flite idol:
Maui I agree with your technique for this example. You are on a STAR passing FL160 with a constraint of FL100 at the next waypoint. ATC instructs "turn right heading 270 expedite descent to FL80 for traffic." Heading is selected and 8000 dialed into the MCP. If VNAV is used for the descent then the FMA will indicate VNAV SPD and then capture VNAV ALT at FL100 until passing abeam the constraint in the legs page and the resume the descent to FL8000. ATC will not be very happy with you and a TA/RA may be the result. I think VNAV looks at the distance along track from the abeam point to the waypoint/constraint to calculate the path but will honour constraints abeam to your postion thus the unintended level segment. I too see this lack of awareness from very experienced pilots and I always select FLCH or V/S when taken off the lateral track by ATC for descents.
Indeed "expedite descent" combined with an off-track heading assignment invalidates VNAV. As you say FLCH or V/S is making the airplane comply with the new ATC clearance. But, can some of the
modern pilots think this through?