Narrative:

Upon single engine taxi (right engine running). Taxied with initial left turn followed by an attempt to turn slightly right. Nosewheel steering would not turn right. This was immediately followed by 'power transfer unit' (ptu) on lower EICAS screen. Checked left hydraulic psi and it showed 1100 psi and recovering back to normal. Called maintenance via radio and could not eliminate EICAS message 'power transfer unit.' maintenance came out to the aircraft on ramp and reset EICAS message with both engines running. Light went out. Again restarted taxi to runway and requested to hold in block while we positively verify that the ptu was in fact operating correctly. Turned off both left engine driven and electric left hydraulic boost pumps and again received 'power transfer unit' EICAS message. Now the left hydraulic psi was zero '0.' I had in the back of my mind a few instances this past few weeks with hydraulic problems had we had one of the previously mentioned problems in-flight, we may have had a real problem. Incidentally, this problem was additionally addressed at ZZZ1, however not fixed. Simply automatic canceling the problem does not fix it. We returned to ZZZ gate as aircraft was OTS for replacement of power transfer unit (ptu). Callback conversation with reporter revealed the following information: a reset of the EICAS warning is no fix and the reporter suspected the power transfer unit was not operating normally. The left engine was shut down and with the right engine running the power transfer system failed to power the flaps, slats, landing gear, and nose steering.

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Original NASA ASRS Text

Title: A B757-200 TAXIING WITH L ENG SHUT DOWN FOUND THAT THE NOSE GEAR WOULD NOT TURN R. HAD EICAS WARNING 'PWR TRANSFER UNIT.' MAINT RESET EICAS WITH BOTH ENGS RUNNING. SHUT DOWN L HYDRAULIC PUMPS FOR PWR TRANSFER TEST. BACK TO GATE WITH PWR TRANSFER SYSTEM INOP.

Narrative: UPON SINGLE ENG TAXI (R ENG RUNNING). TAXIED WITH INITIAL L TURN FOLLOWED BY AN ATTEMPT TO TURN SLIGHTLY R. NOSEWHEEL STEERING WOULD NOT TURN R. THIS WAS IMMEDIATELY FOLLOWED BY 'POWER TRANSFER UNIT' (PTU) ON LOWER EICAS SCREEN. CHKED L HYD PSI AND IT SHOWED 1100 PSI AND RECOVERING BACK TO NORMAL. CALLED MAINT VIA RADIO AND COULD NOT ELIMINATE EICAS MESSAGE 'POWER TRANSFER UNIT.' MAINT CAME OUT TO THE ACFT ON RAMP AND RESET EICAS MESSAGE WITH BOTH ENGS RUNNING. LIGHT WENT OUT. AGAIN RESTARTED TAXI TO RWY AND REQUESTED TO HOLD IN BLOCK WHILE WE POSITIVELY VERIFY THAT THE PTU WAS IN FACT OPERATING CORRECTLY. TURNED OFF BOTH L ENG DRIVEN AND ELECTRIC L HYD BOOST PUMPS AND AGAIN RECEIVED 'POWER TRANSFER UNIT' EICAS MESSAGE. NOW THE L HYD PSI WAS ZERO '0.' I HAD IN THE BACK OF MY MIND A FEW INSTANCES THIS PAST FEW WEEKS WITH HYD PROBS HAD WE HAD ONE OF THE PREVIOUSLY MENTIONED PROBS INFLT, WE MAY HAVE HAD A REAL PROB. INCIDENTALLY, THIS PROB WAS ADDITIONALLY ADDRESSED AT ZZZ1, HOWEVER NOT FIXED. SIMPLY AUTO CANCELING THE PROB DOES NOT FIX IT. WE RETURNED TO ZZZ GATE AS ACFT WAS OTS FOR REPLACEMENT OF PWR TRANSFER UNIT (PTU). CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: A RESET OF THE EICAS WARNING IS NO FIX AND THE RPTR SUSPECTED THE PWR TRANSFER UNIT WAS NOT OPERATING NORMALLY. THE L ENG WAS SHUT DOWN AND WITH THE R ENG RUNNING THE PWR TRANSFER SYSTEM FAILED TO POWER THE FLAPS, SLATS, LNDG GEAR, AND NOSE STEERING.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.