Flight Crew Air Transport Pilot (ATP); Flight Crew Instrument; Flight Crew Multiengine
Flight Crew Last 90 Days 111
Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy
Air Traffic Control Provided Assistance; Flight Crew Landed in Emergency Condition; Flight Crew Returned To Departure Airport; General Flight Cancelled / Delayed; General Maintenance Action
Aircraft; Procedure
Departing Runway XX Captain was the flying pilot and I was Pilot Monitoring. It was a normal takeoff until the Captain asked for the autopilot. About 3-6 seconds later we got a 'AP Trim is NU' caution message. Since we were only 700 feet above the ground the Captain turned off the autopilot and tried to trim the plane himself. He was putting in nose down trim but nothing was happening. During this time we got direct to ZZZZZ and climb to 7;000 ft. We started the turn and kept the plane climbing. During the turn I tried to put nose down trim to see if it would work on my side. It did not. Since we were not getting any messages on EICAS and the trim was still not working; we disconnected the stab trim and Mach trim and then reconnected. The plane was already in a nose high pitch attitude and once we reconnected; the nose high trim increased by an additional .4 units while the captain was trying to trim down. Once we confirmed that we disconnected the the stab trim and Mach trim at that point the Captain transferred the controls to me and he took the radios and started running the QRH. I started to slow the rate of climb of the aircraft and we asked ATC if we could stop the climb and maintain the altitude we were at. ATC gave us clearance to do so; the altitude was 5;000 feet. We were still heading towards ZZZZZ. Then we advised ATC and ask for delay vectors to return back to ZZZ. The Captain did the checklist for the stab trim caution message and we also did our check lists for landing. We then asked ATC for vectors to land on [Runway] XX. We landed with the trim at 8.8 and took off with a trim setting of around 6.8. During the whole fight we never put trimmed up. It was a successful landing in ZZZ. At we taxied to the gate without incident. The stab trim was not responding to our inputs.
First Officer reported a Mach Trim malfunction caused an Air Turn Back and precautionary emergency landing.