Page 21 - Volume 11 Number 8

rudder pedals, on top of the cockpit floor, gets sucked to the blower through a hole in the pressure bulkhead, then gets expelled through the evaporator plenum and back through the pressure bulkhead beneath the cockpit floor into the mixing plenum.
Well and good, but where is the heater?! It’s gone!
Sadly, Beech made a little boo-boo here. They looked at the higher compression ratio on these big PT6A-41 engines that had never been used before and concluded that bleed air heat would always be sufficient for cabin comfort, no supplemental heat would be required.
Wrong! In cruise flight, sure, there is plenty of heat. But the design team must have totally overlooked the fact that the bleed air is not very compressed and hence is not very hot on the ground with the engines at Idle. Before the flight test program concluded, it was recognized that something had to be done to provide cabin heat on the ground ... or even to supplement the bleed air heat in flight, if ever needed. By this time, however, the environmental air distribution design had been finalized, with no provision for any type of grid heater in the ductwork.
What do they do? They come up with the rather puny and ineffective “Radiant Heat Panels” in the cabin
overhead. Better than nothing? Absolutely! As good as an electric grid heater? Not even close! But at least they got rid of the potential for leaks in our now non- existent crossover duct. It was not until the model 300 appeared in 1984 that the design was modified to include both a forward and aft grid-type heater buried in the distribution ductwork. This same design was incorporated into the 200-series beginning with BB- 1444 in 1993. Nice!
There you have it: More than you ever wanted to know about the infamous crossover duct. It is still in the latest C90GTx-series, but never in any 200- or 300-series, including the B300 (known as the 350).
May all your leaks be small! KA
King Air expert Tom Clements has been flying and instructing in King Airs for over 44 years, and is the author of “The King Air Book.” He is a Gold Seal CFI and has over 23,000 total hours with more than 15,000 in King Airs. For information on ordering his book, go to www.flightreview.net. Tom is actively mentoring the instructors at King Air Academy in Phoenix.
If you have a question you’d like Tom to answer, please send it to Editor Kim Blonigen at kblonigen@cox.net.
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AUGUST 2017
KING AIR MAGAZINE • 19