M20K 231 Questions

[LevelWing]

Now that I know I can get away with a M20K that will meet the family requirements, I've begun researching them. Due to price point, I've been focusing most of my research on the M20K 231 models. There are several differences that I've noticed and wanted to get some feedback from everyone here. The forum's search tool has provided a lot of great information, in particular this thread:

I have a few specific areas I'd like to focus on:

Engine:

The original 231 came with the TSIO-360-GB and most of them appear to have since been upgraded to the TSIO-360-LB. However, there are still a few aircraft floating out there with the TSIO-360-GB. What issues will result from this, if any? The horsepower rating is the same and the speeds should be relatively the same.

Is a 231 that has an engine life sitting around 1,000 an issue with the TBO at 1800? That's around ~800 hours of engine life, assuming it makes it to TBO, which is ~6-8 years depending on how much is flown per year.

Turbocharger:

My understanding from the above thread is that two of the largest differences were in the way the turbocharger operated from the 231 to the 252. Most of the 231's I've been looking at have been upgraded to include the Merlyn waste gate and the intercooler. I also understand from reading that the Merlyn waste gate is still available while the intercooler is not. What other issues should I be looking at or concerned with when it comes to the turbocharger?

Other Items:

What other differences should I be looking out for when it comes to the 231 vs. the 252? I've thus far not found anything to be a show-stopper to prevent me from looking for and purchasing a 231, especially if they have the Merlyn waste gate and the intercooler upgrades.

Examples:

Here are some examples on Controller and Trade-A-Plane. I'd like general thoughts on them with regards to how they're equipped and their current asking prices. There are others on these sites as well, some with much lower engines, but some may not have a WAAS GPS or may not have the waste gate/intercooler upgrades.

https://www.controller.com/listing/for-sale/163159477/1981-mooney-m20k-231-piston-single-aircraft 

https://www.trade-a-plane.com/search?category_level1=Single+Engine+Piston&make=MOONEY&model=M20K+231&listing_id=2385509&s-type=aircraft

[kortopates]

If you at all have the means, and assuming you can't afford a 252, you should be considering a 262 - which is the 252 engine in a 231 12V airframe. The most economical means to get true turbo performance with the -MB engine.

The key thing to understand is that the Merlyn wastegate and aftercooler does not change a 231 into a 252 as you may think. The 231 originally came with a fixed bolt in the exhaust  to create a fixed "wastegate" to route part of the exhaust into the turbo. As a result the engine's critical altitude was only 15,700 DA (from memory). The Merlyn replaces the fixed bolt with a manual variable pneumatic wastegate.  Its primary benefit is to significantly raise the engines critical altitude. Doing so will improve its ability to fly high with better climb rates. However much of Merlyns marketing is over stated in my opinion and it is still not a hydraulic VAPC (Variable Absolute Pressure Controller) type of controller that is a "set and forget type" used in the 252. You will still have the issue of being careful not to overboost and still have the need to adjust MAP while changing altitudes and changing mixture etc. But it's still a huge improvement over the bolt fixed wastegate and extends the altitude of the engine. The 252's fully automatic wastegate makes flying a Turbo almost as easy as a NA aircraft while the 231 is still a handful to manage. 
The intercooler cools the induction air temp back down after being warmed by compression translating into cooler CHTs up high and improved detonation margin. For a number of years these have not been available but Turboplus, one of the past providers, has been under new ownership and has brought them back into production. But If you don't fly high, its probably not going to benefit you enough to warrant the cost.

[Niko182]

That second one looks pretty good. On average, being flown 55 hours a year is decent. The split seats in the back that fold down is a major plus IMO. If you can afford it, I'd say go 252 encore. If you can't, that second one should be great. Ask for the useful load.

[gsxrpilot]

I seem to remember your original issue was around useful load. (I might be mistaken). And while the M20K Encore has one of the best useful load numbers of any Mooney made, the M20K 231 does not. So be sure to check the UL of every example you look at. You might find that most 231's have less useful load than your M20C.

The 252's have shitty useful load as well. Except that they are eligible for an Encore conversion which includes the UL increase. In other words, to get the Encore's excellent UL, you can either buy an Encore or buy a 252 and convert it to an Encore.

