Large Transport
Flight Crew Air Transport Pilot (ATP)
Human-Machine Interface
Aircraft Equipment Problem Less Severe; Deviation - Speed All Types
Flight Crew FLC Overrode Automation
Aircraft; Human Factors
We were in stable conditions on arrival; Lorel 4Q to RWY 22 at Stansted UK. Between Bookmans Park (BPK)  and Barkway (BKY) instructed to maintain heading and descend to 4000 feet. Aircraft was at 245 knots;  descending at 900 FPM. After selecting heading (lateral) and maintaining VNAV (vertical) guidance; autopilot engaged.  As aircraft passed abeam BKY; a sudden descent to greater than 6000 FPM began. I (pilot flying) took control of the aircraft; disengaged the autopilot and began a smooth back pressure pull to arrest the descent to below 1000 FPM.  During the initial descent with the autopilot; the airspeed increased to approximately 268 KIAS.  After the aircraft was back below 250 KIAS and approximately 1000 FPM descent; I engaged the autopilot.  ATC did not make any calls regarding the airspeed increase.  We continued the vectored arrival to an uneventful ILS approach and landing. The aircraft was in VNAV; while in Heading and as we passed an FMS point on the arrival the autopilot seemed to not interface with the FMS regarding speed control (the airspeed window was closed in VNAV).  The arrival has a holding circuit built into the FMS arrival.  I assume the built in circuit is to descend in holding.  The pilot monitoring (per my request) removed the holding circuit from the FMS string after we were vectored off the arrival. After that command was executed in the FMS; the flight path descent initially maintained the correct path.  The descent path (on NAV display) went away and massive descent began. The arrival should be modified to not include the holding circuit at BKY (to Lorel) or make an additional arrival  that does not include the hold.  As it is currently published and per normal operations; as vectors begin; the FMS does not properly calculate the constant descent approach from the 'path' altitude.
We continued vectors to a normal ILS 22 approach and landing.While we were in Heading select and in VNAV my first officer asked me to sequence a waypoint on the FMS in order to start cleaning up the display in preparation for the approach. A few seconds after I executed the command in the FMS the aircraft suddenly increased the rate of descent. I don't know if it was a coincidence or if sequencing the waypoint had something to do with confusing the FMS. I think the only thing we could have done better was to click the autopilot off sooner to prevent exceeding 250 knots below 10000 feet.
Large Cargo aircraft flight crew experiences a sudden pitch over while being vectored for arrival and using VNAV for the descent path. The pitch over occurred just after a waypoint; with a charted holding pattern and located behind the aircraft; was removed from the legs page of the FMC. The autopilot is quickly disconnected and the aircraft returned to a reasonable descent rate and airspeed.