Wet Heads
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Wet Heads
The following is from a Mooney M20M TLS flight evaluation.
It seems that perhaps due to 'aggressive leaning' to the 1750°F TIT limit the original TIO-540-AF1A often couldn't get passed 3-400 hours without cylinder work, but Lycoming installed oil-cooled valve guides, called it the TIO-540-AF1B and.. I don't know, was this the end of the problems? Also, if it was such a problem with this engine why don't Chieftans etc also have wet heads, the article seems to indicate not. The fix was specific to the Mooney and had been also used on the Beech Duke before.
It seems like they set a very high TIT limit, told pilots to lean to it in their POH and then had to introduce an engineering solution for a problem they sort of created themselves. Seems like it would have been easier to reduce the TIT limit at high power settings.
Full report: M20M Evaluation
It seems that perhaps due to 'aggressive leaning' to the 1750°F TIT limit the original TIO-540-AF1A often couldn't get passed 3-400 hours without cylinder work, but Lycoming installed oil-cooled valve guides, called it the TIO-540-AF1B and.. I don't know, was this the end of the problems? Also, if it was such a problem with this engine why don't Chieftans etc also have wet heads, the article seems to indicate not. The fix was specific to the Mooney and had been also used on the Beech Duke before.
It seems like they set a very high TIT limit, told pilots to lean to it in their POH and then had to introduce an engineering solution for a problem they sort of created themselves. Seems like it would have been easier to reduce the TIT limit at high power settings.
And fast it is. The engine is a version of Lycoming’s parallel-valve; 540-cubic-inch opposed six with a single AiResearch Turbocharger and carries a maximum power rating of 38 inches manifold pressure and 2575 RPM. Two controllers operate the single wastegate to regulate pressuri ze d air to the induction system. With this system, the engine is capable of maintaining its full 270 hp up to FL210 under standard conditions. With this high 21,000-foot critical altitude, the TLS is able to achieve very high cruise airspeeds. At FL250, as high as you can legally fly in the M20M, it’s still possible to utili ze maximum continuous power – 34” and 2400 RPM, which is about 240 hp or 89%!
Of course all this speed comes at a cost - about 18 gallons of Avgas per hour of cost to be specific. And that is if you lean aggressively to the 1750-degrees-Fahrenheit turbine-inlet temperature (TIT) maximum, one of the highest certified TIT's in piston aircraft. We will discuss leaning in detail again later in this report.
One thing did become evident during the first few years of production. With the airplane delivering such high cruise airspeeds, at high altitude with high power settings, many TLS owners discovered that their airplanes needed engine work at 400 to 500 hours.
In retrospect, this could be a predictable situation. Here we have a large-bore engine producing 270-horsepower and in the hangar next door may be another variation of the TIO-540 in, say a Navajo Chieftain, producing 350-horsepower. It would be easy for the average TLS operator to think his engine was a de-rated version of the 350-hp monster. At 270 hp it’s not working as hard as the engine in the Piper, and using nearly 90% of that number for cruise should pose no major problems. Unfortunately, this premise is incorrect. The two engines have little in common. The engine used in the TLS is rated at 260 hp without the turbocharger – at a higher compression ratio and RPM – and is really a lightweight version of the Navajo’s 540. As a result, exhaust valve guides bore the brunt of the wear from these operations.
There was a predictable rise in oil consumption and dropping compression readings during as little as 300 hours of operation. Fortunately, these excessive wear indicators did not lead to any catastrophic failures. But many owners discovered that a quarter of the way to overhaul, top-end engine work was necessary. One of the factors in this increased need for unexpected maintenance could have been the high TIT red line. Most turbocharged Lycoming engines of that day had a more conservative limit of 1650 degrees F, and they had a better maintenance picture than the early TLS’s.
Mooney and Lycoming came up with a solution for the TLS top-end wear problems. A process that Lycoming used successfully to cure similar problems on the TIO-541 Beech Duke engines – oil cooled exhaust valve guides – was accomplished on the TLS. The illustrative name “wet head” was given to the conversion that was accomplished on all production engines that resulted in a designation change to TIO-540-AF1B. The change from the AF1A to the AF1B brought about the nickname for the engine, the “Bravo mod”, and in fact the TLS became the TLSBravo and has since been shortened to just the Mooney Bravo.
