Air carrier EMB-175 flight crew reported loss of FMS sensors in cruise flight.
Synopsis
Air carrier EMB-175 flight crew reported loss of FMS sensors in cruise flight.
Narrative
We showed up at ZZZ1 Gate X with the aircraft warmed up for the passengers to board. The FO conducted a preflight walk around; the flight attendants prepared the cabin for boarding; and I conducted all of the first flight of day checks. We prepared the aircraft for departure; verified the flight plan; briefed and entered all of the performance information necessary for the flight. We also decided to deice the aircraft due to the freezing temperatures and a drizzle was starting. We complied with all checklist and procedures during preflight; before start; deice; taxi; take off and climb.Shortly before our level off at 34;000 ft.; an aural alert informed us that our auto throttle had failed.We immediately took control of the aircraft and attempted to reconnect it but failed it again. We looked at the EICAS and it showed an amber 'AT (Auto Throttle) FAIL and FD (Flight Director) VERT MODE OFF'. FMS speeds; AT; VNAV were unavailable and not working. We disconnected the Auto pilot and auto throttles and requested to exit the RVSM airspace and flew the aircraft with conventional navigation and manual speeds. We notified ATC and Dispatch that we might have a computer issue. We leveled off at 28;000 ft. After further scanning the aircraft systems; we noticed an incredibly high green dot which represented our L/Dmax with our speed bug below it. We decided to run the unreliable airspeed checklist. We informed the flight attendants of the situation and continued our scan in the cockpit. We found out that the FMS had dropped all of our performance numbers and vertical performance information in our flight plan. We figured this was the reason why FMS speeds and auto throttles didn't work. We requested ATC that we fly with heading and altitude assignments for the rest of the flight. We reengaged the auto pilot and it worked with basic functions like heading; ALT; and manual speed.We attempted to reenter the flight information in the FMS several times; however the FMS rejected the entries. Upon further review of the FMS; we found that both FMS 1 and FMS 2 sensors had failed.The message read on the maintenance section of the FMS; FMS 1 & 2 MAINTENANCE FAILED SENSORS'. We also noticed an erroneous fuel indication in the fuel management page; at one point indicating more than 28;529lbs of fuel. We crosschecked this information with the fuel indication on the EICAS and determined the information on the EICAS was correct.We decided to continue to ZZZ and began planning our landing into the airport. We had about 1.5-2.0hrs of fuel left with about 30-45min left to our destination. Using all of our available printouts and other resources; we determined we would have enough fuel to fly to the airport; be under max landing weight; and have enough for a go around or an alternate if necessary. We were also periodically crosschecking our airspeed; heading; and altitude with ATC.After reading and discussing the emergency section in the FOM; we decided to [request priority handling]. The flight attendants were notified of the situation and informed them not to evacuate until further instructed; we would taxi into the gate like normal. We attempted to contact Dispatch through ACARS but determined the signal was unreliable. We also attempted to contact them but was unlucky after trying 2 frequencies. We [requested priority handling] with ATC and were prioritized direct ZZZ. We had about 30min. to our destination.We verified that our ILS frequency was identifiable and reliable and informed approach that we will be able to conduct the approach. We landed the aircraft safely; followed SOP; and complied with FAA regulations. We were met and followed by emergency vehicles to Gate XX and deplaned the passengers. There were no injuries; no physical aircraft damage; the landing was conducted within limits. Cause: FMS 1 MAINTENANCE FAILED SENSORS and FMS 2 MAINTENANCE FAILED SENSORS. These sensors failed shortly before level off at 34;000ft and dropped all of the performance numbers and VNAV information in the FMS. FMS fuel management was giving erroneous fuel readings reading more than 28;529lbs of fuel at one point making the green dot unreliable. Because of this; none of the FMS autopilot functions worked and certain information were erroneous and unreliable. Flew the aircraft raw data with conventional navigation.Suggestions: If this situation happens in the future; we suggest to fly the plane by conventional navigation. Don't discard any useful printed information that will help you calculate information manually. This definitely helped us having it quick and accessible. I thank my FO for doing a phenomenal job as a pilot. I thank the company for preparing us and providing us with all the resources used to conduct this flight safely. It would be wise to include an FMS failure situation in the simulator. Do not get rid of land-based navigation like ILSs and VORs. this helped us with the absence of the FMS and GPS functions.It's better to have 2 pilots than one. There was a period where the autopilot was not available and needed one pilot to fly while the other pilot dug into the systems and checklist to figure out the issue.Always fly the aircraft first we have precious souls on board."
Second reporter narrative
Enroute; before leveling off at 34;000 ft; auto throttles disengaged. Immediately taking control staying stable; we attempted to reconnect; but it disengaged again. With EICAS messages of AT (Auto Throttle) FAIL and FD (Flight Director) VERT MODE OFF. After running checklist and trying to reset data it would either not take or completely drop information. At this point we went to all conventional navigation and disconnected auto throttles and auto pilot and requested out of RVSM. Descending to 28;000 ft.Observing all vertical guidance; performance numbers (either completely dropped or inaccurate); green dot was unreliable (showing L/Dmax). We crossed checked our position and altitude with ATC; and requested heading and altitude assignments. At this point we [requested priority handling]. Verifying conventional navigation was still reliable we flew vectors into ZZZ for landing on the ILS XXR.Cause: FMS 1&2 MAINTENANCE FAILED SENSORS. With these sensors failing before leveling off and losing the information that coincides (vertical navigation and performance data). Additionally; the FMS fuel management was calculating high fuel readings; resulting in green dot being unreliable. At this point we flew the aircraft raw data with conventional navigation. Suggestions: With this scenario; being able to work as a crew and taking in information a piece at a time without making big changes was vital. One pilot; running checklist; communicating and troubleshooting; and the other flying the aircraft. Captain did a great job of communicating and guiding through the process; which allowed us to work smoothly through the situation.
Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.