accompanying fig1 a and 1b are exterior perspective views of a forward section of an aircraft fuselage 10 equipped with a bottom - hinged , clamshell - type airstair door 12 shown in closed and opened positions , respectively . as is conventional , the airstair door 12 is equipped with a hinge assembly 14 located at the lower end 12 a of the door . a series of steps 12 c are provided between the lower end 12 a and the upper end 12 b of the door 12 which allow passengers and crew to board and disembark from the aircraft fuselage 10 when the door 12 is in its opened position ( i . e ., when the door 12 is pivoted in the direction of the arrow a d in fig1 a so the upper end 12 b of the door 12 is near the ground as depicted in fig1 b ). according to embodiments of the present invention , the airstair door 12 is operatively connected to a counterbalance mechanism 20 as will be described in greater detail with reference to accompanying fig2 - 5 . in this regard , the counterbalance mechanism 20 is depicted in fig2 - 3 with the door 12 in a closed position ( i . e ., as shown in fig1 a ), while the counterbalance mechanism 20 is depicted in fig4 - 5 with the door 12 in an opened position ( i . e ., as shown in fig1 b ). the counterbalance mechanism 20 includes an operator handle 22 which operatively connects the counterbalance mechanism 20 to the lower end 12 a of the door 12 by a hoist rod 24 . in this regard , the hoist rod 24 is pivotally connected at its upper end 24 a to a connection boss 22 a of the operator handle 22 located between the handle knob 22 b at the free end 22 c of the handle 22 and its opposite fixed end 22 d thereby establishing an upper hoist rod pivot axis 24 - 1 . the opposite lower end 24 b of the hoist rod 24 is similarly pivotally connected to the lower end 12 a of the door 12 to thereby establish a lower hoist rod pivot axis 24 - 2 . a lower bearing block 26 is fixed to the aircraft fuselage frame 10 a adjacent the doorway entrance 11 ( see fig1 b ) and includes a lower bearing block shaft 28 which defines and rotates about a shaft axis 28 - 1 . one end 28 a of the bearing shaft 28 is fixed to the lower end 22 d of the handle 22 while the opposite end 28 b of the bearing shaft 28 is fixed to an end 30 a of a first bellcrank 30 . the opposite end 30 b of bellcrank 20 is pivotally connected to one end 40 a of a link arm 40 so as to be pivotal about a pivot axis 40 - 1 . the opposite end 40 b of the link arm 40 is pivotally connected to an end 50 a of a second bellcrank 50 so as to be pivotal about pivot axis 40 - 2 . an upper bearing block 60 is fixed to aircraft fuselage frame 10 a and includes an upper bearing block shaft 62 which defines and rotates about axis 60 - 1 . the opposite end 50 b of second bellcrank 50 and an end 70 a of third bellcrank 70 are fixed to shaft 62 so each of bellcranks 60 and 70 rotate as an integral unit with the shaft 62 about the axis 60 - 1 . the first , second and third bellcranks 30 , 50 and 70 , respectively , and their associated shafts 28 and 62 are thus connected by the linkage arm 40 to establish an inverse parallelogram linkage mechanism between the operator handle 22 and a force accumulator assembly 80 as will be described in greater detail below . the force accumulator assembly 80 includes a piston assembly 82 having a piston rod 84 . a lower end 82 a of the piston 82 is pivotally connected to the aircraft fuselage frame 10 a while an opposite end 82 b of the piston rod 84 is pivotally connected to end 70 b of the third bellcrank 70 . the end 82 a of the piston 82 is thus pivotal about the axis 80 - 1 while the end 82 b of the piston rod 84 is pivotal about the axis 70 - 1 . a compression spring 86 coaxially surrounds the piston 82 and piston rod 84 and is captured between lower and upper end caps 86 a , 86 b , respectively . in use during a door opening cycle with the door 12 initially in the closed position as shown in fig1 a , 2 and 3 , an operator inside the fuselage 10 may operate the interior door release handle 16 or ground crew outside the aircraft may operate the exterior door release handle 18 so as to release the door and allow it to be pivoted about hinge 14 ( arrow a d in fig1 a ) to its opened position ( see fig1 b ). once the door 12 has been released , the operator inside the fuselage 10 will then apply a generally downward counterclockwise force ( as viewed from the right in fig2 and 3 ) on the handle 22 which will urge the door 12 to pivot about the hinge 14 by virtue of the hoist rod 24 being connected between the handle 22 and the door 12 . this counterclockwise movement of the handle 22 will be translated into concurrent counterclockwise movement of the first bellcrank 30 about the axis 28 - 1 due to the fixed connection of the handle 22 and bellcrank 30 to bearing shaft 28 at their respective ends 22 d and 30 a . the counterclockwise pivotal movement of the bellcrank 30 will in turn cause the second and third bellcranks 50 , 70 , respectively to pivot as a unit with the bearing shaft 62 in a clockwise direction about the shaft axis 60 - 1 ( as viewed from the right of fig2 and 3 ) by virtue of the operative connection between the bellcranks 30 and 50 provide by link arm 40 ( i . e ., since the structures collectively form an inverse parallelogram linkage mechanism ). the clockwise movement of the third bellcrank 70 will thus retract the piston arm 84 of the accumulator 80 against the force of the compression spring 86 . thus , as the door 12 pivots about the axis 14 - 1 ( see fig2 ) in the direction of arrow a d ( see fig1 a ), its weight and the force of gravity will be counterbalanced by the continual loading of spring force provided by the compression spring 86 thereby providing a continual mechanical counterbalance against such door weight . the closure cycle of the door 12 when in the opened position as shown in fig1 b , 4 and 5 is reverse to that described above . that is , with particular reference to fig4 and 5 , an operator will manually apply a generally upward clockwise rotational force ( as viewed from the right of fig4 and 5 ) to the handle 22 thereby provide a lifting force to the door by virtue of the hoist rod 24 being connected to the door 12 . this movement of the handle 22 will in turn cause the bearing shaft 28 and the first bellcrank 30 to rotate in a clockwise direction thereby urging the bellcranks 50 and 70 to rotate as a unit with the shaft 62 in a counterclockwise direction by virtue of the link arm 40 being connected pivotally between the bellcranks 30 and 50 ( i . e ., since the structures collectively form an inverse parallelogram linkage mechanism ). the counterclockwise movement of the third bellcrank 70 will thus cause the piston rod 84 to extend thereby unloading or dissipating the spring force that had previously been accumulated or loaded by the compression spring 86 during the door opening cycle of operation . the spring force of the compression spring 86 will thus provide force assistance as a mechanical counterbalance during the pivotal movement of the door 12 from its closed position to its open position ( i . e ., in a direction opposite to arrow a d in fig1 a ). those skilled in this art will appreciate that various equivalent modifications and / or alterations may be made to the embodiment described above . for example , a tension spring or other similar biasing mechanisms may be employed instead of the compression spring 86 described previously , in which case the structures could be modified to accumulate the biasing force of such devices to yield similar and substantially equivalent functional effects to those described previously . therefore , while the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment , it is to be understood that the invention is not to be limited to the disclosed embodiment , but on the contrary , is intended to cover various modifications and equivalent arrangements included within the spirit and scope thereof .