# Reliability of WW2 fighters.



## marshall (Dec 13, 2007)

I'm interested in how reliable were WW2 fighters but I couldn't find any good info about it. Which one was the most reliable fighter and which one was the least? By reliable I mean everything engine, guns, airframe, how much work it took on the ground to get it ready to fly, how much beating it could take in the air, etc.
How do you guys think it was with this reliability?


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## Sgt. Pappy (Dec 13, 2007)

Depending on the theater of war, the aircraft could be of different reliability.

In England, the Spitfire was relatively to repair - the Hurricane was even easier. The small, simple, single-engines fighters were at the top of their game in the Battle of Britain and were doing well. When they were shipped overseas to the likes of the Mediterranean, China-Burma-India and Pacific Theaters of War, they experienced many problems... fouled oil/fuel lines, faster engine break-down rates due to sand etc. After all, they were designed for nice, English weather.


Also, it's not just the era that determines reliability.

I.e. Some may argue that the P-47 Thunderbolt was far more reliable than the much more recent F-14 in terms of engine break-down rates. 

It is widely believed that a radial engine aircraft is more reliable and is easier to fix than an aircraft with an inline engine. The Corsair was powered by a radial, Pratt Whitney R-2800 18-cylinder engine that was air cooled. 

The Spitfire was powered by an inline 12-cylinder, Rolls-Royce Merlin that was liquid cooled. The arrangement of their cylinders necessitated their respective cooling methods. Thus, the Spitfire mechanics had to tinker with coolant lines and radiators and the Corsair mechanics did not. Also, the glycol used to cool the Spit's engine was highly flammable... not so good in combat. But the Spitfire wasn't designed to take hits... rather to dodge them.


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## magnocain (Dec 13, 2007)

> Over half the Spectres' regular armed combat destructiveness comes from shrapnel.... Where the shrapnel gonna come from with those pretty little twinkling lasers??


I agree. I dont think water-cooled, inline engines can even be called "easy mantinance".


> Over half the Spectres' regular armed combat destructiveness comes from shrapnel.... Where the shrapnel gonna come from with those pretty little twinkling lasers??


Wasn't the f-14 taken out of service because it was hard to maintain?


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## Crumpp (Dec 13, 2007)

All of them have very similar TBO's.

The BMW801 for example requires "overhaul" at 200 hours. This could be done 5 times before the motor had to be sent to depot maintenance for rebuild.

"Overhaul" consists of changing oil, clean oil screen/filter, fuel filter change, vacuum system filters, checking compressions, clean/replace plugs as needed, and inspect/repair as needed.

All the best,

Crumpp


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## marshall (Dec 14, 2007)

So how often an engine could break-down during a flight? I mean was it 1 per 100 flights of a given type of plane or maybe more like 1 per 1000 or 1 per 10000 flights?

How often guns jammed? I heard that 20mm cannons were less reliable than machine guns, is this true?

Were there planes, on both sides, that were considered to be very reliable or maybe there were some planes that had a bad reputation in this matter?


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## FLYBOYJ (Dec 14, 2007)

magnocain said:


> Wasn't the f-14 taken out of service because it was hard to maintain?


I knew guys who worked on the F-14 - according to them, compared to the F/A-18 it was a pain in the @ss.

As far as the F-14s retirement - I think it was more or a matter of operational costs compared to newer aircraft that could do the same job - F/A-18.

As far as WW2 - radials over in-line, definitely! I read that engine changes could be as early as 25 hours and as high as 500 hours. Be advised that on all sides during the war you had a lot of "expressed trained" pilots who often abused engines by running them too rich or lean or at the wrong MP settings.

Systems wise I think they were all similar. I read on here somewhere a story about the Manchester with some less than desirable airframe systems.

