What was found
AFTER TAKEOFF, A VMO GEAR DOWN EICAS MESSAGE INCIDATION APPEARED AND THE CREW WAS UNABLE TO INCREASE AIR SPEED OVER 270 KNOTS. AN EMERGENCY WAS DECLARED, THE AIRCRAFT RETURNED TO DEPARTURE, LANDED SAFELY WITHOUT FURTHER INCIDENT. MAINTENANCE REPORTED FINDING NO MAINTENANCE MESSAGE OR STATUS IAW FIM 31-50-F1-14A-421A, AND ACCOMPLISHED A RESET OF THE OVERSPEED WARNING SYSTEM AND RETURNED THE AIRCRAFT BACK TO SERVICE.