 

[LevelWing]

3 hours ago, kortopates said:

If you at all have the means, and assuming you can't afford a 252, you should be considering a 262 - which is the 252 engine in a 231 12V airframe. The most economical means to get true turbo performance with the -MB engine.

The key thing to understand is that the Merlyn wastegate and aftercooler does not change a 231 into a 252 as you may think. The 231 originally came with a fixed bolt in the exhaust  to create a fixed "wastegate" to route part of the exhaust into the turbo. As a result the engine's critical altitude was only 15,700 DA (from memory). The Merlyn replaces the fixed bolt with a manual variable pneumatic wastegate.  Its primary benefit is to significantly raise the engines critical altitude. Doing so will improve its ability to fly high with better climb rates. However much of Merlyns marketing is over stated in my opinion and it is still not a hydraulic VAPC (Variable Absolute Pressure Controller) type of controller that is a "set and forget type" used in the 252. You will still have the issue of being careful not to overboost and still have the need to adjust MAP while changing altitudes and changing mixture etc. But it's still a huge improvement over the bolt fixed wastegate and extends the altitude of the engine. The 252's fully automatic wastegate makes flying a Turbo almost as easy as a NA aircraft while the 231 is still a handful to manage. 
The intercooler cools the induction air temp back down after being warmed by compression translating into cooler CHTs up high and improved detonation margin. For a number of years these have not been available but Turboplus, one of the past providers, has been under new ownership and has brought them back into production. But If you don't fly high, its probably not going to benefit you enough to warrant the cost.

Thanks for the break down. The explanation of what the waste gate does actually helps a lot. I'm not sure I'll be flying into the FL's that often so I suppose the intercooler wouldn't be a deal breaker. Unfortunately I'm not sure that a 252 is in the cards right now, though I suppose that depends on what pops up.

2 hours ago, Niko182 said:

That second one looks pretty good. On average, being flown 55 hours a year is decent. The split seats in the back that fold down is a major plus IMO. If you can afford it, I'd say go 252 encore. If you can't, that second one should be great. Ask for the useful load.

Thanks. Like I said above, I'm not sure a 252 is in the cards right now, which is why I've been researching the 231 a lot. It doesn't seem like it's too much to manage, though it's obvious there will be more to manage than on a 252.

1 hour ago, gsxrpilot said:

I seem to remember your original issue was around useful load. (I might be mistaken). And while the M20K Encore has one of the best useful load numbers of any Mooney made, the M20K 231 does not. So be sure to check the UL of every example you look at. You might find that most 231's have less useful load than your M20C.

The 252's have shitty useful load as well. Except that they are eligible for an Encore conversion which includes the UL increase. In other words, to get the Encore's excellent UL, you can either buy an Encore or buy a 252 and convert it to an Encore.

 

You're correct about needing useful load. I went back and re-read your post, and I apparently added a comma in my head (wishful thinking, maybe?) where you wrote, "...F, K Encore, and the Eagles." I read that as the K and Encore being two distinct aircraft. My mistake. That said, a lot of the 231's I've been looking at have indicated useful loads in the 1100lb range, though I'd have to ask to be sure on each one. The fact that so many others are making E's, F's, J's, and K 231's work with a family is also encouraging. 

Also, I don't see any Encores available on the major sites right now though I'm sure they pop up from time to time. A 252 with a low time engine would likely stretch the budget, but again it depends on what the final numbers would be.

[Mike A]

I have a stock 231, have owned it for for 7 years now, and love it. It has a useful load of 965 lbs and I typically operate it between 10k and 15k feet. Minus some minor bootstrapping from time to time, I have not seen any of the issues that people commonly gripe about on this site. I only recently got GAMIs installed and will say it’s a game changer. I operate between 10.7 and 11.3 GPH LOP and regularly see true air speeds in the upper 160 knot arena, after climbing to my cruise altitude at over 800 fpm the whole way near gross. 
 

I have undertaken some projects on it and dealt with the aftermath of a not great A&P that I used for the first few years of ownership (you live and learn) but my next upgrade will be an intercooler as it does get difficult to keep CHTs under 400 degrees at times in the heat of summer here in the south without putting the cowl flaps in trail. 
 

