Air carrier flight crew reported aircraft navigational system comparison monitor showing an abnormal situation. Flight crew returned to departure airport.
Synopsis
Air carrier flight crew reported aircraft navigational system comparison monitor showing an abnormal situation. Flight crew returned to departure airport.
Narrative
While taxing we had an intermittent Electronic Flight Instrument System (EFIS) comp mon fault. We slew the heading to fix the problem. The message would disappear for a length of time then reappear. We thought it was due to normal ground interface. After departure and during the normal clime out we noticed the heading and map information was inaccurate. Due to the inaccurate information. We were unable to use the autopilot and First Officer hand flew while I reference the QRH. During the trouble shooting we had slight speed and altitude deviations with quick corrections. We were unable to trouble shoot the problem and we [requested priority]. ATC gave us gyro vectors to back to the field. At one point it look liked my PDF was giving accurate information; and I took the controls from the First Officer (FO) and landed at the airport without any further incident. Suggestions: I'm not sure what the cause was; however after this situation I realize that EFIS Comp Mon can be more than a nuisance message. I will make sure I will give it greater attention in the future. As I will with all nuances messages.
Second reporter narrative
While at the gate; EFIS comp mon Cockpit Access Security System (CASS) message appeared. We assumed this was due to the building in the surrounding area; and continued on with normal operations. We continued our taxi and the CASS message distinguished. During our taxi; we continued to receive intermittent EFIS comp mon CASS messages. It would appear momentarily and then extinguish for an extended amount of time; then reappear. As we were cleared onto the runway; we slewed the heading and continued on with with takeoff. The message was still intermittent. I incorrectly believed that because EFIS comp mon was classified as a nuisance message in the manual; we were allowed to depart with the intermittent message. However; I acknowledge; after reading manual section X.X.X (EFIC Comp Mon Magnetic Heading Disturbances Emanating During Ground Operations) that I was incorrect on my assumption. On departure; ATC gave us radar vectors. They soon contacted us and asked if we needed assistance; and informed us that they assigned us a heading and we were not adhering to it. We thought we were; but quickly realized there was an issue with both the captain and I's HSI's. I took radios and controls and instructed the Captain to reference the QRH for EFIS comp mon. As both HSI's were inoperative; I hand flew. During my hand flying; there were several altitude deviations approximately plus or minus 100 feet; with a quick response to correct. ATC asked if we would like gyro vectors; to which we accepted. We requested to return to ZZZ and prepared for our diversion. We were instructed to descend to 9;000 ft; and I mistakenly overspeed the FAR requiring 250 knots under 10;000 ft. I quickly corrected my mistake. I soon [requested priority] once I realized both the captains and I's task saturation that ultimately caused several airspeed and altitude deviations. After the Captain troubleshooted; he determined that his side seemed most accurate and conducted the approach via hand flying. He had a smooth landing and we did not require further assistance. Suggestions: The manual states 'If the caution message remains and it is known that the aircraft has been removed from all sources of magnetic anomalies; a system failure has occurred.' I understand that even though com mon fault is classified as a nuisance message; it doesn't mean we can depart with an intermittent message. This was a tremendous learning moment for myself and I believe I am a better pilot because of it. Because I have taken the time to sit down and read what nuisance messages are; I am now equipped to handle a situation like this in the further. In the future; I will ensure there is no intermittent CASS message before departing.
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Source: NASA Aviation Safety Reporting System (public domain). Reports are voluntary submissions and are not verified by NASA.