777 Not descending for next waypoint?

[sddjd 117]

I've run into this issue occasionally and can't figure out what I've missed in the approach setup. I'm approaching OMDB (can't remember the STAR I selected) and the next waypoint PEDOV is listed /3000A. Immediately following that by 3NM is DB657 with a hard entry of /3000.

From 40 miles out after T/D my 77W simply refuses to descend past 15k. Altitude is confirmed dialed to 3000, but no matter what I do I can't get the aircraft to respond to the upcoming waypoints' altitude requirements... at 5mi from PEDOV I finally intervened, but obviously I"ve missed a step. Any thoughts on where I might check? Thx!

[downscc 1,327]

Those arrivals into OMDB usually have a very shallow descent during most of the intermediate segments.  It is normal for the aircraft to remain at mid-level altitudes for as long as practical before starting a normal descent.  Now, if there is not a normal descent to 3000 then you are missing something.  In other words, instead of a long descent at 150 ft/nm the aircraft will remain at altitude until a normal descent of 300 ft/nm can be established.  It is better to stay high.

[sloppysmusic 169]

Did you level off at any point during descent after T/D? Or did the plane just descend using VNAV PATH and stop at 15000 and wait there?

[sddjd 117]

Thanks both, I followed the "normal" steps prior to T/D including resetting the ALT target to 3000 (PEDOV listed at 3000A). The plane began the descent as expected, but leveled out at 3kagl on its own. 

I'll see if I can recreate the flight from out prior to T/D; as this was a long one from FIMP it won't be as simple to jump in again to the exact scenario 😎

I think without exception my problems with the 777 have been self-inflicted, and suspect this is no different. I've bumped into this one occasionally but am not aware I'm doing something different than other successful descents so thought maybe it might be an equipment option or item I hadn't anticipated. 

[Captain Kevin 586]

6 hours ago, sddjd said:

Thanks both, I followed the "normal" steps prior to T/D including resetting the ALT target to 3000 (PEDOV listed at 3000A). The plane began the descent as expected, but leveled out at 3kagl on its own. 

So let me see if I have this right. You set the altitude at 3,000, the altitude restriction at PEDOV is at or above 3,000....but you're surprised that the plane leveled off at 3,000? Two reasons why the plane won't go below 3,000. The first is the altitude restriction at PEDOV is at or above 3,000, so it won't go below that. Second is you set the altitude to 3,000, so even if the altitude restriction wasn't there, it still won't go below that.

[scandinavian13 5,192]

Gents - this is a very, very, very important lesson, and one that I harp on all too often:

You are the brains of the operation. Do not ever let it become otherwise. I get that the automation can get a bit confusing and complex, and understanding it requires a bit of a doctorate in flying airplanes and/or coding, but logically step through anything you do on the deck.

The first item is that you are responsible for everything that goes on with the aircraft. As such, you need to be aware of everything that you allow it to do. Note the way that I wrote that: everything that you allow the aircraft to do - not everything the aircraft does. If the aircraft does something stupid, it is because you allowed it to - period - end ( - full stop, for you British types).

Thinking back to the origins (think cloth wings), we as pilots change altitudes to meet the demands of the operation: weather, ATC, terrain, and performance. Every time you climb and descend, you have some sort of limit - be it performance-based, rule-based, or physical - that you will observe. The most common one is the ATC constraint: "climb/descend and maintain X." In the past, you watched the altimeter roll up until you got near X, and then slowly leveled off by hand. Later on, we used the AP to do this by adjusting the climb wheel and then engaging ALT HOLD. Later still, we used IAS or VS mode with ALT SEL and pre-selected an altitude at which the AP would stop the climb/desc. Later on, we added in VNAV, which helps us determine a nice climb profile between our other factors, and it can also hit intermediate altitudes between our current altitude and our limit altitude (the one ATC gave, etc).

In all of these cases, if you bust an altitude, it is your fault. If ATC says "climb via the SID, maintain FL350," and VNAV misses a 14000 foot limit at FIXAA for some reason and it causes a loss of separation, that is your issue to answer to. One way to mitigate this risk is that many operators (and manufacturers) recommend setting the MCP altitude at each limit altitude because VNAV will stop at that altitude (climb or descent). VNAV can theoretically fly you all the way to the ground. This in mind, it is always a good idea to set some sort of limit to the power of automation to be sure that it doesn't automatically put you somewhere you don't want to be (such as into the ground).