I have contemplated upgrading to a 252 from time to time when nice ones come on the market (that I can afford), but it’s hard to give up the value that you get from a 231 (performance for purchase price and operational costs).  Also that UL is great when you are loading up the family and luggage for a vacation.  I have about 550 lbs to work with after filling the tanks all the way up. 
 

You can’t go wrong with anything in the K family. Make sure you get yourself a good prebuy and do your research on this site about operational settings. Then have a blast. Once you go turbo you’ll never go back. 

[whiskytango]

I have a 231 with the -LB engine, Merlyn UDC, Intercooler and GAMIs.  The one thing that I wish I had is a vernier throttle.  Even with the Merlyn, very small movements of the conventional throttle will produce significant changes in MAP. 

[LevelWing]

Thanks for the feedback. What is your useful load, @whiskytango?

[ziggysanchez]

I have loved owning and flying my 231. I've flown it close to 900 hours in the last 3.5 years. Useful load in mine is somewhere around 930 lbs which is not too bad and has the Merlyn wastegate but no intercooler. I loaded my family in it right after my panel upgrade and flew them to Maine from Oklahoma. It was a little tight width wise but that might be more from me being 6'3" and 280 lbs than the Mooney being too small...lol. Maintenance costs on mine have been extremely low. The 231 is a great airplane. I'm about to trade it to move into an Eagle that has TKS. If you wanna pull the trigger this week and buy mine before I trade it, it would definitely be a nice one to have.

Edited August 26, 2020 by ziggysanchez

[LevelWing]

Just now, ziggysanchez said:

I have loved owning and flying my 231. I've flown it close to 900 hours in the last 3.5 years. Useful load in mine is somewhere around 930 lbs which is not too bad and has the Merlyn wastegate but no intercooler. I loaded my family in it right after my panel upgrade and flew them to Maine from Oklahoma. It was a little tight width wise but that might be more from me being 6'3" and 280 lbs than the Mooney being too small...lol. Maintenance costs on mine have been extremely low. The 231 is a great airplane. I'm about to trade it to move into and Eagle that has TKS. If you wanna pull the trigger this week and buy mine before I trade it, it would definitely be a nice one to have.

Very nice!

How much fuel do you put in there with all four seats loaded? Luggage?

[whiskytango]

2 minutes ago, LevelWing said:

Thanks for the feedback. What is your useful load, @whiskytango?

The annual is being done now and my A&P/IA has all of my paperwork.  I will let you know when I get the plane back.

[ziggysanchez]

For the trip to Maine we did 40 gallons and just stopped as necessary for fuel. We carried 50 lbs of bags in the plane and sent the rest with family that were meeting us there.

[carusoam]

Classic Mooney family pic, Ziggy!

Go Mooney!

Best regards,

-a-

[LevelWing]

21 minutes ago, whiskytango said:

The annual is being done now and my A&P/IA has all of my paperwork.  I will let you know when I get the plane back.

Thanks!

20 minutes ago, ziggysanchez said:

For the trip to Maine we did 40 gallons and just stopped as necessary for fuel. We carried 50 lbs of bags in the plane and sent the rest with family that were meeting us there.

Thanks for the info. If I can find a 231 with the right useful load this may work after all. 

[chrisk]

Not much to add, but my 231 has a useful load of ~975lbs (from memory).  I'm not sure how folks are getting useful loads in the 1100 lb range.

[jlunseth]

There are a lot of questions embedded in what you asked. One major difference between the various iterations of the 231l and then stepping up to the 252 is cooling, and along with cooling (or lack of it), higher maintenance costs and less ability to get into the teens and flight levels.

The original 231 engine, the GB, ran hot. That was the reason for the change to the TSIO360LB, which was somewhat better. The 231 engines have a Compressor Discharge Temperature limit of 280 dF. CDT is the temperature of the induction air as it exits the turbocharger. It is pretty easy, on a hot dayl say out west, trying to climb to the high teens or the flight levels, to see that temp. The CDT limit is there to protect the engine from detonation caused by too high induction temps, so you really don’t want to exceed it.  As Paul mentioned, the original factory GB and LB also had a fixed wastegate, and because of that a critical altitude of 15,500 (more or less, it varies with the day temp and pressure). That means that after 15,500 the engine can no longer make 100% HP, and that means the climb rate rapidly becomes anemic. I don’t have the GB, I have the LB with the Merlyn and intercooler, which is a cooler running set up, but frankly I don’t see how the factory was able to get the aircraft certified to a service ceiling of 24k. On anything but a really cold day the lack of climb and CDT issues would not let you get there, at least from what I can see in my own engine.

