Thoughts about the RR250 Series?

[OEI-Dave]

My question to all your rotorheads is: What do you guys think of the Rolls Royce 250 series Turbo-shafts? I know they come in a wide range of aircraft. I'm sure many of you have a great deal of experience with them, as well as the other common powerplants used in rotorcraft. I only have exposure to the 250 series (I work at an overhaul facility) and I think highly of these engines. However, they are becoming a very old design and I'm surious as to everyones experience with them. Recently Robinson has announced that the 250 has been chosen to power the new r66 so maybe they're not as out dated as I think?

Comments and input appreciated. :}

[g-mady]

I thought the new RR300 was the power plant for the R66?

I think the general consensus is that the 250 is actually a cheap gas turbine engine. It is however a brilliant design and well proven. Provides excellent power/weight and reliability.

MADY

[army_av8r]

I have been flying the 250-C30R/3 for 5 years now. I have flown it in civil and combat environments with zero complaints. the incorporated FADEC makes the engine very reliable and "almost" impossible to droop the rotor during flight withing aircraft envelope. when maintained properly it provides very rapid and predictable power and fuel burn. i wouldnt mind shedding some aircraft empty weight to take advantage of the power. but even running her at the 650 SHP limit, she runs smooth!

[OEI-Dave]

The RR300 is "technically" a new engine. But the majority of the components are already in use in the smaller frame engines. :ugh:

The C30R/3's are mean engines. They make some GOOD power.IMO they are under rated from Rolls Royce. :E

[engine-eer]

As with any engine there are good points and bad points. The RR 250, when properly maintained and flown is a reliable piece.

The good points are that it is well developed and runs just fine.

The bad points are that it has a lot of piece parts and these can wear and cause problems. You need to make sure the engine is overhauled properly and not just pulled apart and the apparent bad parts replaced. The turbine disks have a finite life and you have to respect those life limits lest you burst one and ruin your day.

The engine is old technology and sucks a lot of fuel for the power you get out of it.

The RR300 has a new compressor, but it has a lower flow and pressure ratio so even though the comrpessor efficiency is improved the SFC as % of power isn't better than the older engines. At the power point used in the R66 it is probably better than a 250 would be at that power, but it still sucks a lot of fuel for what you get out of it.

I don't know if they are still using the single channel FADECS. When those were working they were great, but the "reversionary mode" on the single channel system was dangerous.

[jonwilly]

The original idea of Compressor separated by the Engine Gearbox from the Turbine is a good practical idea for maintenance at base.
Either side can be inspected and repaired without disturbing the other half.
Old idea and I understand the latest models now have a single stage centrifugal compressor which saves much wieght from the axial/centrifugal of early days.
In fix wing applications the engine Hanging from the wing supports to the ER/GB certainly help maintenance.

john

[EN48]

What else is there for light helicopters except possibly Turbomeca? An old design, yes, but this means proven, with something like 30,000 engines built to date and 200 million flight hours. And parts and service widely available.

[krypton_john]

What's the difference between C30P and C30R ?

[RVDT]

What's the difference between C30P and C30R ?

The airframe it fits in?

C30P 206 L1 (retro) thru L4

C30R/3 OH58D with a kind of Electronic governor?

Civil here (http://www.jet-engine.net/civtsspec.html)

Military here (http://www.jet-engine.net/miltsspec.html)

Knock yourself out!

All small turbines will always use a lot of fuel. Even supposedly modern engines have similar SFC to the old 250 series. Problem is in the tip clearance of such a small engine relative to its size. Yes you could improve it but at what cost and for how long?

As the engines get bigger they have a better SFC but it tends to be reflected in the price. Check out a GE T700/CT7 SFC.

[OEI-Dave]

The C30P is a GA application

The C30R/3 is a military variant.

The C30P is a non fadec traditionally regulated turbine.

The C30R/3 has FADEC

Has anyone had an uncontained faluire of these turbines yet? I know an AD was issued for a containment ring, AD 2005-10-13. This only applies to the series 2 engines (ie, C20,B17,C20R ect). Which in theory, should have "contained" a falure of the hot section. Somehow I doubt this after inspecting a containment ring and doing some math on the rotating inertia of the turbine section.
Anyone had anything with the series 3 and 4?

[engine-eer]

All small turbines will always use a lot of fuel. Even supposedly modern engines have similar SFC to the old 250 series. Problem is in the tip clearance of such a small engine relative to its size. Yes you could improve it but at what cost and for how long?

