VNAV issues (solved - not understanding AT override modes)

[Nixon 30]

Hi everyoneAm not sure if I have a problem with the VNAV logic. Just now I completed a flight from Innsbruck to Heathrow. Hand flew the departure from runway 26 doubling back towards RTT. Selected VNAV when I was near the magenta line and then selected the AP. The plane climbed perfectly in VNAV. Was cleared down into EGLL just before TOD selected FL260 and as the plane passed TOD she descended in VNAV from FL360, at FL 340 she stopped descending. The FMC command speed was still 0.8M. The descent profile on the ND screen showed that I was not descending quickly enough and was rapidly very high over the intended profile. I flew for about 8nm and she remained at FL340. Then I selected FLCH keeping the speed at 0.8M and deployed the speedbrakes to max. She still did not descend so I choose VS and kept the speedbrakes deployed and the descent started. I then selected FLCH and turned off the AT and pulled the throttles back to idle and descended as if in IAS mode (similar to the Jetstream 41). When I got to 15,000ft I reselected the AT and the throttles stayed in idle and descended successfully into Heathrow. I would have thought that she would have kept to the profile in VNAV and if the descent got high would have flagged up "add drag" in the ND (as happens in the 747X). This did not happen. Am not sure about FLCH mode logic but I assumed that it was similar to the IAS mode in that the throttles go back to idle and pitch is used to control speed. Is this assumption wrong as even with the speedbrakes deployed at max the throttles did not retard. The samething happened yesterday on the same flight so am wondering, is it me or is it a bug or is it in service failure of the AT. BTW I had the freeze problem and have installed the hotfix.

[RYR738 9]

Hi everyoneAm not sure if I have a problem with the VNAV logic. Just now I completed a flight from Innsbruck to Heathrow. Hand flew the departure from runway 26 doubling back towards RTT. Selected VNAV when I was near the magenta line and then selected the AP. The plane climbed perfectly in VNAV. Was cleared down into EGLL just before TOD selected FL260 and as the plane passed TOD she descended in VNAV from FL360, at FL 340 she stopped descending. The FMC command speed was still 0.8M. The descent profile on the ND screen showed that I was not descending quickly enough and was rapidly very high over the intended profile. I flew for about 8nm and she remained at FL340. Then I selected FLCH keeping the speed at 0.8M and deployed the speedbrakes to max. She still did not descend so I choose VS and kept the speedbrakes deployed and the descent started. I then selected FLCH and turned off the AT and pulled the throttles back to idle and descended as if in IAS mode (similar to the Jetstream 41). When I got to 15,000ft I reselected the AT and the throttles stayed in idle and descended successfully into Heathrow. I would have thought that she would have kept to the profile in VNAV and if the descent got high would have flagged up "add drag" in the ND (as happens in the 747X). This did not happen. Am not sure about FLCH mode logic but I assumed that it was similar to the IAS mode in that the throttles go back to idle and pitch is used to control speed. Is this assumption wrong as even with the speedbrakes deployed at max the throttles did not retard. The samething happened yesterday on the same flight so am wondering, is it me or is it a bug or is it in service failure of the AT. BTW I had the freeze problem and have installed the hotfix.

Did you have a lower altitude set in the MCP ?The MCP takes precedence over the FMC. You could even set your missed approach altitude before T/D and Vnavwill fly you all the way down to it regardless of the waypoints along the way. Fred.

[crvm 0]

Hi everyoneAm not sure if I have a problem with the VNAV logic. Just now I completed a flight from Innsbruck to Heathrow. Hand flew the departure from runway 26 doubling back towards RTT. Selected VNAV when I was near the magenta line and then selected the AP. The plane climbed perfectly in VNAV. Was cleared down into EGLL just before TOD selected FL260 and as the plane passed TOD she descended in VNAV from FL360, at FL 340 she stopped descending. The FMC command speed was still 0.8M. The descent profile on the ND screen showed that I was not descending quickly enough and was rapidly very high over the intended profile. I flew for about 8nm and she remained at FL340. Then I selected FLCH keeping the speed at 0.8M and deployed the speedbrakes to max. She still did not descend so I choose VS and kept the speedbrakes deployed and the descent started. I then selected FLCH and turned off the AT and pulled the throttles back to idle and descended as if in IAS mode (similar to the Jetstream 41). When I got to 15,000ft I reselected the AT and the throttles stayed in idle and descended successfully into Heathrow. I would have thought that she would have kept to the profile in VNAV and if the descent got high would have flagged up "add drag" in the ND (as happens in the 747X). This did not happen. Am not sure about FLCH mode logic but I assumed that it was similar to the IAS mode in that the throttles go back to idle and pitch is used to control speed. Is this assumption wrong as even with the speedbrakes deployed at max the throttles did not retard. The samething happened yesterday on the same flight so am wondering, is it me or is it a bug or is it in service failure of the AT. BTW I had the freeze problem and have installed the hotfix.

If you were cleared for an early descent (you stated you re-selected FL260 prior to T/D) why would you continue in VNAV without using DES NOW through the FMC or ALT INT feature? I don't understand how you were cleared "DOWN TO EGLL" yet set the MCP to FL260. Let's say you simply asked for descent clearance (normal OP) and reset the MCP (normal OP) and let he aircraft des in the profiled VNAV path. What's at FL260? Something you neglected to inform us on? You might get clearance to start your descent down but there are various restrictions (i.e. FL100 -sid restrictions, landing clearance etc)

[Tabs 2,790]

Nixon, The thrust mode goes to RETARD and then to ARM in the initial part of a VNAV PTH descent or in a LVL CHG descent. The fact that you said "I then selected FLCH and turned off the AT and pulled the throttles back to idle and descended as if in IAS mode" is telling. Your throttle was not actually at idle like it should have been during the VNAV and LVL CHG attempts. What likely happened is you didn't physically pull the throttles back, then went to ARM (which in the real airplane is manual throttle control - AT servo disconnects) and then something bumped or jittered your throttle and your thrust got increased to higher than it should have been. Set your AT override mode to NEVER in the options and this won't happen. Always a good idea too to pull your throttle physically back to idle as you reach T/D so that it can't be bumped back up.

[Nixon 30]

I should have said cleared to descend down to FL260. I checked the descent page for the DES NOW prompt but it was not there. The throttles were no way near idle and I quote from page 4.19 of the FCTM concerning Descent "The path is built from the lowest constraint upward, assuming idle thrust" So am at a loss as why the throttles were not at or near to idle. That is why am confused as why this is happening and am wondering if it has anything to do with the Hotfix and the issue it was designed to cure. Edit to add: Thanks Ryan I have just seen your post and it makes sense so please ignore the above