I would say the GB is fine if you stay in the mid-teens.

An issue with both the GB and the LB is that they generally need a top overhaul at 1200, so expect some cost coming up on an engine that has 1,000 hours. We know more now about engine management than what was available when these aircraft came out.  You can bet the prior pilot ran the engine hot and hard just because that was what was recommended,  CHT max, for example, is 460. 460! Geez, your cylinders would turn into puddles before that, but that is what is in the POH and too many people ran the engine hot.  You may need to do a top within a few hundred hours on an engine that has 1,000 on it now.

The Merlyn wastegate is a good thing to have. Its main purpose is to prevent bootstrapping, but it also has the effect of raising the critical altitude quite a bit. I usually run into critical altitude at 22,500, as opposed to the CA of the factory engine.   I can generally maintain a good climb rate up to nearly the service ceiling. The rate falls rapidly after CA, to around 300 fpm max, and the engine starts to heat up pretty quickly. Until you have experienced it, you can’t really understand how poor cooling becomes once you are at or above 18,000, the air is just to thin to help much.

The intercooler helps quite a bit. CDT is the induction air temp after the turbo and before the intercooler.  Induction Air Temp is the temperature after the intercooler. The intercooler, in high altitude cruise, will drop the IAT to 100 or more degrees less than the CDT. CDT, as a practical matter, no longer becomes a limiting factor. The temperature of the air entering the engine is well under 280 dF, usually under 200. So the engine generally runs cooler and you no longer need to worry about CDT as a limiting factor preventing the aircraft from climbing at a good rate. It is also better for engine longevity.

High altitude operations are only part of the picture though.  As a general rule, if you are going west you will be flying into the winds, and the winds aloft in the teens and flight levels will be so strong that they cancel out the benefit of a higher TAS that you get from flying up there. It is no longer worth the climb.  Going east, getting up high to catch those winds is a great thing, you will see some amazing ground speeds. But a climb of, say, 19,000 feet takes quite a bit of time.  I climb and descend as a general rule at 500 fpm, don’t want to hurt any passenger ears. So a climb of 19,000 will take 38 minutes, during which my ground speed will be down because of the climb. So it usually doesn’t pay to climb that high unless a flight is a couple of hours.

I spend a lot of time flying in the range of 6-16,000 feet even though the plane is perfectly happy at FL210. For that purpose, a GB or LB either one is just fine. I definitely like my LB with its ability to get up to 23k or so if I really need it, but I don’t use that ability very much. I did when I first got the plane, but the novelty wore off.

The 252 has two major advantages over the 231, even though it has the same basic engine, the TSIO360. The designers did a much better job of cooling the MB engine than they did the LB or GB, and that means lower maintenance costs and less likelihood of the mid-life top overhaul. The SB engine is even better. Second, they used a fully automatic wastegate. So with the GB or LB the pilot needs to manage the MP during an ascent or descent. With the MB or SB you just put in a power setting and the wastegate controller manages the MP to that setting.  

Another thing about the 252 engine that you can’t underestimate is that it supports a standby alternator. The alternator of the 231 is a weak point, actually, the “coupler” or clutch that connects the alternator to the engine is the weak point. It can fail, leaving you with no charging, and because of that it is imprudent in my opinion to replace the vacuum system and instruments with the more modern electrical instruments. A half hour or 1 hour backup time is not enough to get down from the flight levels, penetrate the layer you were flying over, and fly and approach to landing.  

My useful load is 980, and full fuel is 447 lbs. That leaves a payload of 533. It depends on how big you are, how many people that will handle.  I can generally carry two men up front, a woman in back and a little luggage, or me, my girlfriend and pretty much unlimited luggage, or a man and woman in front and two kids in the back with a little luggage.

One thing worth mentioning is that all of the TSIO360s, set up properly, can be operated lean of peak. Your fuel flow will be in the 10-11GPH range depending on how much power you want out of the engine. That is a lot of range with a 75 gallon capacity. You can afford to leave some out for some trips.  I do a fair amount of Angel Flights and for those, have to manage all kinds of loads, the aircraft does that well.

