Page 22 - Oct24

ASK THE EXPERT
King Air Electrical
Systems Evolution
by Tom Clements
Obtaining a firm grasp on how a King Air’s electrical system operates is one of the more difficult tasks that any new King Air pilot faces. Throughout the course of my more than 50 years of King Air pilot instruction, teaching the electrical system always ranked high on the difficulty scale, right up there with reversing propellers and their governors. Nevertheless, most pilots are indeed able to learn the system satisfactorily, so that they have a solid understanding of what to do when system abnormalities or emergencies arise.
Although all King Air models share many electrical system similarities, there are quite a few differences. I have often been asked why a particular model exhibits a certain design that is not carried over into other models. The following article addresses questions such as this by following the evolution of the electrical system in King Airs from the early to the later years. I believe my King Air readers will find it interesting and serve to deepen their understanding of this important aircraft system.
Introduction
The original King Air, the 65-90 model that was introduced in 1964, was basically a Queen Air with the Lycoming engines exchanged for the first version of the Pratt & Whitney PT6A powerplant and with 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 significant improvements, including the following:
= A totally redesigned cockpit layout that included an annunciator panel;
= Offering reversing propellers as an option (and to the best of my knowledge no A90 was manufactured without that popular option);
= Driving the pressurization system’s air compressor (supercharger) off the left engine’s accessory case, mechanically, instead of via a hydraulic motor driven by a hydraulic pump on that same engine;
= A redesign of the electrical system into one
that continues, with various modifications and improvements, as the design still in use on current production King Air 260.
The following is an overall view of the King Air’s electrical system while elaborating 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 continued until model 90 production ended in 2021 while continuing today in the 360 series.
There are four sources of DC (direct current) electric power in a PT6-powered King Air: A single battery housed in the right wing’s center section, in front of the main spar; two identical engine-driven generators; and—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 (NiCad) without provision for air cooling. In the mid- 1970s, the standard factory-installed battery became a 20- cell NiCad and the battery box included 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
20 • KING AIR MAGAZINE
OCTOBER 2024