BTW - my family recently attained an accident report from my wife's grandfather, a B-24 pilot with the 7th AF. Shortly after the war he was on a training mission and while taxiing out his aircraft suffered a MLG collapse. The aircraft had 1,200 hours on it, in aircraft age especially for a large multi-engine aircraft, that's not a lot of time.

I think the ruggedness of most WW2 combat aircraft made them reliable but at the same time many of them were also designed with the mindset that the said aircraft may only last 10 hours in combat.

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## Crumpp (Dec 14, 2007)

This might help. This is the CONUS figures during the war. What is interesting is the fact this the maintenance required for normal operations. No combat is involved.

You can see that a considerable number of the engines simply became scrap with no hope of overhaul or repair:

http://afhra.maxwell.af.mil/aafsd/aafsd_pdf/t113.pdf

Airframes, generally airframes are pretty resilient and it takes quite a bit to turn them to scrap.

http://afhra.maxwell.af.mil/aafsd/aafsd_pdf/t112.pdf

This breaks down the man-hours spent on specific engines. There is not much to choose from as all aircraft engines operate at 100% capacity and above. No matter which engine you choose, they are all being very stressed just by the fact they are an airplane engine. Power to weight is the most important airplane engine characteristic. Maximum power means very little if the power to weight is not within a certain range. Many early aviation pioneers made this mistake when attempting heavier than air flight. They went for the most powerful engine available instead of the engine that produces the most power per unit of weight.

http://afhra.maxwell.af.mil/aafsd/aafsd_pdf/t114and115.pdf

All the best,

Crumpp

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## Thorlifter (Dec 14, 2007)

Probably the least reliable was the Me-262. If memory serves, the engines were only good for about 12 hours of service, then had to be replaced.


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## szczezne (Dec 14, 2007)

You have to remeber thay it was the first jet in serwice and they were trying to fix many things and also were learning how to fight with it. This was the end of war and things wer not going good for Germany at that point


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## Crumpp (Dec 14, 2007)

> Probably the least reliable was the



I would say the Pratt and Whitney R-2800 series was the least reliable engine ever designed.

At least according to the team that built it:

http://www.enginehistory.org/NoShortDays/Development of the R-2800 Crankshaft.pdf

However it was _developed_ into a reliable aircraft powerplant by the start of the war and went on to join aviation folklore.

All the best,

Crumpp

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## drgondog (Dec 14, 2007)

Crumpp said:


> This might help. This is the CONUS figures during the war. What is interesting is the fact this the maintenance required for normal operations. No combat is involved.
> 
> You can see that a considerable number of the engines simply became scrap with no hope of overhaul or repair:
> 
> ...


Great stuff.

I found one reference in 355th microfilm to a 357FS P-51B-7 that was declared WW and subsequently used only as a hack after 640 airframe hours and two engine changes.. theoretically the highest hour airframe in the 355th FG. This was in October 1944 - for a Mustang that was in the original complement in March.


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## Thorlifter (Dec 14, 2007)

Crumpp said:


> I would say the Pratt and Whitney R-2800 series was the least reliable engine ever designed.



I tried to read the article, but my ADD kicked in and I started arguing with a dust ball. 

But I did read the conclusion and it was practically nothing but praise for the engine.


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## Crumpp (Dec 14, 2007)

> But I did read the conclusion and it was practically nothing but praise for the engine.



And rightfully so, once it was a *developed* engine. The article is about the difficulties experienced in that development.

Facts are all engines start off "unreliable" especially in the days of the slide rule. They are then developed into reliable power plants.

No different than the BMW 801, Merlin series, or any other aircraft engine.

No if you compare the man hours in maintenance for the R-2800 to any other engine you will get a more complete picture.



> I found one reference in 355th microfilm to a 357FS P-51B-7 that was declared WW and subsequently used only as a hack after 640 airframe hours and two engine changes.. theoretically the highest hour airframe in the 355th FG. This was in October 1944 - for a Mustang that was in the original complement in March.



Interesting stuff, Bill! Thanks for posting that. 

