Commercial Fixed Wing
Flight Crew Air Transport Pilot (ATP)
Flight Crew Last 90 Days 150; Flight Crew Total 18500; Flight Crew Type 5000
Aircraft Equipment Problem Critical
Flight Crew Became Reoriented; General Maintenance Action
Aircraft; Procedure
[We were] dispatched with TAT gauge decaled by Maintenance as 50 plus degrees warmer than actual; with the provision that the SAT display must operate normally. It was 95 degrees at departure and the SAT appeared normal on preflight and takeoff; with a correct crosscheck of N1. During climbout; we were climbing slowly; and were queried by ATC. We looked into performance chapters and determined we were climbing at a reduced setting. The climb to cruise was very slow. Once at FL350; we asked another aircraft for their temperatures and they showed a TAT of minus 25 to our 8 plus; and an SAT OF minus 54 TO OUR minus 24. (They were at FL390 and we were at FL350). Using those temperatures; we determined the EPR was normal for .80 mach at 1.19. We sent a message asking about the effect of both SAT and TAT being incorrect and giving those numbers. They came back asking is the auto throttles holding your selected speed? We replied Yes. They came back with do you have any other EICAS messages or warnings? The MEL for the TAT only states SAT indication is available. If you suspect SAT is not correct the auto throttles may not work correctly. Please keep close watch on speed if using auto throttles. We carefully watched autothrottles setting during the flight and they seemed to work fine. We manually computed a GA EPR setting in case of a go-around. We think both the TAT and SAT were incorrect; and decided there was no accurate way to determine when to turn on Anti Ice outside of seeing ice form. The MEL was short of necessary information.
[We were] dispatched with TAT indicator deferred. Write-up (from inbound flight) was that TAT was incorrect; showing 50 degrees warmer than actual. Signed off as Deferred. Static Air Temperature (SAT) display must operate normally; end of MEL Operations Placard. On ground; it appeared that SAT was giving us good information. However; once airborne; it became apparent that both SAT and TAT readings were in error. During climbout; our FMC cruise page steadily indicated lower and lower maximum altitudes (FL260?) until we leveled off at FL350 where it remained steady. Also; about this time we realized that the FMC progress page would not display any fuel predictions at any waypoints displayed. We discussed possible implications for the remainder of the flight and came to the conclusion that we may not receive proper EPR if we were to need to go around at our destination. We looked up in the Flight Manual and found out that we could expect that 87% N1 would be a good cross-check in event of a go-around. Upon departing cruise altitude to begin our descent; we got our FMC fuel indications back at each waypoint on the Progress page. Remainder of the flight was uneventful. After landing; both TAT and SAT indicated 25 degrees C; very reasonable temperatures; considering that the actual outside air temp at the time; according to ATIS; was 20 degrees C. These readings confirmed our (incorrect) pre-flight belief that; on the ground; the SAT appeared to be presenting us with accurate information.
Flight crew reports being dispatched with a malfunctioning SAT gauge with the provision that TAT functions normally. During climb it is determined that normal climb thrust is not being produced by the ATS and that neither temperature gauge is providing accurate information. Flight continues to destination.