Page 26 - Volume 15 Number 10

To truly verify that our two single- engine airplanes are no longer connected, we turn on one side’s windshield heat switch and verify that the loadmeter increases only on that side. Next, turn off that side’s windshield heat switch and turn on the other side to see the other loadmeter increase. Keep in mind that if the windshield’s temperature sensor is already feeling more than 100°F the heat will not activate. Wait for a cooler day!
Next, to verify that each of our single-engine airplanes has a functioning voltage regulator, we check the reading on the Left Gen and Right Gen positions of our six- position voltmeter in the overhead panel. Both should show correct, 28.25 ± 0.25 volts. (As if we can read it that accurately!) Voltage regulation is one of the many functions of the left and right Generator Control Unit (GCU). We are making sure that function works correctly even when the GCUs are not trying to parallel their generator’s output loads.
The next step is to move the GEN TIES switch from OPEN back to NORM. Both Bus Tie Open annunciators should disappear and the loadmeters should nicely parallel. Momentarily turn on either side’s windshield heat and verify that both loadmeters show a slight increase. Good! Our two single- engine airplanes have become one twin-engine again.
Now we reach for the switch just to the left of the GEN TIES switch, the one labeled BUS SENSE. It is spring-loaded to the center, normal position. Down is labeled TEST and up is labeled RESET. Tap the switch down to TEST.
What is being accomplished here is making the Hall-effect devices (HEDs) experience a magnetic field similar in strength to the field they would feel when 275 amperes or more passes through them. These HEDs – we have three of them – are the protective devices that will open an associated relay to stop the
flow of excessive electrical current in the “wrong” direction. Unlike a fuse, current limiter or circuit breaker, these protective devices do not react to heat buildup. As you have experienced driving your old car under a high-power set of wires with the AM radio playing, electric current through a wire creates a magnetic field around the wire. The stronger the current the stronger the field. Mr. Hall, of Johns Hop- kins University, was the fellow who first quantified this relationship. Not until the 20th century were de- vices invented that could measure current flow extremely accurately by measuring the magnetic field strength. When the price of these devices made them within reach of aircraft designers – Voila! – HEDs appeared on some airplanes’ elec- trical systems, including King Airs. They provide an increased level of safety since they react so fast, before the temperature even begins to rise to the level needed to melt a fuse or to pop a circuit breaker.
24 • KING AIR MAGAZINE
OCTOBER 2021