Turbine starting, Burn or Bang
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Turbine starting, Burn or Bang
Question.
I am not sure if this is the same for all turbo prop aircraft, but when starting the engine, usually when hot one of many things happen. My question is only about two of them.
Which is more detrimental to the engine and why.
I know how to get each type.
Does this happen with all turbines or just some.
By the way I have experienced this mainly with the PT6
Thanks in advance
Scenario 1:
Engine is warm, hit the starter, engine winds up, high NG,dump in the fuel, big bang when the fire lights and the max ITT is low, well within limits.
Scenari 2:
Engine is warm, hit the starter, engine winds up, low NG, dump in fuel, no bang, smooth spool up with higher max ITT, but still within max start limits.
I am not sure if this is the same for all turbo prop aircraft, but when starting the engine, usually when hot one of many things happen. My question is only about two of them.
Which is more detrimental to the engine and why.
I know how to get each type.
Does this happen with all turbines or just some.
By the way I have experienced this mainly with the PT6
Thanks in advance
Scenario 1:
Engine is warm, hit the starter, engine winds up, high NG,dump in the fuel, big bang when the fire lights and the max ITT is low, well within limits.
Scenari 2:
Engine is warm, hit the starter, engine winds up, low NG, dump in fuel, no bang, smooth spool up with higher max ITT, but still within max start limits.
I would have thought that neither event should be more damaging to the engine (especially the HP turbine) than the other, given that ITTs are staying under limits.
Logically, one would imagine the smooth start (without bang) is a little gentler, but since ITT is a critical parameter and spool-up is taking place normally (resulting in cooling gas-flow through blades etc, as designed) then either start should be pretty much the same in terms of engines making it to claimed TBO. Provided the hot-end isn't thermally over-stressed I don't think there would be much in it.
BIK_116.80's (tongue-in-cheek) method will produce spectacular results visually (I have such a start on a little Allison on videotape...the flames are about three feet long!) but I wouldn't be surprised if the engine afterwards sounded like a Hoover vacuum cleaner - and developed about as much power. There is a lot to be said for a start-sequencer on just about all gas turbines but doubtless there are also good reasons why they are not fitted.
I might ask my tame engine chappie about this...he wrenches for QF; he ought to know.
Logically, one would imagine the smooth start (without bang) is a little gentler, but since ITT is a critical parameter and spool-up is taking place normally (resulting in cooling gas-flow through blades etc, as designed) then either start should be pretty much the same in terms of engines making it to claimed TBO. Provided the hot-end isn't thermally over-stressed I don't think there would be much in it.
BIK_116.80's (tongue-in-cheek) method will produce spectacular results visually (I have such a start on a little Allison on videotape...the flames are about three feet long!) but I wouldn't be surprised if the engine afterwards sounded like a Hoover vacuum cleaner - and developed about as much power. There is a lot to be said for a start-sequencer on just about all gas turbines but doubtless there are also good reasons why they are not fitted.
I might ask my tame engine chappie about this...he wrenches for QF; he ought to know.
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Turbines unlike recips operate in an abundance not lack of air.
ITT = level, not amount of heat usually an average of what has already happened.
FF = amount of heat energy available.
Ng = relative stability and energy/velocity of gas path.
low Ng with a given FF = relatively unstable gas path, potential for "hotter spots" not revealed by hotter "indicated = average" ITT and relatively slower spool up through critical gas path and resonant nodes.
hi Ngwith a given FF = relatively more stable gas path, lower potential for "hot spots" with coller "indicated = average" ITT and relatively faster spool up through critical gas path and resonant nodes.
I think I got that right??
Good quick cool starts reduce the thermal stress which is where the wear occurs.
Cant remember the number exactly but something like 80-90% of the wear and tear in a gas turbine engine occurs during start and low Ng ops.
There are industrial gas turbines that run over 100,000 hrs within spec as they are run continuously and under load.
ITT = level, not amount of heat usually an average of what has already happened.
FF = amount of heat energy available.
Ng = relative stability and energy/velocity of gas path.
low Ng with a given FF = relatively unstable gas path, potential for "hotter spots" not revealed by hotter "indicated = average" ITT and relatively slower spool up through critical gas path and resonant nodes.
hi Ngwith a given FF = relatively more stable gas path, lower potential for "hot spots" with coller "indicated = average" ITT and relatively faster spool up through critical gas path and resonant nodes.
I think I got that right??
Good quick cool starts reduce the thermal stress which is where the wear occurs.