For the readers, the times are ridiculously low in combat with too many "unknowns" to quantify a good picture of reliability from a maintenance standpoint. 

Happy Holidays!

Crumpp


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## drgondog (Dec 14, 2007)

Crumpp said:


> I would say the Pratt and Whitney R-2800 series was the least reliable engine ever designed.
> 
> At least according to the team that built it:
> 
> ...



Gene - I think you are dead on not just for every new design - but also for major evolutionary step - whether Rolls, BMW, DB, Pratt or Allison. A lot of airframes (i.e B-29) were delayed because of major engine gestation issues..


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## marshall (Dec 15, 2007)

So generally, can we say that WW2 fighters were very similar in terms of reliability and the newer the type/model the less reliable it was but with time it was geting better and better of course in terms of reliability? There was no planes with bad reputation?

One more question, was it often that someone was on a sortie and suddenly engine is dead, broken, and he becomes a glider? But not because of enemy fire just because engine broke.


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## machine shop tom (Dec 15, 2007)

Seems to me that Methanol/water used as a coolant is flammable. Glycol/water is not.

tom


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## Crumpp (Dec 15, 2007)

> There was no planes with bad reputation?



Certainly. Look at the Brewster F2A series which had a horrible reputation as being a horrible dogfighter.

Or the P39 series repurtation as a very poor fighter as well.

Just don't ask the Finns or the Russians their opinion.

Point is reputation is perception based. I am sure you have heard the expression, perception becomes reality.

All the best,

Crumpp


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## Crumpp (Dec 15, 2007)

> Seems to me that Methanol/water used as a coolant is flammable. Glycol/water is not.



Glycol used in WWII fighters is flammable. 

ETHYLENE GLYCOL

You are thinking of Propylene glycol which is non-toxic, non-flammable and commonly used in automobiles today.

Butylene glycol is used in many modern aircraft.

http://www.sciencelab.com/xMSDS-2_3_Butylene_Glycol-9923183

All the best,

Crumpp


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## DOUGRD (Dec 16, 2007)

magnocain said:


> Wasn't the f-14 taken out of service because it was hard to maintain?



The F-14 wasn't removed from service for maintenance issues. There were engine issues back in the mid '70's with the TF-30's. The TF-30 loved FOD. (FOD for those folks unfamiliar with the term means Foreign Object Damage. In the U.S.Navy FOD is also a noun referring to any stray nuts or bolts or used safety wire or other items that can present a hazard to aircraft systems including engines, flight controls, landing gear, etc.) When I was onboard the U.S.S. America (CV66) the F-14's were losing so many engines to FOD that we had to do a "FOD walkdown" along the entire flight deck before each launch evolution which meant about every 45 mins. during fight quarters. Tha A-7's which also had TF-30's didn't have nearly the problems and we figured it was because of the intake shape and airflow volume. Once the F-14's got the GE F110's the problems went away.


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## pbfoot (Dec 16, 2007)

Not to be picky but FOD means foriegn object debris as opposed to damage


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## renrich (Dec 17, 2007)

I would bet that of all US fighters in WW2 the Hellcat would have the best servicability record. It had a reliable engine and most Grumman products were known in those days to be designed for ruggedness and reliability. The R2800 did have many problems during it's development. In the book, "Whistling Death" by Boone Guyton which was the story of the design and development of the Corsair there were many mentions of engine and to a lesser extent prop problems. In fact, the lengthy development time the Corsair required was to some extent caused by the fact that the Corsair was designed for and was one of the first AC to use the R2800 and a new Hamilton Standard prop.


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## davparlr (Dec 18, 2007)

pbfoot said:


> Not to be picky but FOD means foriegn object debris as opposed to damage



FOD has become its own word meaning both foriegn object damage and foriegn object debris. I was in the aerospace industry for about 35 years both in the Air Force and in industry and I don't every remember the term foriegn object debris used in any of the endless lectures on the subject (FOD was always defined as foreign object damage and the debris was always just called FOD).


