The first non-pressurized Turbo Mooney was introduced in 1979. Basically, Mooney Aircraft Corporation capitalized on the aerodynamically improved airframe of the Mooney 201 by installing an engine that possessed a high horsepower to weight ratio, the Teledyne Continental TSIO 360 GB Engine. When they were done with their latest innovation, the Mooney model M20K/231 was born. This combination of engine and airframe gave new Mooney buyers the optimum blend of high altitude flying coupled with incredible speed and performance synonymous with Mooney Aircraft. In 1986 Mooney Aircraft vastly improved upon the Mooney 231 by installing a variant and much improved version of the original engine. This time the engineering folks at Mooney Aircraft got together with their counterparts at Teledyne Continental Motors and "cleaned up" several of the nuisances with the Mooney 231 engine by installing an Intercooler, hydraulically controlled wastegate and controller, more effective oil cooler and other changes internal of the engine. The result, the Mooney M20K/252. The 252 in my opinion is a vastly superior aircraft to the 231. Predominately, my opinion is based on the substantial improvement to the engine. However, the airframe was not overlooked. Inboard gear doors were added and better engine instrumentation to name a few of the significant changes to the airframe. And, as usual with the magicians at Mooney Aircraft Corporation, they were not done perfecting the Mooney M20K. In 1997 the Mooney M20K/Encore was introduced. Once, again, the engineering phenoms at Mooney Aircraft decided to tweak a successful line of aircraft and improve it just that much more. The Encore possess 10 more horsepower that the 252, the latest design/innovation in aircraft interior and instrumentation and most importantly a substantial increase in the always illusive useful load!
All right, enough with the Bar facts. Let's get down to the brass tacks. What should you look for or better yet, look out for when purchasing a Mooney M20K? Lets start from the beginning with the Mooney 231. The Mooney 231 was manufactured first with the Teledyne Continental Motors TSIO 360 GB and then later the TSIO 360 LB. The "LB" version is more reliable and has fewer difficulties than the GB version. Having said this, both engines are prone to service difficulties. The engine time between overhaul (TBO) for the 231 engine is 1800 hours. Any 231 engine that has 1000 hours or more since new or last overhaul is in my opinion on "borrowed time". I do not want to imply that the engine is all done at 1000 hours, however, it has been my experience that this engine begins to develop significant compression and oil consumption problems on or about 1000 hours. There are those people "in the know" who believe all TSIO 360's require a top overhaul at 1000 hours of engine operation. So what does one do when facing the opportunity to purchase a Mooney 231 with an engine that has over 1000 hours since new or last overhaul? Proceed with caution. Review the engine maintenance records to determine the trend of the compression readings. Are they steady or on a steady decline? Are all six cylinders about the same or the compression numbers all over the place? Typically at 1000 hours my observation has been that most Mooney 231 engines will have compression readings somewhere in low 60's. It I were performing a pre-purchase inspection of a Mooney 231 and all six cylinders were somewhere between 6280 through 68/80, 1 would consider this engine to be in average to good condition depending on how many cylinders were closer to 68/80. Conversely, if my analysis of the compression readings were that a couple cylinders were 62/80 and the remaining cylinders were in the 50's, I would expect that this engine is closing in on the dreaded top overhaul. If you find a 231 that appears to be in good condition other than lower than desired cylinder compression readings, do not necessarily pass it up. My point is that I believe you can do far worse than an otherwise good 231 that its only deficiency is low compression. Premature cylinder wear is an unfortunate concession one makes when purchasing a Mooney 231 with either the TSIO 360 GB or LB engine installed.
There are many theories as to why premature cylinder wear occurs. The intention of this article is not to explore those theories. However, I believe the predominant cause of premature cylinder wear and lower than acceptable compression readings are excess heat. Teledyne Continental Motors as well as various "STC Holders" recognized this. That is why the 252/Encore is intercooled with a hydraulically controlled wastegate and turbo controller. This combination of components was designed to lower both cylinder head and exhaust gas temperature values. The result was a much-improved engine in every way. The 252 utilizes a TSIO 360 MBand the Encore a TSIO 360 SB. Both engines, as mentioned, come standard equipped with an intercooler
and hydraulically controlled turbocharging system. While the improvements to the 252 and Encore engines are substantial improvements, the changes made are not perfection. Both of these engines also share the reputation for needing a cylinder or two before reaching TBO. However, the net effect of lowering the engine operating temperatures will result in fewer maintenance difficulties over the life of the engine. And, the best opportunity to making it to TBO.
