Turbine starting and hot starts

[wingisland]

I hope this is the right thread!

I was wondering if someone could explain to me how you start a turbine engine (eg the ever popular PT-6) and how (indeed IF) it differs from starting a turbo fan such as on 737.

Also I was recently talking to a Twotter pilot who said they leave the right engine running on short stops to help when hot starting the left engine, I have heard difficulties of hot starting a turbine engine but never really understood why? Also, if it is such a problem how come ryanair, easyjet, bmi etc dont seem to suffer this on their short turn arounds, or am I just being ignorant and its actualy a continual pain for them!

[low n' slow]

There are probably more qualified persons than me on this forum to answer your question but I guess in the twotters case, doing a battery start might not produce a reduction of ITT to enable them to have a big enough buffer to max ITT during the start sequence. Having the power from a generator running on the right engine will likely produce more juice for the starter so that it can motor down the ITT and therby produce a bigger buffer to max ITT.

The temperature of the turbine (I'm looking at turboprops now, pure turbojets and turbofans may differ) varies very much with the amount of air that goes through the engine. In the initial part of the start sequence when starting a hot engine, the starer is engaged and this will produce a flow of air through the engine. As this ventilates the hot standning air in the turbine section, the ITT (interstage turbine temperature, name may be type specific to the Saab 340) will start to decrease. Different operators may have different temperature limits due differences in engine lease contracts. In our case, the ITT needs to be motored down to 150 degrees C in under 30 seconds. With a poor GPU or bad batteries this might not happen. In that case the start is aborted and the starter allowed to cool before the next attempt. If the limit is made within 30 seconds, we may select the fuel to on, and then we look att the Ng, Fuel Flow and that ITT is rising with normal speed. The ITT will reach it's highest point well before the Ng has established at idle. As the Ng accelerates further, the ITT drops and stabilizes at idle reading (air flowing through the engine increases through out the startsequence up to idle and this counteracts the initial ITT rise).

Selecting fuel on too early will give the ITT a starting point from a higher value and may result in a hotstart as the initial acceleration of air will not be able to counteract the rise in ITT.

/LnS

[oldbeefer]

Originally Posted by wingisland

who said they leave the right engine running on short stops to help when hot starting the left engine, I have heard difficulties of hot starting a turbine engine but never really understood why?

If the PT6 on our 412 helis is anything to go by, it would be to give generator assist to the battery - most (or at least a lot) of hot starts are a result of low voltage.

[cessna24]

going on what low'n'slow said, a hot start is like said, putting fuel mixture in to early causing an high ITT rise. The engine would be shut down straight away. The applicable engine would then have to a an HSI, HOT SECTION INSPECTION. This invovles splitting the engine and inspecting the compresser blades.
low'n'slow has given the best answer for you question though.

c24

[Rotorhead1026]

Let me simplify, Wings

To start a turbine, you basically spin it up (using a starter) to about 25% of max rpm. Fuel is introduced, ignited, and the engine spins up to idle. A fuel control unit regulates engine speed. This is essentially the same for a turboprop or jet.

Jet starters are usually air-powered, and this air (compressed) usually comes from an on-board APU (a small turbine in itself). The jet is relatively easy to start when everything's working right.

On the Twin Otter there is no APU (I think), and the starter is electric. With no engines running, the battery powers the starter. If the battery is old and weak, you'll get a "poor" start; e. g., the turbine may not accelerate properly and start temperatures may rise to too high a value. In any case repeated battery starts are hard on the battery. With one engine (and its associated generator) running, the load on the battery is much less and there's more current to turn the starter.

A "hot-start" is like the "low battery" scenario above; usually you can see the slow acceleration and rapid temperature rise and cut off fuel before the situation gets serious. It has nothing directly to do with a quick turnaround. "Hot-starting" a turbine - a term you used above - really isn't proper verbiage unless you're referring to said malfunction. It's certainly proper when referring to, say, a fuel-injected piston engine (as in starting shortly after shutting it down).

PP's explanations are good; mine might be a little easier to follow. Read theirs after mine and things will make pretty good sense (I hope).