The automation is there as a tool. It is not there to be put on a pedestal such that you're subservient to it. Always know what it is doing, and why. If you don't, then I would strongly suggest punching off the AP, getting the plane under control, and using modes that you better understand until you can come to an understanding of what is going on. Never let the plane get into a position you weren't in mentally well before.

Note that, even before you began your descent, you rolled the MCP altitude window down to a lower altitude. The plane would not descent from cruise unless you had done so. Think that through for a minute. With that in mind, why would you assume that the automation should selectively ignore it - either at cruise, or mid-descent? Further, why would you think that allowing selective adherence would be a good idea? With those both in mind, is it still confusing as to why the aircraft stopped descending, and if so, why?

[BlueStar 323]

1 hour ago, scandinavian13 said:

One way to mitigate this risk is that many operators (and manufacturers) recommend setting the MCP altitude at each limit altitude because VNAV will stop at that altitude (climb or descent)

👍.  A lot of good points Kyle. 

Also a lot of companies use FLCH (or the equivalent) below 10,000. 

Never ever trust the magic or an aircraft any farther than it can be picked up and thrown. 🤣

Grace and Peace, 

 

Edited February 13, 2019 by Bluestar
typo

[sloppysmusic 169]

Some great comments here, thanks! I fly the 3 PMDG Boeing jets I own exclusively outside vnav/lnav. I use those modes to learn all the other systems then when I understand them I take full control of all the flying using only the MCP and VOR/ADF to fly the plane. My route only has departure and arrival airports plus departure runway programmed in. I find this way of flying immensely enjoyable even though unrealistic. If the weather turns real bad or there is an emergency it takes only moments to switch back to FMS Nav and see to the problem. Reading those fcoms is so much fun even though they would attract more readers if they were laid out as a series of advanced tutorials instead. I thoroughly recommend going through them and actually practicing each procedure in the plane. Some real cool tricks in there! FIX is your friend for example!

I used to also occasionally get annoyed when the plane did something unexpected. So I would search the internet, be directed usually here and find the answer was in the manual if I would only read it... Those manuals contains gold...even if you have to dig a little to find it. After all your good money paid for it as it took some time to write so had development costs. 

It really is YOUR plane now. Learn just one new fun fact a day and your flying is much enhanced. Got to go now I have the 747 fcom to get get through! 

🙂

Edited February 13, 2019 by sloppysmusic

[sddjd 117]

23 hours ago, Captain Kevin said:

So let me see if I have this right. You set the altitude at 3,000, the altitude restriction at PEDOV is at or above 3,000....but you're surprised that the plane leveled off at 3,000? Two reasons why the plane won't go below 3,000. The first is the altitude restriction at PEDOV is at or above 3,000, so it won't go below that. Second is you set the altitude to 3,000, so even if the altitude restriction wasn't there, it still won't go below that.

Aha, I see the problem now was my typo in my response - I set the altitude to 3000 in consideration of the 3000A restriction at PEDOV. The aircraft began descent at T/D but stopped descending at 15,000 (per my first post, the 3k was me typing too fast later; nothing like providing the wrong info, sorry!). 

Regarding the responses that follow, agree completely and do tend to follow the steps in the STAR with regard to alt restriction matching on the MCP for each leg. I am still unsure as to what step I may have missed that led to my aircraft holding the descent at 15k.

[sloppysmusic 169]

Any chance you slipped out of VNAV without noticing? Also if you level off at an altitude using the MCP regardless of whether it's in the STAR as a restriction the plane wont start descending automatically in VNAV just because you lowered the MCP altitude. You actually have to tell it descend again. If it's an MCP restriction you are probably in ALT and will need lower MCP altitude and press VNAV again or FLCH and re-engage VNAV once descending. Don't quote me I've been running through the 737/777 and 747 FCOMS so I may get a little mixed up, but logic is similar.