Narrative:

The F flight attendant called the cockpit and advised that lavatory C had a faint 'rubbery' smell that appeared to originate from the overhead vent in the lavatory. The captain instructed me to go to the lavatory and examine. The lavatory had a strong smell of what smelled like electrical and or plastic origin; I tried to locate the source but no heat was apparent and the smell could possibly be from the vent. I returned to the flight deck and advised the captain of my findings. He instructed me to obtain a clearance for mach .83. He instructed a flight attendant to monitor the lavatory for any developments. He contacted maintenance over the satellite phone to get further information about the lavatory ceiling electrical components. A flight attendant then called us back and advised that the source of the smell appeared to come from the bright lavatory light that illuminates when the laboratory door is locked. Maintenance agreed that this could be the source of the smell. The captain instructed the flight attendant to close off the lavatory; but not to lock it. He also instructed the flight attendant to monitor the lavatory for any changes. We looked at our options and decided that [diverting] would be the best option. We sent a message to dispatch [who] concurred with our plan. We got a re-analysis of the remainder of the flight at the new cruise speed of .83. The flight attendant monitored the lavatory and gave us continuous updates - nothing really changed. The smell lingered but wasn't getting any worse. No smoke was present. We were assigned a [distant] gate and discussed getting a better one but; ultimately; decided the original one would suffice since the issue was identified; isolated and mitigated. On arriving at the gate and deplaning we got a call from the galley flight attendant that the light fixture was arcing and smoke was present. He asked us to turn off the lights in the lavatory. The captain instructed me to pull the lavatory lights circuit breakers. I called maintenance on the radio to see if they could assist further with shutting down the light in the lavatory. The galley flight attendant removed the light and the arcing stopped. The captain asked maintenance about removing the light and fixture in flight but maintenance advised against it; since the light was off while the door was unlocked. The captain asked what circuit breakers controlled the lights in the lavatory; but maintenance did not respond. After the passengers deplaned I asked the mechanics about why the lights were on; and they told me that the aircraft had a 'ground mode' where all lavatory lights came on. Had we known this; we would have asked for a different gate and also had the light bulb removed; and or continued to ask for the appropriate circuit breaker to pull.

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Original NASA ASRS Text

Title: A B767-300ER diverted to a closer airport when a flight attendant reported a strong odor associated with lighting in a lavatory.

Narrative: The F Flight Attendant called the cockpit and advised that lavatory C had a faint 'rubbery' smell that appeared to originate from the overhead vent in the lavatory. The Captain instructed me to go to the lavatory and examine. The lavatory had a strong smell of what smelled like electrical and or plastic origin; I tried to locate the source but no heat was apparent and the smell could possibly be from the vent. I returned to the flight deck and advised the Captain of my findings. He instructed me to obtain a clearance for Mach .83. He instructed A Flight Attendant to monitor the lavatory for any developments. He contacted Maintenance over the satellite phone to get further information about the lavatory ceiling electrical components. A Flight Attendant then called us back and advised that the source of the smell appeared to come from the bright lavatory light that illuminates when the laboratory door is locked. Maintenance agreed that this could be the source of the smell. The Captain instructed the Flight Attendant to close off the lavatory; but not to lock it. He also instructed the Flight Attendant to monitor the lavatory for any changes. We looked at our options and decided that [diverting] would be the best option. We sent a message to Dispatch [who] concurred with our plan. We got a re-analysis of the remainder of the flight at the new cruise speed of .83. The Flight Attendant monitored the lavatory and gave us continuous updates - nothing really changed. The smell lingered but wasn't getting any worse. No smoke was present. We were assigned a [distant] gate and discussed getting a better one but; ultimately; decided the original one would suffice since the issue was identified; isolated and mitigated. On arriving at the gate and deplaning we got a call from the Galley Flight Attendant that the light fixture was arcing and smoke was present. He asked us to turn off the lights in the lavatory. The Captain instructed me to pull the lavatory lights circuit breakers. I called Maintenance on the radio to see if they could assist further with shutting down the light in the lavatory. The Galley Flight Attendant removed the light and the arcing stopped. The Captain asked Maintenance about removing the light and fixture in flight but Maintenance advised against it; since the light was off while the door was unlocked. The Captain asked what circuit breakers controlled the lights in the lavatory; but Maintenance did not respond. After the passengers deplaned I asked the mechanics about why the lights were on; and they told me that the aircraft had a 'ground mode' where all lavatory lights came on. Had we known this; we would have asked for a different gate and also had the light bulb removed; and or continued to ask for the appropriate circuit breaker to pull.

Data retrieved from NASA's ASRS site 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.