02-04-2008, 04:43 PM #1
- Join Date
- Oct 2007
OVERSPEED during PM VNAV descent?!
Anyone could tell me why many times during a VNAV descent the speed cue in the PFD does not match the value higlighted by the FMC for the actual leg?.
I know that the FMC value is only a "target value" but many times what happens is that the aircraft start descending and accelerates to OVERSPEED and if i don't swithch off VNAV mode....well....wings will be ripped off (maybe before reaching the point to wich speed would have been reduced?......who knows...)
That's a bit annoying both for pilot and passengers!
02-05-2008, 04:23 AM #2
- Join Date
- Nov 2005
- IJmuiden, Netherlands
I noticed the same, not only with descend but also during climb.
In my legs page , I can see speed/height settings for each waypoint during climb and descend. Sometimes I see the speedcue on the PFD changing when passing a wp. But the altitude cue (is it called that way? anyway I am talking about the magenta coloured altitude the AP is going to) stays the same. Mostly the altitude cue is 10000, that value is in the MCP window. As long as the altitudes for the waypoints are lower in the cdu as the alt set in the mcp window it should use them I think.
To be short, I do not use vnav anymore, only LNAV. I look at the speed/alt indications in the CDU and guide the plane to that via other modes.
If you read this forum you will notice many complaints with VNAV. So better not use it till new releases od the sw.regards
02-05-2008, 02:32 PM #3
- Join Date
- Oct 2007
I agree with you.
My only difference is that, apparently , I have your same problem on altitude only if I reset FS and do another flight without switching off the PCs (itís like PM saves some data from the already completed flight wich prevent the CDU/FMC to work properly during the second flight in VNAV mode)
Having done some more tests in VNAV mode i noticed the following:
ONLY during VNAV descent (during CLB and CRZ everything works finely) the projected speed over the speed bar of the PFD doesn't match the one visualized on the FMC LEGs page for a specific leg.
The problem is that the value showed over the speed bar during VNAV descent (the purple one) is EVERYTIME the MACH value reported in the DES page of the FMC at the TGT SPD line AND NOT the speed value inserted in the LEGS page (right side) for that specific leg.
Being this mach value a more or less FIXED and UNIQUE value, the result is that when I leave a high FL (example FL320) the aircraft will accelerate progressively to overspeed (0.80 at 32000ft is good but 0.80 at 15000 means OVERSPEED for the 737!!!!).
If I change the TGT SPD manually on the DES page to a lower value and the give EXEC, that lower value will be displayed in purple over the speed cue and the aircraft decelerates (but I have to lower the value continuously as the flight level decreases....a little boring!)
Strangely under 10000 ft (more or less) the descent projected speed change from mach to KNOTS and are again in agree with FMC LEGs.
I'm not a 737 pilot but i'm wondering if things work this way in the real one....or maybe it's a sofware bug
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