Hello! I'm new to the forums. Quick question regarding descent in the NGX that I may not be aware of, a little "problem" I am having.

 

In the 737-800 Winglets, let's say I am cruising at FL380, and am about to start the descent into whatever airport. I have LNAV and VNAV armed, I'm 5-10 miles from the calculated TOD, altitude window on the MCP set to 0, and cost index of 50 (if that matters?). I have mach .76 selected on the descent page of the FMC so it slows to that, then starts it's descent at the TOD that the FMC made. Now, when it descends, the pitch is like -5! Is that normal for the aircraft to be pitched down this much? If I go into outside view the aircraft clearly looks pointed down. On VNAV, the plane will adjust itself between like -3 and sometimes -10 pitch down. To get around this problem, I set my vspeed manually in the mcp to like 1600-1800 fpm down. The problem with doing this is I seem to overshoot the airport being very high.

 

My route will consist of a STAR and ILS landing, with the route all connected with no discontinuities on the legs page. What do you guys do to maintain a smooth descent with VNAV? I also tend to be using spoilers often to get my speed under control.

 

Thanks so much!

Nick

No, a negative pitch angle of 10 degrees is rather disconcerting, unless you happen to be behind the wheel of a Vomit Comet. This might have something to do with it: http://support.precisionmanuals.com/KB/a103/aircraft-does-not-want-to-climb-or-flies-with-negative-aoa.aspx. Also, check that your throttles are at idle before you commence the descent.

You are possibly pushing the limits of the FMS's vertical speed constraints in between the legs (e.g, your first leg is FL250, next leg is 6000ft and the two legs are 10nm apart), thus the aircraft can not suitably fulfil your requests.

You are possibly pushing the limits of the FMS's vertical speed constraints in between the legs (e.g, your first leg is FL250, next leg is 6000ft and the two legs are 10nm apart), thus the aircraft can not suitably fulfil your requests.

In that case you would probably get a stream of FMC alerts like "STEEP DESCENT AFTER XXXXX" and an UNABLE for the next descent.

On VNAV, the plane will adjust itself between like -3 and sometimes -10 pitch down.

 

If you have a hardware joystick, does it have an elevator trim axis assigned? If so, make sure to zero that trim assignment out before engaging the AP. If you have a hardware throttle, make sure it is at idle before beginning the descent.

My route will consist of a STAR and ILS landing, with the route all connected with no discontinuities on the legs page. What do you guys do to maintain a smooth descent with VNAV?

 

If you're in the United States, that's a problem.

 

Discontinuities are not evil! They're there for vectored portions of your route, where you're also often told to descend. With a few exceptions, most of the time you're not going to have an unbroken line from airport to airport. To get past the discontinuities, use HDG SEL, and then when you're ready to go direct, line select the fix to the top of the LEGS page and hit EXEC.

 

...and it's an ILS approach. The landing is its own separate thing. Landings are done by hand 99.9% of the time, too. To convey the fact that you're letting the plane do everything, it would be an ILS approach to an autoland.

Also, MCP altitude set to 0.

 

I'd never want to see that below the MDA for the airport or DH for the runway... and would consider an MCP altitude set below the MVA or Class C or E control step for the location I am at to be bad form.

 

I guess when there's no ATC being used (Vatsim or one of the programmed ones or the default one) you would just dial down to the highest of the 3 considerations above in steps. My usual practice if I'm being really lazy is to roll MCP altitude to ILS intercept altitude (somewhere in the vicinity of 2000 to 4000ft above the airport altitude depending on what the chart, (or if I'm being exceptionally lazy, the legs page of the fms) says.)

 

If I know the airport is deftinatley not more than 500ft above sea level, I'll stick 3000ft in sometimes, but that probably means I'm getting so lazy that I should really probably go to bed, go for a walk, or something else other than flying the sim.

 

 

http://support.precisionmanuals.com/KB/a103/aircraft-does-not-want-to-climb-or-flies-with-negative-aoa.aspx is something to look at though.

Thanks guys. I took all your tips into account and had a vector going to my ILS approach at KMDW. I got my angle of attack down and now it's about -1 to -2 degrees on descent. Thanks! Nick

Good to hear!  Glad it helped.