Cant remember the number exactly but something like 80-90% of the wear and tear in a gas turbine engine occurs during start and low Ng ops.
There are industrial gas turbines that run over 100,000 hrs within spec as they are run continuously and under load.
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Turbotter.
Rule of thumb-Get as much air,N2,as possible going through the engine before introducing fuel. The booming lightoff is typical of hot BGT engines,it's only a noise. The cooler the start and operating temperatures the longer the turbine will last.
Ask someone in the know how much a starter motor costs compared to a turbine assembly, you'll get a shock.
Starter motors on large BGT engines now have a starter time limitation of about 15 minutes, thats a big wind up.
Rule of thumb-Get as much air,N2,as possible going through the engine before introducing fuel. The booming lightoff is typical of hot BGT engines,it's only a noise. The cooler the start and operating temperatures the longer the turbine will last.
Ask someone in the know how much a starter motor costs compared to a turbine assembly, you'll get a shock.
Starter motors on large BGT engines now have a starter time limitation of about 15 minutes, thats a big wind up.
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mainwheel
Gets to the nub of it better.
Air, air and more air is the secret.
Fuel works in the opposite sense to a recip.
In a turbine more = hotter as there is an overabundance of air as distint from a recip.
is also good practise on smaller ones if you have a cart or really good batteries when it is hot or after a quick turnaround, just motor the engine for long enough (within its duty time limits) to get the temps way down and stabilise the gas path as much as you can before you turn on the fuel.
PT6 types are easier than the Garrett in this because they are not driving the prop, but the Garrett responds the same way and the starter/gen is designed for and totally up to it assuming that you have been servicing it properly.
On batteries and carts
The last time I spoke to a factory guy, their experience was that more "hotties" result from dodgy batteries dying and carts dying, running out of fuel or the connection dropping out during start than for just about any other reason.
I can't stress enought the importance of "battery hygeine", a battery dying at a critical point in a start can be very expensive.
Battery service is AS important just about anything else.
I am a bit of a fan of "Lead Acid" as being a lot more reliable and I think have a higher discharge rate, they certainly seem to give a harder faster start and quicker recovery and therefore start on the othery.
Any Battery gurus out there who can help me here.??
The cart should get at least the same level of maintenance attention as the aircraft, after all, it becomes part of the aircraft during the start.
The same applies to "that connector plug" and its "receptacle". Most aircraft have the "under" type and if it used routinely on a fleet will become worn and looser with age.
"That connector plug" and its associated cable are pretty heavy due to the high amperage required, so if they are not tight or have a "positive" locking method, guess what.
The beginning of the start process is when the highest demand and necessity for a continuous supply is required, it is also the time of the most vibration, which usually also occurs in the RPM range where the manufacturer has placed the highest "resonance" period and the period when the gas path is still not fully stable and at its hottest and when it has not reached the self sustaining stage and ability to accelerate. .
It is the period when the condition of the cart and the connector are the most critical.
If it is lose or is not securely or carelessly placed, thats exactly when it will fall out, or when it will fail under load, or run out of fuel
In this circumstance the fuel = heat must be removed from the engine IMMEDIATELY either electronically or manually or both, lest it hang and burn itself to death.
Some types have circuit systems that make the ships batteries available to power the "stop" buton, fuel cut off or whatever, others may not.
If it isn't already covered in your SOPs, talk to your CP and Engineers about this scenario.
I would always "guard" an "unlatched" manual fuel lever cut off during this part of the start as the final defence against it all turning to poop.
You may even decide to include a serviceability, fuel and connector check on the cart you use as part of your regular preflight, especially if using an "unkown" cart at an "foreign" outbase or transit FBO somewhere.
Which brings us back to how well the ships batteries are serviced.
As in recips where the cheapest thing you can put through it is fuel and oil, in turboprops it is the state of starter/gen, the batteries and the cart if you use them.
Subtle difference is recip maybe $50,000 , turbine maybe $500,000 for starters.
Makes a $25000 starter/gen or start cart or spare set of batteries to keep you going whilst the other is being serviced look sorta cheap doesn't it.
Gets to the nub of it better.
Air, air and more air is the secret.
Fuel works in the opposite sense to a recip.
In a turbine more = hotter as there is an overabundance of air as distint from a recip.
Starter motors on large BGT engines now have a starter time limitation of about 15 minutes, thats a big wind up.
PT6 types are easier than the Garrett in this because they are not driving the prop, but the Garrett responds the same way and the starter/gen is designed for and totally up to it assuming that you have been servicing it properly.
