To start at the begining.....

An airstair is a passenger staircase that is built in to an airliner — often, though not always, on the inside of a clamshell-style door. The stairs can be raised or lowered while the aircraft is on the ground, allowing passengers and ground personnel to board or depart the aircraft without the need for a mobile staircase or a jetway.

Some piston-era airliners were equipped with airstairs, including the A/B-26 Invader and the On Mark marksman series of aircraft, and versions of the Douglas DC-3 specially modified in the 1940s by Southwest Airways. Airstairs have become less common because of increasing airport infrastructure, but they are still popular on small regional airliners and aircraft which operate into less-well equipped airports.

The airplane hijacker D. B. Cooper escaped via an airstair. Subsequently, Cooper vanes were installed to keep the airstair from being deployed in flight.

Ventral airstairs incorporated into the Boeing 727 and McDonnell Douglas DC-9 designs were particularly efficient from a ground handling perspective, for as passengers were deplaning aircraft, cleaners could be servicing the aft lavatories and moving forward, maintaining the aircraft cabin and facilitating more thorough and quicker aircraft turn arounds. Quicker turn around benefits airline, as this allows for greater daily aircraft utilization and thus potentially more profits. Modern airline executive accountants analyze cost benefit penalties of the fractionally increased overall aircraft weight which such designs impose.

Airstairs provide aircraft with a degree of independence from ground services that can be useful in special circumstances.

As the Invader developed and evolved into a popular executive VIP transport serving the civil aviation industry, ease of access for passengers became more of an issue and drop down hatches with steps or stairs as well as air stairs were built into the fuselage utilising areas of space usually reserved for bomb loads.

These early hatches/air stairs were small and situated on the underside of the aircraft and formed a segment of where the bomb bay used to be and thus installation did not jeapardise the integrity of the airframe. 

But passengers still had to crouch beneath the fuselage of the invader to gain access and were manually hinge operated without the need for power assistance. 

 

Many a suits and dresses ended up with oil smears down the back, as a result of the passengers not quite bending down far enough.

The next big step came when companies such as On Mark, L.B. Smith Company, Lockheed Air Service (LAS), Rhodes Berry, and Rock Island Oil & Refining Co, began to dramatically modify the invaders fuselage, thus enabling the addition of drop down air stairs that formed an integral part of the structure.

As these Air stairs formed such a large part of the airframe structure, it became increasingly necessary to strengthen and modify the airframe around the door so as to accomodate the streeses met when in flight and of course when the Air stair was dropped down.

Multipoint locking systems around the door were designed to sustain fuselage strength along the longitudinal axis of the fuselage.

These Air stairs were usually electro-hydraulically operated.

 

Access to the aircraft cabin was made much easier and was a great selling point for these companies, who boasted wide and easy access to their executive transport aircraft.

 

 

Richard E Fulwiler wrote:

All of the conversion companies had essentially the same type of articulated / folding step door of their own design, but subtle differences could be seen.

As these doors were the gateway to the cabin, that section would be appropriate to show the photos captioned to reflect the conversion company ( ie : Rock Island, On Mark, etc. ). Even the way On Mark produced their Airstairs changed a little over time, early ( N40Y ) to late ( N7079G ) Marketeer doors, then to the requirements for the pressurized Marksman.

As companies that developed the Invader became more adventuress with the aircrafts fuselage, new designs of air stair were created as can be seen below and these designs went on to be patented and utilised by other aircraft manufacturers for incorporation into their own aircraft.

 

The two shots below show how the Invader Air stair developed from a side access point, to an almost under body access point, due to the increased size of the newer and taller fuselages on the On Mark Marksman and Rhodes Berry Silver 60, the latter of which could carry up to 16 passengers, so access had to be quick, easy and comfortable.