As the engines get bigger they have a better SFC but it tends to be reflected in the price. Check out a GE T700/CT7 SFC.
Actually the issue is the height of the blade passage as a function of compressor pressure ratio and size. As the engine pressure ratio gets higher, the height of the blade passage gets smaller in the final stage of compression. This limits the amount of pressure ratio you can obtain. Big engines get low SFC from big pressure ratios. Small engines, if you limit the pressure ratio to 8 or 9 are going to have an SFC of about .52 lbs/hp hr no matter what you do. You could, theoretically, add an additional spool and bring the diameter of the compressor down and increase the blade height. Problem is that you have to put a shaft through the second compressor and that actually limits how small you can make a multi-spool engine.

There have been two reseach engines that I know of that made around 750 hp and had an SFC at full power of under .45 lbs/hp hr (which is actually as good as the T700). The technology is out there to do it, and one of those engines was a low cost solution. Do some reasearch on the Army SHFE program.

All it takes is money to finish the development and certify the engines, but the technology is there.

[Ian Corrigible]

RVDT,

Have disagree with you on RR250 vs. T700/CT7 pricing. The T700 offers considerably more bang for the buck, especially now that GE Lynn and CCAD have got the TBOs under control.

T700-701Cs (and Ds) typically sell for $650K (www.acq.osd.mil/.../USA002235-10_ Complete%20Foreign%20Military%20Sales.pdf) to the DoD, which equates to about $330 per shp. Rolls charges the DoD $560K (http://www.defense.gov/contracts/contract.aspx?contractid=4160) for the 250-C30R/3, which equates to about $780/shp. Rolls' commercial pricing for spare C20/C30/C47 engines is even worse...not that Turbomeca is much better.

The sad thing is that we're still using what is at heart a late 1950s engine design. The NASA GAP (gltrs.grc.nasa.gov/reports/2008/CR-2008-215266.pdf) program (remember that?) was supposed to give us a 250 successor in the shape of the TSX-2, but the technical problems encountered by Williams on the lead FJX-2 (FJ22/ER22) (http://www.airspacemag.com/flight-today/engine.html?c=y&page=5) fan engine (mainly related to scaling issues and the complexity of the three-spool layout) killed the program.

Honeywell's Small Heavy Fuel Engine (http://www.flightglobal.com/articles/2007/02/27/212326/honeywells-demonstrator-shfe-engine-set-for-testing.html) seemed like a potential successor-to-the-successor-to-the-250, and Honeywell says the engine has met most of its goals (testbed achievements including a 57% increase in power:weight, a 24% reduction in cost and an initial 13% reduction in SFC), but the DoD doesn't seem to be in any hurry to put the engine into production.

I/C

[RVDT]

Problem is in the tip clearance of such a small engine relative to its size

re-phrase - Problem is in the tip clearance relative to the blade size in such a small engine?

falls off sharply when the average tip clearance becomes greater than about one percent of the blade height

[FH1100 Pilot]

Although RR is calling their new engine the "RR300," it is on the same type certificate as the 250 series. Kind of the same thing as Hughes calling the 369 the "500." Just some differences is all. I've got tons and tons of hours logged underneath 250's, and I'd fly 'em to the ends of the earth. And back. And I know they'd get me both ways.

Now, the engine I'm flying behind is a big, round, belching, farting, roaring, noisy, uncorrelated 700 h.p. seven-cylinder beast, and I dearly miss my relatively quiet little RR 250 sewing machine. The things we do for a living :rolleyes:

[OEI-Dave]

7 cyls. Radial I'm guessing?

[Savoia]

.
Thoughts about the RR250 Series
It will always be the Allison 250 series to me (old habits etc.) but .. one of the all time great general aviation turbines along with the PT6.

[FH1100 Pilot]

7 cyls. Radial I'm guessing?
You guess correctly: Wright R-1300.

[Kulwin Park]

I think the RR250 technology is well past its used by date, but like Cessna, they build the same reliable airframe and keep on going, and get orders!

But preferably, I would rather operate the Turbomeca engines for their ease of start & reliability, and operate LTS101's for their cost effectiveness & simplistic approach of keep costs down during maintenance. Good performance figures for money too

Cheers, KP

[riff_raff]

The small GA aircraft turbine engine market is a tough business. It's expensive to develop, certify and tool up for a new engine design. The small turboshaft segment (ie. the RR250) is very few numbers and relatively low price, so it is difficult for an engine OEM to recover their non-recurring costs. That's why you see little significant (non-military) development occurring with these engines.

