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Turbine starting and hot starts

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Turbine starting and hot starts

Old 22nd Feb 2009, 20:39
  #21 (permalink)  
Join Date: Feb 2005
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Originally Posted by ernie blackhander
...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...

I think you may not be allowing for the fact that the temperature limits during starting are far, far higher than the temperature limits during normal operations... in other words, you might be making the very common error of assuming that if the T5 indications rise up into the amber or red markings during start, that implies a 'hot start'.

Consider a worst-case situation: You land, part with about a 10 knot tailwind (which would tend to keep residual heat in the engine), then 60 seconds after shutdown, you have to start the engine again.

T5 at the beginning of the cranking cycle will probably be around 500. After about 8 seconds of cranking (which is necessary to bring the Ng from zero up to a stabilized speed at which you can introduce fuel), indicated T5 will probably have dropped to about 300. That is approximately 250 higher than a "cold" engine.

If the engine normally peaks at (for example) 650 when it is started from cold, the peak during this starting cycle will be no more than 250 higher (viz, 900), and that peak is well within the allowable starting temperature limits. Certainly, it is a far higher peak than is normally seen, but considering the circumstances, there is an easily understood explanation for that peak, and reaching a temperature of 900 during that particular start cycle would not be considered anything to be concerned about... you could forecast that the peak starting temperature would be significantly higher than the usual before you even put your hand on the start switch.

If you reached a 900 peak during a normal start of a cold soaked engine, certainly that would be something you would want to investigate for cause, even though the peak was (again) well within starting limits. But, if you know ahead of time that the start is going to be warmer than normal, either due to retained heat in the engine, or a poor state of battery charge - that's when you need to be familiar with exactly what the starting temperature limits are, in order to avoid a "Chicken Little - the sky is falling!" type of response to the higher than usual T5 indications.
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Old 22nd Feb 2009, 21:37
  #22 (permalink)  
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V1 sorry must not have made it clear about the temperatures. when i was talking about bringing the engine down to an acceptable temp the procedure i have been taught is below 200 deg c for an engine hot start. i know operational redline is 750 deg c and start limit is 1090 or 1100 because if you can differentiate 10 deg on that guage your a better man than me. company procedure at the moment is to pull it at 850 deg c on start for reasons that they have devised in all their wisdom. I was just trying to point out that im sure there are two deffinitions of a hot start
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Old 24th Feb 2009, 10:18
  #23 (permalink)  
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The advice our CP received from P&W when doing the trend course was that even though 1000o starts can be well under the limit they do reduce the 'real' life of a number of components significantly.

Indicated T5 is air temperature not metal temperature. Whilst motoring the engine following a short shutdown will reduce the indicated T5 and gas temperature, the actual metal will not cool anywhere near as much during a 'short' motoring.

How valuable extended motoring is for cooling the engine to a 'nominal' T5 figure before introducing fuel to reduce start temps is a bit debatable IMHO - if a battery is already down the additional drain incurred may well result in slower acceleration during the later parts of the start and a hotter start than if the fuel was introduced earlier.

Our company policy is to pull the fuel if ITT reaches 600o before 25% Ng - if it is that high that early in the start there has to be a reason - and it is cheaper to pull the fuel earlier rather than too late.
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Old 25th Feb 2009, 11:39
  #24 (permalink)  
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Hello Werbil:

Although I don't doubt the sincerity and good intentions of your chief pilot, I have a lot of concern about how much credibility gets attached to "My grandpappy told me that..." and "My chief pilot heard it from an instructor who heard it from someone else..." type of anecdotal comments.

I'm a pretty simple guy, I only read the black ink in the manuals. Pratt & Whitney has made over 50,000 PT6A series engines now, and if they thought that the starting temperature limits needed to be lowered, they would have lowered them. They have not done so, and the starting temperature graph is pretty simple to comprehend - for a -27, temperatures as high as 925 are acceptable without time limitation during a start.

As I said before, if an engine normally peaks at 600 when starting, and suddenly, without apparant justification, the T5 takes off heading well north of 600, sure, it would make sense to abandon the start attempt. My point, though, is that if we know before we even engage the starter that the start will be warmer than normal - either due to retained heat in the engine, low starting voltage, or some other abnormality - then we should have sufficient comprehension and understanding of the published engine limitations to know that a higher than normal start temperature is not, a priori, a problem.

