A B757 First Officer's gyro failed during preflight and an incorrect MEL was applied which would apply to other B757's in the fleet but not this particular one so the flight was completed with an inoperative item.
Synopsis
A B757 First Officer's gyro failed during preflight and an incorrect MEL was applied which would apply to other B757's in the fleet but not this particular one so the flight was completed with an inoperative item.
Narrative
We had a failure of the First Officer's attitude indicator while on the ground. I switched from First Officer to AUX on the Flight Instrument Control panel. This corrected the problem. I contacted Maintenance Control. We checked the MEL and it said we could proceed as long as we had an extra gyro. The Flight Instrument Control panel indicated we had a Captains; First Officers; and an Auxiliary gyro installed; (CA; FO; AUX). The panel did NOT say CA; FO; ALT. This would have indicated to us that by switching from FO to ALT (alternate) this would have only been switching the First Officer to the Captain's gyro meaning that there were only two gyros installed. But since the panel said CA; FO; AUX meant to me and Maintenance Control that the AUX was an auxiliary gyro giving that airplane three gyros making it legal to dispatch as MEL'ed. Maintenance Control informed us that we could defer the First Officer's gyro and gave us an MEL number. I accepted Maintenance Controls decision based on the fact that the panel said CA; FO; AUX. and felt we were in compliance with all regulations and airline policies.
Second reporter narrative
We were preparing to depart when I noticed the First Officer's ADI was showing a 5-7 degree bank while parked on the ramp. This aircraft had a recent history of First Officer Gyro discrepancies; and had been documented in the aircraft log book by previous flight crews. We notified maintenance about the issue and the possibility of deferring the the Gyro so we could continue to our filed destination. There were no 'flags' in view on the ADI; so we believed the discrepancy was the First Officer Gyro. When selecting the 'Aux' position on the overhead panel; the ADI erected itself and displayed straight-and-level. The Mechanic contacted Maintenance Control after initially troubleshooting the problem; and returned to the aircraft saying it could be deferred. He arrived with the MEL number assigned by Maintenance Control; and then proceeded to defer the Gyro IAW air carrier procedures. At that point; we departed and arrived uneventfully. There are three contributing factors to this event: the overhead switching panel on our aircraft; the fact that maintenance control issued a MEL number saying it was deferrable; and the pressures of an on-time departure. The aircraft we fly have a 'switching panel' on the overhead panel that allows either the Captain's ADI or First Officer's ADI to be switched to a third or 'Aux' Gyro in case of failure. This configuration can be deferred; and we thought this was what we had installed on the aircraft. The panel is different from the other aircraft we fly; and those aircraft are labeled with 'Alt' and cannot be deferred. After further research; it appears switching panels have differences to them. In regards to the MEL number assigned by Maintenance Control; I felt at that point we were operating IAW air carrier policies and our deferral program or a MEL number would not have been issued. Finally; as an aircrew member we are always under the pressures of departing on-time and maintaining reliability. We were doing our best to depart on schedule; but did not jeopardize safety in any way just to maintain our reliability
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.