A B757's spoilers were fully extended with the spoiler handle down and no EICAS alert after gate arrival. The following day the aircraft landed and the spoilers did not extend; again without an EICAS alert or other malfunction notification.

Narrative

This event report regards a major flight control malfunction: uncommanded; full deployment of the spoilers/speed brakes with the control lever fully stowed (and; apparently; with no flight deck warning). In the evening [the aircraft] taxied in and parked with all spoiler panels fully deployed; as noted by myself and a mechanic. Upon entry onto the flight deck; we both were very surprised to see the spoiler handle fully stowed. The arriving crew's First Officer returned to confirm they had competed their checklist and all hydraulic pumps were off and the spoiler handle was full forward. Our mechanic proceeded to pressurize all three hydraulic systems and cycled the spoiler handle. Despite three full cycles; the spoilers remained fully deployed and did not move; as confirmed through visual inspection. Finally; a fourth cycle of the handle had the spoilers all dropped and operate normally from that point on. Neither of us recall seeing any EICAS or other messages for a spoiler abnormality. I mulled over the implications of this malfunction during my exterior preflight of the aircraft and upon my return I informed the Captain that I was uncomfortable with the airworthiness of the aircraft and that I could not operate our flight that evening. He concurred and after hours of attempted diagnosis by multiple mechanics; the aircraft was declared 'out of service' and we were released back to the hotel; arriving after midnight. The next morning we were told that nothing specific was found to be wrong so all the appropriate spoiler control relays were replaced as a precaution. Prior to takeoff we confirmed with Tower that our spoiler panels were down. As a human factors engineer by education; I am highly concerned that the 757 appears to have a failure mode which can result in the full deployment of all spoilers without the handle being deployed; and potentially without any notice to the crew. This has profound flight safety implications; so much so that I recommend removal of that 757 FDR and FOQA data as soon as possible for analysis by Boeing and the FAA. If this failure were to repeat in flight; the successful outcome of the flight would be unlikely if not impossible should it reoccur at an inopportune time.