In this conversion (and on subsequent production engines) oil is fed under pressure to a gallery in each cylinder head adjacent to the exhaust-valve guide. The guide it self has a small groove cut into the outer diameter allowing oil flow to surround the guide and draw away heat. From the valve guide, this additional oil flow is routed through the rocker boxes where it helps to draw away additional heat and then through the normal drains back to the sump. The modification is easily seen as a bunch of snaking external oil lines leading to each cylinder head. Our information is that all US airplanes have been modified (Mooney paid a substantial part of the Service Bulletin), however some foreign TLS’s may have been missed.
Of course all this speed comes at a cost - about 18 gallons of Avgas per hour of cost to be specific. And that is if you lean aggressively to the 1750-degrees-Fahrenheit turbine-inlet temperature (TIT) maximum, one of the highest certified TIT's in piston aircraft. We will discuss leaning in detail again later in this report.
One thing did become evident during the first few years of production. With the airplane delivering such high cruise airspeeds, at high altitude with high power settings, many TLS owners discovered that their airplanes needed engine work at 400 to 500 hours.
In retrospect, this could be a predictable situation. Here we have a large-bore engine producing 270-horsepower and in the hangar next door may be another variation of the TIO-540 in, say a Navajo Chieftain, producing 350-horsepower. It would be easy for the average TLS operator to think his engine was a de-rated version of the 350-hp monster. At 270 hp it’s not working as hard as the engine in the Piper, and using nearly 90% of that number for cruise should pose no major problems. Unfortunately, this premise is incorrect. The two engines have little in common. The engine used in the TLS is rated at 260 hp without the turbocharger – at a higher compression ratio and RPM – and is really a lightweight version of the Navajo’s 540. As a result, exhaust valve guides bore the brunt of the wear from these operations.
There was a predictable rise in oil consumption and dropping compression readings during as little as 300 hours of operation. Fortunately, these excessive wear indicators did not lead to any catastrophic failures. But many owners discovered that a quarter of the way to overhaul, top-end engine work was necessary. One of the factors in this increased need for unexpected maintenance could have been the high TIT red line. Most turbocharged Lycoming engines of that day had a more conservative limit of 1650 degrees F, and they had a better maintenance picture than the early TLS’s.
Mooney and Lycoming came up with a solution for the TLS top-end wear problems. A process that Lycoming used successfully to cure similar problems on the TIO-541 Beech Duke engines – oil cooled exhaust valve guides – was accomplished on the TLS. The illustrative name “wet head” was given to the conversion that was accomplished on all production engines that resulted in a designation change to TIO-540-AF1B. The change from the AF1A to the AF1B brought about the nickname for the engine, the “Bravo mod”, and in fact the TLS became the TLSBravo and has since been shortened to just the Mooney Bravo.
In this conversion (and on subsequent production engines) oil is fed under pressure to a gallery in each cylinder head adjacent to the exhaust-valve guide. The guide it self has a small groove cut into the outer diameter allowing oil flow to surround the guide and draw away heat. From the valve guide, this additional oil flow is routed through the rocker boxes where it helps to draw away additional heat and then through the normal drains back to the sump. The modification is easily seen as a bunch of snaking external oil lines leading to each cylinder head. Our information is that all US airplanes have been modified (Mooney paid a substantial part of the Service Bulletin), however some foreign TLS’s may have been missed.
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Atkinson that is 
I should also point out that agressive leaning is the wrong phrase to use. It should be called abusive leaning. Agressive leaning would be on the far LOP side of peak EGT/TIT and would result in far lower cylinder pressures and CHT's.
The problem is also found in the new Turbo 206's where in flight failures have occurred here in Australia. One of this countries beter engine shops has had them in for rebuild. Their comment was also about valve guides and Dukes
.
