When I discussed the PT6 engine function, the compressors were already spinning, the fuel burning steadily in the annular combustion chamber, and the combustion products blasting out through the compressor and power turbines. But if the flow of air is required to contain the fuel, the burning fuel is required to turn the compressors, and the compressors are required to create the flow of air, how does all this start?
Early models took advantage of the wheel-like properties of the turbines and employed squirrels to achieve the initial NG. Unfortunately the high T5 immediately after lightup rendered each squirrel a single-use component, leading to rapid squirrel depletion at popular landing sites1. A more sustainable starting technique was called for.
The current technique, rumoured to have been proposed by a member of the squirrel pool2, is common to many turboprops. Each engine is already equipped with a generator, and as a generator and a motor share most of the same parts, it's not especially cumbersome to use the generator as a starter motor. Adding to the efficiency is the fact that electrical power isn't required to be generated during start, and a starter motor is not needed after start. So the airplane is equipped with two starter-generators.
The first step is to spin the compressor up to sufficient speed to maintain airflow. The preferred way to achieve that is using someone else's electricity, connecting the airplane to an external power supply. It should be able to supply 28 V at a minimum of 800 A. It should also fit the plug receptacle in the airplane. There's nothing in the manual about lugging around one of those plug converter things, so perhaps the international standardization of electrical equipment is better for aviation than for toasters. I know some of these airplanes were sold in England, but perhaps, like English toasters, they were delivered to the consumers without the plugs, and the end users had to install their own plugs3.
It's also possible to start the engines using the battery. Either way the procedure is the same, with the exception of the position of one switch, the EXTERNAL/BATTERY switch. Completion of the rest of the prestart checklist leaves the power levers at idle, the propellers selected to full fine (but lack of oil pressure means actual blade angle will be either feathered or latched at one degree, depending on the modification), DC electrical master on, the boost pumps on, but the fuel levers off. The warning light for the generator should be illuminated, because, with the engine not running, the generator is not functioning.
The pilot grabs the start switch and engages it. The generator light goes out, and the engine starts turning.evidenced by rising NG and Np. The oil pressure too should be rising, but will probably not reach 40 psi, because more air flow is required to seal the bearings. While the pilot is monitoring NG, Np and oil pressure, she should also sneak a look at the battery, because if output drops below 17 V, there may not be enough juice to start the second engine without recharging. Oh, and don't let go of the starter switch yet.
Once NG has stabilized at a value of 12% or higher, the fuel lever should be moved to on. If NG stabilizes below 12%, then don't turn the fuel on. If external power is being used, it may stabilize as high as 23%. The fuel is turned on by moving a lever with a yellowish-orange knob on it. It's next to the propeller lever. Don't let go of the starter switch.
Activating the starter switch not only energizes the motor that spins the engine, but provides power (from an auxiliary battery, so there's enough juice for both) to energize the glow plugs. A glow plug is a hot electrical coil, sort of like the cigarette lighter that occupies a car 12 V receptacle when you first get the car, before you take it out and plug in something more useful, like your GPS or your cellphone. There are two glow plugs in each engine, at the eight and four o' clock positions, if that's important to anyone. When the fuel enters the engine and contacts the glow plugs, the fuel should start burning. We call that engine lightup or lightoff. Don't let go of the starter switch.
Whoosh! The fuel starts burning. You can't hear the whoosh, so you have to watch the internal turbine temperature shoot up, the oil pressure increase, and the NG continue to rise. If T5 doesn't increase, then the fuel failed to light. If T5 skyrockets or NG stabilizes around 30-40% you have a hung start and the air isn't flowing through the engine properly. In either case shut the fuel lever off, but don't release the starter switch. Let it run for another 10 seconds.
Assuming NG rises as it should, it should stabilize at idle: 48% if the propeller is feathered and 52% if it's not, higher at higher altitudes. T5 should drop back from a peak value of not more then 1090 degrees celsius for no more than two seconds. Then you may release the starter switch. The generator light should turn on again.
If there's enough battery power to start the second engine, the generator should be left offline, the second engine started immedately. Otherwise, the power lever should be advanced to idle (whatever that was) plus 15% and the generator on the running engine may be selected on to charge the battery. It's sufficiently charged when the battery charge current load is 0.4 or less. Both generators must be offline to start the second engine.
After start the power should be advanced to idle plus 15% before the generators are selected on, and not reduced below that until the generator load is 0.5 or less. I'll blither later about things that could go wrong during start, but that's all I have to say for now. Except for the footnotes.
1. The bits about the squirrels may not be completely accurate.
2. In fact, the bits about the squirrels are total lies.
3. This hypothesis is only slightly more likely than the bits about squirrels.