A Cape Air plane from Maine landed at Logan Airport in Boston Wednesday afternoon after declaring an emergency.A spokeswoman for Cape Air said the plane was coming into Logan when the top of the clam-shell door opened.The pilot called the tower for an emergency and was given priority for landing.The door was open for 10-15 minutes, the spokeswoman said.The plane landed safely at 4:17 p.m.There were nine passengers on the plane.The cabin was not pressurized because the plane flies at a lower altitude.No one was injured.