ACR FLC STOPS TAXIING ACFT ABRUPTLY WHEN KNOCK ON DOOR NOT PROPER CODE.

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DURING INITIAL TAXI FROM GATE; HEARD COCKPIT DOOR KNOCK OF 2 PLUS 1. AUTHORIZED KNOCK WAS 2 ONLY. MADE SUDDEN STOP TO DETERMINE PROBLEM; FEARED POSSIBLE HIJACKER ATTEMPTING COCKPIT ENTRY. HIJACK SECURITY PROCS AND STERILE COCKPIT PROCS NEED FURTHER EMPHASIS WITH INFLT DEPT. SUPPLEMENTAL INFO FROM ACN 193885: APPARENTLY THE FLT ATTENDANT FOUND HER SEATBELT CAUGHT IN THE COCKPIT DOOR. 2 FLT ATTENDANTS WERE KNOCKED OFF THEIR FT AND SUFFERED MINOR INJURIES. THE PROBLEM ARISES BECAUSE OF A CONFLICT BTWN SOP AND REALITY. DURING 'CRITICAL PHASES OF FLT' INCLUDING TAXI; THE COCKPIT IS 'STERILE' AND FLT ATTENDANTS ARE NOT SUPPOSED TO CONTACT THE FLC. IN REALITY; THEY FREQUENTLY DO FOR MATTERS SIMILAR TO A SEATBELT CAUGHT IN THE DOOR. HERE; THE CAPT ASSUMED AN EMER WAS IN PROGRESS AND STOPPED THE ACFT SUDDENLY; RESULTING IN INJURIES. SUPPLEMENTAL INFO FROM ACN 193998: DURING TAXI OUT FROM THE GATE AT ALBANY A KNOCK ON THE COCKPIT DOOR PROMPTED THE SO TO CRACK THE DOOR OPEN. THE FLT ATTENDANT SITTING AT THE DOOR NEEDED HER SEATBELT THAT WAS CAUGHT IN THE DOOR. THE CAPT IMMEDIATELY STOPPED THE ACFT AS THE COCKPIT DOOR WAS CLOSING. THE PARKING BRAKE WAS SET AND THE CAPT TURNED TO DETERMINE THE REASON FOR OPENING THE DOOR. THE FLT ATTENDANT THAT KNOCKED TOLD US THE FIRST FLT ATTENDANT HAD FALLEN DURING THE STOP. THE CAPT ASKED THE FIRST FLT ATTENDANT TO COME INTO THE COCKPIT AND SHE SAID SHE HAD FALLEN AND ANOTHER FLT ATTENDANT IN THE AFT CABIN HAD FALLEN. THEY SAID THEY WERE OKAY AND THE FLT CONTINUED TO THE DEST. ENRTE THE 2 FLT ATTENDANTS REQUESTED TO GO TO MEDICAL UPON REACHING O'HARE. A FLT SVC REPRESENTATIVE MET THE ACFT AND THE FLT ATTENDANTS DEPLANED AT THE END OF THE FLT.