A Line Lead Mechanic reported he misread the allowable damage limits noted in ATR-42 Service Bulletin (S/B) NP-158862-001 during a re-inspection of a First Officer's forward side window and signed-off the window inspection. The aircraft was later grounded after the Records Department noted the damage limits had been exceeded.

Narrative

I performed a re-inspection of the First Officer's forward side window on an ATR-42 aircraft In Accordance With (IAW) a fleet Engineering Directive (ED). This task was completed at around XA:00 pm local time after spending the day completing some unscheduled maintenance. The aircraft had to be powered-up and ready by XA:30 pm local time. I found damage and recorded it on the Inspection Result Sheet of the Service Bulletin NP-158862-001. I however; misread the allowable discrepant conditions and signed-off the window as within allowable limits with a re-inspection required in 50-flight cycles; when in fact they were out of limits per Page-5; Paragraph 4 of the Service Bulletin. The aircraft departed ZZZ to ZZZ1; on its normal flight and returned the next day without incident. Two days later; I was notified by Maintenance Control that the Records Department had found the mistake and we would be required to change the window prior to further flight. The window has since been changed and the aircraft is back in service. The Engineering Directive is confusing and hard to follow; unless sufficient time is used to review and familiarize yourself with the information. Also; a brief description of the reason why the window is being re-inspected; in the work order would be useful.