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turbine to its outlet. From station 4 – Compressor Turbine Inlet – to station 5 – the Interstage Turbine location, downstream of the CT and upstream of the PT. Since nearly two-thirds of the combustion product’s energy is used to drive the CT, the exhaust gas loses a lot of temperature as energy is being extracted. That’s why ITT – T5 or Interstage Turbine Temperature – tends to run in the 700°C range, and the temperature probes have a relatively good life. But it is also why ITT should not be used as the primary power-setting instrument since it is not monitoring the most critical locale in the engine. Torque and propeller speed (Np) is the correct way to set cruise power, based on graphs and tables that are based on pressure altitude and OAT.
Back to the Model 87: This was a Queen Air powered by PT6s. The first flight occurred in May 1963. It would go on to become the U-21 “Ute” line of airplanes that the U.S. Army would buy and operate for many years in many different roles. Soon thereafter, the first flight of the King Air, the model 65-90, took place in May 1964. It is amazing that the airplane received its type certificate in just a few months and customer deliveries began before the end of the year. The number “65” was the official name for all Queen Air models – the 65, 70, 80, A80 and B80 – all were added to the original FAA type certificate. The “improvements” that rapidly came with the pressurized King Airs were, for a long time, simply add-ons to the 65 type certificate. That explains why the Model 90 and the Model A90 King Airs are officially designated as 65-90 and 65-A90. When the B90 model and subsequent King Airs appeared, the 65 prefix was deleted.
Take a Queen Air, pressurize the fuselage a little, put on PT6A-6s, throw in the extra gauges and switches wherever they will fit on the panel and voila! – we have a King Air 90! At the time, the 90 was a very hot item. Looking back, however, it was quite primitive. The maximum differential pressure was a lowly 3.1 psid. At 20,000 feet, the cabin would already be above 10,000 feet. Realistically, the airplanes cruise speed rarely exceeded 200 knots. The “Straight” 90 saw seven being delivered into customers’ hands in 1964, the same year it received FAA certification. Sixty-nine airplanes were delivered in 1965 and another 36 in 1966 for a total of 113 airplanes, LJ-1 through LJ-113. If you add the yearly delivery figures presented here, they total 112, not 113. What’s wrong? LJ-76 became the prototype for the A90 and is considered a 65-A90 instead of a 65-90.
The A90 is a huge improvement over the straight 90! Although the dimensions are the same, and they both have a 9,300-pound maximum gross weight limit, that’s about all they share. The PT6A-20 engine was installed, with ITT replacing TIT. The pressure vessel was strengthened and tested to be certified with a 4.6 psid maximum differential. Now the airplane could climb to 25,000 feet before the cabin reached 10,000 feet. However, perhaps the best feature was a cockpit redesign to make it much more ergonomically satisfying.
18 • KING AIR MAGAZINE
APRIL 2022