Even in the large commercial turbofan engine market, the OEM's don't make much profit on the sale of new engines. Most of their profits come from sales of spare/replacement parts. (http://www.westchesterdevelopment.com/pages/News/files/issues/issue_index219.pdf)

[AdamFrisch]

But isn't this all a bit of inflating one's importance with turbines? Let's be honest here - they can charge these kind of money because they're on the type certificate as the only engine that particular aircraft can use. It's what is called a 'captive market'.

Engineering-wise, there is nothing today that suggests they would be that hard to make. The tolerances are far lower than in many other industries where none such price hiking appears. Let's not forget they were certified and approved when they were still hand made on lathe's and manual mills. Today we have robotic CNC vertical mill stations that have more productivity and much higher tolerances. Neither are the materials used very exciting or expensive. I don't believe for a second these aren't downright cash cows for RR, GE and P&W. Nobody can with a straight face tell me a 250 or a PT6 costs $600.000 to make. The R&D and the certification costs were recouped decades ago. What we're left with is corporate bullying.

The basics of capitalism apply here as everywhere else. The reason they're not selling very many of these engines and why their R&D costs are high per sold unit is - surprise! - because they're too bloody expensive. We could have had a myriad of small turboprops in every LSA and Cessna since the 60's, but they chose to ignore that because the times were good and the military could be relied on to overpay for their stuff - ever wondered why every aerospace manufacturer ever devised has gone after military contracts? One doesn't have to be a genius to figure that one out.

Make a certified 100-300shp turboprop and sell it for the same as a piston engine and you'd see massive sales. Bitching about how they don't make any money on their $30 million a pop GEnx engines for the 787 - excuse me if I don't immediately shed a tear and buy too much into that one (if you take future sales into account). If the RC guys at Jetcat, Wren and AMT can make working turbines in their sheds and sell them for $3000, the most certainly RR and the good old boys can make money selling theirs slightly bigger ones for $600K a pop. Yes, I know there are differences between the examples, but they're more similar than they are different.

[500e]

Well put Adam.
I feel that the containment ring on C20 series is another rip off cost, I am not saying it is not a good idea but the cost is crazy, because they can.
Engineering-wise, there is nothing today that suggests they would be that hard to make. The tolerances are far lower than in many other industries where none such price hiking appears. Let's not forget they were certified and approved when they were still hand made on lathe's and manual mills. Today we have robotic CNC vertical mill stations that have more productivity and much higher tolerances. Neither are the materials used very exciting or expensive. I don't believe for a second these aren't downright cash cows for RR, GE and P&W. Nobody can with a straight face tell me a 250 or a PT6 costs $600.000 to make. The R&D and the certification costs were recouped decades ago. What we're left with is corporate bullying. "
OEI-Dave
I have some pics of a failure (pre ring) SCARY.
Has anyone had an uncontained failure of these turbines yet? I know an AD was issued for a containment ring, AD 2005-10-13. This only applies to the series 2 engines (ie, C20,B17,C20R ect). Which in theory, should have "contained" a failure of the hot section. Somehow I doubt this after inspecting a containment ring and doing some maths on the rotating inertia of the turbine section.
Anyone had anything with the series 3 and 4?
"But isn't this all a bit of inflating one's importance with turbines? Let's be honest here - they can charge these kind of money because they're on the type certificate as the only engine that particular aircraft can use. It's what is called a 'captive market'.

[EN48]

you'd see massive sales


To paraphrase Slick Willie, it depends on what the meaning of "massive" is. Looked lately at total small aircraft produced annually with piston engines? Not exactly a "massive" number in the conventional meaning of the word. A few thousand at best. :confused:

Consider that in the 50+ year history of the 250 series, only about 30,000 engines have been manufactured, or about 500 per year, and this includes all applications: new acft, replacements, etc. For perspective, something like 60 MILLION motor vehicles (cars, truck, motorcycles) are produced annually, each with a piston engine.

If you examine what per centage of the selling price the engine cost represents in a new light acft, either piston or turbine, if this cost went to zero, it would make only a modest differencce in acft price and therefore sales volume. Examples: new Cessna Skyhawk about $300K, Lyc piston engine about $40K; new B206 about $1.6 million (when you could get one); new 250-C20 about $350K. (Aircraft piston engines benefit to some degree from what is sometimes called "shared technology" among all piston engines.)

