AN ACR A320 HAD TO RETURN LAND WITH A FUEL LEAK. DIVERSION TO ALTERNATE.

Narrative

I DISCOVERED SIGNIFICANT DISCREPANCY BTWN ESTIMATED FUEL CONSUMPTION AND ACTUAL FUEL CONSUMPTION GOING BY FSD. I TRIED TO DETERMINE IF IT WAS AN INDICATION PROB OR AN ACTUAL LOSS OF FUEL. I INSTRUCTED FO TO GO BACK INTO THE CABIN AND TAKE A LOOK OUT OF THE WINDOWS TO CHK FOR FUEL VENTING; ETC. NO VISIBLE FUEL LEAKAGE RPTED. FOUND PROB TO BE ACTUAL LOSS OF FUEL FROM L SIDE (L TANK/L ENG). I CONTACTED DISPATCH AND RETURNED TO MINNEAPOLIS. XFED FUEL TO KEEP WITHIN BAL LIMITATIONS. I RETARDED #1 THROTTLE TO IDLE AND TURNED OFF L TANK PUMPS. THIS SEEMED TO SLOW FUEL LOSS. I ELECTED TO KEEP L ENG RUNNING AT IDLE JUST IN CASE #2 WERE TO QUIT. AFTER LNDG; CAUSE OF LEAK WAS FOUND TO BE FAULTY SEAL ON HYD-MECHANICAL UNIT; L ENG. SUPPLEMENTAL INFO FROM ACN 338924: WE BROUGHT BACK THE #1 ENG TO IDLE AND TURNED OFF THE FUEL PUMPS TO THAT ENG (FUEL WILL STILL GRAVITY FEED WITH THE PUMPS OFF). IMMEDIATELY THE FUEL LEAK (ACCORDING TO THE FUEL GAUGES) DRAMATICALLY DECREASED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR FLIES THE A320. THE 'HMU' IS A HYD-MECHANICAL UNIT IN THE ENG POD THAT METERS FUEL. THE LEAKAGE WAS VENTED THROUGH A VENT PIPE BUT WAS SO SMALL THAT IT COULD NOT BE SEEN FROM THE ACFT. THE FUEL CHK THAT WAS PERFORMED IS A NORMAL PROC DURING CLB AND CRUISE. THE ACR HAS HAD A LONG SERIES OF BAD HYD- MECHANICAL UNITS. THE REPLACEMENT HYD-MECHANICAL UNIT ALSO LEAKED. THE RPTR DOES NOT KNOW IF IT WAS NEW OR REBUILT. THE RPTR STATES THAT THE OLD FLT MANUALS FOR THE A320 GO INTO GREAT LENGTHS ABOUT THE HYD-MECHANICAL UNIT. THE NEW MANUALS DON'T EVEN MENTION IT. 'HMU' IS ONE OF THE FRENCH TERMS THAT HAS BEEN PURGED FROM THE MANUALS.