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## Crumpp (Dec 18, 2007)

> I would bet that of all US fighters in WW2 the Hellcat would have the best servicability record.



I would bet it is not much different than any other airplane of similar type.

All the best,

Crumpp


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## Crumpp (Dec 18, 2007)

You can see by examing this document that the R-2800 offered no advantage in number of maintenance hours over any other aircraft engine.

http://afhra.maxwell.af.mil/aafsd/aafsd_pdf/t114and115.pdf


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## renrich (Dec 18, 2007)

During the 1944 Pax river fighter conference when voting on the engine that inspired the most confidence, 79% of the pilots voted for the R2800.


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## AL Schlageter (Dec 18, 2007)

Crumpp said:


> You can see by examing this document that the R-2800 offered no advantage in number of maintenance hours over any other aircraft engine.
> 
> http://afhra.maxwell.af.mil/aafsd/aafsd_pdf/t114and115.pdf


That is for average man-hours per *major overhaul* at depots.


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## renrich (Dec 18, 2007)

From what I understand the original engines in the F14, the P&W engine, was not designed or suited for a lot of abrupt throttle changes and inputs. In other words, the usage occurring in an ACM context. When the GE engines were substituted, most of the engine problems ceased. The F14 technology as far as electrical circuits was concerned was 1960s and 70s vintage and the AC began to have a lot of problems with the miles and miles of electrical wiring which made the AC a maintenance hog. I forget the exact number but it was a lot of hours maintenance for one hour flight time. In addition many of the AC were beginning to be risks because of metal fatigue. I believe many F14s were severely G limited near the end. I had an F14D driver tell me one time that the Tomcats that were taken to Top Gun school almost invariably came back "bent out of shape" and would never fly right again. Sounds nutty but maybe plausible. Having said that I still believe a newly manufactured strike Tomcat would have been a better purchase than the F18E-F.


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## Crumpp (Dec 18, 2007)

> That is for average man-hours per major overhaul at depots.



Yes it is and do you understand what that means?

I don't think you do if your intent is to call into question the fact it is a representation of maintenance hours.


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## Crumpp (Dec 18, 2007)

> During the 1944 Pax river fighter conference when voting on the engine that inspired the most confidence, 79% of the pilots voted for the R2800.



Yeah, there is nothing like the feeling of that big radial thumping up in front of you to inspire confidence.

There is also the mistaken belief that aircooled engines are more reliable than water cooled. Air cooled engines are simpler and require less parts.

All in all, they are about the same due to the fact all airplanes engines are designed with the physics of flight in mind.

All the Best,

Crumpp


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## Crumpp (Dec 18, 2007)

You might get a kick out of this one and some insight into aircraft engines:


_View: https://www.youtube.com/watch?v=EkcX0KGIBwk_


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## AL Schlageter (Dec 18, 2007)

Crumpp said:


> Yes it is and do you understand what that means?
> 
> I don't think you do if your intent is to call into question the fact it is a representation of maintenance hours.


Do you?


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## renrich (Dec 18, 2007)

I think the notion that air cooled engines are more reliable than liquid cooled is based somewhat that they are more resistant to battle damage. That is the reason the US Navy ruled out liquid engines prior to WW2.


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## davparlr (Dec 18, 2007)

Crumpp said:


> You can see by examing this document that the R-2800 offered no advantage in number of maintenance hours over any other aircraft engine.
> 
> http://afhra.maxwell.af.mil/aafsd/aafsd_pdf/t114and115.pdf



I am not sure this tells us anything about the availability of aircraft at the squadron level but only tells us how long it takes to overhall the engine at depot. I am not too knowledgeable but I suspect that depot overhall manhours starts with the engine out of the aircraft and probably does not include the cooling system on the liquid cooled engines. As such, the hours are similar for work accomplished, 11.8 hr./cylinder for the V-1710 to 13.4 hr/cyl for the R-2800 (average, 1945).

