PT6 Starting question
 

So all the helicopters I have flown with the exception of the S61 do not have a separate igniter switch, meaning the starter button did both the starter and the igniters simultaneously. My Nephew is just getting into a turbine with an AirTractor for crop spraying and it has a PT6. This FW has a separate ignitor switch and the procedure in the manual is to activate the starter and at 10-12% activate the ignitor switch just prior to inducing fuel. Why would this be different for basically the same engine as a 212/412/Aw139? Seems like an unnecessary step to me.

Any P&W reps out there that can explain the reason for that?

Maybe the PT-6 equipped choppers have FADEC as well? My S-76B did.

It could be a requirement set by AT, rather than P&WC and if the case, I suspect to reduce the likelihood of a hot start, as it allows the pilot to ensure they have good NG speed prior to lighting it up. I was always a fan of holding back on opening up the fuel tap, until the NG was higher than published minimum data, so as to get a much cooler start. Of course, a good battery and preferably external power makes this a lot easier.

Additionally, the AT has a nose high attitude, so unburnt fuel that does not drain out, is likely to run down the GG case and sit around the exit of the lower diffuser tubes, just waiting for the next start.

Ag machines are pretty rudimentary.

START - dry motoring run

START + IGNITER ON - start

IGNITER CONTINUOUS - self explanatory used in inclement weather conditions

Igniter can be left in ON position after start as it only will operate with starter. Maybe the selection at 12% avoids a hot start from wet combustion section? AG Model PT6’s have high TBO’s and some run on diesel in certain circumstances.

Lots of helicopter PT6 out there without FADEC. 212/412?

AC did you mean “The Big Fella’s” B model?

RVDT: Yup.

The Quest Kodiak has a PT-6 and a separate igniter switch.

(Note: When doing a dry motoring run don't forget to pull the igniter CB or switch off the igniters before hitting the start button - I heard there was a bit of excitement dry motoring an AW139 in the hangar that wasn't so dry somewhere this week...ooops).

So not just the AT, another FW with this separate ignitor switch..... definitely a FW thing I guess

I agree that the 12% could prevent that from happening, but if thats the case then why isn't the same when installed in a 212/412 for example.

AG PT6's only have high (on condition) TBO's in the US and 212's are routinely operated on winter diesel in northern Canada.

I would also guess that it is just an additional safeguard against hot starts due to premature or residual fuel in the combustion chamber. Maybe AT found it prudent to add this step while Helicopter manufacturers didn't. On the AW119 the igniters also fire as soon as the starter engages.

Never flew a turbine helo that had a ignition switch. Never has trouble ever starting a 212 or 412 (non fadec PT-6), no tendency to over temp, but that was all sea level operations.

Fixed wing manuals I have powered by PT-6 all have separate starter and ignition switches, one single engine even has a caption warning if the ignition is on when the engine is running with the advice if not flying in rain then turn it off. Outwest, the start on the Thrush is exactly as you describe for the Air Tractor.

King Air system.

The engine ignition system is a high-energy, capacitance type consisting of a dual-circuit igniter box and two igniter plugs in the combustion chamber. The ignition system is divided into starting ignition and autoignition.

STARTING IGNITION
A three-position lever lock switch for each engine controls this system. The switch is located on the left switch panel. It has three marked positions: ON–OFF–STARTER ONLY. The ON position (UP) is lever locked and it provides for engine cranking and ignition operation. The STARTER ONLY position is a momentary (spring loaded to center hold down) position and it only provides for engine motoring. In this position, the igniters do not function.

AUTOIGNITION
The autoignition system is controlled by a two-position switch for each engine marked ARM and OFF. Turning on an AUTO IGNITION switch arms the igniter circuit to an engine torque switch that is normally open when the engine is developing more than 400 foot-pounds of torque. The system must be armed prior to takeoff and for all phases of flight, and it should be turned off only after landing. If engine torque drops to 400 foot-pounds or less when the autoignition is armed, the ignition system will energize to prevent engine flameout if the loss of power was caused by a momentary fuel or air interruption.