Thus far, we have discussed the anomalies with premature cylinder wear associated with the M20K series of aircraft. Because cylinder compression testing can be subjective from one maintenance facility to another, I believe a discussion about the "science" of cylinder compression testing would be in order. The acceptable compression readings of a Continental engine will typically be much lower than those of Lycoming engines. Teledyne Continental has printed Service Bulletin M84-15 which dictates the approved procedure and acceptable compression readings for their engines. In this Service Bulletin they discuss the difference between "static leakage" and "dynamic leakage". Static leakage is air leakage past the valves and dynamic leakage is air leakage past the compression ring. In Service Bulletin M84-15, Continental states that zero static air leakage is acceptable. However, dynamic air leakage is another story. Teledyne Continental will allow incredibly low differential compression readings if the low reading is due to dynamic air leakage. How low is acceptable? Well that varies depending on the tester you are using. Let me try to explain this to you. In the aforementioned Service Bulletin, Continental specifies a special tool (part number XXXX) that must be used to calibrate the differential compression tester the Technician uses to perform the differential compression tests. Upon application of the special tool with the differential compression tester, a Technician will then know the lowest acceptable value for differential compression tests performed on Continental engines. For example, at East Coast Aviation both of our calibrated differential compression testers are at or about 48/80 P.S.I. Thus, according to Teledyne Continental Motors, differential compression readings as low as 48/80 P.S.I. are acceptable so long as the air leakage is past the compression rings and not the valves.
Now, after that long-winded explanation on what Continental considers acceptable let me explain what our standards are at East Coast Aviation. First of all, if performing an Annual Inspection we wouldn?t return to service any engine that had a cylinder or cylinders with differential compression readings of 48/80 P.S.I. How low is too low depends on a few factors.
1. The total time of the cylinders.
2. The relationship of all cylinder compression readings compared to each other and what the compression readings were the last time a differential compression check was performed. Also, when comparing the differential compression readings subsequent to the current readings, the amount of flight time that has elapsed is important.
All of these factors are important when evaluating the health of six individual cylinders.
OK, I agree, enough already about cylinder compression tests/issues with the Mooney M20K Series aircraft. What else should my Maintenance Technician be looking for when performing a pre-purchase inspection on my next new Mooney?
Staying within the engine nacelle for now ....
·231/252/Encore: Perform an oil and filter change. Ensure the oil filter cartridge is checked for excess metal. Request previous copies of oil analysis if available. If they are unavailable, ask why.
·231/252/Encore: Check the metal engine mount for severe rust, specifically the front left and right corners. In the case of the 231, inspect in the vicinity of the turbocharger for excess rust/ deterioration of the engine mount. The reason the engine mount tends to rust in these areas is because of heat erosion of the paint applied to the engine mount caused by the close proximity of the engine exhaust system.
·231/252/Encore: Check the metal cooling air baffles for cracks. Typically you will find some cracks and several of the metal cooling air baffles will have been repaired. This is common place for this aircraft/engine installation. Eventually, decision must be made whether to repair or replace an individual piece of the cooling air baffles.
·231/252/Encore: While we are discussing the cooling air baffles ... make sure the flexible cooling air baffles are in good condition and are not limp or blown backwards. In my opinion, there is not enough emphasis placed on the importance of flexible cooling air baffles being in good condition.
·252/Encore: Carefully check the induction tubes where they attach at the cylinder heads for the 252/Encore engine for cracking. You may notice black soot or streaking emanating out from the flange that secures the induction pipe to the cylinder head. Teledyne Continental has issued Service Bulletin 98-8 which addresses this exact problem. If this service bulletin has not been complied with the likelihood of cracked induction pipes is very high.
·231/252/Encore: Check the Adel clamps that secure the turbocharger inlet and outlet oil pressure check valves. These Adel Clamps are a special type that can be recognized by the yellow color of the rubber that surrounds them. If the color of the rubber is red or black the check valves must be removed and inspected for excess chaffing. Also, ensure the check valves are properly "oriented" with the side marked "hinge" in the up position.