[Cyclone733]

Starting a turbine engine on the type I operate is easy enough from a control point of view. The engine is selected and the start button pressed. This energises the starter/generator, which starts winding the high pressure compressor and turbine. When Nh is indicated on the engine display, the condition lever is moved up into the start/feather position. This introduces fuel into the engine. This also starts the igniters in the engine.

The starter motor keeps the driving the turbine RPM up to self sustaining speed at which point it will cut out and leave the combustion to drive the engine.

If for any reason the engine doesn't reach self sustaining speed the start is referred to as a 'hung' start and indicated by a low turbine RPM

A 'hot' start on the other hand is where the temperature in the engine rapidly rises. Possibly due to engine damage or unburnt fuel. In the case of unburnt fuel, the start is aborted and the engine cleared of fuel by running the starter motor to pull air through the engine.

Other starting methods include using high pressure air either from the other engine or a ground unit to rotate the turbines.

I've not heard of any real issues with hot starts on short turn arounds on the type I operate, but the Twotter may have issues as I'd imagine it's not a FADEC engine.

I'd guess the main reason for keeping the engine running is to keep the right engines generator online, allowing for a normal start as opposed to battery starting or getting a ground power unit attached to the aircraft. The issue with battery starts is the high load placed on the batteries which on very short trips eg Island hopping may not allow the batteries to fully recharge before the aircraft is shut down which may cause starting problems such as a hung start. Nothing as annoying as being unable to leave a small airfield due to a lack of GPU or spare battery

The larger aircraft get around this through the use of the APU which provides electrical power for the starter motors or possibly bleed air to spin the turbines.

[CJ Driver]

Start Limitations

At the basic level all turbine engines start the same way - you spin them up (using an electric motor, an air source, whatever) until they are going fast enough that there is a stable airflow through the engine in the right direction, then you introduce fuel and ignite it which accelerates the engine to the normal idle state.

The design of the starting cycle is a compromise between the weight and cost of the starter (which may be an APU, a battery and starter motor, whatever) and the ability of the engine to start reliably and at a reasonable temperature. The temperature is the most obvious constraint, because all turbine engines are also effectively air cooled - if you light them up when the RPM is too low, then the innards will overheat.

Onto your specific example - the PT6. The PT6 invariably uses a straighforward electric starter motor to spin it up for starting. In the absence of ground power, starting the first engine is always a battery start. In some aircraft types, starting the second engine can then be helped by power from the first engine alternator. Spinning up a turbine engine for the start takes a lot of energy, and in either case, after the start sequence you will find that (1) the starter motors are seriously hot and (2) the battery will have taken a beating. Most PT6 aircraft I am familiar with (I should say that list does not include the Twotter) have a start limitation, such as three start sequences per hour, or something similar. This phenomenon is not a PT6 feature - the smaller Citations have similar limitations - it is a feature of starting a turbine engine on a battery.

The limitation is therefore - how long was it since I last started the engines? No matter how fast the turn-around, if it comes at the end of a 90 minute flight, it is unlikely to be an issue (EasyJet, Ryanair, etc). Twotter pilots like your friend often find themselves doing very short trips - a 20 minute flight would be quite reasonable. A 20 minute flight to a short turn around could face the pilot with a second engine start sequence less than 30 minutes after the previous one. Hence, tricks to avoid doing the whole sequence are common.

As an aside, at small airfields you often see piston aircraft start up, taxy to the fuel pumps, shut down, refuel, start up, taxy to the parking area, shut down, load passengers, start up, and depart. You'll never see a turbine aircraft do that!

[wingisland]

Thanks everyone, makes an awful lot more sense now, I thought that a hot start was where the engine hadn't been given enough time to cool, rather than a lack of power in the starting system.

Leading on, if your flying an SET, caravan, porter etc, isn't it vitaly important to get the engine running first time, and also to have a suitable trip distance to recharge the batteries in order for the next start? I've got a few friends doing bush flying in those types and they tend to do short trips, or have manufacturers taken this into account?

[Rotorhead1026]

If the battery is new and healthy things are fine (as the long service of there airplanes would indicate). The constant discharge / charge sequence will wear the battery out faster. I've never flown these, but I'll bet they've got a high-capacity battery option.

