Hmmmm....
Placard limit reversion can effect itself in slightly different ways depending on whether the AFDS and AT are in/out.
In a VNAV PTH descent the first FMC advisory message when speed is fast is at +10kts - DRAG REQUIRED. I normally ignore that one.
At +15kts you get an OVERSPEED DISCONNECT message and there is a mode reversion which, if
Lackof747 is correct, is to VNAV SPD or LVL CHG. I thought it was the later - that might just be Update 10.2.
Similarly, if low on the programmed descent speed, the AT annunciation changes, I believe, to FMC SPD to acquire the correct speed again before returning to ARM.
The FMC Guide indicates that VNAV PTH is
never supposed to exceed the barbers pole, but that in the time it takes to pitch up, the clackers may be set off.
It also suggests that in Update 10.2, 7kts prior to Vmo/Mmo, reversion to LVL CHG takes place.
I believe the important point is, however, that there is a degree of protection (albeit
I would not rely on it myself - the time constants in some of the FMC feedback loops are far too large in my opinion!) built into a VNAV PTH descent in that it respects a placarded limit.
In this respect it is the same as a V/S descent where the thrust levers are on the idle stop. Our company SOP prohibits this, although in my opinion, accepting that a diligent pilot should always be observant of what the aircraft is doing, in principle there is no difference between such a form of descent and a VNAV PTH descent. Both will chase a particular trajectory through space at the expense of
small deviations in airspeed. The only difference is a FMC advisory message or two.
I used VNAV SPD a lot when I flew the 737, because it remembers the 250 KTS speed restriction below FL 100, something LVL CHG does not thus demending me to be awake
At the risk of being a pedant, you're one of the guys I'd catch out then by removing the restriction surreptitiously when he wasn't looking!
In VNAV path, an optimum path, that is, line in space (but definitely not a speed) is generated within the FMC. This path will change with CI ansd weight, but not 'constantly' unless you 'constantly' change the CI on the Perf Init page.
Again, to be a pedant, the FMC uses
cruise winds at cruise altitude linearly interpolated to zero wind at the destination to compute the path. The linear model can be made more accurate by entering winds on the DESCENT FORECAST page of the FMC although I'm still not sure of exactly how the modification works. However, any change to the route in the FMC, such as a simple speed change, a route modification, an altitude constraint will force a recomputation of the path using
current winds. We see this everyday and it leaves us either high or low on the previously computed path depending on the circumstances.
I always try and use VNAV as much as possible, but like everything in the digital world:
Garbage in, garbage out
