Date: 201206
Local time of day: 0001-0600
Location: ZZZ.Airport, US
Weather elements / visibility: Rain
Light: Night
Aircraft: B737-800
Operator: Air Carrier
Operating under: Part 121
Mission: Passenger
Flight phase: Parked
Aircraft component: Pitot-Static System
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Technician
Reporter qualification: Maintenance Airframe; Maintenance Avionics
Human factors: Situational Awareness; Troubleshooting; Communication Breakdown
Anomaly: Aircraft Equipment Problem Critical
Detector: Person Flight Crew
When detected: Pre-flight
Result: General Maintenance Action
Contributing factors: Aircraft; Human Factors; Logbook Entry; Manuals; Procedure
Primary problem: Aircraft

Narrative: Sent out to B737-800 aircraft with Standby Altimeter reading approximately 600' feet higher than the Captain and First Officer (F/o) altimeters; at departure gate; myself; my Lead and the Captain and First Officer. Upon pre-flight the Flight crew noticed the Standby Altimeter was considerably out of tolerance. It was a driving rain that night and [rain] had filled the outer Alternate Static ports with water. As the Barometric Pressure changed; the pressure in the Alternate Static System did not; creating a pressure differential seen only on the Alternate Static System instruments. Inspected Alternate Static ports for the well known; above condition. Found water as expected. Called Lead Mechanic on radio to start looking for Maintenance Manual (M/M) reference.  Broke surface tension of water mass on Alternate Static ports by applying thumb to static ports and removing quickly. Observed water being pulled from ports due to low pressure induced by removal of thumb. Returned to flight deck and checked altimeters; all within less than 20' feet of each other. Had brief conversation with [Flight] crew to the effect of the Airworthiness of the aircraft and the Maintenance Actions that also had to still be performed. The Captain wondered if this condition would re-occur as they taxied out; I assured him just the airspeed alone would shear the water out of the ports and I had never had one return once cleared as above. Went to office to retrieve Maintenance Manual (M/M) references; but [instead]; ended up looking them up and printing them myself as they were not available as I had hoped they would be; given the amount of time between the request and my arrival at the office.  With the proper manuals; returned to aircraft; examined Alternate Static port drain for moisture as referenced in MM 34-11-00-680; Paragraph D.(1).  Finding no moisture; did not proceed to Paragraph D.(2); therefore did not 'break into' Alternate Static System. Returned to flight deck with MM 34-13-00-730 and determined by Table in Paragraph 'G' that the Standby Altimeter would indeed be within tolerance for dispatch. However;[I] poorly worded [information] on Log page; did not perform test per MM 34-11-00-680; only referenced test in resolution of discrepancy. Did not 'break into' any Pitot Static System. Did not require RII Inspection. Recommend more properly describe steps taken in examination of Static System drains; more properly word resolution as to reference of Table in the test for Air Data Instruments Test.




Synopsis: A Line Avionic's Technician describes a simple check for determining how water mass accumulation on outer Alternate Static ports had contributed to a B737-800 aircraft Stanby Altimeter reading 600' feet higher than the Captain and First Officer Altimeters while on the ground.  Technician noted the water accumulation at Static ports is well known.