Turbine starting and hot starts [Archive]

low n' slow

28th Jan 2009, 11:50

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

28th Jan 2009, 11:51

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

28th Jan 2009, 11:57

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.:ok:

c24

Rotorhead1026

28th Jan 2009, 12:02

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

28th Jan 2009, 12:09

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

28th Jan 2009, 12:10

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

28th Jan 2009, 12:22

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

28th Jan 2009, 12:31

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

28th Jan 2009, 12:49

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 :ok:

mwaugh

28th Jan 2009, 14:01

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

28th Jan 2009, 17:56

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

28th Jan 2009, 21:13

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

28th Jan 2009, 22:42

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

20th Feb 2009, 05:27

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

20th Feb 2009, 05:37

...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

22nd Feb 2009, 07:20

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

22nd Feb 2009, 10:49

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

22nd Feb 2009, 10:54

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

22nd Feb 2009, 20:27

...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.

V1... Ooops

22nd Feb 2009, 20:39

...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...

Ernie:

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.

ernie blackhander

22nd Feb 2009, 21:37

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

werbil

24th Feb 2009, 10:18

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.

V1... Ooops

25th Feb 2009, 11:39

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.

werbil

25th Feb 2009, 23:50

V1Ooops,

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.

werbil

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?

V1... Ooops

26th Feb 2009, 08:50

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.

werbil

27th Feb 2009, 11:31

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. :{

Regards
werbil.

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.

WriterGuest

21st Mar 2010, 10:36

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.

Fangio

22nd Mar 2010, 21:55

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

finncapt

23rd Mar 2010, 07:49

Fangio

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).

rudderrudderrat

23rd Mar 2010, 11:22

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.

P.Pilcher

23rd Mar 2010, 22:40

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.

V1... Ooops

25th Mar 2010, 03:44

...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... :E

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.

onehotflyer

20th Sep 2014, 19:18

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

Mecaniquito84

20th Sep 2014, 20:14

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 15°C 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.

Mecaniquito84

20th Sep 2014, 20:22

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.

barit1

20th Sep 2014, 21:04

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! :E

Mecaniquito84

20th Sep 2014, 22:27

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.

onehotflyer

21st Sep 2014, 03:07

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.

framer

21st Sep 2014, 03:17

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.

Dream Land

21st Sep 2014, 04:08

On the Garett engines if your ground time is short, you can spin the prop, thereby bringing in cool air to cool things down, on the Pt6 I recommend that just after shutdown, you engage the starter and crank the engine for 30 seconds, works great.

flyhardmo

21st Sep 2014, 05:17

on the Pt6 I recommend that just after shutdown, you engage the starter and crank the engine for 30 seconds, works great.

That drains your battery so your next star is a hot start. I wouldn't recommend it unless you have a GPU.

JammedStab

21st Sep 2014, 07:28

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.

Don't know about the modern 737's but there are some similarities between starts on the PT-6(at least older versions) and the old 737's even though one is using an electric starter while the latter uses an air starter.

The similarities are that if you have enough starting power(psi of air pressure or electrical power) engaging the starter and then normally waiting until getting a stable and steady rpm, then and only if you have the required minimum rpm, selecting fuel on and holding the fuel lever until it is certain that there are no exceedences and that the start is normal. As well, releasing the start switch at the appropriate time. Many minor differences though.

Mecaniquito84

21st Sep 2014, 13:13

Technically you are right, but the point here is, again, to follow established procedures. Manufacturer clear indicates to shutoff al others consumers like packs and MINIMUM air pressure for engine start. Should air pressure is below that level, you are not allowed to start engine

Dream Land

21st Sep 2014, 23:13

That drains your battery so your next star is a hot start. I wouldn't recommend it unless you have a GPU. I am currently operating a TBM 850, we replace batteries every two years, after a typical one hour flight, operating the starter for 30 seconds has no detrimental affect on the start, a typical voltage indication during a start is still 21 volts.

glendalegoon

22nd Sep 2014, 01:35

dear original poster

I think you are mistaken in the meaning of a HOT START. A hot start is a bad thing in which the engine gets too hot and may damage the hot section/turbines.

A hot start is not a start of an engine after only a short time of being shut down.

flyhardmo

22nd Sep 2014, 02:18

Dreamland

Try doing it on a twin and see where the voltage ends up especially on cold mornings. One of these days you'll have to explain to your pax that you can't go because you have drained the battery unnecessarily cranking over a PT6 after shutdown. Just park the aircraft into wind for shutdown/startup and if you really want to you can motor the engine an extra 5sec after max motoring speed before you introduce fuel. Try it and let me what the difference is in starting ITT/EGT.

I never flew Garretts but an engineer told me the reason for spinning the engine after shut down was to stop the main shaft from flexing once it had cooled down. I'd be happy for some experienced chap/chapette to confirm or deny this.

barit1

22nd Sep 2014, 12:53

flyhardmo:an engineer told me the reason for spinning the engine after shut down was to stop the main shaft from flexing once it had cooled down. I'd be happy for some experienced chap/chapette

Sounds like a phenomenon common on several engines. After an engine is shut down 10 to 60 minutes (depends on engine size & rotor mass), a convection stratification occurs within the case, meaning that the segment of rotor that stopped at 12:00 is a bit hotter than at 6:00.

This makes the rotor shaft bow slightly. :uhoh: After another hour or two, everything cools down and temps equalize. :)

But if you attempt a start during this bowed-rotor period, it's gonna be out of balance for a few minutes. I've heard of pilots "tickling" the starter while shut down to suck a little cool air through, and to randomize the rotor position.