Page 29 - Volume 15 Number 6

Model 90s were being operated from small airports with relatively short runways.
Another concern was the airplane’s pressurization system. Although a distinct improvement over the days when nonpressurized cabins required passengers and crew to use supplemental oxygen at higher altitudes, the pressurization systems was limited to 3.4 pounds per square differential (psid). Customers wanted that number increased to take advantage of the Model 90’s ability to climb to and cruise at altitudes above 20,000 feet.
During 1965, Beechcraft engi- neers addressed these as well as other technical, performance and systems issues to create the Model A90. Although the airplane was es- sentially identical to the Model 90 externally, the majority of upgrades were transparent to the owners/op- erators and resided entirely within the airframe and engines.
More powerful Pratt & Whitney PT6A-20 engines, each rated at 550 shaft horsepower (shp) for takeoff, 538 shp for climb and 495 shp for the cruise phase flight, replaced the PT6A-6 powerplants of the A90’s predecessor. To address the concern about runway landing distance, the A90 was the first of the King Air series to receive fully reversing propellers. These units proved highly effective at reducing landing rollout, particularly when operating on rough, unimproved landing strips.
As for the aircraft’s pressurization system, maximum differential was increased to 4.6 psid that provided passengers and crew with a sea level cabin at 10,000 feet altitude and an 8,000-foot cabin at a cruising altitude of 21,000 feet. The upgraded system finally matched the cabin to the airplane’s performance capabilities and provided a welcome increase in comfort.
First flight of the A90 occurred Nov. 5, 1965, and following Feder- al Aviation Administration (FAA)
The U.S. Army began acquiring military versions of the versatile A90 King Air as early as 1967. Designated as U-21 or RU-21 “Guardrail,” these airplanes served as effective airborne platforms for special mission operations that included, among other roles, battlefield communications, surveillance and electronic countermeasures. (Special Collec- tions and University Archives, Wichita State University Libraries)
MAY 2021
KING AIR MAGAZINE • 27