·231/252/Encore: Check the two cabin air heat shrouds for cracking and overall condition. Also, check the flanges of the exhaust pipes that they enshroud for cracking. The cabin air heat shrouds are like the metal cooling air baffles mentioned above, if they have numerous crack patches and are cracked once again, eventually they need to be reduced to Coke Cans and replaced with new units.
·252/Encore: Inspect the cam/roller bearings that actuates the variable poppet valve for the wastegate controller for excess play. We have found this oft overlooked item to cause variations in the maximum manifold pressure indication at take off.
·231/252/Encore: Oil pressure. All three engines are prone to declining oil pressure values as the engine ages. Mahlon Russell of Mattituck Aviation has told me that he believes this is because the oil pump and system were designed for the Teledyne Continental Motors 0
·231/252/Encore: Magnetos. Both manufacturers of magnetos require a special inspection every 500 hours of operation. At East Coast Aviation our experience has shown that a better interval is every 250 hours of operation. For some reason, we have had many more problems with magnetos on the 231/252/Encore prior to the 500 hour special inspection than on other Mooney Aircraft. We believe that it has something to do with the magneto pressurization system pumping in moisture, but have never really consulted with anyone on this topic. Anyway, it is our recommendation on the M20K Series aircraft, pull the magnetos every 250 hours of operation for the special inspection.
·231: If the "old style" alternate air door is still installed make sure the "new style" alternate air door kit is installed as soon as possible. The latter is vastly better than the former.
·252/Encore: Inspect the magnet that keeps the alternate air door closed to ensure it has not loosened up. We have to periodically replace these items from time to time. We have even seen the magnet missing, sort of scared me where that thing went!
·231: The hot section of the turbocharger can develop extensive cracks. There was an AD that came out years ago that dealt with this issue. Basically it required replacement of the hot section of the turbocharger. Upon compliance, the "new style" housing relieved compliance with the AD. Bottom line, the new ones can crack also, that is the bad news. The good news is I have never seen one actually fail. The only way you can see these cracks is to remove the exhaust transition housing at the turbocharger and visually inspect.
·231/252/Encore: Make sure your pre-purchase inspection technician knows how to pressurize the intake and exhaust system utilizing shop air or the exhaust of a fairly strong shop vacuum cleaner. With either system pressurized, they can inspect for leaks at seals, gaskets, o-rings or cracks of components.
·231/252/Encore: Check the "coming in speed" of the gear driven alternator. On a 231 this is the only type of alternator installed, on a 252 this will be the number one alternator, on an Encore this will be the right alternator. The "coming in speed" for the sake of this article is the engine RPM required to extinguish the high/low alternator warning light. This should occur on or about 1200 RPM with a "typical" electrical load. If it takes 1600 RPM or better to get the warning light to extinguish, suspect a bad alternator or alternator drive clutch. Either way, both are expensive components and their deterioration can be easily identified. If you are considering buying a rare 252 that has only one alternator (most had two), be careful. This aircraft will go through a lot of batteries. For those MAPA members that know Mr. Bill Lamb, just ask him, he will tell you. Bill is convinced that from the proceeds of the many batteries he purchased from East Coast Aviation, my son will be able to enjoy a Harvard education! Getting serious again, the 252 with one alternator is just not sufficient enough to keep the battery properly charged at lower engineRPM's. In my previous articles we have discussed performing pre-purchase inspections on the Mooney TLS/Bravo and the Ovation. The oldest of these two types of aircraft is 1989 which is when the TLS was introduced. As mentioned, the M20K Series was introduced in 1979. This presents more significant airframeissues because of the potential high airframe total time in service as well as extensive calendar time. At this point the first Mooney 231's were introduced 20 years ago! This presents some new and different problems that we have not discussed previously. Such as tubular structure corrosion, airframe corrosion, metal fatigue, windshield and side window crazing/distortion, accumulation of debris/soot/exhaust residue old flexible brake hoses, worn out lock discs, etc. Since the basic airframe of the 231/251/Enclore is the same as the TLS/Bravo and Ovation there will be many items discussed that I have mentioned before and there will be some new items indigenous to the M20K Series aircraft.
·231/252/Encore: Carefully inspect the fuel tank area for leakage. Obviously, as a Mooney aircraft ages so does the fuel tank sealant. If you purchase a 20-year-old 231 and the fuel tank sealant has never been replaced, get ready cause it's coming.