I guess what gets me going on this soapbox is that I have seen so many examples of accidents or near-accidents caused by pilots who do not comprehend engine limits and instead rely on hand-me-down folklore. Consider a Twin Otter, for example: It has a 680 HP engine that has been flat rated to 620 HP for airframe reasons (not related to the engine). This means that a 50 PSI torque takeoff - that being the redline on the engine - is only demanding 91% of the power that the engine is designed to produce. But, how many times have you seen someone take off (at ISA) using only 45 PSI torque, because they think that doing that will somehow "be nice" to the engine, or lengthen the TBO or the time between hot sections? Truth is, it has been well demonstrated that such actions do not increase TBO or time between hot sections, but they do greatly increase the liklihood of accidents, due to longer ground runs or less than adequate single engine climb performance.

As professionals, we need to be critical of what information we give credence to. Like I said, I only read the black ink in the manufacturer approved publications.
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Old 25th Feb 2009, 23:50
  #25 (permalink)  
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I will talk in terms of the 114A in the C208 as that is what I fly.

Truth is, it has been well demonstrated that such actions do not increase TBO or time between hot sections
TBO and HSI inspection schedules are to designed by the manufacturer to ensure the engine is serviceable. What is found at these inspections is determined to a large extent by how the engine is operated - ie the cost of the overhaul is determined by what components require replacement - some of these will be time / cycle lifed, others will be replaced on inspection. I believe P&W set a TBO of 3,600 hours. In Australia this is extended to 5,000 hours provided additional requirements specified by CASA a met. In the US the FAA only permits operators to extend the TBO based on their company's history with the engine - I believe some operators now have TBOs up around 7000 hours - they all start at 3,600. The only differences between different operators (unless they're not complying with the published documentation) is the different flight profiles and how the levers are pushed.

P&W and Cessna introduced climb torque limits in Rev 7 of the C208 POH to extend the life and reliability of the engines. In addition to the ITT and Ng limits, there are cruise and climb torque limits for the C208 which take into account temperature, altitude and by implication airspeed. Operational experience is that these torque limits result in a cruise itt below 700o (cruise limit is 740o) and a climb itt below 740o (climb limit 765o). By following the current manufactures data these limits have been lowered.

Re the soapbox - limits are limits. If we need the power we use it (and that includes rough water to reduce airframe loads). If we don't (good water and no obstructions) we keep the itt down to or below the published cruise limits by limiting torque usually to our normal initial climb setting 1700ftlb (limit is 1865ftlb), and possible even as low as 1500ftlb (cruise setting) if we're empty. We probably average six sectors per flight hour - certainly not the flight regime that the engine was designed for. There are a number of statements in the poh to the effect that operating close to the climb itt limit WILL shorten the life of the engine (I'm not talking TBO here).

No argument with the paragraph about anticipating higher start temps. However if the engine starts or runs hotter than normal there has to be a reason. Our experience is that pilots notice problems by different temperature characteristics which have been investigated by engineering and a problem found and corrected. Early intervention is far cheaper in the long term than persisting until the limits are reached or exceeded.


PS The information to our CP came from a P&W employee at a P&W engine course. Who do you suggest would be better to listen to?
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Old 26th Feb 2009, 08:50
  #26 (permalink)  
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Hi Werbil:

I think that you and I are pretty much converging to agreement on this limits topic.

In your most recent post, you mentioned climb limits that were recently introduced by PWC and Cessna. I can well appreciate the rationale behind this is, because my own experience with the small PT6A family (which is with the -27, a pretty close cousin of the -114A, which I believe is a slow-turning version of the -34) is that crews are far more likely to inadvertently exceed engine limits during the climb phase of flight than during any other phase of flight.

It sounds to me like this problem was happening on your aircraft type in the past, based on your comment that now you are observing margins of between 25 during climb and 40 during cruise when operating at the (new) maximum calculated power limits.

The drum I have been beating all along is that crews need to be fully educated concerning what engine limitations are, and they have to know both sides of the story - in other words, there is no benefit arising from an operator emphasizing 'restrictive' limits but not fully explaining and fully permitting 'permissive' limits. The starting limitations I quoted earlier are 'permissive' limits, in the sense that they exist to allow the crew considerably more operating freedom than might be first apparent from looking at the gauge.

