I understand that in a VNAV climb, when setting up in the FMC an altitude constraint with a B, the plane should remain below or not go higher than this altitude, and provided than the altitude set in the MCP and that the cruize altitude are higher than the altitude constraint, the VNAV mode should change from VNAV SPEED to VNAV PATH when reaching this altitude constraint. This is actually what is happening when setting up a simple constraint without a B. But as soon as the B is added, the altitude constraint will just be ignored; when reaching it, the plane will continue his climb above, for about 300 feet; then the error message "UNABLE NEXT ALT" will be displayed on the FMC and instead of changing to the VNAV PATH, the mode wil change to VNAV ALT and further automatic VNAV process of the route will be blocked till manual action is taken.Am I doing anything wrong or is it a bug ?RegardsEric L.

>I understand that in a VNAV climb, when setting up in the FMC>an altitude constraint with a B, the plane should remain below>or not go higher than this altitude, and provided than the>altitude set in the MCP and that the cruize altitude are>higher than the altitude constraint, the VNAV mode should>change from VNAV SPEED to VNAV PATH when reaching this>altitude constraint. This is actually what is happening when>setting up a simple constraint without a B. But as soon as the>B is added, the altitude constraint will just be ignored; when>reaching it, the plane will continue his climb above, for>about 300 feet; then the error message "UNABLE NEXT ALT" will>be displayed on the FMC and instead of changing to the VNAV>PATH, the mode wil change to VNAV ALT and further automatic>VNAV process of the route will be blocked till manual action>is taken.>>Am I doing anything wrong or is it a bug ?>>Regards>Eric L.Hi Eric,I'm not sure what you mean by the letter B but I don't think you have a bug. I don't think you get a VNAV PATH when you reach an altitude constraint. You only get a VNAV PATH when you reach your cruising altitude that's programed in the FMC. You will get a VNAV ALT when the auto-pilot captures the altitude that's entered in the altitude window of the MCP. I understand you to say that you leveled 300 feet above the altitude that's entered in the MCP. What altitude are you flying when this occurs? Did you set the altimeter to the current pressure setting if you're using Real Weather? Ken.

Hi Eric,I have had the same problem with the 744 when entering an "at or below" constraint. I posted this issue a few months ago when PMDG was using the alternate forum. I didn't receive a response to my post though.For now, I just end up entering the altitude as a hard constraint and that works fine.-Christian

Hi, KenBy "B", I mean the constraint "at or below" the altitude.Sorry to insist, but if your cruise altitude and MCP altitude are higher than the FMC defined altitude constraint, VNAV SPEED should change to VNAV PATH when reaching the altitude constraint (I tested it on PS13. Once the waypoint with this constraint is reached, the climb should automatically, without manual action, resume in VNAV SPEED.The problem happens whatever the altitude is.I am not using Real Weather.Eric

Hi, ChristianThanks for advising. I also figured out that, since there is no problem with it, a hard constraint is the best temporary solution till this likely bug is fixed. At a sim level, the possible few feet more would not make such a difference.Eric