FMS setup for descent before ToD point?

[BluesmanEP]

Hello!

I recently ran into an issue while flying online.  I was at cruise FL200 and got a cross XXX waypoint and maintain 15,000, well before the FMS ToD point.  I tried to enter an altitude restriction for that point, thinking the FMS would re-calculate the descent point to meet the new altitude.  The FMS accepted the restriction, but the next waypoint went back up to the cruise altitude setting.  It seems the FMS was never in the 'descend' mode.

At that point I lost all use of the FMS altitude restrictions, and had to fly the rest of the route and approach using only manually set altitudes and VS or FLC commands.

 

What's the procedure for small altitude changes (descents) well before the calculated ToD point?  Should I have entered the altitude as a new cruise altitude?

 

Thanks!

[Litjan]

Yep, enter a new cruise altitude and if you dial in that altitude on the MCP before EXECuting, the plane should even fly a cruise descent (with -1000fpm) to the new altitude.

Cheers, Jan

 

[BluesmanEP]

Perfect! I'll give it a try tomorrow.

Thanks!

Sent from my iPhone using Tapatalk

[BluesmanEP]

That worked great, thanks!  It did bring up a few more questions though

 

I think the problem I was having with the ToD point is that I was flying a route without a STAR, so the calculated ToD was actually pretty close to the airport.  By getting the "cross XXX at 15000" I basically wanted to start the descent at a point to capture that as a crossing restriction, not a new cruise altitude.  Is there a way to have the FMS go into 'descent' mode on command?  Rather than waiting for the calculated point?

 

Also, on the CRZ and DES page, I think I should be able to specify a cruise speed (the TGT SPD box), but I can only enter mach numbers, I can't enter a value in knots.  Additionally on the DES page, if I want to enter a new descent speed I enter /270 and click TGT SPD.  I get an EXEC prompt, but the entered value doesn't show up.  It's still the value it was previously.  The top of the DES page shows "280kt PATH DES 1", even though I entered a restriction of 270 knots.

I assume I'm doing something wrong... but can't figure out what.

 

Thanks!!!!

[BluesmanEP]

so it seems that you can only edit the descent parameters while the descent mode is active.  Once I reached the calculated ToD, I was able to change speeds etc.

That makes it all the more confusing to me.  I guess my general questions is, how do I setup an 'ad-hoc STAR'?  When flying a real STAR the FMS knows that the ToD point is actually quite far from the airport, and there may or may not be various level-off points along the way.  Once in descent mode, you're able to edit the descent parameters, and add / delete crossing restrictions etc.

However, if my first required altitude is at the IAF of an ILS approach (say 3500') and I'm cruising at FL200, I may be given by ATC several "descend and maintain", or "cross XXX at 8000" commands.  The calculated ToD point is such that I fly a straight descent to 3500'.  It seems the FMS isn't happy when flying below cruise altitude before the ToD point.

So my question is, how to I fly these step-down legs?  Do I just keep setting a new, lower cruise altitude in the FMS?  If I do that I always get a 1000 fpm descent rate, but no guidance on when to start that descent to cross a fix at a specified altitude.  Is there any way to set a crossing altitude restriction, and have the FMS calculate the ToD to reach that point, even if the FMS isn't yet in 'descent mode'?

 

Does this make any sense?  haha... I'm very solid on flying a route with a SID and a STAR, but I seem to be having trouble understanding how the FMS wants to fly a route in a more manually managed descent mode.

Thanks!

Edited September 29, 2016 by BluesmanEP

[Litjan]

To tell you the truth, handling a multi-restriction arrival is not the strength of our current iteration FMS... The real plane does handle that a bit better - and we are planning to get there.

To save yourself the frustration for now, do what real pilots do - don´t try to micromanage your descent with the FMS, but do the math in your head and fly the airplane, instead of having it fly you.

This may sound like a lame excuse, and I understand that simulator pilots are obsessed with the fully managed flight - it is my fault that we have built the plane with the real pilots priorities in mind, and not the simulator crowd. We fully plan to flesh the VNAV part out in the future - but rest assured, if you get 100 real 737 pilots into a discussion about what VNAV will do during the descent, 98% will sigh, roll their eyes and twitch their shoulders up . And I was one of the 2 remaining ones that had at least a vague idea...

Fly, don´t code!

Cheers, Jan

 

Edited September 30, 2016 by Litjan

[BluesmanEP]

Haha that makes sense

 

I'm comfortable flying manually calculating ToD and managing my own crossing restrictions, I just didn't think that's what you were 'supposed to do'.  So from cruise, I should just set a new altitude on the dash, and use FLC / VS to descend - not worrying about re-programming the FMS at all?  I assume as I get closer to the airport the FMS and my manual descent will 'sync' up again, once I pass the FMS calculated (but wrong) ToD point.

 

Thanks for your help!

[Litjan]

16 hours ago, BluesmanEP said:

Haha that makes sense

 

I'm comfortable flying manually calculating ToD and managing my own crossing restrictions, I just didn't think that's what you were 'supposed to do'.  So from cruise, I should just set a new altitude on the dash, and use FLC / VS to descend - not worrying about re-programming the FMS at all?  I assume as I get closer to the airport the FMS and my manual descent will 'sync' up again, once I pass the FMS calculated (but wrong) ToD point.

 

Thanks for your help!

Exactly right. And even if the FMS "thinks" that you are somewhere else, both vertically and laterally, this does not affect the airplane at all. You could program in a flight from Beijing to Hanoi and then fly from San Francisco to Seattle and the plane would just fly perfectly (although you would need to disregard all information on the CDU´s and fly VOR-VOR).

The FMS is just an "aid" - but not a required device like the FMGC on the Airbus fleet. In fact it is perfectly legal to fly the 737 with the FMS (and even the AP!) totally failed.Sure, some restrictions would apply (RVSM, RNP-1, CAT I only). It may also be an interesting fact that it is not legal to base your fuel estimation on the FMS - it is purely an additional information device.

So even if your vertical path is totally haywire, as long as you fly the vertical profile in a selected mode and verify that the magenta line makes sense (no huge extrawide turns due to erroneous radius calculation) you are fine.

Jan