737NG VNAV desc
Here is a Boeing ISAR concerning this issue :
BOEING 737-600 IN-SERVICE ACTIVITY REPORT 98-01
FEB 04/98
98-01-3462-00 (737-700) FLIGHT MANAGEMENT SYSTEM - PATH DESCENT SPEED
Initiation of the overspeed clacker has been reported by Boeing Flight Test pilots during vertical navigation (VNAV) path descents on the Model 737-700 airplane. The economy (ECON) target speeds incorporated in the Update 10 Series FMC are closer to VMO/MMO (maximum operating velocity/maximum operating mach number) than for previous FMC versions. As a result, the Model 737-700 will have more tendency to initiate the overspeed clacker than on Model 737-300 to -500 airplanes during VNAV path descents. When a VNAV path descent is being flown, the FMC is controlling to a vertical speed to maintain the desired path. This issue is not applicable when flying a VNAV speed descent where the FMC is controlling to a specific target speed.
Boeing is formulating options to address this issue in a future update to the Update 10.0 FMC software. (Status: Open)
This report is a little old, the 737-800 CBT (having the update 10.2A for the FMC) states that in a path desc :
“if you let speed increase to 10 knots or more than the target, the FMC shows the scratchpad message DRAG REQUIRED”
and also when u are below the airport restriction altitude “if you do not use the speedbrake when the DRAG REQUIRED message shows, VNAV disconnects when the speed is more than 5 knots above the airport speed restriction. The FMC shows the scratchpad message OVERSPEED DISCONNECT” and u’ll see the amber [CWS P] annunciation on the FMA.
I think this is a sufficient overspeed protection.
To avoid these messages and the speedbrake, the best thing I would do is to put in all available and most accurate data (winds, temp, weight…), if nevertheless the FMC still calculates a late ToD in a notable number of occasions, I would cheat on the wind and put in a more tailwind(or less headwind) than the forecast, I think this would be better(with practice) than use of speed desc, because of the workload and the non-economical speed desc.
If anyone has something on this pls post a message
A.H. (possibly a 737NG pilot in the near future)