Narrative:

We were at FL350 approximately 45 minutes into the flight deviating around weather in the vicinity and with engine anti ice on at approximately 208;000 pounds. Simultaneously the following EICAS messages appeared: rudder ratio; mach trim; and a ...disagree message that was only present for a second or two and neither of us caught it as it was the third on the list. This was followed very closely by the autopilot disconnecting and us noting an airspeed disagreement of over 120 KTS between the captain's and first officer's airspeed indicators and low to moderate acrid smell in the cockpit. I told the first officer to declare an emergency and get us a lower altitude. We were cleared to FL290 and started down immediately but smoothly. As the first officer was getting out the QRH for the rudder ratio and mach trim (without being asked or prompted); I asked her to seat the flight attendants and that we would get back to them. She read the QRH's quickly and efficiently and then called the flight attendants. By the time this was done which was less than a minute and I don't believe we were even through FL350 yet all the EICAS message were gone; the airspeed was back to normal; and the acrid smell had decreased. We deployed our O2 masks to clear our nostrils and attempt to locate where the smell was coming from. It seemed to both of us that the smell was stronger towards my instrument panel. We leveled at FL290 and talked to the flight attendants to see what additional information they might have. None of them saw; heard; smelled or even thought anything was wrong; but a passenger in seat xxa reported smelling something burning. No circuit breakers were out that we could see; the engine anti-ice was turned off due to being VMC; and the autopilot was functioning properly. I then sent a detailed message to dispatch with the info and requested a fuel burn for FL290. The dispatcher was right on top of it for us; almost anticipating my request! After 30 minutes or so at FL290 and all systems functioning normally; we decided to go back to FL330 for the remainder of the flight. I put a lengthy write up in the maintenance log book and we debriefed the oncoming crew at the top of the jet bridge. When I got to the hotel maintenance control called and we further discussed the situation with the main emphasis of the conversation on the passenger report for xxa. I relayed to him that I didn't know if that had a bearing on what they might need to do; but I put it in the write up to give the maintenance personnel as much info as I could so that they could locate and solve the problem. I am thinking that maybe there was wiring; a relay; or a transmitter in the vicinity of xxa that may have had something to do with the situation.

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Original NASA ASRS Text

Title: A B757 had RUDDER RATIO; MACH TRIM and DISAGREE EICAS messages present for a several seconds before disappearing. The autopilot disengaged and a 120 KT airspeed difference developed along with an acrid burning smell. All EICAS messages and smell disappeared with a minute.

Narrative: We were at FL350 approximately 45 minutes into the flight deviating around weather in the vicinity and with engine anti ice on at approximately 208;000 LBS. Simultaneously the following EICAS messages appeared: Rudder Ratio; Mach Trim; and a ...disagree message that was only present for a second or two and neither of us caught it as it was the third on the list. This was followed very closely by the autopilot disconnecting and us noting an airspeed disagreement of over 120 KTS between the Captain's and First Officer's airspeed indicators and low to moderate acrid smell in the cockpit. I told the First Officer to declare an emergency and get us a lower altitude. We were cleared to FL290 and started down immediately but smoothly. As the First Officer was getting out the QRH for the Rudder Ratio and Mach Trim (without being asked or prompted); I asked her to seat the flight attendants and that we would get back to them. She read the QRH's quickly and efficiently and then called the flight attendants. By the time this was done which was less than a minute and I don't believe we were even through FL350 yet all the EICAS message were gone; the airspeed was back to normal; and the acrid smell had decreased. We deployed our O2 masks to clear our nostrils and attempt to locate where the smell was coming from. It seemed to both of us that the smell was stronger towards my instrument panel. We leveled at FL290 and talked to the flight attendants to see what additional information they might have. None of them saw; heard; smelled or even thought anything was wrong; but a passenger in seat XXA reported smelling something burning. No circuit breakers were out that we could see; the engine anti-ice was turned off due to being VMC; and the autopilot was functioning properly. I then sent a detailed message to Dispatch with the info and requested a fuel burn for FL290. The Dispatcher was right on top of it for us; almost anticipating my request! After 30 minutes or so at FL290 and all systems functioning normally; we decided to go back to FL330 for the remainder of the flight. I put a lengthy write up in the Maintenance Log book and we debriefed the oncoming crew at the top of the jet bridge. When I got to the hotel Maintenance Control called and we further discussed the situation with the main emphasis of the conversation on the passenger report for XXA. I relayed to him that I didn't know if that had a bearing on what they might need to do; but I put it in the write up to give the maintenance personnel as much info as I could so that they could locate and solve the problem. I am thinking that maybe there was wiring; a relay; or a transmitter in the vicinity of XXA that may have had something to do with the situation.

Data retrieved from NASA's ASRS site as of April 2012 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.