A320 flight crew experienced Auto Thrust malfunction during approach to Runway 28 at ORD. Auto Thrust added thrust while the flying pilot was attempting to slow. Control was regained by disconnecting Auto Thrust and retarding the thrust levers.
Synopsis
A320 flight crew experienced Auto Thrust malfunction during approach to Runway 28 at ORD. Auto Thrust added thrust while the flying pilot was attempting to slow. Control was regained by disconnecting Auto Thrust and retarding the thrust levers.
Narrative
We were inbound and cleared for approach to 27L at ORD with AP and AT engaged on the ILS. ATC had assigned a speed of 250 KTS since we were still a good distance out. ATC offered us sidestep to Runway 28. We said yes and they assigned us a heading of 10 degrees left to join the approach for 28. While on the new heading; the new approach was entered into the MCDU. At this point; the Auto Thrust (AT) advanced so the plane started to accelerate. The pilot flying quickly reset the the FCU speed select window to a speed lower than 250 (actually 180). The AT did not pull back the engines. The pilot flying then clicked off all automation and hand flew. By this point the plane had accelerated to about 275 KTS. Plane slowed with commendable pilot technique; Continued in for normal landing on Runway 28. Event written up in logbook. We spent a lot of time deconstructing the event during a post flight debrief. Also chatted with the mechanic and 2 other pilots. Various ideas put forth; but neither of us can specifically identify why the automation behaved this way.
Second reporter narrative
Cleared to intercept 27L LOC about 30 miles out 10000 FT with assigned speed 300 KTS. Captured LOC 27L and cleared to descend to 8000. Selected speed 250 then descended to 8000. ATC switched us to Runway 28 with an initial 10 degree heading left/south; cleared to intercept LOC. Pulled heading and switched approach to Runway 28 in the MCDU. Armed approach to intercept 28 LOC. Intercepted 28 LOC. Cleared visual 28 and pull VS to descend into glideslope for 28. Thrust remained more than 50%. ATC assigned 180 speed. Selected speed 180 and deployed speed brakes. FMA had LOC green and MCDU was in approach mode. Pushed managed speed to get the thrust to come back to reduce speed. Thrust instead increased. Pulled speed button and reduced it to 180 but thrust didn't come back. With speed increasing to over 270 KTS; manually pulled throttles back to about 40% and pushed AT disconnect button. Thrust decreased and pulled back more and then to idle. Shut off FD and continued with a visual approach and normal landing to Runway 28.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.