A MD88 was released for flight with an IRU MEL'ed but it was later determined the aircraft was not airworthy because an IRU is not deferrable. The pilots; dispatchers and maintenance did not catch the error.
Synopsis
A MD88 was released for flight with an IRU MEL'ed but it was later determined the aircraft was not airworthy because an IRU is not deferrable. The pilots; dispatchers and maintenance did not catch the error.
Narrative
While en route at cruise altitude the command bars on the Captain's flight director disappeared while the auto pilot disconnected. The Captain hand flew and noticed the #1 IRU flashing 'align'. After two to three minutes; the IRU and flight director returned to normal operation. Auto pilot was re-engaged and a normal arrival was conducted. While parked at the gate the Captain placed the #1 IRU to quick align and it would not align in the expected time frame. We had a flight mechanic onboard and advised him. The logbook entry was; '#1 IRU will not align'. He called Maintenance Control and they came up with deferring the FMS. Prior to pushing back for the return flight the #1 IRU did align and all systems were normal. We were pressed for time for the return flight and as we pushed back; we realized the paperwork was left as is. The return flight was normal. Looking back; the correct log book entry should have been; '#1 IRU intermittent' and the proper corrective action should have been 'Ops check normal' and any MEL's should have been removed.
Second reporter narrative
Aircraft landed and pilot advised me #1 IRU would not align. Called Maintenance Control and advised them I could not find a MEL for above system and we would need to source one. I was told to give them 15 minutes. After 15 minutes I was told that the flight and maintenance department heads had got together and wanted to MEL the FMS MEL 34-42 and the pilot would fly on charts and a new release would be issued. I questioned if they were sure this was correct because the IRU was not specifically listed in the MEL and was told by Maintenance Control the Directors were sure and it would cover it. I put the FMS system on MEL and informed the pilot of the situation and told him to call Operations Control.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.