Originally Posted by
sgs233a
So, I looked in the PWC PT6-34AG Maintenance Manual, and this is what is has for engine start:
PWC PT6-34AG MM Jan 18, 2021
PWC PT6-34AG MM Jan 18, 2021
This is from the Air Tractor Maintenance Manual:
Air Tractor AT-502A/502B/504 Maintenance Manual - Mar 14, 2019
So, interestingly, the PWC MM mentions 12% as the minimum Ng for a successful light....and that number is referenced in the AT pilot training course and official checklists as the value for when to turn on the ignitors. My guess is that they figure it's best not to introduce spark until past the bare minimum Ng that won't torch the engine, in the event that some fuel is present? It's also interesting that the AT MM lists 15% as the value to turn the igniters on.
I'd be curious to see the "official" answer from Air Tractor.
That might be on the money in regards to residual fuel and having no ignition source until at a N1 value that can accept ignition. Given Helicopter PT6 requires dry run if failing to light off after introducing fuel. Perhaps the angle the engine sits at on the AT makes this more susceptible.