B757 flight crew experienced asymmetrical spoilers during descent. When speed brake handle is stowed right spoiler panel remains up. Second attempt causes left spoiler to extend and both spoilers to retract when stowed.
Synopsis
B757 flight crew experienced asymmetrical spoilers during descent. When speed brake handle is stowed right spoiler panel remains up. Second attempt causes left spoiler to extend and both spoilers to retract when stowed.
Narrative
We were descending using VNAV and LNAV; with the left autopilot engaged; from approximately 20000 FT to cross BEANO at 11000 FT and 250 KTS. Our speed was about 315 KTS. We were slightly above the descent profile; so the Captain selected half or less than half flight spoilers. The drag on the aircraft felt unusual and the autopilot commanded a left turn to maintain wings level. The Captain took control as the autopilot disengaged automatically. He maintained left aileron to keep the wings level and slowed the aircraft to 250 KTS. We had a spoilers light on the overhead panel and on the upper EICAS screen. The Captain re-selected 50% spoilers and the asymmetry disappeared. He put the spoiler handle full forward (fully retracted) and the spoilers retracted symmetrically. We referenced the QRH; flight controls; but it did not apply. The Captain flew the rest of the descent and approach with the autopilot off. We chose Runway 08 as it's longer than Runway 10. We did not arm the auto spoilers for landing but they did deploy properly when we went to reverse. At the gate; we talked to Dispatch via phone to get them up to speed on the situation. San Juan Maintenance checked the spoiler computer and the autopilot computers. They told us the left autopilot indicated a fault.
Second reporter narrative
Descending through FL200 at 315 KTS; left autopilot engaged; LNAV and VNAV engaged; I extended speed brakes to 50% to maintain vertical path. 20 to 30 seconds later; the aircraft banked to the right and the autopilot put in left aileron but was unable to maintain course and then disconnected. These conditions were accompanied by both an EICAS message: 'SPOILERS' and an overhead annunciator panel light 'SPOILERS.' I stowed the speed breaks in attempt to alleviate the asymmetry to no effect while also slowing to 250 KTS and again extended the speed brakes to 50% and the asymmetry disappeared. Next; I stowed the speed brakes and the symmetry remained; all spoilers were now stowed. The First Officer consulted the QRH in 'FLT-C and WARN'. Neither applied to an asymmetry for spoilers. I did not arm the spoilers for automatic deployment for landing. After applying reverse thrust at landing; the spoilers deployed normally. I contacted Dispatch after the parking checklist and consulted with Maintenance. They found that there were no faults in the spoiler system but the #1 (left) autopilot did have a fault.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.