737NG VNAV question: geometric->idle descent

[singletracker]

I'm brand-spankin-new to the 737NG, and for the most part, love it. However, the FMC still baffles me in some respects, mainly 'cuz I keep looking for things that I could find in an instant on my old jet, even though they ain't there on the Boeing.

My biggest issue is that I don't know how to convert a geometric path to an idle path descent. For example: you're cruisin' along at FL410 (in VNAV ALT, of course). The legs page has two waypoints ahead, each with an altitude restriction: XXX at FL300, then 50 miles later, YYY at FL280. ATC gives you "cross XXX at FL300." Cool, no problem, you dial in 30000 into the window, and when you hit the T/D, the throttles come to idle and you descend in VNAV PTH. So you arrive at XXX at FL300. Now for the tricky bit: you cruise along for 10 or 20 miles past XXX, and you notice that the VNAV path <> is going below you, as you would expect, since the FMC is now in descent mode, and has constructed a geometric path descent from XXX to YYY – a very shallow one, you note.. Eventually, ATC says "cross YYY at FL280." Well, if you blindly dial in 28000 into the window and mash ALT INTV, you get a idle descent for a few hundred feet to join a really shallow path...not very smooth or ATC-friendly. What I really want it to do is make the box act like it's still in the cruise mode: maintain FL300 in VNAV ALT until the T/D for an idle path down to FL280 at YYY. I suppose I could bring up the VNAV cruise page, which for the moment shows [][][][][], and put FL300 in there. But I've noticed, especially at lower altitudes, that the econ cruise speed will drop considerably from what you were doing before. Not a very clean solution.

So what does everyone else do in this case? I've seen guys re-select cruise, change the next waypoint altitude to your current cruise altitude then back again, maybe something about changing the cost index by 1, and maybe other stuff. What works the best/cleanest?

[alistomalibu]

My answer will not help you very much...I am sure someone will provide you what you are looking for.
But I will suggest to engage other descent modes. You want to descent only 2000' (It will take you 6 miles more or less depending on the wind). Maybe you could let the VNAV profile as it was scheduled and use FLCH or VS modes (You could use the green arc as reference)

Alejandro

[Three Wire]

New VNAV Descent

With a crossing ALT requirement and a long level segment to the next crossing requirement, the FMC (on all Boeings) behaves exactly as you describe.

Once the FMC crosses the first T/D it enters descent mode. It then blindly continues to calculate paths to the next altitude restraint, and then navigates them with thrust as required.

Try entering your new level (in your example F280) into the CRZ page. the FMC will return to cruise mode and recalcualte an idle descent to the next restraint.

Hope this helps

[Rainboe]

The NG FMS is a bit of a disaster area in the descent. I normally prefer manual control of the autopilot and use VS quite often to control the descent better. In your case, to do it cleanly and prevent those weird speed reductions, input a new cruise alt at FL300, ensure the speed is what you want or reset it, re-engage VNAV, and wait for your new TOD to hit YYY at FL280. Untidy, but leaving VNAV sometimes produces some bizarre speed reductions that I haven't worked out yet .

The 737 VNAV descent program is a disaster compared to the 747 one which is far better.

[salamanderpress]

is it that difficult?

When you dial in FL 300 on the MCP,as you would HAVE to do at Top of Descent short of XXX, the new MCP altitude (30000) automatically appears on the scratch pad in Cruise page, just begging you to enter it into the cruise altitude line. This done, and the altitude at XXX set at FL 300 as you describe, would get you an idle thrust descent to XXX/ FL 300 and subsequently another idle descent to YYY/ 280.

Or do I have it wrong somehow.. A new NG driver myself.

[gyni]

Level off at FL300, select DIR TO next waypoint on FPLN. FMC recalculates path and gives you a new top of drop for next restriction.

[RYR-738-JOCKEY]

Geometric path descents only work now and then. My solution in your case is; after you reach FL300 at the first waypoint, delete that restriction and enter FL300 as new CRZ alt with whatever speed you like. That'll give you a new T/D with idle descent after that.
In RYR we've reverted back to the 10.7 FMC update now, because of all the bugs on the 10.8, which also includes this geo. path rubbish.

[busz]

Or just as you are about to reach XXX, select LVL CHG and delete the 300B at XXX from the FMC. The ac will then ALT ACQ at 300 and an idle descent will now be calculated to cross YYY at 280. You will capture the path from below whilst cruising level at 300.

Alex

[CaptainSandL]

The VNAV geometric path descent was so unpopular that Boeing included an option in U10.7 to go back to idle descent and just retain geometric path descent in approach phase only. This was one of the main reasons why we decided to upgrade to U10.7.

You can see if you have this option enabled by checking MAINT > FMCS > PERF FACTORS. This page will show GEO DES FULL/APP at line 1L, where either FULL or APP is highlighted in reverse video. It would be great if in U11 (or the recently announced 10.8A) this option could be line selectable by crew in-flight.



RYR 738 Jock,
The reason all U10.8 operators have been told to rollback to 10.7 is because of a serious bug in geo path desc in the approach phase in which VNAV can command the airplane to descend below the expected path on approach by 200ft !!!
See my write up at FMC Update 10.8 and the imminent 10.8A service pack.