Be it poor design or not, with "abusive leaning" on the rich side of peak EGT, and modern glass panels alowing accurate levels of abuse, then the cylinder pressures and thus temperatures become a problem far sooner or far more often than they otherwise would.
The TIT limit is not the actual problem, so correlation of the two is not correct. For example you could create a very high TIT and have much lower ICP and CHT by retarding the spark timing. In doing so you could have kinder load on the cylinders but be at the TIT limit.
The TIT limit is a point where the turbine can operate all day and not suffer from creep problems temp & RPM), it is permissible to exceed it for short periods to establish peak and even then the time frame is generous. TAT have deliberately had to retard timing to do extended TIT testing for FAA certification projects and despite such treatment they live happily. Tough little critters!
TIT is being used to measure a point along a curve, and the famous set of curves by Advanced Pilot Seminars is worth noting with respect to the EGT/TIT relationship to both power and CHT. Study this for a while to get the concept.
As for the POH, I have not read that particular one, and if you wish to send me a copy I would happily study it for comment, but it is likely to follow in the footsteps of so many pathetically and scientifically wrong POH's of the past. (Not opinion, just sad fact).
I should also point out that agressive leaning is the wrong phrase to use. It should be called abusive leaning. Agressive leaning would be on the far LOP side of peak EGT/TIT and would result in far lower cylinder pressures and CHT's.
The problem is also found in the new Turbo 206's where in flight failures have occurred here in Australia. One of this countries beter engine shops has had them in for rebuild. Their comment was also about valve guides and Dukes
.Be it poor design or not, with "abusive leaning" on the rich side of peak EGT, and modern glass panels alowing accurate levels of abuse, then the cylinder pressures and thus temperatures become a problem far sooner or far more often than they otherwise would.
The TIT limit is not the actual problem, so correlation of the two is not correct. For example you could create a very high TIT and have much lower ICP and CHT by retarding the spark timing. In doing so you could have kinder load on the cylinders but be at the TIT limit.
The TIT limit is a point where the turbine can operate all day and not suffer from creep problems temp & RPM), it is permissible to exceed it for short periods to establish peak and even then the time frame is generous. TAT have deliberately had to retard timing to do extended TIT testing for FAA certification projects and despite such treatment they live happily. Tough little critters!
TIT is being used to measure a point along a curve, and the famous set of curves by Advanced Pilot Seminars is worth noting with respect to the EGT/TIT relationship to both power and CHT. Study this for a while to get the concept.
As for the POH, I have not read that particular one, and if you wish to send me a copy I would happily study it for comment, but it is likely to follow in the footsteps of so many pathetically and scientifically wrong POH's of the past. (Not opinion, just sad fact).
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Akro - I've been reading that thread, very educational but I thought this may be thread drift so started another.
So is the wet head a standard TIO thing these days or was it specific to Mooneys and Dukes? Why was this problem only showing itself on a seemingly de-rated 540?
I understand TIT temps are arbitrary when talking about mixture settings, but you'd have to assume seeing as it is a Lycoming that they'd be recommending ROP operations in a turbocharged engine at FL's that were probably not rich enough, hence the high TIT's on the wrong side of Jabawocky's graph, high ICPs etc.
This sought of ties into another question I have, why did the Piper Malibu get discontinued? Apart from the fact that the then TCM had to pay out US$32m to a pilot injured in an engine failure related incident (wikipedia) what was the real weakness of the TSIO-520BE. If anything it should have been a more reliable engine than the TIO-540 that replaced it, as it was meant to be run LOP only. Sort of flies in the face of the science.
So is the wet head a standard TIO thing these days or was it specific to Mooneys and Dukes? Why was this problem only showing itself on a seemingly de-rated 540?
I understand TIT temps are arbitrary when talking about mixture settings, but you'd have to assume seeing as it is a Lycoming that they'd be recommending ROP operations in a turbocharged engine at FL's that were probably not rich enough, hence the high TIT's on the wrong side of Jabawocky's graph, high ICPs etc.
This sought of ties into another question I have, why did the Piper Malibu get discontinued? Apart from the fact that the then TCM had to pay out US$32m to a pilot injured in an engine failure related incident (wikipedia) what was the real weakness of the TSIO-520BE. If anything it should have been a more reliable engine than the TIO-540 that replaced it, as it was meant to be run LOP only. Sort of flies in the face of the science.