Another good example of a 'permissive' limit is the acceleration limits that are published for torque and T5. In the case of the -27 (and probably the -114A as well), the pilot has a very generous torque (68.8) and T5 (850) acceleration limit to allow him or her to make a rapid forward movement of the power lever in the case of a go-around, windshear, or failure of one engine on a twin. Despite the presence of these acceleration limits, I have observed many pilots very carefully and very gingerly advancing power levers during a go-around or during windshear avoidance because "they don't want the pointer to go into the red". These observations have been made over the course of about 5,500 hours of full motion simulator instruction that I have given in a -27 powered aircraft.

It is interesting to note that the -27 engine operator who continues to hold the 'world record' for highest TBO on that engine - 9,500 hours TBO, based upon PWC incrementing the TBO over the 20 year history of that operator using a fleet of 25 of those engines - made every single takeoff at full calculated power. This is why I reject any suggestion that 'babying' the engine, or forcing pilots to abide by arbitrary limits imposed by company management (as opposed to the airframe manufacturer or the engine manufacturer) is wrong.

You wrote: "The information to our CP came from a P&W employee at a P&W engine course. Who do you suggest would be better to listen to?" I suggest that we only listen to what is written in the publications issued by the airframe and engine manufacturers. I have no doubt that whatever the PWC employee said was sincerely meant, but, if it is absolutely correct and if it is germane to operating their engine, hey, PWC would have put it in the book. I have been in a similar situation to that employee in the past - I wrote the FlightSafety training manuals for the aircraft type that I specialize in, and I currently write, edit, and maintain the AFM for that same aircraft type. I have my own personal opinions, as is obvious from what I have written in this thread, but in the end, the only opinions that matter are the ones that I manage to get approved by the regulatory authority and published in the AFM for the aircraft. The same thinking applies to PWC.
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Old 27th Feb 2009, 11:31
  #27 (permalink)  
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V1... Ooops,

The torque limits for cruise, and torque limits for take off always existed in earlier revisions of the POH, it's just the climb limit that has been added. The T5 limit and all the other data in the limitations section of the POH remain unchanged apart from the additional requirement not to exceed the torque for climb limit described in the performance section. Is your comment about the climb exceedances from observation or has that come from another source? There was a huge amount of discussion on the caravan pilots forum when those limits were introduced - the general consensus was that if you approached the T5 limit before you reached the new torque for climb limit the engine needed to visit the shop.

Yes the -114A in the C208 also has permissive transient limits - particularly the torque limit. Whilst I've been told that there are additional maintenance requirements depending on the length of time and peak torque I have not found any reference to it in the POH. As I've never seen the maintenance manual I can't verify whether the above statement is true.

I am assuming that the 'world record -27' holder is flying DHC-6s which you have already advised have airframe power limits nearly 10% lower than those permitted by the engine manufacturer. The -114A as installed in the C208 operates at the engine manufacturers rating - this engine was derived from the 600HP -114 (I think the change was to the compressor section). Additionally, the profile of the operation (altitudes for take off, temperatures and cycles, engine temperature before start) determines the T5 patterns. Even at rated takeoff power none of our engines reach the climb T5 limit during sea level takeoffs (99.5% of our operation is at low level - the only freshwater our floats usually see is out of a hose or the sky). It is for these reasons that I still treat this particular argument with a good deal of skepticism.

One of the other changes in REV 7 of the POH was the addition of recommendations to restrict T5 and oil temperature to a narrower range than the documented limits to ensure the longevity of the engine. When I get a chance I'll dig out the actual wording - it might take a while as a fracture in my right foot has me flying a desk at the moment.


PS What I've seen of the 400 series looks great. Are there any decent photos of her on the water - it would make a great desktop for the PC. Now if only the company could justify putting one on line ...... but then they'd never be able to get rid of me.
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Old 21st Mar 2010, 10:36
  #28 (permalink)  
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Twin Otter Query

Hi Guys

I'm a professional screenwriter and I'm looking for some expert opinion. I'm writing a project at the moment that involves a scene where a twotter pilot is giving hurried instructions on how to start a Twin Otter and taxi it along the runway. (No take-off necessary.) These instructions are being dealt over the radio to someone who's never been behind the controls before and it's quite a dramatic scene. Obviously I'd like to get the tech right, and to do that I would need help.

