Is there a way to change the VNAV rates in the FMC for the Boeing 737 MAX 8? During climbs and while cruising, the plane has been overspeeding when VNAV is active, and uses excessive V/S during descents which then also causes overspeeding.
I’m also wondering if there is a way to configure the FMC to be V/S priority during VNAV descents so that way I can tell the computer to follow a specific V/S. I think that would make speed management easier.
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No, there is no way to override the airplane’s calculated rate of descent in VNAV as that would be in direct contrast to the entire purpose of VNAV – which is to track your preferred speeds and altitudes throughout descent and keep it as efficient as possible by waiting to descend at the last possible moment on idle thrust.
In normal operations, the vertical speed is managed by the VNAV system to maintain the optimal airspeed (depends on the cost index, usually between 290-310 IAS for climb or descent and Mach 0.79 for cruise) and passenger comfort, typically ranging between 2,500-3,500 FPM while also getting the aircraft to it’s cruising altitude in the most time and fuel-efficient way.
In an emergency, such as a rapid decompression, the pilots will perform an emergency descent, during which the aircraft will descend at maximum speed (Mach 0.82). To reach 10,000 feet as soon as possible, the pilots will extend the speedbrakes, which results in a much higher vertical speed – potentially exceeding 6,000 FPM – and passenger comfort is a very small factor in this case.
It appears the Asobo 737 MAX has a bug in its VNAV mode, in which it does not account for tailwind. So you can end up too high on profile, resulting in the MAX going full-on “emergency mode” and diving to get back on profile.
However, pilots would never use VNAV mode to initiate an emergency descent. They prefer either the LVL CHG or V/S modes, because VNAV would not descend the aircraft with such a high V/S, as such is not SOP.
It would appear that the Asobo 737 MAX is coded to do a “rapid dive” when losing the calculated path, which is incorrect. The aircraft should would close the thrust levers and notify the pilot to extend the speedbrakes so a higher rate of descent can be achieved.
It is also worth noting that the 737 MAX’s engines are more powerful than the 737NGs, so the airplane has a huge tendency to increase speed during descent. Most pilots would use the speedbrakes liberally, so I suggest you try this during your next flight: just extend the speedbrakes to the flight detent once you see the speed start to rise and keep them out unless the airplane adds thrust for ALT HOLD or it gets too slow. Either way, a thrust increase means speedbrakes away.
If this is a bigger issue than I assume, and the airplane is just bugged and diving for no apparent reason, you can temporarily switch to V/S mode and pop the speedbrakes until the airplane is back on profile and then re-engage VNAV.
If the issue persists enough that you would like to ditch VNAV altogether, you can use the LVL CHG mode for climb once you have no altitude restrictions left.
For descent, you can use the V/S mode. In order to determine your descent rate and point, there is a simple formula for that: for every 1,000 feet of altitude, 3 miles are needed to lose it. Then you multiply your airspeed by 5 – so 300 knots is 1,500 FPM.
So, if you’re cruising at 33,000 feet and you need to lose 30,000 feet to reach to your initial approach fix at 3,000 feet, you’d need to start descent 90 miles and use a V/S of 1,500 at 300 knots, then 1,250 at 250 knots below 10,000 feet. And that is basic VNAV doing it’s thing, just it is the pilot performing the calculations.
You can descend this way and avoid the problematic V/S angles in the Asobo 737 MAX. A bit more hands-on than the way VNAV is intended to be, but effective all the same.
And you should also make a bug report, for whenever they get around to fixing these bugs in 1st-party planes.
Wow, that was a long post 
Take a look at this video. That should help. This guy has a lot of good videos.
Thanks for help explaining! I found a work-around that made speed management easier, as well the tip that @davegranstrom16 gave for utilizing the speed-breaks. Also appreciate the tips in the video that @TinJungle791349 provided!
Hi everyone. I’m no expert, and this might not make any sense. In these cases, I regulate it with the cost index. For example, a cost index of 100 means the descent starts much later (80-100 NM depending on altitude), making the VS (Velocity Score) more aggressive. With a cost index of, for example, 5, it will start descending much earlier (120-140 NM depending on altitude), making the VS smoother and more progressive to prevent the aircraft from accelerating.
Part of my solution is similar to this, except I figured out that I can manually change the VNAV target speeds to be slower on the descent phase. Though the FMC gives me a notice that I don’t have the most economical speeds selected for VNAV.
