737-800 not automatically descending after Top of Descent

Question

Hi there,

I'm new here, and I've been learning in the past week how to fly the default 737-800. I learned how to use the FMC and now I'm flying with it, along with the autopilot. I'm following a Youtube tutorial, but I'm having issues regarding the automatic start of the descent when I get to the Top of Descent. Here is the exact part of the tutorial that I'm struggling with: https://www.youtube.com/watch?v=ylTKY9ZboW0 (24:05 in case the video doesn't start from the right point). 

What is happening for me, is that the plane just does not goes down as it hits the top of descent. I tried different altitude amounts and nothing worked... I then have to manually go down and most of the time I'm too high to land at the runway...it's just frustrating at this point! Pretty much all other settings are what the guy has in the video. 

Did I maybe do something wrote in the FMC setup? 

Any help would be much appreciated! Thanks

Answer 1

Hi andymasc

I'm not with Laminar Research, just a fellow simmer in the UK and I fly the Zibo 737-800X and 737-900U quite a lot.

As you probably already know, when you enter a flight plan in the FMC, an altitude profile will also be set up along the following lines:-

1. Departure and ascent to TOC

2. TOC to TOD

3. TOD and descent to arrival

Before departing, the autopilot altitude display located in the centre upper console has to be set to the maximum flight level shown in your flight plan i.e. the top of climb altitude, say FL 36,000ft.

Importantly though when trying to descend again and before you get to the TOD point, you have to remember to reduce the autopilot altitude by using the rotary control switch in the upper centre console, otherwise the autopilot will simply stay at your maximum flight level.

When flying the aircraft in the real world, ATC would probably vector you down to given flight levels as part of your descent profile and you would then alter the autopilot altitude to the appropriate level each time.

However when flying a simulator without ATC control, all I do is reduce the altitude by using the rotary control switch (before TOD), to the glide slope platform altitude for the destination airport i.e. 2000ft. By so doing, you don't have to worry about adjusting altitude again, as the autopilot will descend the aircraft for you, based on the flight plan altitude profile set in the FMC.

When the aircraft reaches and locks onto the ILS and glide slope, the autopilot will again automatically reduce altitude as required, until you switch off the autopilot and auto throttle for a last minute flare/retard for landing.

I'm currently on vacation so I'm writing this from memory, but I hope it's of some use and even more importantly, I hope that I have understood your question after having writing all of the above!!

Regards

Comments

Hi Greengolfer15, thanks so much for your thorough response! Great job considering it was all from memory :) 

You clarified parts of the process that I wasn't very sure about and you certainly covered my question. In the past few days, since I posed the question, I've been flying at least one 1H flight every day, and that has given me more opportunities to understand the process - mostly by making mistakes, the best way to learn! I successfully got the autopilot to descend until I reach and lock onto the ILS, but once when it didn't happen, I simply descended manually using the altitudes set at the different legs in the FMC and it also worked fine! 

Anyway, thanks for taking your vacation time to respond in a great manner to my question. 

Best

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Hi there

Glad that I was able to help out and if you were happy with my response, it would be great if you could consider giving my answer a positive upvote, as per the suggestion on the Q&A website.

Many thanks in advance and happy trails :-)

Regards