Development
The On Mark Engineering Company was involved in the maintenance and conversion of Douglas A-26 Invaders for both civil and military customers from 1954 to mid-1970s, The first conversions mainly involved the removal of military equipment and replacement with fairings and civil avionics, sealing of the bomb bay doors, soundproofing, and additional cabin windows. The original "gunner's hatch" was replaced with a larger retractable entrance door, room for baggage was provided in the nose section. They had improved brake systems and fuel systems and uprated engines with reversible-pitch propellers.
Around 1957 the company had developed a major modification that replaced the "carry-through"' section of the rear wing spar with a circumferential steel "ring spar" that freed the fuselage space for better passenger accommodation and cockpit access. Other major improvements included a broad-chord metal-skinned rudder, Douglas DC-6 wheels and brakes, an Auxiliary Power Unit, autopilot and additional fuel tanks inside the wing and the addition of wingtip fuel tanks. It also had an extended fiberglass nose for baggage (or a radar) which increased the overall length by about 26".
The typical package of optional improvements was standardized and marketed as the On Mark Executive. One aircraft (registered N40Y) had all the options embodied and was used for development and was renamed in 1957 as the On Mark Marketeer.
Further development continued into the 1960s into what became the On Mark Marksman. The major difference was the addition of full pressurization. Improvements were also made to the cockpit with the incorporation of Douglas DC-6 flat glass windscreens and cockpit side windows. A replacement fuselage roof structure was added from the new windscreens, tapering back to the original tail section. In January 1961 the first Marksman conversion (registered N100Y) was test flown. A Supplementary Type Certificate was issued in January 1961 and deliveries commenced soon after. With the third Marksman conversion (registered N400E), On Mark further altered the upper fuselage by creating a constant interior height to provide a headroom of about 6 ft in the passenger cabin.
By 1964, six Marksman conversions had been carried out for civil customers, the final seventh and eighth being of a special purpose version with terrain-following radar and cargo-dropping hatch for low level air-drops, designed by and delivered to CIA-associated companies.
Read more about this topic: On Mark Marksman
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