The real importance to warplane availability is things like mean time between failure (MTBF), mean time between scheduled maintenance (MTBSM) (like engine overhall), mean time between unscheduled maintenance (MTBUM), mean time to repair (MTTR), etc. These are what planners must use to determine how many of 100 aircraft assigned will be available for combat at any given time. Improvements in these numbers at the weapon system (aircraft) level are force multipliers.

Since we don't have these numbers for the engine systems, predicting weapons system reliability is difficult. However, as a rule of thumb, these numbers are proportional to complexity. As such, when you calculate in the liquid cooling system reliability, it seems the air cooled engine should have fewer failures, thus more reliability. Also, it must be noted that the 18 cylinder R-2800 engine in itself is pretty complex (note that the overhall number for the R-2800 is higher than the smaller cylindered stablemates).


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## Crumpp (Dec 18, 2007)

> but only tells us how long it takes to overhall the engine



That is a very good indicator of maintenance guys. Why do you think Bill, another aircraft owner, thought it was such a good find?

Here is why it is a very good indicator of maintenance requirements: 

_There are several levels of "major overhaul"._ 

*Service limit* - Parts are checked to ensure they fall within service limits. 
This overhaul has little chance of reaching TBO as the standard is the minimum service limits. If a part is allowed a service limit of .010 and it is at .009 when checked the part continues life. It does not need to be replaced in a service limit overhaul. These are the least expensive overhaul because generally, there is less disassembly involved. *Parts are replaced as needed.*

*New Limit Overhaul* - Parts are checked to ensure they fall within *new part* tolerances. Engine time is zeroed and reset. This is the overhaul the US Military most commonly does as it has it's own mechanics to perform the work. The engine logbook continues life. *Parts are replaced as needed*

*Re-manufactured limits *- Performed at the factory by the factory being the major difference between a "New Limit" major overhaul and "re-manufactured Limits". Engine time is zeroed and reset. Factory issues new warranty and new logbooks. *Parts are replaced as needed*.

The common theme with all of the overhauls is, "*Parts are replaced as needed*.".

Therefore we can examine the document and see that in 1943, the R-2800 required considerably more time to overhaul than it did in 1945. Why? The engine became more reliable. The parts were made stronger or the points of stress reduced and the overhauls did not need as many parts replaced.

Partial disassembly and checking tolerances is one thing. Complete disassembly and reinstallation of a new part is quite another.

You can now see that the R-2800 developed into a reliable aircraft engine and that _all aircraft engines are very similar in their maintenance._

All the best,

Crumpp


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## Crumpp (Dec 18, 2007)

Understand too that crank tolerences on most aircooled engines can be checked by removing the barrels while leaving the corncob on the mounts.

Liquid cooled inlines require removal of the engine and disassembly of the case to check the crank. So pick your poison, both engines take about the same amount of time to overhaul. Do you want more barrels to overhaul or do you want to remove the engine?

All the best,

Crumpp


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## drgondog (Dec 18, 2007)

Crumpp said:


> That is a very good indicator of maintenance guys. Why do you think Bill, another aircraft owner, thought it was such a good find?
> 
> Here is why it is a very good indicator of maintenance requirements:
> 
> ...



Gene - slightly off on a tangent but still in the strike zone. While I no longer fly or own my own ship 'the dreaded engine overhaul' to New Limit OR Re-manufactured Limit was by far the worst sound - then and now.

I have a close friend with both a Gulfstream IV and Challenger at Hayward who is having turbine blade replacements for a cool Million and a Half on the Challenger - approximately 20% of the total value of the entire ship.

Has anyone priced out a Re-manufacured Limit $$ for either the R2800 or 1650-7 lately?

This is why Mustangs get sold.. The previous owner got one 25 years ago, flew it very little but one day the dreaded day comes.

