When you push the FLCH mode switch on the MCP, the
airspeed window unblanks to show current airspeed. The
TMCF uses the difference between the present altitude
and the MCP altitude to calculate a vertical speed to
capture the MCP altitude. Airspeed does not change. The
A/T control laws command thrust which produces a
vertical speed to complete the altitude change in 125
seconds. The A/T command can not exceed the thrust
limit mode.
From my engineers training notes
