I've learned a few more things about the PT6, and caught a few misconceptions, so I'm going to revisit the fuel nozzles and the problems it could dump on my beautiful PT6 engines. This entry will be a little rambling, and will repeat some things I said already, but I'll get the obligatory squirrel reference out of the way in the first paragraph.
I've already said that fuel enters a PT6A-20 engine through fourteen nozzles. When you push the fuel lever ON, a valve opens and fuel sprays out of all of them into the burner can. If the glow plugs (or igniters) are functioning properly this should ignite the fuel. PT6A-20 engines tend to be pretty hot at start, which shortens the time to overhaul of the engine, so for early models of PT6A-27 they tried dividing the nozzles into two banks of seven. The fuel supply splits, so that the first seven primary nozzles spray fuel as soon as the fuel is selected on, and the next seven secondary nozzles come online after about 38% NG. That reduced the start temperatures significantly, so in later PT6A-27 and all PT6A-34 engines (both approved, but flat rated to 620 SHP for this airframe) there are still fourteen nozles, but they are arranged as a group of four primaries and ten secondaries. Fuel comes from the fuel control unit through a flow divider. Two pressure cracking valves allow fuel to the primary nozzles at about 12% NG and to the secondary nozzles at about 38% NG. If the secondary cracking valve sticks closed, the engine will never increase above idle NG. That appears to suggest that if the primary valve sticks closed, fuel would not enter the burner at all, and that the cracking valve protects against a pilot who turns the fuel on without verifying that NG has stabilized at or above 12%. Perhaps they don't want us to know, lest we get cavalier with the fuel levers. (We're supposed to call them fuel levers, not condition levers, because condition levers are the ones with an OFF FLIGHT IDLE and GROUND IDLE setting, while these are just ON and OFF).
By the way I mentioned that you could tell if an airplane had igniters or glow plugs installed by checking to see if it had igniter switches. What I neglected to mention is that if it has engine igniter switches, then it contains glowplugs. If it has no igniter switches then it has igniters. There's even a reason for that, not just trying to make things difficult. Igniters or glowplugs are used at start and in turbulence or heavy precipitation, but they are used a little differently.
At start, any delay igniting the fuel could lead to a high T5 at start, and a greater power drain on the battery. Although the battery can be charged from the generator between starting one engine and starting the other, it's not recommended because, like charging your laptop every time you use it, it then doesn't hold as much charge as if it is deep cycled. So at start, whether you have glowplugs or igniters, you want to have them all turned on and as hot and ready as the girls in the telephone dating ads on TV. You don't need to think about that, because with the IGNITION (not the IGNITERS) switch set to NORMAL, both igniters or glowplugs are energized when the start switch is held in the LEFT or RIGHT position. (The person who stenciled the labels on the dashboard only had CAPITAL LETTERS).
During flight in turbulence or heavy precipitation the pilot ought to be concerned (while appearing perfectly calm on the exterior) that the engine could flame out. (That means the flame goes out, like a candle being blown out, not that flames shoot out: we refer to the latter as an "engine fire").1 If the engine flames out, we'd very much like it to relight instantly, so we turn on the igniters or one glowplug in each engine during flight in such conditions. We do this by flipping up the cover over the IGNITION switch and moving it to the MANUAL postion. But if we flew along through heavy rain for a long time with both glowplugs energized, they would degrade and not be as hot and sexy next time we wanted to start an engine. That's where the IGNITERS switches come in. They each have three positions: NO.1, BOTH, and NO.2. While using the glowplugs in flight, you select NO.1 for a period of time and then let the number one glowplugs cool down and select NO.2. I don't know what that period of time is.
1. Can someone who is informed, or at least opinionated about such things please advise me on the proper order of those last three punctuation marks. I think there may be national or at least transatlantic differences.