Of course the stuff that Crump and his guys do on rebuilding an Fw 190 ranging from inspection of spars (and repair/re-manufacture) of 60 year old combat airframes (and engine parts) is unbelieveably expensive and doesn't even fit in the discussion.


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## Crumpp (Dec 18, 2007)

> This is why Mustangs get sold..



You are so right, Bill. The cost of airplanes is staggering.

> The following is a shameless plea for donations!

We live off donations and are non-profit. One of the reasons I am out posting on these types of boards is to help keep up the interest in WWII aircraft and get the word out on the WWII Aircraft Foundation.

Buy some Christmas gifts from our shop, make a donation, or become a member. We need your support and only through that support can these airplanes continue to grace blue skies.

All of you are more than welcome to visit the Museum and the Restoration Shop.

WWII Fighter Aircraft Foundation

All the best,

Crumpp


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## Crumpp (Dec 20, 2007)

Hi davparlr,

There is no mystery to solve. Failure predictions give us the maintenance time schedule. This schedule is posted in the appropriate pubs on the type. Keep in mind too, that as an aircraft, our margins are much smaller than we would find on any other form of transportation.



> The real importance to warplane availability is things like mean time between failure (MTBF),



Time Between Overhauls is a function of time between failures. Most aircraft engines have a TBO of around 2000hrs. For example, both the R-2800 and the BMW801D2 both had 2000hrs TBO's. 



> mean time between scheduled maintenance (MTBSM) (like engine overhall),



All engines have a recommended regular maintained schedule. This schedule is dictated by probable failures of subcomponents. These schedules are very similar in scope and frequency by the fact physics limits our design choices.

The schedule is posted in the engines manuals.



> mean time between unscheduled maintenance (MTBUM),



Impossible to quantify and has little bearing IMHO. Everyone works to reduce this portion. Given a reasonable passage of time, both operators and design teams will reduce this to the point that once again, all aircraft engines are similar.

All the best,

Crumpp


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## davparlr (Jan 10, 2008)

Crumpp said:


> Hi davparlr,
> 
> There is no mystery to solve. Failure predictions give us the maintenance time schedule. This schedule is posted in the appropriate pubs on the type. Keep in mind too, that as an aircraft, our margins are much smaller than we would find on any other form of transportation.



I still do not think that rebuild time tells us anything about reliability of the engine. In a way it corresponds in that a more complex engine typically takes more time to rebuild and a also has a lower MTBF. However, an engine may be very difficult to rebuild but be built to such standards as to be very reliable. In fact, in designing equipment and installation on modern aircraft, this is a criteria. If the part is difficult to repair or is hard to get to, reliability must be built into the component. Still rebuild time is high but reliability is up.



> Impossible to quantify and has little bearing IMHO.



Actually, operationally this is of major concern since all aircraft related aborts are due to unscheduled maintenance (if it could have been scheduled, it would have been). As you mentioned, operationally, this can indeed be quanitified over time and incorporated in aircraft availability planning and corrective action implemented. 

On newly developed engines, it is a large impact, but, as time goes by, fixing these are part of the maturing process and equipment get more reliable. For comparing reliability of engines, only mature designs should considered and MTBUM should indeed have little bearing.


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## Crumpp (Jan 10, 2008)

> On newly developed engines, it is a large impact, but, as time goes by, fixing these are part of the maturing process and equipment get more reliable. For comparing reliability of engines, only mature designs should considered and MTBUM should indeed have little bearing.



Over a fixed and finite time period such as determining the reliability of WWII aircraft engines it is impossible to only consider mature designs.

Hence my statement that MTBUM has little bearing in the context of comparing WWII engines.



> In a way it corresponds in that a more complex engine typically takes more time to rebuild and a also has a lower MTBF. However, an engine may be very difficult to rebuild but be built to such standards as to be very reliable.



An engine with a 1500hour TBO has a shorter MTBF than an engine with a 2000hour TBO.