Would be great if turbine engine prices were much lower, but unless demand is in the million plus units per year range, isnt likely to happen, IMHO.

[Ian Corrigible]

Ironically, the target price for the original 250 shp T63 of 1958 was $4,000 (about $30,000 in today's money). Bill Castle and the Allison design team soon realized just how badly they had low-balled their proposal once the contract was awarded.

I/C

[engine-eer]

All pretty good points. If production was up to a reasonable rate the cost would come down some. While we have a lot better machining technology today, the 250 was made using dedicated machines. That is, each operation had a dedicated machine and that reduced costs a lot. While you can do more nowadays with more capable machines, it doesn't reduce costs as much as you might think compared to production line methods.

RR right now isn't making their cost bogeys for the RR300, and is, consequently, losing money on each engine. This is because they have high costs and low productivity in a union environment and their costs are higher than if they bought all the parts and just assembled the engines. RR gets a big number for the other Model 250 engines and makes a good profit on them.

I'm sure the PT-6 could be sold for less if P&W had some competition, but they don't so the price is what the traffic will bear.

Nobody has mentioned the cost impact of product liability, which is a big deal for engines. You have to factor that in to the price too. It isn't just the cost of making and sending an engine out the door and covering the warranty costs.

Turbines can be less expensive if the engine is designed expressly for low cost production using modern production techniques. But you aren't going to ever see a 300 hp turbine with a cost similar to a 300 hp recip. The reason is that the nickel base alloys in a turbine expensive, and all of these nickel based castings for the hot section pieces are vacuum investment castings and those are expensive.

Don't try to compare model turbojets to larger multi-spool man rated turboshaft engines. Those engines are very simple machines with relatively low mechanical loadings. Consequently the performance is poor and they don't operate at higher temperatures or require high performance and costly materials. They don't have power turbine sections and gearboxes and don't have fuel controls that would ever be capable of being used on a real engine. They aren't man rated and I'm sure the makers aren't carrying the product liability insurance that would be required for a real engine. They are basically toys when compared to real engines, and you are paying the price of a toy for what you are getting.

[OEI-Dave]

500E

Can you post some pictures of the failure? I've only ever seen the engines scrap out from oil deprevation, MIO ect.

[500e]

http://[IMG]http://i98.photobucket.com/albums/l259/500d_2006/IMG_9071.jpg

http://http://i98.photobucket.com/albums/l259/500d_2006/IMG_8912.jpg (http://[img]http//i98.photobucket.com/albums/l259/500d_2006/IMG_8912.jpg%5B/IMG%5D)

[500e]

http://http://i98.photobucket.com/albums/l259/500d_2006/IMG_9071.jpg (http://http//i98.photobucket.com/albums/l259/500d_2006/IMG_9071.jpg%5B/IMG%5D"Lost photo[/URL]


[URL="http://http//i98.photobucket.com/albums/l259/500d_2006/IMG_9071.jpg%5B/IMG%5D)

http://[IMG]http://i98.photobucket.com/albums/l259/500d_2006/IMG_9068.jpg (http://[img]http//i98.photobucket.com/albums/l259/500d_2006/IMG_9068.jpg%5B/IMG%5D)

Had a load more but Pbucket Seemed to loose about 30 picture's from my account there were pics of tail section with damage close to tail pitch rod real close + lots of further shrapnel damage.

[RVDT]

Ugly!

Seen similar on a B206. The pieces went down through the engine pan through the the boot and out the bottom. Missed the T/R control!

I can't remember what the reason was after all these years but there was a conclusion as to why.

A classic one that often gets missed and not implying anything to do with the above but after a module change a start and ground run up to flight and subsequent shutdown is imperative. Do NOT start, ground and then fly. It seems the engine likes to go through a heat cycle just to get everything lined up properly.

Couple of things on the five -

Leaking no.8 U seal will leak air into the oil system when it is running (it will run slightly hot) and oil onto the wheel after shutdown causing shock cooling to the wheel.

Leaking check valve in the oil cooler will let oil drain back through the scavenge and flood the sumps and then onto the wheel. Did it ever leak oil out of the burner drain? The burner drain has to be clean to leak the oil so you can tell! Often they are not!

