Page 16 - Volume 15 Number 1

ASK THE EXPERT
King Air Electrical Systems Evolution
by Tom Clements
This article first appeared in the January 2012 issue of this magazine. It is also a chapter in The King Air Book – Volume II. It is significant enough to merit reprinting here both to enlighten newer readers, as well as
tTo provide a review for seasoned King Air veterans.
he original King Air, the 65-90 There are four sources of DC model that came out in 1964, (Direct Current) electric power was basically a Queen Air with in a PT6-powered King Air: (1) a
the Lycoming engines exchanged single battery housed in the right
The battery is typically wired directly to a bus that is always “hot” with voltage whenever the battery is installed. This bus is named the “hot battery bus” and is usually located in or very near the battery box. Some convenience items as well as some components that were considered most important by the engineers receive power from this bus. These include the door and baggage compartment lights, boost pumps and crossfeed in the LJ-series, and, often, standby fuel pumps and fuel firewall shutoff valves. The fuel-related items that receive power from this hot battery bus, for redundancy, also receive power from their own fuel panel bus after the battery and/or generator is switched on, as we will see.
The generators, which of course do double duty as the starter motors, as well, are made by Lear-Siegler and originally were rated at 200 amperes maximum continuous output. Beginning with the C90- and the 100-series, better cooling ducts were incorporated that allowed the maximum generator output to be upped to 250 amps. Instead of presenting the generator output on a gauge that was marked in amps, the decision was made to mark the gauge with decimal equivalents relating to the maximum rating. For example, with the 250 amp generators,
for the first version of the Pratt & Whitney PT6A powerplant and the fuselage modified and strengthened to allow for a meager pressurization system. Two years later, the A90 model replaced the “Straight 90,” and with it came a number of very significant improvements, including: A totally redesigned cockpit layout that contained an annunciator panel; reversing propellers were offered as an option (to the best of my knowledge no A90 was manufactured without that popular option); driving the pressurization system’s air compressor (supercharger) off of the left engine’s accessory case, mechanically, instead of via a hydraulic motor driven by a hydraulic pump on that same engine; and a redesign of the electrical system into one that continues, with various modifications and improvements, as the design is still in use on the King Air 250. The aim of this article is to present an overall view of the King Air’s electrical system and to elaborate on some significant changes that occurred along the way. I will also discuss the Five-Bus system that first appeared on the F90 model in 1978 and that continues in the current C90GTx and the 350-series.
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wing’s center section, (2) in front of the main spar, (3) two identical engine-driven generators, and (4) sometimes, an External Power Unit (EPU). The B100 model, powered by the TPE331 engine, uses two batteries, one in each wing’s center section, due to the greater starter demand of its fixed-shaft turboprops.
The first battery used was a 19-cell Nickel-Cadmium without provision for air cooling. In the mid- 1970s the standard factory-installed battery became a 20-cell NiCad and the battery box including ram air cooling. This was about the same time that the battery monitoring system was included, with its battery charge annunciator that could indicate the early stage of a thermal runaway. Improved lead- acid batteries first became popular as an STC’d replacement for the more-expensive and potentially more troublesome NiCads, and in the 1990s the factory discontinued the use of NiCads and went to a Concorde VRSLAB (Valve-Regulated, Sealed, Lead-Acid Battery) replacement. The battery charge annunciator is no longer required with a lead-acid battery since that battery does not have thermal runaway potential.
JANUARY 2021