A 2000hour TBO is a 2000hour TBO whether it is on a 4 cylinder engine or a 28 cylinder radial. FYI a Lycoming IO-360 4 Cylinder air-cooled engine, PW R-2800 series, and the BMW801 series all have a TBO of 2000hours. *The MTBF of these engines is comparable and the TBO is a function of that MTBF.* 

Certainly the more complex engines designer worked to increase the MTBF and that workload was higher than the team that worked on a simpler engine. 

The complexity of the engine is irrelevant if both engines last 2000hours and we are looking to compare reliability.

If we want to compare reliability as a function of complexity then your point is valid. The R-2800 required much more development time than the Lycoming IO-360. 

If the engine we are comparing are reliable and shows little or consistent wear, then the Overhaul Man-Hours will remain consistent as well.

If the engine is not reliable, then we will see a large reduction in overhaul man-hours as that reliability is increased by the designers as well as end user’s input.

All the best,

Crumpp


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## davparlr (Jan 10, 2008)

Crumpp said:


> Over a fixed and finite time period such as determining the reliability of WWII aircraft engines it is impossible to only consider mature designs.
> 
> Hence my statement that MTBUM has little bearing in the context of comparing WWII engines.



No problem here. This is what I agreed.



> An engine with a 1500hour TBO has a shorter MTBF than an engine with a 2000hour TBO.
> 
> A 2000hour TBO is a 2000hour TBO whether it is on a 4 cylinder engine or a 28 cylinder radial. FYI a Lycoming IO-360 4 Cylinder air-cooled engine, PW R-2800 series, and the BMW801 series all have a TBO of 2000hours. *The MTBF of these engines is comparable and the TBO is a function of that MTBF.*



No problem here.



> Certainly the more complex engines designer worked to increase the MTBF and that workload was higher than the team that worked on a simpler engine.
> 
> The complexity of the engine is irrelevant if both engines last 2000hours and we are looking to compare reliability.
> 
> ...



All of this is fine.



> If the engine is not reliable, then we will see a large reduction in overhaul man-hours as that reliability is increased by the designers as well as end user’s input.



I am not sure of what this says. But this is my main point.

Say that both the R-2800 and the IO-360 (hypothethically, since I do not know what the TBO of either is), has identical TBO, say 2000 hrs. This would imply that both have similar reliability. However, I am quite sure that the manhours to rebuild is much higher for the highly complex R-2800 than the manhours for the simpler IO-360. In other words, manhours to rebuild is a good reflection on complexity, but a poor reflection of reliability.

Even similar types of engines can have the same reliability but one can be more maintainability friendly and thus have less maintenance manhours to rebuild. I am sure you have seen designs in aircraft and cars that were not maintenance friendly, and, some that were.


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## Crumpp (Jan 10, 2008)

> (hypothethically, since I do not know what the TBO of either is),



The TBO for both engines *is* 2000hours, davparir.



> However, I am quite sure that the manhours to rebuild is much higher for the highly complex R-2800 than the manhours for the simpler IO-360.



True. The much simpler IO-360 requires less man-hours to overhaul.



> In other words, manhours to rebuild is a poor reflection of reliability.




I have edited your sentence. This is not a true statement. *Manhours is a good reflection of reliability.*

I have explained why already. The problem is in how you are viewing the data I believe and what you expect to see from the data. 

You cannot look at the specifics but rather examine the trends. The data is excellent for trend development because it represents the extent of wear under normal usage of the engine as a function of it's MTBF. It's not tainted by combat or prototypical testing.

To use this data for example:

If both our engines are reliable, then the difference in man-hours will remain constant. If one engine is not reliable, then the trend will be our average man-hours decreases as our engine becomes more reliable during it’s developmental lifecycle. This is a function of both the end user’s and design teams working to overcome problems and improve the engine. The reliable power plant will remain generally fixed in average man-hours to overhaul. It’s already reliable within the constraints of physics.