On batteries and carts
The last time I spoke to a factory guy, their experience was that more "hotties" result from dodgy batteries dying and carts dying, running out of fuel or the connection dropping out during start than for just about any other reason.
I can't stress enought the importance of "battery hygeine", a battery dying at a critical point in a start can be very expensive.
Battery service is AS important just about anything else.
I am a bit of a fan of "Lead Acid" as being a lot more reliable and I think have a higher discharge rate, they certainly seem to give a harder faster start and quicker recovery and therefore start on the othery.
Any Battery gurus out there who can help me here.??
The cart should get at least the same level of maintenance attention as the aircraft, after all, it becomes part of the aircraft during the start.
The same applies to "that connector plug" and its "receptacle". Most aircraft have the "under" type and if it used routinely on a fleet will become worn and looser with age.
"That connector plug" and its associated cable are pretty heavy due to the high amperage required, so if they are not tight or have a "positive" locking method, guess what.
The beginning of the start process is when the highest demand and necessity for a continuous supply is required, it is also the time of the most vibration, which usually also occurs in the RPM range where the manufacturer has placed the highest "resonance" period and the period when the gas path is still not fully stable and at its hottest and when it has not reached the self sustaining stage and ability to accelerate. .
It is the period when the condition of the cart and the connector are the most critical.
If it is lose or is not securely or carelessly placed, thats exactly when it will fall out, or when it will fail under load, or run out of fuel
In this circumstance the fuel = heat must be removed from the engine IMMEDIATELY either electronically or manually or both, lest it hang and burn itself to death.
Some types have circuit systems that make the ships batteries available to power the "stop" buton, fuel cut off or whatever, others may not.
If it isn't already covered in your SOPs, talk to your CP and Engineers about this scenario.
I would always "guard" an "unlatched" manual fuel lever cut off during this part of the start as the final defence against it all turning to poop.
You may even decide to include a serviceability, fuel and connector check on the cart you use as part of your regular preflight, especially if using an "unkown" cart at an "foreign" outbase or transit FBO somewhere.
Which brings us back to how well the ships batteries are serviced.
As in recips where the cheapest thing you can put through it is fuel and oil, in turboprops it is the state of starter/gen, the batteries and the cart if you use them.
Subtle difference is recip maybe $50,000 , turbine maybe $500,000 for starters.
Makes a $25000 starter/gen or start cart or spare set of batteries to keep you going whilst the other is being serviced look sorta cheap doesn't it.
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Lanky, you seem to have got me wrong a bit.
A normal engine start using a turbine type starter has a time limit.
If you want N2,AIR, then wait for the starter system to give you that.In my example, you can wait a long time.
Keep it cool!
A normal engine start using a turbine type starter has a time limit.
If you want N2,AIR, then wait for the starter system to give you that.In my example, you can wait a long time.
Keep it cool!
Props are for boats!
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The key is CORE TEMPS. With PT6 Parking into wind and strong cranking rpm NG will give a cooler start temp allowing the core T5 TO DROP BEFORE ADDING FUEL. Make sure you do this otherwise both scenarios you mention will be expensive.
Batteries carts, and GPUS are a must and must be maitained, somethimes you get the odd GPU THAT CANT HANDLE THE LOAD IE. Drops off as your cranking. And voltage was ok before start.
When you say you have a low NG on scenario 2 You talking -20S OR BETTER -27S. bIG dIFFERENCE. Ie the -20s have single stage burners, so if you do what you with them, youll most allways get melt down ie 1090 FOR over 2 secs and thats an instant HSI, AT THE LEAST.
Where -27S you may get a way with it because of the staggered light off, ie, 2 stage nozzles instead of one llike the -20 engine.
So really this topic is type specific. Got no idea on Garrets
Batteries carts, and GPUS are a must and must be maitained, somethimes you get the odd GPU THAT CANT HANDLE THE LOAD IE. Drops off as your cranking. And voltage was ok before start.
When you say you have a low NG on scenario 2 You talking -20S OR BETTER -27S. bIG dIFFERENCE. Ie the -20s have single stage burners, so if you do what you with them, youll most allways get melt down ie 1090 FOR over 2 secs and thats an instant HSI, AT THE LEAST.
Where -27S you may get a way with it because of the staggered light off, ie, 2 stage nozzles instead of one llike the -20 engine.
So really this topic is type specific. Got no idea on Garrets
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Here's a few thoughts on engine operations:
1. The higher the temperature, the higher the damage. And the effect is exponential, a small increase in temp is a very large increase in damage.