I've done some research reading SOP manuals and looking up cockpit control photos, etc., but if you could help me, by maybe posting what you would say in that situation as the pilot giving the commands to follow, i.e. "Lift up the forward boost switch, check the boost pump caution light is off. Now release the parking break. It's the red lever on the left...." etc., I'd be extremely grateful.

Again, thanks so much for any help.
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Old 22nd Mar 2010, 21:55
  #29 (permalink)  
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RR DART 'Hot-Start'

I am surprised that nobody has mentioned starting the RR Dart.
From what I remember from years ago, the starting sequence with an external start using a GPU was as follows. The fuel trimmers were set to the appropriate percentage, depending upon the OAT, the throttles closed, the appropriate engine then selected, the starter motor switch armed, the starter button presssed and the stop watch started. After 5 seconds, the HP cock was opened and the TGT rising confirmed light-up along with rising oil pressure. One finger was kept on the fuel trimmer, and the othe hand on the HP cock, ready to close it if the TGT climbed too rapidly, the HP cock could be 'milked' ie slightly closed,reducing the temperature. the starter button should pop out before 30 seconds had elapsed, if not the button was pulled out manually. If a 'Hot Start' occurred, more often due to a strong tail wind up the jet pipe, the engine was shut down and a 'Blow-Out' cycle was used get rid of unburnt fuel by spinning the engine with fuel and igniters off.

To see a 'Hot-Start' with a RR Dart was quite spectacular, a long sheet of flame from the jet pipe.

It was many years ago since I last started a Dart and maybe someone currently operating Darts will correct me
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Old 23rd Mar 2010, 07:49
  #30 (permalink)  
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Several thousand hours on the 748 and you have brought back memories.

I think I remember it exactly how you say.

I seem to remember that you were not supposed to milk the HP cock but of course we all did - especially on a battery start.

Talking of milking the HP cock, the flight engineer often did it on the Conway (VC10).
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Old 23rd Mar 2010, 11:22
  #31 (permalink)  
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Hi Fangio,

We used to "milk" the early RB211-22B on L1011 TriStars. The procedure was called "Potential hot start below 35%". Provided the N3 was between 25 - 35%, then the Start & Ign switch (equiv HP cock) could be cycled off and on. We could suffer from something called a "rotating stall" (especially with tail winds) and this procedure would keep the starter motor properly engaged, prevent a hot start, and un-stall the turbines. Once the TGT rise was sensible compared to N3, then a normal start continued.

We also had a position called "En-rich" which increased the scheduled fuel flow, should we have a start stagnate. Same rule between 25 -35%, but once above 35%, then if enrich was on - we had to leave it on until starter cut out. Then along came FADEC. Gosh how I miss the old days.

Last edited by rudderrudderrat; 23rd Mar 2010 at 12:01. Reason: + en rich
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Old 23rd Mar 2010, 22:40
  #32 (permalink)  
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Well, I have no experience of the big ones and have only operated PT6's in Twotters and Sheds plus the shaft turbine engines (Allisons?) in the J31.
Apart from sticking to the manufacturer's manual (obviously) the principle I have always been taught is the cooler the peak turbine temberature on start the better. Thus TBO's which in my case have always been determined by computer analasis of repeated recording of engine parameters in flight are maximised. - Well this was the theory. To minimise turbine start temperatures the power supply to the starter motor must be as good as possible. When starting these engines using on-board batteries, the batteries are operated virtually to their limits. Larger batteries than absolutely necesary will of course provide a weight penalty. The ideal way of starting is thus to use a GPU which can easily cope with the starting requirement and minimises the temperature spike on start up. If this is impractical or too expensive, then a GPU start should be used if possible for the first start of the day. This is because the battery voltage will have decayed slightly when parked overnight.
When a battery start is needed, the first engine is started, and once it is up to an appropriate speed the starter-generator is switched to generate to re-charge the batteries. The charging current is monitored and when this has fallen below a specified figure the battery is deemed fit for another start cycle. The charging generator (only on the Twotter if I remember correctly) is then switched off again and the second engine is started.
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Old 25th Mar 2010, 03:44
  #33 (permalink)  
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Originally Posted by WriterGuest
...involves a scene where a twotter pilot is giving hurried instructions on how to start a Twin Otter and taxi it along the runway... These instructions are being dealt over the radio to someone who's never been behind the controls before and it's quite a dramatic scene.
Geez, I can think of a few slipshod operators where the above scenario pretty much describes the "initial training and line indoctrination" that the new hire pilots get...

