For the B737, speed and altitude intervention through the MCP are offered as optional features.
In the B777, when descending in VNAV PTH mode after T/D, speed intervention (i.e., pressing the IAS/MACH selector) results in a mode change to VNAV SPD. Automatic mode reversion from VNAV PTH to VNAV SPD occurs in certain under- or overspeed conditions (before switching to VNAV SPD, the system first allows the aircraft to deviate up to 150 ft from the computed path in an attempt to correct the airspeed). VNAV SPD is also used for early descents ("DES NOW" prompt on the FMS or altitude intervention when within 50 NM from T/D); in that case, VNAV PTH is automatically re-engaged when the computed idle descent path is captured.
In general, descent modes can be divided in two classes: speed-on-pitch and speed-on-thrust. LVL CHG is speed-on-pitch, V/S and FPA are speed-on-thrust. For the VNAV PTH descent mode it is less obvious what's actually happening: in VNAV PTH descent the aircraft basically follows a
computed idle thrust descent path. But, while the initial autothrottle action (closing the thrustlevers) appears identical to what happens in LVL CHG, VNAV PTH is not a speed-on-pitch mode! It only appears that way as long as the conditions on which the FMS based its path computations are identical to the actual flight conditions. In fact, VNAV PTH is basically a sort of speed-on-thrust mode, except that it assumes idle thrust for the entire descent and pilot inputs (additional drag or thrust) are required if airspeed corrections are necessary.
This explains why the system automatically switches to VNAV SPD descent (which is a speed-on-pitch mode, although not always by default based on idle thrust, as in LVL CHG) if the actual flight conditions differ from the planned conditions to such an extent that simultaneously keeping the flight path
and the airspeed within a certain acceptable bandwidth becomes impossible.
See also this paper:
http://human-factors.arc.nasa.gov/IH...Journal1TM.pdf.