Date: 201311
Local time of day: 1801-2400
Location: ZZZ.Airport, US
Light: Night
Aircraft: B757-200
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Cruise
Aircraft component: Hydraulic Auxiliary/Standby System
Reporter organization: Air Carrier
Reporter function: Pilot Flying
Human factors: Confusion
Second reporter function: Pilot Not Flying
Anomaly: Aircraft Equipment Problem Less Severe
Detector: Person Flight Crew
When detected: In-flight
Result: Flight Crew Overcame Equipment Problem
Contributing factors: Human Factors; Manuals; Aircraft
Primary problem: Human Factors

Narrative: After reaching cruise altitude; observed L HYD ENG PUMP EICAS message with PRESS light in associated Left Hyd Engine Pump (EDP) switch. After accomplishing the QRH procedure and consulting with Dispatch; we continued the ETOPS flight with the understanding we still had the Left Hydraulic System operating and the Hydraulic Motor Generator (HMG) available.  The B757 Aircraft Manual; Vol II states that the Power Transfer Unit (PTU) is provided to supplement the Left Hydraulic Electric Motor for several items; including the HMG.  B757 Vol II states the PTU automatically activates with the loss of Left Hyd System or Left Engine-Driven Pump (EDP) pressure. Question: with the POWER XFR UNIT (PTO) status message also displayed; did the PTU activate?  Did we have HMG available in the event of an electrical failure?  I know there are no in-flight crew procedures associated with status messages.  Talking with Maintenance personnel; it is my understanding that with the loss of L HYD ENG PUMP pressure; the POWER XFR UNIT Status message will also come on; leaving one with only the Electric Pump pressurizing the Left Hydraulic System and not enough hydraulic volume for HMG operation. This contradicts our B757 Manual; Vol II.  It is my understanding the PTU did not activate. After touchdown we did get a brief Left System Pressure caution as the spoilers and reversers deployed.  Does the POWER XFR UNIT status message mean we do not have protection of the PTU in the event of Left Hydraulic pressure loss; or does it mean the PTU will activate; but without its own internal protections?  This is important to know. Without the PTU; and subsequent HMG; I would not continue into ETOPS environment. Was safety in the ETOPS environment compromised? EDP failure.

Callback: Reporter stated the Hydraulic Motor Generator (HMG) on #1 Engine was replaced; but did not correct the indicated loss of hydraulic pressure. Further troubleshooting and replacement of a Hydraulic Pressure switch ended up being the corrective action. They use Rolls Royce RB-211 engines.

Second narrative: Does the POWER XFR UNIT (PTO) status message mean we do not have protection of the PTU in the event of Left Hydraulic pressure loss; or does it mean the PTU will activate but without its own internal protections?  This is important to know. Without the PTU; and subsequent Hydraulic Motor Generator (HMG); I would not continue into ETOPS environment. Safety of flight was not compromised; but was safety in the ETOPS environment compromised?  [Recommend] providing a detailed explanation of status messages in respective flight manuals.

Second callback: Reporter stated he was referring to the Hydraulic Motor Generator (HMG) that is hydraulically powered by the PTU. At the time when the EICAS status message appeared; they were unclear just what the message meant. Especially since they were getting ready to go ETOPS and were concerned about being legal to fly under ETOPS rules that required certain equipment to be operating. They later found a better explanation about the status message in one of their manuals; either the Boeing Flight Manual; or the Boeing QRH or perhaps in their air carrier Flight Operations Manual System pages that list status messages.

Synopsis: Two pilots report they were unclear what an EICAS Status message POWER XFR UNIT (PTU) display meant; when the message appeared after observing a previous L HYD ENG PUMP EICAS message on their B757-200 aircraft. They were getting ready to enter an ETOPS category flight and were concerned about ETOPS safety requirements and legality.