Loadmeters and Bus Ties

A twin otter has two wings, two engines, two starter-generators, two main bus bars, and two little electrical gauges on the dashboard. There is one engine on each wing, one starter-generator for each engine, and one bus bar powered by each generator, but don't be so silly as to assume each gauge corresponds to one generator.

The gauge on the left is the DC Voltmeter. The needle on its face deflects from left to right over a scale from 0 to 30 volts. This is an indication of the voltage available at the left bus bar (which is normally connected to the right bus bar, so don't worry about it). The power source suppling that potential could be either or both generators, the battery, or an external power source. Ya, you can plug the airplane into an external power source, like your laptop, but that is only useful while parked, unless you have a really long cord.

The gauge on the right is more complicated. It's the DC Loadmeter. Its needle deflects from the centre to the left or the right. The centre is marked as 0 and the left and right extremes as -1 and +1 respectively. If you don't do anything, the gauge shows the charge or discharge of the battery, with each .1 on the scale representing 10 amperes. It shows negative deflection when you are running off battery alone, positive numbers right after the generators come online, and then spends most of the time sitting around zero.

Next to the gauge is a little selector switch, which gives the pilots something to fidget with in cruise, so the passengers think we're doing something important. The switch automatically goes back to BAT when you release it, but you can twiddle it to the left or the right to select L GEN or R GEN. In that case, the needle indicates the loading on the appropriate generator, with each .1 representing 20 amperes. Full scale deflection corresponds to the full 200A output of that generator. The generator outputs are expected to be within 20A of one another unless something is broken.

You have the option of removing the connection between left and right bus bars, useful in certain abnormal situations, but if one of the generators isn't working (definitely an abnormal situation), opening the bus tie will render some of the electrical services unavailable. As a pilot you have to remember what runs off the left bus and what runs off the right bus, so that you know what you are going to lose when you open the bus tie switch. In general, stuff on the right side is powered off the right bus bar and stuff on the left side off the left bus bar: left and right pitot heat, intake anti-ice, bleed air, windshield heat. But, and look for this sort of thing as a sign of quality airplane design, the lights alerting you to the failure of a generator are powered by the opposite bus bar from the one supplied by the failed generator. I always wonder whether they got that right on the first try.