2. On starting, the combustion chamber flame pattern is unstable, therefore hot spots are common - which is why starting limits are lower than running limits.
3. The temperature indicating system is often slugged (i.e. an average value over the last few seconds is shoen, rather than instantaneous peaks) and always averaged over several sensors around the turbine. So, small rapid changes may not be seen in the cockpit, and neither will hotspots.
4. For engine longevity, quick start cycles combined with low peak temps are in order. However, it may be better for your engine to have a slightly higher temp, but for a much shorter time. Unfortunately, t's impossible to know what is best for your engine without going to the engine manufacturor's product support department - have you tried that yet?
A
1. The higher the temperature, the higher the damage. And the effect is exponential, a small increase in temp is a very large increase in damage.
2. On starting, the combustion chamber flame pattern is unstable, therefore hot spots are common - which is why starting limits are lower than running limits.
3. The temperature indicating system is often slugged (i.e. an average value over the last few seconds is shoen, rather than instantaneous peaks) and always averaged over several sensors around the turbine. So, small rapid changes may not be seen in the cockpit, and neither will hotspots.
4. For engine longevity, quick start cycles combined with low peak temps are in order. However, it may be better for your engine to have a slightly higher temp, but for a much shorter time. Unfortunately, t's impossible to know what is best for your engine without going to the engine manufacturor's product support department - have you tried that yet?
A
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Starting limits lower than running limits ?
In the PT6-34 that I use,
the starting limits are :
- 1090 for 2 seconds
- 925 for 10 seconds
And the running limits are
- 790 normal max.
- 850 "accelerating" (2 second limit)
Here, the starting limits are much higher than the running limits.
the starting limits are :
- 1090 for 2 seconds
- 925 for 10 seconds
And the running limits are
- 790 normal max.
- 850 "accelerating" (2 second limit)
Here, the starting limits are much higher than the running limits.
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With you on that one Bagot
From memory ditto for a Garrett. Max start temp 770C but max for normal ops 650C. Don't confess to being an authority though as only flew with the Garrett in props before moving onto different type. Very little PT-6 experience. Good subject all the same.
Onya
Onya
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This is getting interesting, so I'll qualify my above post.
I was talking generic engine and the comments still hold. What you may find is that the above engines will not start with the lower TgT limit, so they have to certify 'em to start at the higher. A good hint to the damage caused is the v-short time limit on the high temps.
Notwithstanding, the higher the temp, the higher the damage, and the unit increase in damage for a unit increase in temp is exponential.
A
I was talking generic engine and the comments still hold. What you may find is that the above engines will not start with the lower TgT limit, so they have to certify 'em to start at the higher. A good hint to the damage caused is the v-short time limit on the high temps.
Notwithstanding, the higher the temp, the higher the damage, and the unit increase in damage for a unit increase in temp is exponential.
A
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Will not start with a lower limit ?
From my experience, the PT6 will start well below its starting limit.
650 is a good average - well below the normal running limit / well below the take-off limit (790).
Very rarely does it approach its running limit during start - as long as you got a good battery / external power.
650 is a good average - well below the normal running limit / well below the take-off limit (790).
Very rarely does it approach its running limit during start - as long as you got a good battery / external power.
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For PW 120, 123's
Providing it's below start ITT limitations, it does not matter HOW the start went, and is not reportable to Engineering.
KISS principle, folks
KISS principle, folks
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Agree with onya and BCL
PT6-20 Start limit 1090C (2 secs).
Normal Start up to 600-650C, Max Take off 750C, Max Climb 725C
Max Cruise 705C
Garrett TPE 331-2 Start Limit 815C (1 sec)
Normal Start 500-600C
Max Take off Depending on OAT around 582C
Normal Cruise 540C again depending on OAT
PT6 require GG to stabilise for 5 secs before throwing in the fuel min required is 12% but Normally with good ground cart you get around 20% on good aircraft batteries around 18%, more is good!!
PT6-20 Start limit 1090C (2 secs).
Normal Start up to 600-650C, Max Take off 750C, Max Climb 725C
Max Cruise 705C
Garrett TPE 331-2 Start Limit 815C (1 sec)
Normal Start 500-600C
Max Take off Depending on OAT around 582C
Normal Cruise 540C again depending on OAT
PT6 require GG to stabilise for 5 secs before throwing in the fuel min required is 12% but Normally with good ground cart you get around 20% on good aircraft batteries around 18%, more is good!!