On a more practical note, there are several videos posted on YouTube that show the whole starting process for a Twin Otter. Go there and have a look, and that will, I think, provide you with sufficient background knowledge to either write your script, or get hired at some of the lower-end operators in this world.
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Old 20th Sep 2014, 19:18
  #34 (permalink)  
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PT6 inflight shut down and restart.

Not sure if this has been answered but I've heard stories of guys shutting down a caravan engine to see how far it glides then restarting it again. Would that damage the engine in any way? Other than the obvious reason of what if it didn't start again and you only got one engine can anything like shock cooling occur? Would it effect engine life ? Thanks
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Old 20th Sep 2014, 20:14
  #35 (permalink)  
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Hot Start is, by definition, a start in which, max eng turbine temperature (EGT for a Jet or TGT for a turboprop) surpasses a certain limit established by a manufacturer for that type of engine for Start process (there are other Egt limits, for instance Max EGT for TO)..
It happen for several reason, nowdays, with start process being fully automatically controlled by FADECs, the must probable reason is a deteriorated turbine assy hardware (blades) or combustor chambers. Back then when start process was manually, as previously indicated, if fuel was added before or after the correct "fuel-on rpm" you may have a hot start.
Manufacturers give directions in accordance with EGT overlimit severity, example: Max EGT during start is "only" 5 C above the maximun acceptable level, pilot is allowed to continue a flight but should made an entry in the log book. Then maintenance will inspect the engine @ next station including a borescope inspection. If start EGT it's 15C above limit, pilot should shutdown de engine and maintenance action is requiered before next flight.
The events in which there is an EGT, RPM, Vibration overlimits are automatically recorded and sent to maintenance by ACARS.
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Old 20th Sep 2014, 20:22
  #36 (permalink)  
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A/C engines and a/c systems are designed to allow an engine restart on flight should an error eng shutdown had taken place or, for inst., fuel starvation. Nevertheless, a deliberate healthy eng shutdown in flight is not allowed in commercial aviation, and any time and eng shutdown occurs, a report have to be filled and sent to FAA.
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Old 20th Sep 2014, 21:04
  #37 (permalink)  
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Reading through this old thread, it seems evident that two definitions of "hot start" are circulating. Mecaniquito84 is using the engineering definition - an over-limit gas temp - essentially a malfunction resulting from an over-rich fuel-air ratio or poor starter performance (e.g.weak battery...)

But the OP wingisland may have something different in mind - restarting an engine that hasn't had a chance to cool down significantly - that is, quick turnaround. We used to note the "residual" EGT on engines when evaluating the occurrence of hot starts per the above.

So it would help minimize confusion to specify which you mean.

Eschew obfuscation!
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Old 20th Sep 2014, 22:27
  #38 (permalink)  
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Sorry for my part in the confusion.

I was talking mostly about jet turbine with air powered starters. Starter torque will not change in dependance of batteries' power.

And concerning residual EGT values after a short turnarounds, again manufacturers establish the maximun residual EGT value before starting an engine and, if it's a modern eec engine, should computer (eec) senses a higher than normal EGT before start, it will only cranck the engine, without fuel injection and ingnition power thus "blowing" the engine and reducing the EGT for a while, then computer will stop the engine and, after engine is fully stop, a new start attempt, this one with fuel and ignition will follow.

In older engines w/o eec, there are strict residual EGT margin before attemping a new start. Should you follow them and there are not engine malfunctions, start process should proceed flawlessly.
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Old 21st Sep 2014, 03:07
  #39 (permalink)  
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Thanks Mecaniquito84. At least I wasn't totally barking up the wrong tree for disagreeing with people who like to turn their van into a glider for fun.
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Old 21st Sep 2014, 03:17
  #40 (permalink)  
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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.
Good that you have a clearer understanding, but keep in mind, whenever you are thinking of a turbine engine, airflow airflow airflow.....that's all they care about really, so it's not that the power in the starting system is low, it's that the low power results in lower airflow.
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