Narrative:

When we arrived at aircraft; maintenance was working on the standby altimeter and replaced the entire unit. The technician was not sure if it was working correctly as it would slowly process downward a few degrees; after caging it; to indicate a nose-low attitude of about 2 degrees; while the two electronic attitude director indicators (eadi) indicated nose level. He finally signed it off as being within standards and all 3 pilots agreed.during climb out the relief pilot noted that the standby ADI was lagging behind the eadis in pitch and roll and at times it would indicate a 2-3 degree bank when the wings were level on both eadis. It would also vary pitch error up to about 5 degrees nose low from what the eadis indicated. We all felt that the standby ADI was unreliable and giving false indications and that it would be difficult to control the aircraft with no visible horizon using only the standby instruments; if the other horizon instruments were to fail or in the event of an electrical failure.as we were preceding over the ocean at night; we discussed the legality of continuing in ETOPS and class ii nav airspace as well as the prudence of continuing. We then looked through the afm and fom and found very little information pertaining to the standby instruments. There was not a QRH for it either. We then went to the MEL for guidance; as we remembered that maintenance was not going to dispatch the aircraft with that instrument deferred; and found that the MEL said that if the standby ADI was inoperative; the aircraft was limited to day VMC conditions only.as this seemed an important enough safety consideration for the MEL to severely limit the aircraft's operations; we felt that it may be a safety consideration for crossing the atlantic at night should we have an electrical failure. We then called dispatch on the satphone who patched us into maintenance to confer with. Both of my first officer's and I; along with the dispatcher; all agreed that continuing the flight while legal; would not be the safest choice; given the fact that the MEL would not have allowed it at all had the defect been known prior to dispatch point. We elected to return for repair and dispatch and maintenance wanted us to return to our departure airport.we then realized we were over 40;000 pounds too heavy to land and could not dump fuel. I did not feel the situation warranted using emergency authority; and after consulting the fom on overweight landings; as well as talking with dispatch and dispatch over ACARS; we decided to hold for almost 3 hours near the departure airport in order to burn enough fuel to be under max landing weight.we felt that since the aircraft was in safe working order with no defects other than the standby ADI; the weather was clear and forecast to stay that way; and we were able to hold near major airports; the safer course of action was to burn the fuel prior to landing rather than to land overweight by 30-40;000 lbs. With no performance data for landing at that weight we felt we were test pilots if we did that and that the situation was not dire enough. It would have been hard to justify an overweight landing.we held; burned the fuel; and landed just under the max landing weight uneventfully. I felt that we used all resources available to us and made decisions based on safety. My entire crew and I looked at the situation many times over both before diverting back as well as when in holding and we all felt that there was nothing we had overlooked and that we had taken the best and safest course of action.

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Original NASA ASRS Text

Title: Standby attitude indicator became unreliable during flight. Crew elected not to continue the oceanic routing and returned to their point of departure.

Narrative: When we arrived at aircraft; maintenance was working on the Standby Altimeter and replaced the entire unit. The technician was not sure if it was working correctly as it would slowly process downward a few degrees; after caging it; to indicate a nose-low attitude of about 2 degrees; while the two Electronic Attitude Director Indicators (EADI) indicated nose level. He finally signed it off as being within standards and all 3 pilots agreed.During climb out the relief pilot noted that the Standby ADI was lagging behind the EADIs in pitch and roll and at times it would indicate a 2-3 degree bank when the wings were level on both EADIs. It would also vary pitch error up to about 5 degrees nose low from what the EADIs indicated. We all felt that the Standby ADI was unreliable and giving false indications and that it would be difficult to control the aircraft with no visible horizon using only the standby instruments; if the other horizon instruments were to fail or in the event of an electrical failure.As we were preceding over the ocean at night; we discussed the legality of continuing in ETOPS and Class II Nav airspace as well as the prudence of continuing. We then looked through the AFM and FOM and found very little information pertaining to the Standby instruments. There was not a QRH for it either. We then went to the MEL for guidance; as we remembered that maintenance was not going to dispatch the aircraft with that instrument deferred; and found that the MEL said that if the Standby ADI was inoperative; the aircraft was limited to Day VMC conditions only.As this seemed an important enough safety consideration for the MEL to severely limit the aircraft's operations; we felt that it may be a safety consideration for crossing the Atlantic at night should we have an electrical failure. We then called dispatch on the SATPHONE who patched us into maintenance to confer with. Both of my FO's and I; along with the dispatcher; all agreed that continuing the flight while legal; would not be the safest choice; given the fact that the MEL would not have allowed it at all had the defect been known prior to dispatch point. We elected to return for repair and dispatch and maintenance wanted us to return to our departure airport.We then realized we were over 40;000 LBS too heavy to land and could not dump fuel. I did not feel the situation warranted using emergency authority; and after consulting the FOM on overweight landings; as well as talking with dispatch and Dispatch over ACARS; we decided to hold for almost 3 hours near the departure airport in order to burn enough fuel to be under max landing weight.We felt that since the aircraft was in safe working order with no defects other than the Standby ADI; the weather was clear and forecast to stay that way; and we were able to hold near major airports; the safer course of action was to burn the fuel prior to landing rather than to land overweight by 30-40;000 lbs. With no performance data for landing at that weight we felt we were test pilots if we did that and that the situation was not dire enough. It would have been hard to justify an overweight landing.We held; burned the fuel; and landed just under the Max landing weight uneventfully. I felt that we used all resources available to us and made decisions based on safety. My entire crew and I looked at the situation many times over both before diverting back as well as when in holding and we all felt that there was nothing we had overlooked and that we had taken the best and safest course of action.

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.