S&L

[plain-plane]

this is one way to avoid the stupid speeds at your new lower cruise lvl if you enter them as you reach them.

go to RTA page, then LIMITS, there you can enter new min and max speeds for VNAV. ie: min 270/,790 and 271/,791. you do not need the RTA point for this to work, and it is less hassle if you do not put it in.

PS: some drivers out there might enjoy the fact that the FMC will still show ECON for your speed selection when you do this....

[BOAC]

ST - three wire's solution is perfect, but do not forget that sometimes ATC, having given you a 'descend FLxxx', expect you to 'descend' rather than fly level 'until you are ready'. A quick call if in doubt to clarify might be a good idea. Not forgetting that sometimes Alejandro's use of v/s is more comfortable for your salary payers than an idle descent..

[Denti]

@CaptainSandL, did that problem during VNAV Approach only affect the VNAV approach function or IAN approaches as well? Just curious as we have 10.7 (still) on all of our planes. By the way, the 1. VNAV Armed Before Takeoff feature isn't really new, is it?

With the features planned for 11.0, especially route 2, i'm really looking forward to the (hopefully) mostly debugged 11.1 version.

[CaptainSandL]

Denti,
Good question, we don’t have the IAN option so I have not looked into this and I am therefore not qualified to answer. However as the 10.8 anomaly affects approaches constructed from the FMC, and IAN uses an FMC derived glidepath rather than an ILS glideslope, I would prohibit IAN approaches until receiving conformation from Boeing that it was safe to continue with U10.8. I doubt it will be.

Here is some of the info I have. I cant reproduce it all due to copyright:
“Boeing has received reports from several operators regarding anomalies associated with the Update 10.8 (U10.8) FMC VNAV operation. One operator reported 3 events where the FMC displayed the message "STEEP DESCENT AFTER XXXX" on a specific VNAV approach. The XXXX was the FAF (Final Approach Fix) waypoint in the descent approach segment. Upon sequencing the FAF, the autoflight system commanded the airplane to descend below the expected path on approach.
…has determined that an error in the FMC U10.8 software result in the VNAV path being built to the wrong waypoint, specifically to a waypoint with an AT constraint upstream of the waypoint where the steep descent takes place.
… An error in the prediction logic may calculate an incorrect path between the AT waypoints that is lower than the published navigation database (NDB) path. Under these conditions, the published NDB path is not honored.”

The U11.0 features I listed are not confirmed yet, but are hotly tipped. The latest I am hearing is that RTE 2 may not be included in U11.0 but I am not sure.

I did not say that VNAV armed before take-off was a new feature, it has been available since U10.7. However crew were instructed not to use it in FCOM OMB TBC-70, “Arming VNAV on the Ground (FMC Update U10.7)”, dated 11 June 2007. U10.8 fixes this and removes the restriction.

S&L

[Denti]

Thanks for the information and for pointing me in the right direction with the TBC. Got told during retraining that we may use VNAV arm before take off if it seems to work (as tested by selecting VNAV, 10.7 version). Tried it a few times in new planes and it worked, but checking the relevant TBC it seems those training instructions were not really up to the current status of our documentation.

Would be a shame if they do not include route 2, after all its not really a new thing (judging from jumpseating on the airbus fleet) and would really help with those short turnarounds. Granted, datalink route loading would be even better, but i haven't heard anything about that coming within my outfit, just those sub-lease companies flying for us use it.

[singletracker]

Wow guys, thanks for all the responses. Seems that everyone has a different way to tackle this issue. I'm disappointed – and a little baffled – that Boeing or Smiths didn't provide a simple "recalc path" selection, maybe in the descent page. I mean, this is a very common issue, at least with the way ATC works here in the USA. It's almost as if no one asked pilots how they use VNAV day-in and day-out. Oh well, whining won't make it any better, so I'll try everyone's techniques once. I especially like plain-plane's RTA speed limits suggestion. I do wonder, however, what to do when ATC issues a speed assignment...will I be able to mod the descent speeds on the VNAV DES page, or will I have no choice but to open the window with SPD INTV? I'll try it and report.

Keep the ideas coming!

[Capt. Inop]

Boeing could have done something about this years ago (if they wanted to).

We're in for a major update these days, for the better i hope.

[plain-plane]

ps: when you put in the rta limit speeds you will end up flying a bit faster at lower levels, especilly below FL260... as "it" likes to "cruise on mach"... but you will see... and find out what speeds you like to put in...

[Kirks gusset]

I was on the NG for 5 years and as I recall..if you fly past the TD point, the a/c goes into energy compensation mode if in VNAV. When you execute the descent now the a/c initially descends at 1000fpm then tries to capture the path. FMA shows IDLE, VNAV PATH. Obviously it cant capture so it reverts to VNAV Speed and ingores the FMC constraints for speed while trying to recapture path mode. Simple ans is use LVL CH and wind the speed up, when the magenta path deviation is above the the line just select vnav again.

[Jesper]

You just always have to put the level off alt in the crz-page as new crz-alt. I found the fmc really hard to manage in descend at the beginning, but when u realize how it works it's quite easy.

[BOAC]

Kirks G - 'going past TOD' takes us well off topic, but unless a later update has changed things, 'Descend Now' is only available before VNAV TOD. Cruise Speed will begin to reduce towards min clean (to attempt to control total energy) until descent is 'authorised', whereupon Starship Enterprise will dive as fast as it can (unfortunately into V/Mmo land) until either you do something else or 'Path' is regained. Your last sentence is correct.