into the same problem.

The standard profile is 2.5 degrees. Some guys will set 2.0 to assist the system(I don't). Others will ride the brakes, which is what I do. If it's aggressive, I start to slow the jet earlier prior to TOD. If I know the jet is getting behind, I ease in speed brakes early on. The Honeywell systems I use in the Gulfstreams allow you to do a vertical direct to altitude. If I see that the jet is getting a little behind prior to the top of descent, I drop a vertical direct to, which forces it to recalculate the VPATH from my current altitude and position. This is the same as the descend now function in the 757. This in a sense will modify the descent profile making it shallower since you are starting early. I'm in this situation alot because typically, ATC will start you down earlier than your TOD. Sometimes in congested areas, like the North East coast USA, they may hold you up longer past TOD. This forces you to be pro active and request your descent ahead of time.

Will be interesting to see what CS finds with this one. From your description, it sounds like once the STAR/Approach is changed, the system never recalculates or just loses the VPATH all together. I'll take her for a spend and try what you did and see if I can get it to re-engage.