737NG VNAV Descent

[GAJ52 54]

I must be constantly doing something wrong, can somebody please help.When descending from cruise altitude with VNAV engaged, I normally try to descend at Mach 0.76/280kts to FL100, but the FMS appears incapable of maintaining this speed. The speed normally drops to about 260kts or less with the speed bug on the speed strip demanding a higher speed. The speed mode annunciator (Left hand indication on the Autopilot/FD mode annunciator) remains at ARM and doesn't go green. I normally have to go to 'LVL CHG' until the speed settles down, then back to VNAV.The FMS autothrottle maintains cruise speed perfectly then descends at TOD but fails to hold the descent speed as commanded in the descent page of the FMS ???Glen Coyne

[crvm 0]

Simple, VNAV PATH does not keep ANY speed, target speed is for planning purposes only, the AFDS will make no attempt at keeping the speed, that is for you to do byTOO FAST - SpeedbrakesTOO SLOW - Add some manual throttle by holding down the F4 keyNOTE: If you become really slow (maybe 20 knots below target) the AFDS WILL increase the throttles. I believe on the real bird it brings you back to target then reverts back to ARM. I'm not sure on the PMDG though, try to slow it and find out ;-)[h4]Randy J. Smith[/h4]

[Guest Hawkeyeted]

Glen, to expand on Randy's point, VNAV decent is one of two things: A) PATH decent (default) or, :( SPEED decent. It does not do both at the same time as most are used to, as is the case wth the Microsoft default aircraft. And, since VNAV is a bit of an enigma to most new users, here is a quick explination.PATH decent most closely resembles the glideslope of an ILS. It is the most economical decent, regardless of speed. If the aircraft gets under (below) the path, the aircraft will slow as nose pitch increases to stay in the "slot". Autothrottle will then increase to slow the speed of decent to get you back in the slot (just like when you are on a ILS G/S). Conversly, if you get ahead (above) of the decent path (as is most common), the aircraft will increase nose pitch down. Since the aircraft can't shut the throttles any lower than flight idle, the only way to slow is to increase drag, therefore the "DRAG REQUIRED" warning.SPEED decent throws the PATH out the window, and adjusts pitch to maintain speed. If the aircraft gets slow, nose pitch down increases to speed her back up. If she is fast, nose pitch increase. This is a terrible way to decend if you are concerned about being at a given altitude do to an altitude restriction, simply because you have no control over the decent path.You can always trump the FMS by engaging both the V/S and SPEED on the MCP. That way, you control both, if so desired.Hope this helps!Regards,Ted Barkley

[Guest jplieben]

Wow! Every once in awhile a thread on this forum gets the lightbulb in my head to light up with a chime noise (similar to the seat belt chime). Randy and Ted, thanks very much for your insights! I just "got" VNAV descent!Jim Liebenstein

[Guest Hawkeyeted]

Great Jim. I'm glad you now understand since I "chimed" in. Sorry.... =]/R,Ted

[GAJ52 54]

Randy and Ted, many thanks for your replies.One thing still confuses me slightly, when the aircraft gets below the profile, the nose pitch increases and speed drops, Randy advises manually increasing power, but Ted indicates the autothrottle will increase the speed to compensate for the nose up pitch. Can you please clarify this.Many thanksGlen

[Guest Hawkeyeted]

Glen,Randy is correct in that if the aircraft speed gets TOO far behind, you can bump the throttles a little to help. But remember, PATH descent is not interested in speed, so throttles will only increase if you get WAY behind in speed (like in danger of falling out of the sky!). I don't want to tell you to not look at the speed in the descent, because that is bad airmanship. But, speed is not a factor in a PATH descent.Another thought, and I added it to some other threads, is the use of the "DES NOW" function. It's on the descent page, softkey 6R. You will only see it if you are within 50 nm if the calculated descent point.Scenerio: You're cruising at FL350, 35 nm from TOD. ATC tells you to descend to FL210. Enter FL210 in the MCP, and hit the "DES NOW" (LSK 6R on descent page). The aircraft will descend at approximately 1000 f/m until it intercepts the originally calculated descent path.Hope this clarifies a bit./R,Ted

[GAJ52 54]

Ted,I have finally grasped what a VNAV descent is all about, with the help of yourself and Randy.I wrongly assumed the descent was calculated for speed as well as path, thats why were required to put a Mach No/Speed in the descent page of the FMS. I'm not sure this is covered properly in the manual, if anyone can point me to the correct page I would be very grateful.Thanks againGlen

[Guest Hawkeyeted]

Glen,You only put in a speed if you want a SPEED descent that is other then what the FMS pre-calculated for you. If you are in PATH, FMS doesn't care what speed is set.Page 8-45, right column, second paragraph; "TGT SPD: Line 2L contains the descent speed mode information. The descent speed is displayed in large font, in the Mach/Airspeed. Information on this line can be updated manually if desired.It is often misunderstood because folks don't totally understand the difference between a SPEED descent and a PATH descent.Hope this help!/R,Ted

[GAJ52 54]

Ted,Ignor my comment on the manual not covering this subject properly, there is a perfectly good explanation on VNAV descents on the page you suggested and continued on pages 46 & 47, I just didn't read them properly.Many thanks for taking the time to point me in the right directionGlen