As long as the battery and fuel control unit are okay, the engine(s) normally start on the first try. It's not something you'd spend sleepless nights worrying about.

[kijangnim]

Greetings,

PPRUNE is also a time machine, because 28 years ago, I was confronted with the same issue, and batteries were not what they are today, so I use to start the PT6 as follows, I used STARTER ONLY, then around 12.5%, ENGINE AUTO-IGNITION ON, then FUEL RUN, worked like a charm

[mwaugh]

Keep in mind that starting a turbine engine regardless of whether the engine is hot or cold is not such a gamble as starting a piston engine. Most of the time a turbine engines fires on up the first time. Hot starts, hung starts etc. are usually reserved for the simulator. With a piston, turning the key is as much as act of faith as it is a mechanical action, not so for a turbine. If you got two failed starts in a row on a turbine the state of the battery would be the least of your problems.

So first time starting is the usual and the "starter/generator" of airplanes like the Caravan is pretty robust. Most of the time the battery was pretty well charged by the time we powered up for taxi and I'm sure takeoff power took care of the rest.

[Gooneyone]

spelling

As far as I remember (and it's been a while), cross-generator or generator assisted starts were not permitted on the Twotter. The #2 engine was left running to keep the gen on line running all the elecrics and keeping the battery fully charged.
What I have seen on the Do228 is the crew spinning the prop after shut down - I was told that this was to vent residual hot gases out the engine and if it was not done, a hot start would result.

[Junkflyer]

The fans are pretty much the same except you have a starter which is turned by air. It may be apu, another engine, or external cart air to spool it up. The starter is cut out about 50% N2 and the engine should spool up to idle on its own after that. A weak air source can make a hot start more likely though spinning it up to max motor (around 22% on the JT-9) before introducing fuel helps in preventing this.

[Dufo]

On L410 with Walter engines, all you have to do is 1) press start button and 2) open fuel shut-off valves. Everything is done automatically after that..

On Saab340, you have two options - motor the turbine to 20%, open the fuel and switch on the ingniton or alternatively swicth on the ignition, open the fuel and press the start button.

[V1... Ooops]

Originally Posted by wingisland

Also I was recently talking to a Twotter pilot who said they leave the right engine running on short stops to help...

I think the key words in the original question are "short stops". If short stops imply short sectors - and 5 to 10 minute sectors are not uncommon in Twin Otter operations - then the concern of the crew might be that the ship's battery has not been fully recharged since the previous start cycle. In this case, the crew would use the generator of the operating engine to assist starting. The AFM for the DHC-6 describes this as a 'generator assisted start' and states that this is accomplished by initially spooling up the engine to be started using the battery alone, then bringing the generator of the operating engine online to further increase the Ng of the engine being started. It is not desirable to have the generator of the operating engine online right from the get-go because the relay that connects the operating generator to its bus is not designed to cope with very high amperage going through it in the wrong direction (wrong direction being from the generator to the bus).

If that is not the case, then the crew are simply using old, out of date operating procedures that were handed down from someone's grandfather. A contemporary battery (lead acid or ni-cad) should be able to accelerate a small PT6 such as the -20, -27 or -34 used on Twin Otters up to between 16 and 18% Ng. This is more than sufficient Ng to provide a start that is comfortably within limitations, even at hot and high aerodromes.

Another possibility - this being a very common error - is that the crew are mistaking the colour markings on the Twin Otter T5 gauge for the temperature limitations applicable for starting conditions. For a normal Series 300 Twin Otter equipped with -27 engines, the top of the green arc is at 695°, this corresponds with the maximum climb and cruise temperature. The top of the yellow arc (equal to the bottom of the red arc) is at 725°, this corresponds with the maximum takeoff and maximum continuous power limitation.

Starting temperature limitations are not marked on the face of the T5 gauge, save for one tick at 1090°. The actual starting temperature limitations for a -27 engine installed in a Series 300 Twin Otter are as follows:

Up to 925° for the entire duration of the start (equal to what would be green)
Up to 980°, as long as temperature declines below 925° within 10 seconds (equal to what would be yellow)
Up to 1090° as long as temperature declines below 980° within 2 seconds (equal to what would be red).

