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........

So if you are unable to unable to feather the prop then all the engine components keep spinning? I could see why you would want to remove an ignition source for a piston engine then. Perhaps it’s an issue for a fixed turbine engine too or a regulatory throw back.
I’m just theorising here, hopefully someone will have a solid answer.

Company procedure on the idle stop.

The start isn’t a big deal especially once used to it. Rather my point is the more you add to the process the more likely hood of something going wrong. I think it’s good we do not also have to engage the igniters although I’m sure it could be managed.

No. It's a free turbine. The only items driven by the Power Turbine on most PT6 fixed wing are the prop gov, PT ovspd gov, and PT tach/gen. Everything else is driven by the Compressor Turbine. My guess the reason for the AT igniter delay is more airframe electrical system related than anything else.

It's worth noting as well that the Air Tractors (at least the 502/504 that I'm familiar with), are VERY simple airplanes, with quite basic systems...and this seems to be baked into the design philosophy. Might be as simple as separate starter and igniter switches were seen as the simplest way to implement the start.

The igniter switch is a 3 position switch - start/on - off - on igniter continuous. In start/on, the igniters only come on with starter switch held on. In on igniter continuous, they're on continuous - there is no controller or other logic in the system as in many other airplanes.

I understand it is a free turbine. My question related to fixed turbines/pistons and if seperate ignition was a carryover from those designs.
I suspect your point about the airframe electrical system is correct though.

I can understand the logic of having a separate ignitor switch for use in water washes, etc. In a helicopter we have to pull the CB for these or other reasons when we don't want ignitor activation with the starter. What I don't get is the need to turn that switch on at 12%, why not select it to Start/on, hit the starter and go from there just as we do with a RW installation.

So, I looked in the PWC PT6-34AG Maintenance Manual, and this is what is has for engine start:
https://cimg3.ibsrv.net/gimg/pprune....8f6ae65c9a.png
PWC PT6-34AG MM Jan 18, 2021
https://cimg4.ibsrv.net/gimg/pprune....91e51528df.png
PWC PT6-34AG MM Jan 18, 2021



This is from the Air Tractor Maintenance Manual:

https://cimg5.ibsrv.net/gimg/pprune....52c023bc47.png
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.

It’s been a while, but I think that procedure brings risks. With the idle stop open you don’t know if you opened the throttle passed it. If you have and get a hot start, Murthy’s law dictates that the stop will close as you try and close the throttle, and attempts to release the stop will fail because you’re jamming it still trying to close the throttle! If you learn to feel the pressure of the cam, you will always know you have not passed it and can immediately snap the throttle closed. I’ve had hot starts and you need to be quick! Some weren’t ‘real’ though and were due to faulty ITT compensators.

I have emailed AT to see if they have an answer. As I said earlier it was demonstrated to me once that a PT6 will not light until 12% as thats when the flow divider opens. I see their thought process in not turning the ignitors on until then in case there would be fuel in the can, but again if that is such a big concern with a PT6 then why is that not how it works in a RW installation. I really hate to say it, but I think that when AT switched from piston to turbines the test pilot was new to turbines and just thought this was a good idea with no real justification.
Just re-read the posting by sgs233a and see that for some unknown reason the P&W MM says to turn the ignitors on with a dash 34. Strange that in a 212 with a 3 or 3B engine they don't do this. Is there a difference in FCU? Flow divider? I would think other than power output and installation an old school PT6 is a PT6.

As you know when ever there is a new Tech pilot/chief pilot, etc he or she has to introduce their own procedures which are not always based on fact or good practice. The company I was with also had this same idea that when the starter is engaged there might be so much draw on the batteries that there would not be enough left to actuate the idle stop release. So they said roll past idle stop, back to the stop, hit the release and roll back just slightly over the cam so if it goes hot you could just close the throttle. Not saying a good idea or bad, just that if you wanted to comply with SOP's sometimes you had to do stuff that you may or may not have agreed with.

