Another NVAV Question

[Guest celestar]

Hi PSS team,I just bought the PSS 777 package today, and took it for a little flight VHHH-WSSS .. as usual, I armed the LNAV and VNAV before take off. I set in the VNAV page on the FMC FL380 (optimum is FL375). The after takeoff, passing 400ft or 500ft (which VNAV should have taken over) VNAV did not take over at all, and the IAS/MACH window became active (the VNAV light still on though). I had to reset the altitude in the FMC to FL360, and also in the MCP,I got succeded. The VNAV got activated. However, when I set to FL380 in the FMC then MCP, VNAV got de-activated again, and the mode became SPD ALT in the FMA. Is it only me or anybody else having this problem? Looking forward to your responses, PSS Team.Regards,Dany Setyadihttp://www.precisionmanuals.com/images/forum/supporter.jpg

[Guest D17S]

Seems like the TO sequence is OK. With Vnav armed, pitch stayed in TOGA till 500 feet, then MCP airspeed window blanks and airspeed bug hops to the FMC's V2+10 with VNAV SPD and THR REF annunciated in the FMAs. Acceleration altitude occurred at 1000 feet as witnessed by the airspeed bug hopin' up to 1st speed limit . . . then keeps hoping up as flaps are raised. Airspeed followed, via pitch. Good.A little more realism could be obtained by modeling a lift-off or averaged FMC V2 (in addition to the FMCs' V2+10) for that first vnav capture bug hop . . . but that's just for the purists. All in all, the TO sequence looked OK. But, I never got to cruise. The PDF/ND panels were flashing back to indicate my pre-roll,

[baw716 0]

Dany,There are a number of issues that people have with VNAV. However, some of the issues with VNAV have to do with the complexity of the logic of this FMC. I made a rather large posting on this subject; but the short of it is that if you set an altitude in the MCP that is higher than the optimum, VNAV will not engage. You must set an altitude that is lower than the optimum on the MCP.Also, when you do the pre-departure setup, the altitude setup is done in the INIT REF page. You don't touch altitude on the VNAV pages before departure. Once you are airborne, if you want to make altitude changes, you can make it on the VNAV CRZ page, so long as you are within the optimum parameter set out on the VNAV CRZ page.Third, very rarely, in fact almost never, is the initial MCP altitude setup for your cruise altitude. Usually, your clearance is for anywhere between 3000-23000 feet. Once you are airborne, you continue to increase the MCP altitude and VNAV will adjust the climb accordingly to give you best performance in the climb.Fourth, as a rule, I don't arm LNAV/VNAV on the ground. Once you hit TOGA, the aircraft will follow the ground track and once off the ground will continue to fly the same track. Above 400, you can engage LNAV/VNAV; however, often many pilots will not use VNAV until after flap retraction is complete. Usually, the IAS is set to V2+20kts, then out of 400 feet, you hit FLCH and then you increase your speed in the IAS as you bring the flaps up. Once the flaps are up and you are clean, then hit VNAV and it will hold your speed at 250kts (what is put in the speed/alt restriction page on the VNAV CLB page) until 10000 at which time VNAV will speed you up to your programmed climb speed.There are some issues (I won't call them problems...since the occurance is not always the same) with VNAV. Certain actions by the pilot will force VNAV out of the current mode into either CRZ or DES mode. While this is a issue that must be watched carefully in your climb, it is not a disaster. You can switch to FLCH managed climb or SPD+V/S (however, be careful with V/S because there is no altitude protection unless you have something set in the MCP altitude and even then, you need to adjust your V/S down so you don't bust through the altitude (this is especially important on the descent).A number of us have brought this issue in the VNAV logic to the PSS team and they are trying to work out a solution. The principal difficulty here is that the failure is inconsistent. We know it fails under certain circumstances, but for others, it fails intermittently. Its a bugger, since unless the failure can be reproduced consistently, its difficult to fix. In my post on VNAV flying, there are some work arounds if you get into the situation where VNAV craps out on you. The beauty of this aircraft is that you don't NEED VNAV to fly the airplane. The flip side of that is that you probably won't get as efficient fuel burn and you will have to watch speed and altitude like a hawk.I hope this rather long winded answer gives you some sense of the issue, that we know about it, PSS knows about it and what some of the work arounds are until the VNAV logic is corrected. Since it is an exceptionally complicated problem, I don't hold a great deal of hope that it will be solved in the very short term, since there are some other issues (specifically the CDU crash), which are more pressing and that need to be fixed as soon as possible.If you have more questions, please feel free to post them and if PSS can answer then, terrific; I have a lot of resource material on the 777 FMC (the real one), so either myself or one of the real T7 drivers on this forum can answer the question for you. There are some real fine resources here...all you have to do is ask.Good luckDave Lambbaw716

[baw716 0]

Sam,VNAV will advance to DES mode if the flaps are inadvertently extended in the climb. If you are using the keyboard to make flap retractions, make very certain you hit the button squarely. There's been more than once I've hit F6 to pull the flaps up and accidently hit F7 and caused myself some grief.However as you said, this is why we have pilots in the cockpit :)Your description of the PFD/ND reverting back to its original state is odd; I've not heard that reported before.Give it a go again and if it repeats, you might want to offload the T7, run a reg cleaner, spyware check and defrag then reload the T7. I have found that in some cases, there were bad reg entries and by clearing it and reloading it, I was able to solve the problem.Good luckDavebaw716

[Guest D17S]

Thanks for taking the time for the reply. Good to hear from anther Vnav dog. It

[baw716 0]

According to Leo A. and a couple of other T7 retired drivers, VNAV is not suppose to do anything if you inadvertently extend the flaps. It should remain in climb mode. This is definitely a bug in the FMC logic, but there are other factors involved in the VNAV issue. This is why it is such a bugger to fix.Regards,Dave/baw716

[baw716 0]

Sam,To answer this question, I think I need to defer to my T7 drivers. What I know and learned about VNAV (and this applies to the 747-400 as well), is that any change in the MCP altitude will affect VNAV. The degree of impact is minimal if it is a 2000 ft change over, say 50 miles. However, if you are at the start of your flight and you have step climbs programmed into your flight and you have to change MCP altitude, say for weather, it is my understanding that each of the legs that are affected by the change are modified to adjust to the step climb level that is entered based on the adjusted weight of the aircraft. Again, I stress this is my understanding, don't take it as gospel. I will ask my sources and get an accurate answer for you.Regards,Dave/baw716

[FOD 23]

Sam & DaveAs discussed in detail prevously. In the PSS T7 the system works perfectly - and is fail safe - in CRZ and DES modes. Only in the CLB mode is it both illogical and dangerous, and would never be allowed rw.Thus - IMO - a genuine bug and I know that the teanm are well aware.Regards to allJohn REditDave - just seen your other post on this and Norman's reply - as we expected of course. This bird is going to be a real winner when PSS have finished with her!