The On Mark Airstair
This is an depth description of the Airstair cabin access door as
designed and installed on On Mark executive conversions of the A-26 Invader.
This write-up is based on an observation of the Airstair cycling
open on a Marksman ( ZS-CVD at On Mark prior to customer delivery ) in 1962, a basic understanding of things mechanical, and
the close study of photographs.
The basic ( patented ) design by On Mark varied somewhat over time,
however, the overall operation was essentially the same from early to late, and for the pressurized Marksman. This analysis is
only concerned with the starboard side articulated step Airstair version, and not the belly mounted folding step entrance
The Airstair door consists of three main component groups;
1) the door, 2) the steps, and 3) the handrails and actuation linkages.
1) The door :
The Airstair door is a sophisticated component
that is built with sufficient strength to support the weight of an adult male when in the open position. By climbing the steps
that open outward from the inside of the door, cabin access is achieved. The outer skin of the door is contoured to match
the surrounding fuselage side and is completely flush when closed. A viewing port / window is located in the upper portion,
and is in the same alignment with the fixed cabin window (s) aft of the door. A " Piano " style hinge is located along the
bottom edge, allowing the door to open outward and down, swinging through nearly 150 degrees to the fully open position. Links
of chromed telescoping tubing that double as handrails support the door when open, and compress and fold into a narrow space
between the door and doorframe as the door retracts.
The double layer door shell is reinforced
by an inset peripheral box-sectioned structural frame on the inside surface. This frame provides rigidity and mounting points
for the steps, along with their associated linkages. The outer flange formed by the inset framing mounts the door seal,
the door end connection points for the handrails, and the retraction cable. Below the window, on a reinforced panel, mounts
the door latching handle that connects to tapered "plunger-type" latch pins. The door in the up and secured position
is received within a structural door frame with an inner flange that mounts and conceals the door linkages and handrails.
2) The steps :
There are four steps, mounted on pivots
to the inside of the door between the top-to-bottom structural frame rails. They stack flat along the inner surface of the
door in two groups, separated by a fixed upholstered panel, paired one atop the other with the door closed. The steps pivot
and move to equally spaced stair treads with the door in the folded down, open position. For reference, I will number the
steps 1 through 4, counting from the door hinge downward, with the door open. All four steps retract, folding with the
outer step edges moving towards the door hinge as the door is closed, leaving the door latching handle and window area unobstructed.
Step #2, #3, and #4 tread panels are extended on arms to provide for correct position and spacing with the door open. Step
#2 and #4 are upholstered on their lower surfaces, and with the door closed, face into the cabin providing a finished appearance.
Step #1 has a fixed pivot and folds directly into a well between the door's structural frame rails. Step #2 is connected to
step #1 with a dedicated link ( on the forward side ) and pivots on a moving hinge ( controlled by an articulation linkage
on the aft side ) to stow covering step #1 when the door is closed. The fixed upholstered panel spanning across the door,
separates the closed step #1 and #2, and the step # 3 and #4 stacks.
Similar to step #1 and #2,
steps #3 and #4 operate as a pair, having a separate dedicated connecting link between them ( again on the forward side ).
Step #4 pivots on another moving hinge ( also controlled by the articulation linkage on the aft side ) to stow covering step
#3 when the door is closed. As the size of step #4 is slightly smaller than step #3, an upholstered strip is mounted on the
under side, just behind the outer edge of step #3. This strip fills the space remaining when the door is closed. When viewed
from the cabin, the closed and secure door presents a series of closely spaced finished panels from the floor up to the door
latching handle area, concealing nearly all of the mechanisms beneath.
3) The handrails and articulation linkages :
Twin chrome, telescoping, tubular handrails
hinge in the upper, inner corners of the door frame. The smaller diameter segments extend downward from inside of the upper
portion, nearly doubling in length as the door swings open. The lower end of each telescoping section is hinged and receives
the upper end hinge of the handrail struts. The opposite end of the handrail struts hinge on brackets on the door flange near
the top outer corners of the door. Two radius link struts pivot at each side of the door frame at the lower corners, and fold
down and outward as the door opens. The opposite end of the link struts attach by hinges to double hinged intermediate links,
connecting to the lower ends of the telescoping handrails. The outboard end of the aft side radius link carries a roller guide
that the door retraction / tension cable routes across. This cable leads from the retraction mechanism inside the fuselage
aft of the door to a pulley in the door frame, across the roller guide at the end of the radius link, to terminate at a pivot
bracket on the door flange. Tension supplied to the retraction cable pulls the door upwards for closure. Door movement is
hydraulically dampened to slow and control opening, while the linkages prevent over extension and support the open door.
The step articulation control linkages are
installed along the forward side of the door's aft, top-to-bottom, structural frame rail. Although the exact operation of
the control links are speculation on my part, the end result of their function is understood. The position and movement
of these links are controlled by a " J " shaped arm. The end of the curved portion pivots on a hinge mounted inboard of the
door frame threshold. The opposite end would connect to the control links by a center pivot toggle. Because of the difference
in the hinge planes between the door and the " J " link, the free end of the " J " link moves toward the top of the door as
the door closes, and closer to the door hinge as the door opens. With the " J " link connecting to the step articulation linkage
through the center pivot toggle, relative motion of the control linkage is opposite to that of the " J " link. As the door
closes, the control linkage is drawn towards the door hinge. Connected to the control linkages are steps #2 and #4, pivoting
on moving hinges. Simultaneous folding and movement towards the door hinge of step #2 and #4 continues as the door closes.
As step #2 is linked to step #1, and step #4 to #3, movement is synchronized until the steps are completely folded flat and
stacked as previously described. The process is reversed as the door opens until the steps are fully unfolded and extended.