Because these engines almost always have peak starting temperatures below 725 (the painted red arc corresponding with the maximum takeoff and maximum continuous power limitation), it is not uncommon for the crews to mistakenly assume that the coloured arcs also apply to starting conditions. They do not.

[V1... Ooops]

Originally Posted by C J Driver

...Most PT6 aircraft I am familiar with (I should say that list does not include the Twotter) have a start limitation, such as three start sequences per hour, or something similar.

The Twin Otter does have a limitation similar to what you describe, but this would not be a factor because the starter cooling limitation only applies to ground operations. Once the aircraft takes flight, all the timers get reset, so to speak, because the limitation only exists because the generator is very tightly cowled and it is very slow to cool down on the ground at zero airspeed.

Once the aircraft takes flight, the generator is more than sufficiently cooled by ram air. Thus, even if the crew were making five minute sectors with ten minute turnarounds, they would not be restricted by the generator cooling limitation.

The limitation for Twin Otter starter use is:

25 seconds on, one minute off;
25 seconds on, one minute off;
25 seconds on, 30 minutes off.

but, as mentioned earlier, as soon as you take flight, everything gets reset to zero again.

[werbil]

I suspect the reason they keep the engine running in this case is to reduce the number of cycles on the engine - and subsequently the long term maintenance costs. It is my understanding that some of the engine components in the PT6 are life limited by cycles which include both a take off and an engine start. Cut out the start and it reduces the number of cycles.

With the PT6 114A, typical ITT temps we see in our operation are 660oC idle, 780oC peak start, 720oC take off, 680oC cruise - each engine has its own characteristics. After a short shut down, the ITT indicates about 250oC with no air moving through the engine. The temperature differences between idle, take off and cruise (typically 60oC variation) are much less than those involved in starting a hot engine (typically 500oC variation). Whilst the metal has thermal mass and doesn't change temperature instantly, significant thermal stresses occur during start and shutdown that don't otherwise occur. With the PT6, it is very important to shut down from a constant, stable temperature (min 1 minute) otherwise tips of the turbine blades can rub.

Other components, especially the battery and the starter/generator also work hard during every start.

[ernie blackhander]

I think "Hot" start has two entirely different meanings. The first hot start which we all dread is the "um gee im sorry boss but we need a new engine cause i just melted the other one" or a hot start just after shut down, when there is still quite a bit of residual heat still in the engine. The second hot start is not that much to worry about and is quite easy to do but as mentioned it does have a fair ammount of current drain on the batteries due to the extra motoring of the engine to bring it down to an acceptable turbine temp before inducing the fuel to avoid the 1st example of a dreaded hot start, hence why on short turn arounds, you or your company may decide to leave the right eng running so you get that extra kick to help it over the hump. Honestly on a pt6 powered aircraft, with todays batteries i have not yet had a hot start (1st), and even when doing a non assisted hot start (2nd) the difference between the peak itt of the two is usually 20 to 40 deg c. As for the twotter knew of companies that would transpose the engines from left to right at halve life ttis so that the engines did not cycle out due to one engine being constantly started and shut down all the time. As for it being the right engine that is left running, most turbine twins that i know of load from the left with exception of the co pilot. Hot start (2nd) on a garrett is a hole different ball game.

[ernie blackhander]

The applicable engine would then have to a an HSI, HOT SECTION INSPECTION. This invovles splitting the engine and inspecting the compresser blades.

Um, the compressor blades are located in the "cold" section of the engine, you would really be inspecting the compressor turbine blades, the can, ducts, stators and the power turbine

[V1... Ooops]

Originally Posted by werbil

...I suspect the reason they keep the engine running in this case is to reduce the number of cycles on the engine...

That is a good thought, but I believe (I am not 100% certain of this, but reasonably certain) that each takeoff and landing is counted as an engine cycle for the purpose of calculating component life, regardless of whether or not the engine was left running or shut down.

I recall seeing a service letter that P&W Canada circulated about 20 years ago in which they provided a formula for operators to use when they were making frequent short flights without shutting down the engine. Whether this formula affected TBO or just certain cycle-limited components (e.g. turbine wheels) I cannot recall.