[engine-eer]

The biggest issue with the 250 is the limited low cycle fatigue life of the turbine wheels.

The wheels are integral castings. That means that the wheel is made up of a disk that is cast in one piece with the blades. The blades are not removeable. What happens is that on startup and as the engine accelerates, the rim of the disk gets hot quicker than the bore (the center of the disk). This causes the rim to yield in compression. When the engine is shut down, the disk is hot and the rim cools quickly and the disk rim now goes into tension and cracks.

To help reduce the amount of thermal stress in the disks, it is very important to cool down the engine after you land and before you shut it off. This allows the disk bores to cool off and minimizes the temperature difference between the disk rim and the bore. The shutdown procedure includes two minutes at flight idle before shutting down the engine, and to get the most out of your engine it should be followed to minimize the damage to the disks. Going from a high power hover to a quick shutdown is a really bad thing to do to these engines.

If you don't properly shut down the engine the cracks, which grow from the rim of the disk downward toward the bore, can progress to the point where the disk will fail and you will burst the disk.


The disks crack and the crack growth is well understood, so there are time limits on the disks and periodic inspectons. In all it is a safe system when the procedures are followed correctly.

[Shawn Coyle]

The manual says to cool for two minutes at idle. But if you notice the TOT at flat pitch with the throttle full open, it's the same and perhaps lower than at idle.
Why don't they let you count time at flat pitch in the cooling time?

And if two minutes is good, three minutes isn't better.

[FH1100 Pilot]

Shawn, I *believe* (meaning: maybe I read it once...somewhere...) that there is a high-wind shutdown procedure for the 250 in which the cooldown is done at 100%. I mean, I agree with you, it's TOT that makes the engine hot, right? And if the TOT is at its lowest point, why DON'T they let you count the time at 100%/flat pitch?

Me, I'm old school. I start my cooldown when the N2 gets to idle, like I was taught many, many, many years ago, no matter how badly I have to pee by that time (and usually I do!). Old habits and whatnot.

Engine-eer, it's interesting that you point out the temperature issues with the wheels themselves. I would imagine that most 250 drivers believe the cooldown to be *only* an issue of oil temperature and coking.

I have a young friend who flies a 206. He knows EVERYTHING about it, or so he says. He believes he has learned more in his 500 hours of 206 time than I have in my 30 years of flying them. Whatever. Perhaps he has. This youngster apparently views the 2-minute time as a maximum, or perhaps just a "suggestion" from Allison/RR, because I have never seen him do a complete 2-minutes; I have however seen him give it as little as a minute and a half. Perhaps he starts his countdown clock from some point prior to throttle roll-off, as Shawn posited. But hey, it's his ship (or his boss's), not mine and he views any input from this old-timer as gratuitous criticism of his fine piloting skillz. Some fights just aren't worth fighting.

Then again... I have heard from sources I consider fairly reliable that modern engine oils (like Mobil 254), can allow a shorter cooldown time than 2-minutes. One "expert" did actually suggest that 1.5 minutes is fine for a 250, and maybe my young friend also heard that advice and took it to heart.

So I don't know. Sometimes the tales we hear from those old wives are wrong. I'll just keep giving RR-250s a proper 2-minute cooldown until I'm officially told by RR that it's not necessary anymore.

No matter how badly I have to pee.

[EN48]

And if two minutes is good, three minutes isn't better.


Been told that more than 2 minutes has no benefit. Maybe from an old wife - someone here will know the truth!

[engine-eer]

It's turbine INLET temperature that matters, not turbine outlet temperature. The cracks start at the leading edge of the rotor. If the main rotor is taking out work (even though it is in flat pitch it has some drag), then the turbine is doing work and the inlet temperature is higher than the outlet temp. When you are at idle and doing no work the turbine inlet temp is pretty much equal to the outlet temp.

You don't measure burner outlet temperature since that would toast thermocouples, but generally the less fuel you are burning the lower the combustor outlet temp is.

It's possible that the engine could be more efficient (compressor and turbines into a more efficient operating range) that the temperatures could be cooler at a higher idle. I've seen some small turbojets that had a flat EGT curve from 20% speed up to 50% speed, so it's possible, but you want the combustor exit temperature to be as low as you can get it and that usually happens at a lower idle point, except in a high wind situation where you have the wind acting to back pressure the exhaust.

