VNAV descent in PMDG 737 NGXu

[737Andi 6]

Hi,

I was flying on Vatsim this evening and the controller told me to level off at FL 110 at my next Waypoint GIVME which was approximately 30 nm away.

At that moment I was flying at cruise level at FL 210.

My plan was to use VNAV descent for that job. So I changed the altitude of GIVME in the FMC to FL 110 and pushed the "Des Now" Button on the DES page of the FMC.

The plane started to descend with a V/S of exactly 1000 fpm. The controller asked me if this will work. Since I was 10000 feet above GIVME and the distance was approx. 30nm I thought the FMC will do everything correctly. 

I think the formula is 21000-11000 = 10000 feet

10000/300 = approx. 30nm

However the V/S of 1000 feet/min turned out to be to low.

Can anyone tell me what I did wrong?

Thanks!

Best regards Andi

[Ray Proudfoot 8,944]

Andi, you don’t mention speed. Either current or that required at GIVME. 1000fpm is the default descent rate rather than the rate that would get you to the required FL in time.

It’s possible the aircraft had to slow before descending at the relevant descent rate. The arc depicted on the Nav display will show where you will reach the altitude in the MCP at the current descent rate.

One trick to get down quickly is to engage FL CH. Dial in a high speed and the aircraft will pitch down to achieve that speed. Keep throttles to idle.

Once at the required FL you may be too fast but should have enough time to slow by the time you reach the waypoint.

[Snowfalcon 18]

Andi,

You can go down or slow down but not both. NGXu will slow first before descending which may leave you above your target flight path. You don't mention which version you in -600, -700, -800, -900, -900ER. That can affect things as well. The -800 is notorious for being hard to slow down and can cause problems with being fast coming into final approach. The RL pilots on the beta team have stated that the 800 does not always like "descend now" directive if it hasn't been slowed down already.

Also consider how much room you have after reaching the next way point to slow down. Depending on what plan you loaded in ACARS you should see your deceleration points on the display. If you hit one of those the aircraft might level off to slow before resuming it's decent.