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The weakness of the TSIO 520 was the fact despite it being mandatory to run them LOP they were not. This was most likely the poor education at the time but the old habits and some high profile media articles of the day describing how it performed better at something like 50 ROP, and poorly tuned injectors.....problems a plenty.
If John Deakin or Walter Atkinson see this thread they will surely give a much more accurate history lesson as they lived right through it.
As for the Lycoming problems, it is worth a chat with the folk at Brisbane Aero.
If John Deakin or Walter Atkinson see this thread they will surely give a much more accurate history lesson as they lived right through it.
As for the Lycoming problems, it is worth a chat with the folk at Brisbane Aero.
Why was this problem only showing itself on a seemingly de-rated 540?
Also, I wonder if baffle condition might be an issue. It seems to me that baffle condition doesn't get due attention from many Australian LAME's.
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baffle condition is critical to the longevity of your engine. ....but what would I know 
I just drink the occasional scotch and fly an old engine...
I just drink the occasional scotch and fly an old engine...
Hey Jack!.....I think you are referring to towel heads...
. Oops.....here we go again . I got banned for being a racialist once . I could not understand why since I hate everyone equally....Try not to hijack the thread but welcome back.....
What was the thread about again?.......
. Oops.....here we go again . I got banned for being a racialist once . I could not understand why since I hate everyone equally....Try not to hijack the thread but welcome back.....What was the thread about again?.......
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So pilots incorrectly assumed they could run this engine hard because after all it's 'only a derated Chieftan engine' and
This seems to me a non-sequitur.
It's indeed a mystery, wish there were more engine builders scouring forums on a Sunday!
As a result, exhaust valve guides bore the brunt of the wear from these operations.
It's indeed a mystery, wish there were more engine builders scouring forums on a Sunday!
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Piper Malibu Engine problems
Jaba said:
"Lived through it?" <Harumph> First we lived through the extinction of the dinosaurs, then we saw the creation of all that oil, then we saw airplanes come along, then there was the Malibu - and then there wasn't.
(That's called humor, er humour! You may laugh now.)
That deserves a repeat! The LIMITATIONs section of the POH clearly stated that LOP operations were MANDATORY, and ROP operations were forbidden!
Yes. It was customary in those days for FLYING MAGAZINE to publish articles in almost every issue with a pilot report on the "Airplane of the Month." Pilots all over would paste these articles on hangar walls, and discuss them, and use them to sometimes buy their airplanes. Good idea, but often poorly executed, often by the Editor.
So, they did the usual thing on the Malibu, a lovely airplane, if a little bit cramped for the tall pilot. He took it out, and reported that they set up cruise power at LOP, as per the LIMITATIONS section. He further reported that the airplane met the factory specifications for performance, and listed all the parameters. So far, so good.
Now, you've got to realize, in those days (and in certain circles these days), "Leaner is worse, Richer is better." Leaning into the LOP range was quite a leap, unknown, unfamiliar territory, and it was somewhat controversial.
Then, as an offhand, unthinking comment, he said that the engine was just a little bit rough at that setting, but this could be corrected by enriching the mixture just a little. No details, just push that li'l ole red knob in a bit. That would, to his mind, made the mixture "better" and smooth the engine out.
That one offhand remark was wholly responsible for many Malibu engine failures, accidents, and deaths. What no one knew (then) was that the LOP setting was critical, and that LEANER would have been better (but rougher), and RICHER was moving squarely into the APS "Red Box!"
And there, my friends, is the classic example of engine abuse by the ignorant. You must, must, must, at about 60 to 65% power and above, operate RICH ENOUGH, or LEAN ENOUGH. To get LEAN enough, you must have good FUEL DISTRIBUTION TO THE COMBUSTION EVENT, and the ignition system has to be in very good condition. If those conditions are not met, you are doomed to operate ROP, where things can be, and usually are, far more sloppy.
