The aircraft took off from Dublin Airport for a check flight following the completion of “C” check maintenance at Team FLS. After take-off, significant airframe vibration was encountered. The crew then deduced that both airspeed indicators were under-reading significantly. Following declaration of an emergency, and trouble-shooting by the crew off the east coast of Ireland, the aircraft returned safely to Dublin. After landing it was discovered that the flap system had suffered damage. It was found that the static drain ports in the Avionics and Electrical (A&E) bay, connected to both the Captain’s and the First Officer’s instruments, were left open after maintenance. This resulted in both airspeed indicators under-reading by a significant amount.