I agree Megan, so again why the difference.....if the FW installation is concerned about fuel in the can when the starter is engaged then why not on a RW installation? As we all know the procedure to start a 212 is to roll the throttle on above about 12% and wait for light off. Many years ago an 212 pilot I was flying with said that was not necessary because the flow divider won't open until 12% anyway. He said I'll show you, well I thought its his a## if it goes hot, but he rolled the throttle to ground idle and hit the starter, sure enough nothing happened until it went past 12% and lit off normally. Now I'm not suggesting to try it, but it sure surprised me.

I appreciate all the comments from everyone but I would really like a P&W rep to explain the difference, any reps out there?

Yup I have seen the PT-6 start with the throttle at idle and, yes, it works because of the flow divider , but why do it? You could do the same thing with the T53 on the 205.
Trying it on a 250 and you are buying a new hot section…you will never be fast enough to close the throttle before the needle pegs. More than one 250 has been “Boat Anchored” due to the pilot not checking the throttle position of carrying out the throttle check incorrectly.
One fellow returned from lunch and found the throttle in the full open position, someone had entered the cockpit and fooled around. Fortunately we had a policy of turning the fuel valve OFF, pulling the ignitor CB and then.disconnecting the battery before leaving the aircraft unattended.

Some folks get confused between a cooling run to lower residual temp before a start with ignitors on and a drying run to blow residual fuel out of the engine if the darned thing didn’t light off on the first attempt. ( Happened from time to time with a cold soaked engine and Jet-A ) in which case the ignition CB would be pulled to avoid loud noises startling the engineer. I miss good old Jet-B.

I have no knowledge as to why but if all/most fixed wings have seperate ignition circuits perhaps it hearkens back to removing electrical power from a piston engine propellor free wheeling in the slipstream or turbines with fixed power sections? Do turbine aeroplanes have freewheeling units?

I’m certainly glad they don’t have that system on the helicopter PT6. Opening and closing the throttle then modulating and using the starter and also activating the idle detent is already a bit of a dance of the hands.

It is an interesting question.

Yes, exactly why I'm asking this. My nephew tells me of a couple ways different pilots have told him on which finger/thumb combination to use to accomplish this dance. I told him I would not do this digital gymnastic and risk my finger slipping off the starter part way thru the start after light-off.

Really? I never found it a big deal. Why are you activating the idle stop? We only opened the throttle against the cam and not past it.

Hah, yeah. The other thing with the Air Tractor setup is that it requires right hand for the panel mounted start and igniter switches, sort of braced so that the finger on the spring loaded starter doesn't accidentally come off it, but so that you're able to also actuate the igniter switch once past about 12% Ng. The left hand is required for the condition lever. So.... In the unlikely event that an inflight relight using starter was to be required, and there was enough altitude to accomplish it, the stick must be held with one's thighs....

Realistically, there are very few cases where an inflight relight with starter should be required in that airplane, and fewer where there will be enough altitude for it. Most likely need for it would be fuel mismanagement on a ferry flight using fuel in the hopper, I think. On the ground though, with both hands available it's quite easy.

Yes, so why not just turn the ignitor switch on before hand and concentrate on keeping the spring loaded starter switch engaged for the start. Sure seems like a recipe for a F#$Kup if you ask me.....

As I recall, the igniter switch requirement/lack of is based on airframe interface and engine type (A,B,C,E,T, etc) as I believe a AW119 has a separate igniter switch for its PT6B. Keep in mind, while the core engine is all the same on the PT6, the accessories/systems are very different in how things are controlled especially between a fixed-wing and rotor-wing application. As for the 10% delay in actuating the switch that would be an Air Tractor process. Perhaps shoot them a mail as they have a good support team.

I do believe you are correct that this is an AT procedure as opposed to a P&W one. I'll see if AT will have an explanation.

No........