(A footnote to this. The Malibu (and a few others) have Inconel rotors in the superchargers, which allow TITs of 1750℉/955℃. The TURBOS are happy all day long at 1750, and these TITs may be exceeded for a short time (a couple of minutes) for finding peak and other tests. But a few (most) airplane manufacturers chose the cheap route in supplying exhaust PIPES that were too thin, or not good material, and many of them failed prematurely after running for many hours, giving the Turbo or the TIT limit a bad reputation.)
(A bad reputation like the Pit Bulls of today, and the T-Rex of yesteryear. I used to raise T-Rex's, and as long as you kept 'em fed they were very gentle. They preferred virgins, which was a problem around Walter, who had other uses for them, and who raised Diplodocus's. Why, I remember the time...but that's a story for another day.)
Best...
John Deakin
If John Deakin or Walter Atkinson see this thread they will surely give a much more accurate history lesson as they lived right through it.
(That's called humor, er humour! You may laugh now.)
The weakness of the TSIO 520 was the fact despite it being mandatory to run them LOP they were not.
This was most likely the poor education at the time but the old habits and some high profile media articles of the day describing how it performed better at something like 50 ROP, and poorly tuned injectors.....problems a plenty.
So, they did the usual thing on the Malibu, a lovely airplane, if a little bit cramped for the tall pilot. He took it out, and reported that they set up cruise power at LOP, as per the LIMITATIONS section. He further reported that the airplane met the factory specifications for performance, and listed all the parameters. So far, so good.
Now, you've got to realize, in those days (and in certain circles these days), "Leaner is worse, Richer is better." Leaning into the LOP range was quite a leap, unknown, unfamiliar territory, and it was somewhat controversial.
Then, as an offhand, unthinking comment, he said that the engine was just a little bit rough at that setting, but this could be corrected by enriching the mixture just a little. No details, just push that li'l ole red knob in a bit. That would, to his mind, made the mixture "better" and smooth the engine out.
That one offhand remark was wholly responsible for many Malibu engine failures, accidents, and deaths. What no one knew (then) was that the LOP setting was critical, and that LEANER would have been better (but rougher), and RICHER was moving squarely into the APS "Red Box!"
And there, my friends, is the classic example of engine abuse by the ignorant. You must, must, must, at about 60 to 65% power and above, operate RICH ENOUGH, or LEAN ENOUGH. To get LEAN enough, you must have good FUEL DISTRIBUTION TO THE COMBUSTION EVENT, and the ignition system has to be in very good condition. If those conditions are not met, you are doomed to operate ROP, where things can be, and usually are, far more sloppy.
(A footnote to this. The Malibu (and a few others) have Inconel rotors in the superchargers, which allow TITs of 1750℉/955℃. The TURBOS are happy all day long at 1750, and these TITs may be exceeded for a short time (a couple of minutes) for finding peak and other tests. But a few (most) airplane manufacturers chose the cheap route in supplying exhaust PIPES that were too thin, or not good material, and many of them failed prematurely after running for many hours, giving the Turbo or the TIT limit a bad reputation.)
(A bad reputation like the Pit Bulls of today, and the T-Rex of yesteryear. I used to raise T-Rex's, and as long as you kept 'em fed they were very gentle. They preferred virgins, which was a problem around Walter, who had other uses for them, and who raised Diplodocus's. Why, I remember the time...but that's a story for another day.)
Best...
John Deakin
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Ohhh Deakin....you are a funny man. My eyes are watering once again.
But for those learning something, stop laughing at the humour and study Johns words VERY carefully.
And as for Atkinson and the virgins.......I think he appreciated the Virgin Australia Premium Economy.
But for those learning something, stop laughing at the humour and study Johns words VERY carefully.
And as for Atkinson and the virgins.......I think he appreciated the Virgin Australia Premium Economy.
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ssshhhhh just coz John is a tights airline guy...and Walter thought it was Business
Hey Bro, be in LAX when I am as JD and I are doing lunch. I am sure you and he would have fun catching up again.
Hey Bro, be in LAX when I am as JD and I are doing lunch. I am sure you and he would have fun catching up again.
