Counterbalance mechanism for inwardly and upwardly opening plug-type airplane cargo doors

A counterbalance mechanism for countering the weight of a door in an airplane, the door being hingedly mounted to the fuselage of the airplane, wherein the door rotates upward and inward relative to the fuselage along a hinge axis, the counterbalance mechanism comprising:

• (a) a guide tube rotatably connected to the fuselage and extending toward the door, the axis of rotation of the guide tube being located approximately mid-length along the guide tube and generally parallel to the hinge axis of the door, the guide tube including:
• (i) a rod extending outwardly from the guide tube in a direction that is toward the airplane door, with the rod passing generally along the longitudinal axis of the guide tube and being partially contained by the guide tube;
• and
• (ii) at least one spring contained in the guide tube and mechanically linked for opposing outward movement of the rod along the longitudinal axis of the guide tube;
• and
• (b) a linkage assembly including:
• (i) a bell crank having first and second arms, the first arm of the bell crank being connected to the rod of the guide tube;
• and
• (ii) a push rod having first and second ends, the first end being connected to the second arm of the bell crank, and the second end being for connection to the door.