Incident Description
During engine start while being pushed back, both engines unexpectedly increased to a high power setting. The resulting forward movement of the aircraft caused a pin to shear, the tow bar to bend, and the tug to be struck.
Maintenance Testing
Prior to engine start, maintenance personnel had conducted an automated check that included automatic movements of the control wheel and throttles. The flight crew returned to their seats while the test was in progress. Upon completion of the test, the airplane was returned to service and maintenance personnel left the cockpit.
Flight Crew Actions
The captain initiated the BEFORE STARTING ENGINES checklist without either pilot first completing a cockpit flow setup, which was required by company procedures. The checklist was then interrupted by a cockpit visitor, leading to a non-pertinent conversation lasting 3½ minutes. When the checklist was resumed, several items had been missed. As the checklist continued, the first officer called out "THRUST LEVERS," and the captain correctly responded "CLOSED."
Pushback was initiated, and the captain verbalized starting the first engine as required by the checklist. During the start of the first engine, the captain began another non-pertinent conversation with the first officer. The second engine was then started without the required verbalization per company checklist procedures.
Checklist Procedure Deviations
Company checklist procedures made both pilots responsible for ensuring a normal engine start. However, neither pilot detected that the engines were accelerating above normal idle. Follow-up testing revealed that the engines accelerated to a power setting corresponding to the last position of the throttles at the completion of the automated test conducted by maintenance.