Understand?

Now when you get to a certain level, the complexity of the engine is academic and it becomes a matter of just picking your poison. There is little to choose in the complexity of a V-1650 and R-2800.

We can examine the data and see that the R-2800 experienced a ~286% reduction in overhaul man-hours during the war and the V-1650 experienced a ~170% reduction in overhaul man-hours.

That tells me that the R-2800 started out as considerably less reliable a power plant than the V-1650 series.

We can also conclude by examining the overhaul man-hours that by the end of the war, the R-2800 had developed into at least an equally reliable power plant as the V-1650. It probably reached the extent of its reliability within the constraints of physics.

All the best,

Crumpp


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## pbfoot (Jan 10, 2008)

I may be off track here but from what I've gleaned from various sources is that after 200hours of combat flying they rotated the Spits back to OTUs or moved them to less strenuous duties


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## davparlr (Jan 11, 2008)

Crumpp said:


> You cannot look at the specifics but rather examine the trends. The data is excellent for trend development because it represents the extent of wear under normal usage of the engine as a function of it's MTBF. It's not tainted by combat or prototypical testing.
> 
> 
> Understand?
> ...



Aaaah, I see!

Let me make an editorial change to your editorial change to my comment that I think correctly reflects your position,

"Manhours *trends are *a good reflection of reliability."

I agree with this! Good observation. 8)


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## drgondog (Jan 11, 2008)

pbfoot said:


> I may be off track here but from what I've gleaned from various sources is that after 200hours of combat flying they rotated the Spits back to OTUs or moved them to less strenuous duties



That does suprise me. The Mustangs in the 8th AF generally trended to 500 airframe hours before being declared War Weary and assigned to OTU or in Group Clobber Colleges. From memory the highest time P-51B I found for that category in the 355th was 640 hours and it flew steadily from March 1944 through October 1944 before declared WW.

The A/B/C/D/K Mustangs were designed to 8g limit and 12g ultimate which I believe was higher than the Spits. The P-51H was designed to 7 1/2 and 11.25 to reflect more to Brit standards to conserve weight. 

I suspect the lower limit load envelope may have something to do with the lower airframe hours?


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## Crumpp (Jan 12, 2008)

> I agree with this! Good observation.



Thank you.



> I suspect the lower limit load envelope may have something to do with the lower airframe hours?



It does, Bill. The lower load limits means the airframe is experiencing more wear and tear in normal flight. Gust and density effects of the atmosphere cause the normal "bumpiness" of the air to exert more relative stresses on the lower limitations of the airframe. This in turn causes increased wear.

I suspect the RAF's decision to remove the Spitfire from the line at 200hours has more to do with Logistical system of the RAF than airframe design. The CRO was a major source of repair for RAF airframes. This loose organization did an outstanding job and was a key factor in the RAF victory in the Battle of Britain. Being a loose organization of civilian repair shops the organizational answer to the slight loss of control on repair quality control would be to reduce the airframes frontline service time. 

I imagine too this removal was nothing more than an airframe overhaul to serviceable standards with placement right back to service.

It is a possibility that the aero elasticity of the thin wing of the design affected the airframe limits as well but I would tend to be skeptical of such major departures from aircraft norms until I saw definitive proof.

All the best,

Crumpp


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## Kurfürst (Jan 12, 2008)

Never particularly understood these comparisons of prescribed overhaul periods, usually put forward to claim superior quality of some sort.

It`s odd, because generally there was not too much difference between the 'lifespan' (or really, the amount of time between major inspections) of contemporary engines and airframes, and statistically, airframes and engines were lost or damaged in combat well before being worn out would become a concern..


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## Crumpp (Jan 12, 2008)

> It`s odd, because generally there was not too much difference between the 'lifespan' (or really, the amount of time between major inspections) of contemporary engines and airframes,



Exactly. It's a function of the physics of aircraft.


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