[rotor-rooter]

An interesting story regarding RR catastrophic failures here.

http://abcnews.go.com/Business/wireStory?id=12349267

[OEI-Dave]

As bad as the failures seem I still think these engines are quite robust. We've had engines come in from foreign militaries for low power only to find that there was a bullet hole in the occ and pieces of copper/lead in the cubustion case and on nozzle leading edges. The even crazier thing is that they put 100hrs on the engine before they decided to pull it!:ugh:

[nigelh]

One of the well known manufacturers did thousands of engine offs in flight with no cool down . When the engines went in for maintenance i am told there was no difference or damage at all . Not advocating ignoring the 2 min ......

[RVDT]

Probably the most significant issue with cooldown time on the 250 engines is the 6 & 7 oil drain which goes through the lower support strut. If you shut off with the strut still hot the oil "fries" in the strut and eventually blocks it. This will lead to flooding of the bearing cavity and the oil exiting through the labyrinth seals onto the wheel. Not good to quench the wheel and also stop the oil flow to the bearings. From memory the 6 & 7 sump should be removed periodically and the flow checked and the strut eyeballed to see that it is clean. The No. 8 "hook" jet can also suffer the same.

Some oils are better than others and results have varied over the years with supposedly the same oil but different manufacturing batches.

Another area that also used to suffer was the PT coupling shaft. Carbon buildup would lathe off the "pea shooter" shaft and disconnect the compressor!

Definitely guaranteed to make your eyes water in one way or another............

[that chinese fella]

I recall seeing an Allison chart showing the core temp of the turbine wheels and after 1 min 45 secs the temp had stabilized so I can only assume the published Bell 206 '2 minutes' added a little margin for those challenged by time-keeping..

[engine-eer]

Some oils are better than others and results have varied over the years with supposedly the same oil but different manufacturing batches.

Oils with better scorching resistance result in much better performance in this engine. You could make the case that the engine is marginal in this regard, but the fact remains that if you use a better oil you will go a long way towards avoiding a costly situation.

In addition you should alwasy use the newer 3 micron oil filter. Life of a number of components in the gearbox and sumps was drastically improved by going to the 3 micron filter. In addition, by filtering out more particles from the oil, there was a lower tendency to produce deposits in critical parts of the system.

[OEI-Dave]

The new super-finish gears help alot with gearbox life aswell :hmm:

[lamanated]

I'm fresh out of a week recurrent training from an overhaul shop, the rr 300 is the old technology back end with a centrifugal compressor up front.the reason for the cool down is to get the core below 450 degrees which is where carbon forms. I like 3 minutes on our R2 as we had lock up at times with 2 minutes cooldowns, and none on the 3. making sure the 6&7 struts upper and lower are cleaned well on the inside helps also , also the oil tube with the stem out the top is the new style and thus the 200 cc oil flows we see..

[Shawn Coyle]

lamanated:
Did you submit an SDR on the lock up?

[ericferret]

Oil manufacture as opposed to type can make a difference.

In the early 80's Hughes 500D's were delivered standard with Mobil Jet II.
In the UK at the time the standard oil used by most operators in the Bell 206 was Aeroshell 500.

A number of operators changed to AS 500 in their 500D's for commonality.
It became apparent that although both oils were to the same Mil Spec AS500 made far more carbon than JET II. The oil filters would come out dripping carbon at scheduled maintenance. This was then coupled with a number of engine failures all on AS500
aircraft.

This problem only occurred with the 500D the early C model did not seem to have any carbon issues.

The C30 in the S76 was even worse. Again AS500 was the oil in use. One UK operator used to remove both turbines from one of it's fleet of six every weekend for a workshop strip and decoke. This was a continual process in an attempt to prevent turbine bearing failure.
At this time Sikorsky added armour plate to the engine bays to protect the airframe, about 20 kgs a side.

In the end they operated all six with a different oil as a research programme, bit of a logistical nightmare.. The cleanest oil was Mobil 254 and that was fixed as the company standard.

An earlier post refers to a graph issued by Allison. This is actually a peak soak temperature plottted against idle time. The graph was issued in 1976 and I got my copy on my course in
1979!!!!!!

To cut it short the hottest area is the power turbine strut.

0 seconds at idle 371 degrees centigrade
30 secs 304
60 secs 260
90 secs 238
120 secs 227

At 2 mins the graph goes more or less flat so no further gain from continued idle running.
A drop in core temperature of about 150 degrees C over the two minutes and enough to give the oil a fighting chance!!!