Wednesday, July 13, 2005

Starting the PT6

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.


Avimentor said...

Interesting to note the differences between the PT6 you're describing and the one in the Caravan.

We don't have a fuel lever - ours is called a condition lever.

The starter-generator in the 'van doesn't become a generator until the starter switch is put in the OFF position. The downside is that if you accidentally bump that switch into the START position in flight, the generator goes off line. And if you have a one fuel selector turned off to cure a fuel imbalance when the starter is set to START, you'll get a very loud warning horn. D'oh!

The starter switch in the 'van doesn't have to be held in the start position; it stays there until you move it to OFF or MOTOR. No need to repeat the last sentence. ;-)

When the ITT (analogous your T5) temperature skyrockets, we silly 'van drivers call that a "hot start." If that happens, we move the starter switch to MOTOR for at least 30 seconds.

An in the 'van, you really can hear the WHOOSH when the engine lights off. I've heard it it the King Air, too. The whoosh is particularly pronounced if you happened to have bumped the power lever out of the idle position prior to starting. D'oh! ;-)

Aviatrix said...

Not as different as all that. The fuel levers are called condition levers, I just left out that bit of terminology. I use "T5" and "ITT" interchangeably: the gauge is labelled T5, so I picked that one today.

The starter generator doesn't actually come online until selected on, but the generator offline warning light turns off while it's being a starter. I'll have to listen for the whoosh.

dibabear said...

Aw now I'm all confused. I thought a turbo-prop was a normal recip engine with a turbocharger and related plumbing bolted on to the manifold. And that the PT6, being a real turbine, was a prop-jet.

Aviatrix said...

Dibabear: the terminology may vary from country to country, but where I fly turboprop always means a turbine engine turning a propeller. The prefix "turbo" is used to indicate turbocharging of a piston engine, especially if a model is available in normally aspirated and turbocharged options, as in "turbo Seneca." If the aircraft comes in a piston and a turbine model, someone might call the turbine one turbo, e.g. a "turbo Beaver" has a PT6 in the front instead of the radial reciprocating. Except with the Beaver, I think "turbine" is more common than "turbo" for that meaning, e.g. "turbine otter."

In the case of the PA31 , the PA31-300 is noramlly aspirated, the PA31-310 "turbo-Navajo" has turbocharged piston engines, while the PA31T uses PT6s (popular engine, eh?).

I've never heard the term prop-jet anywhere.

Anyone else want to weigh in on turbine/turbo/propjet?

Anonymous said...

When Calm Air was advertising service to compete with Westjet they were calling the SAAB 340 a Jet Prop. They stated that they too had jet engines but their jets where turning a prop. Other then that I don't think it is too common for that term to be used in Canada.

Comparing the Caravan and in the case of Aviatrix the Twotter, I think most start generators are the same. On the van the switch below 46 NG is a starter and after that just kicks off the gen, on other PT6 aircraft the GCU won't let the Generator come on line till after the set NG (around 46-50). It can spin an engine up to 6000 RPM and generate DC power at the same time for obvious reasons.

As for being complicated...if I had to go start a hot piston twin I would have more problems then a PT6. Its simple....starter, oil pressure, NG (12% min), Fuel on (fuel flow), T5 (ITT same deal)...Done. The glow plugs you are talking about are they specific to one type of PT6. Everyone PT6 I have flow they are ignitors.

Anonymous said...

Sorry previous post to read can not spin up an engine to 6000 RPM....

Aviatrix said...

I believe the igniters vs. glow plugs thing is a mod that became standard with later serial numbers. The manual says the best way to tell if your airplane has igniters or glow plugs is to look and see if there is an igniter switch.

The automatic switchover to generator above 46% makes good sense for a single, but as both generators should be offline to start the second engine, it wouldn't be appropriate for the twin. I like the way I'm learning more about this aircraft by hearing about others.

dibabear said... aviation background is strictly south of the 49th parallel even if my backside is parked in Germany. You go where the work is.

A C-130 jockey corrected me when I said that that plane is a turboprop. He said no, it's a propjet. Not sure what that distinction is now.

Anonymous said...

Would you have any tips on starting a TSIO-520 SER when the engine is hot? I just started flying a Cessna 210 and I always hear these crazy stories about vapor lock. I just want to know what to do if I ever run across it.


Anonymous said...

Good write up, but what happend to the high and low volume fuel pumps?

Dean Sibley said...

I've got 4500 hours and no turbine, in two days I have a sim ride with a Fedex subcontractor flying the supercargomaster. I love that I can get this info on the net. If it's got wings I can fly it, but somebody else would might have to start it for me. I've got 1000+hrs 210/206 with the IO 520 as for the last post "When in doubt flood it out." had some great tips on the fuel injected hot start.

Anonymous said...

Anonymous said:
Would you have any tips on starting a TSIO-520 SER when the engine is hot?

Probably the same techniques I use on my BE36 with a IO-520BA.

1. (TCM recommended) Mixture Idle cut-off, Throttle open, Aux fuel pump to high. This will pump cold fuel from the tanks into the plumbing cool it down and return it to the tanks via the overflow. After 45 secs, turn fuel pump off. Then do a normal start.

2. Leave the throttle where you left it at shut down, mixture rich, crank the engine. It will catch briefly, turn on the aux fuel pump, until it catches again, let go of the starter.

3. Do a over-prime start: Mixture full rich, throttle wide open, fuel pump on for about 15secs. Mixture idle cut-off, throttle open. Crank the starter. When she catches, advance throttle and increase mixture ...

I usually try 1. first. If it doesn't work straight away, I go to 3 and it does the trick 99% of the time.

2. works if it hasn't been sitting for too long - ie you've just thrown out the pax and are back in within about 5 minutes...

Keith_J said...

Turbines are no harder to start than a carburetor reciprocating..well, almost. 30 minutes of instruction for HSLD types.

Now, propjet? Well, most have significant exhaust thrust. 8 pounds of air per second at 800 F at near the speed of sound has a bit of shove to it. Lancair used it to differentiate between their TSIO-550 powered LIV and the Walter 601E variants.

And on that last name-dropping statement, I believe the market is large enough for PWC to have a competitor with Walter now that GE has infused those crafty Czechs with capital. Yes, the GE H80 looks interesting, if not for the reverse rotation.

Unknown said...

i m a corporate pilot flies beech 200gt,,my airplanes left engines is always above from right engine in cruise any reason?????please do reply,,thanx

Unknown said...

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