AIR CONDITIONING UNIT HOSE SUCKED INTO ENG DURING ACFT PARKING PROC.

Narrative

ACFT WAS PARKED AT THE GATE IN DTW. THE R ENG WAS SHUT DOWN AND L ENG AT IDLE AWAITING EXTERNAL PWR AVAILABILITY ACCORDING TO PROCS. ACFT HAD BEEN CHKED AND PARKING BRAKE WAS STILL ON. EXTERNAL PWR SHOWED AVAILABLE AND WAS SELECTED ON BY FO. I WENT TO SHUT DOWN L ENG FUEL CTL SWITCH. DURING MOVEMENT OF L ENG FUEL CTL SWITCH; I HEARD A POP SOUND FOLLOWED BY ACFT VIBRATION. MOMENTS LATER; I SAW DEBRIS FLOATING OUTSIDE COCKPIT WINDOW. PRIOR TO ENTERING GATE AREA; ALL EQUIP WAS CLR OF YELLOW SAFETY LINES. ALL COMPANY PROCS WERE FOLLOWED. PARKING AND SECURING CHKLISTS WERE COMPLETED. WE LEFT FLT DECK TO INVESTIGATE WHAT HAD HAPPENED AND SAW AIR CONDITION UNIT HOSE WRAPPED IN THE BLADES OF THE L ENG. COMPANY POLICY ENCOURAGES PLTS TO NOT START APU IF GATE HAS AIR CONDITIONED CART AND EXTERNAL PWR AVAILABLE. THE L ENG IS LEFT RUNNING UNTIL EXTERNAL PWR IS HOOKED UP AND SELECTED. JETWAYS ARE POSITIONED TO DOOR 2L WITH L ENG RUNNING AND PEOPLE ARE DEPLANED; WITH ENG STILL RUNNING; WHILE WAITING FOR EXTERNAL PWR TO BE HOOKED UP. I BELIEVE THAT THE APU SHOULD BE RUNNING PRIOR TO ENTERING GATE AREA AND THAT BOTH ENGS SHUT DOWN AS SOON AS PARKING BRAKE IS SET TO PREVENT THIS TYPE OF OCCURRENCE FROM HAPPENING. ONCE EXTERNAL. PWR IS AVAILABLE; THEN TURN OFF APU IF NOT NEEDED FOR ELECTRICAL OR AIR.