Page 22 - Volume 15 Number 1

however, are exclusively located on the left sidewall, where the fuel panel is located. Unlike in the past, the CBs on the left sidewall would include some items not associated with the fuel system: Flaps and ignition, for example.
Also, the 200-series moved the main buses from the cockpit pedestal to a location in the cabin aisle, just aft of the main spar. A neat panel is located there under a clear plastic overlay that is painted with labels showing exactly what’s what. As in the past, two spare 325-amp isolation limiters are installed near the main buses to be available for on-the-road replacement.
The Five Bus System
It might be said that the model 200 was a Super 100 since it had the same cabin size but offered a tremendous increase in overall performance. The Beech engineers were given the directive to create a similar change for the 90-series: To develop a Super 90 that would be head and shoulders above its C90 and E90 siblings.
Off to the drawing boards they go. Wow! Here it is 1978 and we get to modify what was first designed in 1963! What improvements we can make! Let’s redo the nose profile to eliminate the “flat face” of the King Air and go with a sleek profile like that used on the Model 60 Duke. Let’s go with a wet-wing system and eliminate the bladder tanks. Let’s go with a whole new wing but using the
landing gear from the 100-series, to allow for a higher gross weight. Let’s redesign the electrical system to make it more modern, copying a lot of what Boeing did on the 737. Let’s use the T-Tail Rudder Boost system, and cockpit layout of the 200, along with its automation of the fuel transfer system. Lastly, let’s use the 200’s stronger cabin door and dual-pane cabin windows so that we can increase the pressurization differential. We’ll call this model the F90.
Not many of my readers prob- ably remember those days of the 1970s. The Carter presidency saw nearly runaway inflation and prices were soaring dramatically. After the Beechcraft financial analysts reviewed the changes that were planned, they concluded that this proposed F90 would need to cost more than its big brother, the 200, to account for these major changes that were to be paid for in late-1970s dollars! Oops, that cannot be! So, the engineers were sent back to the drawing boards with orders to tone it down, keep the price in line with reasonable expectations.
The pressurization increase, the T-Tail, the 200-style cockpit layout, the automated fuel transfer, the landing gear from the 100-series, and the 737-like electrical system were retained. The rest was tossed. So, let’s talk about this new electrical system that first appeared on the F90 but continues in the C90A, C90B, C90GT and all its variants, as well as on the entire 300-series.
Although this article cannot go into the depth required to really “teach” this new system, I will say that one of the few similarities to the older system is the presence of left and right generator (Main) buses. Instead of subpanel buses, we now have a triple-fed bus, a center bus, and, as before, a hot battery bus ... five buses in total, with the airplane’s components distributed among them appropriately. Of special interest is the fact that three Hall Effect Devices (HEDs) are included. These devices – not yet available back in the ’60s – allow excessive current flow to be terminated instantaneously, based on the increase in magnetic field around the wire, instead of waiting for the heat buildup to melt a fuse or blow a CB ... a safer design.
Also, the new system includes “Automatic Load Shedding.” Previ- ously, in the rare situation in which both generators were lost, it was incumbent upon the pilot to rap- idly turn off the unnecessary, heavy load, components so as to prevent the battery from being discharged too rapidly. In the new system, when the second generator fails, bus tie relays open automatically to dis- connect the generator buses from the battery. Hence, the battery only supplies those items located on the hot battery bus, the triple fed bus, and the center bus, and thereby prolongs its life a great deal. (It is nearly impossible to explain this satisfactorily, but the heavy load items on the center bus – electric heat and air conditioning motor – are also “shed” in this scenario.)
Because of the bus that is fed from the left generator, the right generator, and the battery, too, with the name triple-fed bus it seems that this new system is often called the “Triple-Fed Bus System,” which is OK. Some, however, leave out the “Fed” and say the “Triple Bus System.” Huh? But there are five buses! I try to be consistent and always refer to this as the Five Bus System. (The 350 kinda, sorta, has a sixth bus, the dual fed bus, but it really acts as an extension of the hot battery bus.)
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20 • KING AIR MAGAZINE
JANUARY 2021