If it is a matter of budget, I would suggest the 231 with the LB engine, intercooler, and Merlyn. Check the ability to upgrade to an MB at TBO, there is a serial number cutoff for that. If you have a high enough SN, then when it becomes time to replace the engine you can get most of the way to a 252.  If budget is not as much of a concern then by all means get a 252 or even better, the Encore.

[carusoam]

Way to go JL!

Thanks for sharing all those details.
 

Best regards,

-a-

[CharlesHuddleston]

I have nothing of use to add to the above... I, too, fly a 231 with the intercooler and Merlyn. It is a great airplane. I have loaded it with my wife and 2 sons, and flew to the beach... No issues.

Loaded with my wife, my best friend and his girlfriend, and flew to The Bahamas... no issues.

I believe my current UL is in the 950# range, and the plane has better range than my bladder, so we usually make a stop if our flight looks like it will be more than 4 hours if flying with everyone on board. You will love it! Just like anything else in aviation (and life in general for that matter), it all comes down to "how much money do you want to spend??!!"

[LevelWing]

For those that load their plane up with people and luggage, how close are you getting to max gross weight for takeoff?

@CharlesHuddleston loading up with four adults and light bags had to put you near max gross weight, I assume? 

Edited August 27, 2020 by LevelWing

[tmo]

Mine has been known to be loaded pretty close to the limit (as in, I've left air in the tanks on purpose to make the W&B work). No performance issues, even on ISA++ days.

[J Bird]

On 8/25/2020 at 10:00 PM, ziggysanchez said:

For the trip to Maine we did 40 gallons and just stopped as necessary for fuel. We carried 50 lbs of bags in the plane and sent the rest with family that were meeting us there.

With a bathroom break, grabbing lunch, stretching, 40 gallons is right in line with some passengers bladder.

[N231BN]

For those that load their plane up with people and luggage, how close are you getting to max gross weight for takeoff?
[mention=17163]CharlesHuddleston[/mention] loading up with four adults and light bags had to put you near max gross weight, I assume? 

A 231 at 2900lbs is no big deal.

[gsxrpilot]

2 hours ago, LevelWing said:

For those that load their plane up with people and luggage, how close are you getting to max gross weight for takeoff?

@CharlesHuddleston loading up with four adults and light bags had to put you near max gross weight, I assume? 

My K flies at gross weight pretty much every time my wife and I go on a trip. Neither of us are small, nor is the dog. And then there's the luggage. I keep a small luggage scale on the hat rack and so we know how much gets us to gross weight. It flies just fine, takes off and climbs out without any issues.

I've heard rumors of 252 owners loading their plane to Encore weight which is about 200 lbs over gross.

[jlunseth]

Gross weight and high altitude takeoffs are a non-event in the turbo Mooneys. I fly quite a bit for Angel Flight and almost always have a passenger load in the 300-400 lb. range, plus myself at 200, so I often leave out ten or fifteen gallons to make it right under 2900 pounds at takeoff. The K has plenty of HP to handle that. The handling characteristics of the aircraft are a little different on landing at GW but it is nothing major, you will usually be a little more pitch up and have a greater fall rate right above the runway that you will have to arrest, but as I said, nothing at all major.

I have been to Leadville. It was ho hum, so what. Kind of disappointing actually, after everything you read about taking off from there. The takeoff and landing distances are a little greater out west, but then the runway lengths are generally longer at the high altitude airports than they are here in the midwest, but again, nothing major. A high altitude takeoff is just handled as a normal takeoff.

The aircraft makes 100% horsepower at ground level at any airport in the US including KLXV at 10,000 (rounded off a little). It feels a little like cheating to get the certificate for that one.

What we have to manage carefully are engine temps during a long climb out to the high teens or flight levels out west on a hot day. Again, nothing major if the aircraft fuel flow is set up correctly. You want to be full rich full power, cowl flaps open and with airspeed up around 120, not lean out like you might with an NA. 

 

[LevelWing]

Thanks for all of the feedback and thanks to @jlunseth for the detailed write up.

What is the impact of not having the inter cooler? I've found a few that have the Merlyn waste gate but not the inter cooler.