The specifics of the Ignition system for single-engine aircraft are very similar for either fixed or rotary-wing. When you have only one engine, there are operational circumstances that might increase the risk of engine failure or flame-out, and there are manoeuvering phases of flight that may have potentially disastrous outcomes that may be mitigated by the use of continuous ignition to relight an inadvertently flamed out engine. I have operated Caravans, MD500D's and B206's that all had continuous ignition options, and all specifically because they only had one engine.

In the normal modes, the ignition system is energized and operates in conjunction with the engine start switch, de-energizing when the start switch is disengaged. There is an additional selection that enables energizing the ignition system to run continuously unrelated to any other engine control selection. Typically, this selection would be made during take-off and landing, air starting, operation in blowing snow, heavy rain, icing, turbulence and low fuel conditions.

Without overthinking this, it is to allow single-engine aircraft a resource that will automatically relight the engine in the most likely scenarios that this could occur, or for emergencies when single-pilot workloads can be reduced by having this system already selected on.

If you only have one motor, you need to preserve it at all costs.

Outwest,

I do see your frustration of different procedures but in my lifetime, I've heard of nearly 10 x FCU/pilot related issues in PT6T helicopters (FCU to Manual Mode usually) and only one hot start in a PT6A (related to a Fast Start system on a 802 and an inattentive pilot). Bottom line is the fire isn't going to start unless the ignitors are on. This system works just fine and encourages a larger safety margin for your average fixed wing spray pilot. If there was a Manual Mode on PT6A's, Air Tractor would never encourage pilots to move a switch just a few inches from the Manual Mode switch (as Bell does).

For mine, the PT6A is far more likely to be hot started than suffer a flameout requiring an air start. Air Tractor has employed a procedure that gives the largest margin for error for the most likely scenario.

Best of luck getting an answer from Olney!

Doubtful. Direct drive turbines didn't necessarily have igniter switches and comparisons to piston are moot as they need spark to run vs a turbine once lit. As I mentioned I believe the AW119 has igniter switches as well and follows the single engine app route theory.

The company SOP is to roll throttle open at 12%n1 close to the detent activate idle stop and roll throttle just behind the gate. Modulation normally not required unless the engine is hot. I understand this practise is widespread. The company I work for would be in the 6 digits of hours operating 412s so have a bit of an idea on how to do so.
It could of course be argued that it’s not the flight manual procedure exactly but if I get a hot start following SOPs I’ll still be employed.
Incidentally I like this process as it gives an exact throttle position every time including on different airframes and you are always behind the gate closing not opening the throttle. I will say however it is a little awkward until you are used to it.

PT6s are definitely not all created equally. Just look at the differences between a PT6A and PT6C. While the FCUs/GOVs are very different between aircraft types most of the basic core parts are the same. For example, I believe the start flow dividers are similar but rigged differently. But another thing to keep in mind on the start side is an airplane has a very light prop to spool up vs the xsmn/MR of say a 212. It's still the most bulletproof engine type out there in my humble opinion.

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.

Not going to suggest not following SOPs - just giving an explanation why they might not be ideal. The company I worked for had a bit of 212 experience too.

No misunderstanding there and thanks for your explanation on the procedure you use.
I was just giving you a explanation on why I use a process that is not exactly as per RFM. My experience is on the 412 so maybe there are differences between 212/412 that make that start less viable.
lots of ways to skin a cat…. Discussion is always good.

well I can tell you from experience that the difference in spinning a prop or a rotor in regards the start has nothing to do with it. Once while troubleshooting a vibration in a 212, maintenance disconnected the drive shaft and had me start the engines. I was very concerned that with no drag on the engine it would be some sort of crazy start and possibly over speed. Well to my surprise the start was exactly the same as with the transmission connected. Uneventful.

The FCU acts as the Ng Governor, by venting Py air when Ng is higher than required.