·231/252/Encore: Inspect the tail for excess play at the tail trim link mechanism. Also, check for excess play at the two hinge bolts.
·231/252/Encore: The two elevators and rudder utilize several rivets, call, "Cherry Max" rivets. Inspect these rivets for signs of loosening. Occasionally, we have seen these rivets start to loosen or "work" as Technicians call it in the field. You can identify this by a trail of black soot aft of the rivets. Technicians in the field call this, "smoking rivets".
·231/252: Check the Cherry Max rivets at wing station 145.7 of either wing for the same condition, "Smoking rivets". This is a fairly common condition, One tip here, if the Cherry Max rivets are smoking, ask your maintenance technician to try and replace them with as many solid rivets as possible.
·231/252/Encore: Check the topsides of the wing flaps for excess chaffing. All Mooney wing flaps will chafe to a certain degree. However, if the flap attaching and actuating hardware are not maintained to near new tolerances, excess wing flap chaffing will occur.
·231/252/Encore: Check the landing gear shock disc's for excess compression. The maximum allowable compression is 5/8". Also, if you jack the aircraft and can rotate the shock disc's one half hour later, the shock disc's require replacement.
·231/252/Encore: Check the landing gear rigging and play amongst all of the actuating rod ends. If the aircraft was maintained by well meaning Technicians that are not very familiar with the M20K Series landing gear system, there may be problems. Pay particular attention to the roll pins that are used to secure each rod end to its respective push pull tube. We note play in this area from time to time. Also, because of the potential for high airframe total or calendar time, several individual sub-components may be worn out. Such as bolts, bushings, retraction links, etc.
·252/Encore: Carefully check the inboard gear door attaching rivets for looseness or play. Several times I have found loose rivets that attach the inboard gear door to the round stock metal.
·231/252/Encore: Inspect the nose gear steering linkage for excess play. A component called the nose gear steering horn needs to be properly shimmed. If allowed to develop excess play it will wear out. Then you get to spend lots of your hard
·231/252/Encore: Perform a careful evaluation of the tubular structure. If I suspect rust exists, I will remove the interior side panels so that I can get a good look at the entire tubular structure, less the overhead tubes which requires removal of the headliner. Most aircraft up through the late eighties were coated with Zinc Chromate primer. This primer base does not have sufficient staying power. In other words, rust will set in over time. How much time depends on the environment in which the aircraft has been exposed? This is the perfect segway to discuss the infamous Mooney Aircraft Service Bulleting 208B. Proper completion of this Service Bulletin is essential to corrosion prevention. Please keep in mind that complete compliance with this service bulletin is rarely accomplished. Most owners choose to complete a modified portion of this service bulletin, however, owners, brokers, etc. will advertise that Service Bulletin 208B is complied with. Be wary, educate yourself as to what complete compliance means. Then, position your questions to determine if complete compliance has occurred. Post 1986 Mooney Aircraft started using an Epoxy based primer which is vastly superior to the Zinc Chromate primer. This type primer seems to have superior rust resistant properties. However, it is not perfect. I have seen rust exist in the presence of the Epoxy based primers. Typically, it has been only a minor amount.
·231/252/Encore: Another trap that is more subtle than the items mentioned above and typically found on aircraft that have extensive calendar time as well as several previous owners are major alterations, repairs and modifications. Make sure this type of work is properly documented and the paperwork is in order. Once you own it, you are responsible for ensuring all proper documentation is in place. I can not tell you how many times I have discovered a major alteration, repair or modification has been made and not properly documented. This can become a sticky wicket when one Maintenance Technician must inspect and sign off an alteration, repair or modification performed by another. Each and every time I have written a pre-purchase inspection article I always feel uneasy that I left out something important. After thinking this through, I realized there is just no substitute for product knowledge. I have stated it before and I will state it again, make sure you have very experienced Mooney Aircraft Maintenance Technicians execute your pre-purchase inspection. Especially when you are contemplating the purchase of a 20-year-old aircraft. Just because an aircraft has 20 years or 4,500 hours since new doesn't mean it isn't a good aircraft, worthy of the asking price. However, it does mean you should approach the transaction in the right frame of mind. An older 231 or 252 offers Pilot's an excellent opportunity to own, operate and fly one of the best high flying, high speed single engine values in the industry. Comparatively, the Turbo Mooney is second to none!