Narrative:

During level cruise flight at FL380 we received an interphone call from the flight attendants stating that passengers seated in rows 20-21 on the left side of the aircraft had seen a flash of light and heard/felt a bump. We had not seen anything unusual and all engine and system indications were normal. The captain was reluctant to send me back to inspect at that time. Several minutes later a flight attendant asked to come forward and speak to us. Once on the flight deck she indicated that there was something that looked unusual at the junction of the left engine cowl and pylon that could be damage. Still; [we had] no unusual indications and nothing seen; heard or felt on the flight deck out of the ordinary. However; based on the flight attendant's concern the captain sent me back to inspect. At rows 20-21 abc inspection revealed no visible damage. The location at the pylon/cowl junction that the flight attendant was concerned about was simply the normal configuration of sheet metal that she had not noticed before. I moved around so as to change my angle of view out the window and see as much area as possible. No damage was visible. After reassuring the passengers that there appeared to be no damage; and that all indications were normal on the flight deck; I returned to my duty station. However; given the level of concern on the part of the passengers (six of whom reported seeing; hearing and feeling something); the decision was made to contact maintenance control and make a logbook entry. The captain had me contact maintenance control through commercial radio with the dispatcher in the loop; and we were advised that line maintenance would meet the aircraft. About 30 minutes after the initial call from the flight attendants; we distinctly heard and felt a second bump; and a call from flight attendants indicated that this second bump was noticed throughout the aircraft. Although unclear if this second bump was in any way related to the initial report; it was decided to append this information to the discrepancy write-up. As a precaution; the captain reduced airspeed during the descent; and slowed and configured early for the approach. However; flap extension and aircraft handling was normal throughout the descent; approach and landing. Line maintenance met the aircraft on arrival and significant damage to the left; most inboard leading edge device was discovered. The led was dented and torn through. This damage was located very near the leading edge wing root and so not visible from a cabin window view. I called maintenance control four days after the incident to follow up on what might have caused the damage. I was informed that mechanics found a forward lavatory drain shut off valve leaking slightly. The exit for that valve is on the lower left of the fuselage in the vicinity of the nose gear doors. They believe ice built up and broke off during the flight causing the damage. I always do a thorough preflight and certainly did not notice any leaks during my walk around. Additionally while our manual calls for checking all drains and masts; I was previously unaware of this drain exit and thought that the only drains for the potable water systems were the heated drain masts for forward; mid and aft lavatories; and the system drain located at the potable water service panel at the tail of the aircraft. I don't recall this item ever being mentioned as something to check specifically. After the fact; I went through the 'walk around slides' training aid on our company's training site. The slides do not point out this drain exit. I have to acknowledge that it's possible this drain exit was leaking when I did my preflight and I simply failed to notice it. It is also possible that the leak began when the airplane was at cruise for some time and the aircraft cooled and contracted from being at high altitude and cold temperatures for a period of time. Regardless; I'll be paying extra attention to this area in the future.

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Original NASA ASRS Text

Title: A B757 left inboard leading edge device was damaged in flight by ice which built up from a forward lavatory drain shutoff fluid leak that subsequently broke off.

Narrative: During level cruise flight at FL380 we received an interphone call from the flight attendants stating that passengers seated in rows 20-21 on the left side of the aircraft had seen a flash of light and heard/felt a bump. We had not seen anything unusual and all engine and system indications were normal. The Captain was reluctant to send me back to inspect at that time. Several minutes later a Flight Attendant asked to come forward and speak to us. Once on the flight deck she indicated that there was something that looked unusual at the junction of the left engine cowl and pylon that could be damage. Still; [we had] no unusual indications and nothing seen; heard or felt on the flight deck out of the ordinary. However; based on the Flight Attendant's concern the Captain sent me back to inspect. At rows 20-21 ABC inspection revealed no visible damage. The location at the pylon/cowl junction that the Flight Attendant was concerned about was simply the normal configuration of sheet metal that she had not noticed before. I moved around so as to change my angle of view out the window and see as much area as possible. No damage was visible. After reassuring the passengers that there appeared to be no damage; and that all indications were normal on the flight deck; I returned to my duty station. However; given the level of concern on the part of the passengers (six of whom reported seeing; hearing and feeling something); the decision was made to contact Maintenance Control and make a logbook entry. The Captain had me contact Maintenance Control through Commercial Radio with the Dispatcher in the loop; and we were advised that Line Maintenance would meet the aircraft. About 30 minutes after the initial call from the flight attendants; we distinctly heard and felt a second bump; and a call from flight attendants indicated that this second bump was noticed throughout the aircraft. Although unclear if this second bump was in any way related to the initial report; it was decided to append this information to the discrepancy write-up. As a precaution; the Captain reduced airspeed during the descent; and slowed and configured early for the approach. However; flap extension and aircraft handling was normal throughout the descent; approach and landing. Line Maintenance met the aircraft on arrival and significant damage to the left; most inboard leading edge device was discovered. The LED was dented and torn through. This damage was located very near the leading edge wing root and so not visible from a cabin window view. I called Maintenance Control four days after the incident to follow up on what might have caused the damage. I was informed that mechanics found a forward lavatory drain shut off valve leaking slightly. The exit for that valve is on the lower left of the fuselage in the vicinity of the nose gear doors. They believe ice built up and broke off during the flight causing the damage. I always do a thorough preflight and certainly did not notice any leaks during my walk around. Additionally while our manual calls for checking all drains and masts; I was previously unaware of this drain exit and thought that the only drains for the potable water systems were the heated drain masts for forward; mid and aft lavatories; and the system drain located at the potable water service panel at the tail of the aircraft. I don't recall this item ever being mentioned as something to check specifically. After the fact; I went through the 'walk around slides' training aid on our company's training site. The slides do not point out this drain exit. I have to acknowledge that it's possible this drain exit was leaking when I did my preflight and I simply failed to notice it. It is also possible that the leak began when the airplane was at cruise for some time and the aircraft cooled and contracted from being at high altitude and cold temperatures for a period of time. Regardless; I'll be paying extra attention to this area in the future.

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.