A B737-300 flight crew reported loss of 'a' hydraulic system; but they were confused initially becau...

Narrative:

Climbing out after takeoff both 'a' system hydraulic low pressure lights illuminated. The 'a' system showed good quantity but no pressure. Checking the 'B' system it showed good pressure but empty on the quantity gauge. We checked circuit breakers and consulted the QRH. We determined the failure to be an 'a' system failure even though we couldn't explain the quantity gauges. We contacted dispatch regarding returning to [departure airport] and asked them to see if maintenance control had any explanation for the gauges. Maintenance had no guidance regarding the gauges. We informed dispatch we would be making an overweight landing and asked them to coordinate with the station for a tug to bring us in from the runway. All QRH procedures were followed. The flight attendants and passengers were advised of the situation and we talked to operations. We declared an emergency and returned for a normal landing. The fire department noted the brakes smoking after landing but it was determined to be hydraulic fluid on the brakes. We were towed to the gate with the fire department following. Logbook entries were made for the system failure and the overweight landing. Maintenance met us and took the aircraft from us. Upon inspection in the wheel well the gauges there showed empty 'a' system and normal 'B' system quantity. The mechanic informed us the gauges were cross wired to the cockpit. The cross wired gauges led to initial confusion identifying the malfunction. It may be necessary for maintenance to check other aircraft for cross wired hydraulic quantity gauges.

Original NASA ASRS Text

Title: A B737-300 flight crew reported loss of 'A' hydraulic system; but they were confused initially because the 'A' system showed good quantity; with the 'B' system empty. It was later determined the gauges were cross wired.

Narrative: Climbing out after takeoff both 'A' system hydraulic low pressure lights illuminated. The 'A' system showed good quantity but no pressure. Checking the 'B' system it showed good pressure but empty on the quantity gauge. We checked circuit breakers and consulted the QRH. We determined the failure to be an 'A' system failure even though we couldn't explain the quantity gauges. We contacted Dispatch regarding returning to [departure airport] and asked them to see if Maintenance Control had any explanation for the gauges. Maintenance had no guidance regarding the gauges. We informed Dispatch we would be making an overweight landing and asked them to coordinate with the Station for a tug to bring us in from the runway. All QRH procedures were followed. The Flight Attendants and passengers were advised of the situation and we talked to Operations. We declared an emergency and returned for a normal landing. The fire department noted the brakes smoking after landing but it was determined to be hydraulic fluid on the brakes. We were towed to the gate with the fire department following. Logbook entries were made for the system failure and the overweight landing. Maintenance met us and took the aircraft from us. Upon inspection in the wheel well the gauges there showed empty 'A' system and normal 'B' system quantity. The Mechanic informed us the gauges were cross wired to the cockpit. The cross wired gauges led to initial confusion identifying the malfunction. It may be necessary for Maintenance to check other aircraft for cross wired hydraulic quantity gauges.

Data retrieved from NASA's ASRS site as of April 2012 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.

Attributes
ACN	914729
Time
Date	201010
Local Time Of Day	1201-1800
Place
Locale Reference	ZZZ.TRACON
State Reference	US
Environment
Flight Conditions	VMC
Aircraft 1
Make Model Name	B737-300
Operating Under FAR Part	Part 121
Flight Phase	Climb
Component
Aircraft Component	Indicating and Warning - Hydraulics
Person 1
Function	Captain
Qualification	Flight Crew Air Transport Pilot (ATP)
Experience	Flight Crew Last 90 Days 152 Flight Crew Type 13000
Person 2
Function	First Officer
Experience	Flight Crew Last 90 Days 197 Flight Crew Type 5500
Events
Anomaly	Aircraft Equipment Problem Critical Deviation - Procedural Maintenance

A B737-300 flight crew reported loss of 'a' hydraulic system; but they were confused initially because the 'a' system showed good quantity; with the 'B' system empty. It was later determined the gauges were cross wired.