Early m20k 231 model

[MooneyNoob]

Hi all,

I'm considering buying a 1979 m20K 231 and wanted to see if there are any specific gotchas that the thearly 231s were prone to, whether it be specific to the turbo or just related to the earlier airframe. FYI, this one has a merlyn wastegate and intercooler installed already but any advice on what to look out for would be appreciated!

 

 

[LevelWing]

I started a thread asking similar questions when I was on the hunt for a 231. @jlunseth is full of incredibly valuable information, among many others on this board.

Having a waste gate and intercooler is helpful. Do you know what engine is in it?

[jlunseth]

What would you like to know? I can tell you that if you run the aircraft right, it is fast, sips fuel, and can travel incredible distances without a fuel stop. Minneapolis to Boulder, for example. It was Mooney’s first attempt at a turbocharged piston so there are some things that were improved in later models, but invariably at the expense of useful load. 

[MooneyNoob]

@LevelWing The engine is TSI0360LB1, installed in 2007.

@jlunseth What was improved later is exactly what I wanted to know...who do I watch out for (or upgrade) when looking at a 1979 1st generation m20K if I don't mind losing some useful load?  It's probably going be be 320lb worth of humans max, MAYBE another 50lb of baggage, so I have weight to spare.   There are currently two vacuum systems in there and I'm probably going to get rid of both of them (and a whole lot of wiring) as soon as I upgrade the panel to glass, so that should save a bunch of weight too.

[MooneyNoob]

@jlunsethI just read your awesome, detailed discussion on the topic on the other thread.   Two questions:

- Where can I find out if I can upgrade to an MB engine? Is that based on the airframe's SN or my current engine's SN?

- Is there a path to curing the alternator / coupler weakness?  I do want to move to glass, so fixing this issue will wind up high on my list.

Thanks for any words of wisdom.

[tmo]

The eligibility to upgrade to the MB engine is primarily determined by sn of airframe; see the Type Certificate Data Sheet (TCDS).

Quoting from there:
TSIO-360-MB1 (S/N 25-1000 and 25-1999).
TSIO-360-MB2 (S/N 25-2000 thru 25-2012) See NOTE 21.

NOTE 21: M20K S/N’s 25-1000 thru 25-1230 and 25-2000 thru 25-2012 may be retrofitted to TSIO-360-SB2 engine and
gross weight increase to 3130 Lbs. when complied with M20K Gross Weight Increase Retrofit Instructions.

 

So yours (and mine) will be limited to at most the MB1. (overly optimistic me edit)

As far as the alternator, if you remove the vacuum, you can probably explore the secondary alternator that goes in place of the vacuum pump. IIRC there are some threads on here.

[MooneyNoob]

Thanks, @tmo, it looks like I have to stay with LB.  My A/F SN is way below 25-1000.

Thanks for the info!

[jlunseth]

I was able to cure the coupler issue by going directly to Mooney for the part several years ago. The aftermarket couplers we tried, all failed in a very short time and one failed spectacularly when a non-MSC installed it, left out a bushing, and it fell off into the running engine. However, the Mooney supplied part has lasted for four or five years without incident. I want to say it was in the neighborhood of 3-3,500 but I was happy to use it because it worked.

I am trying right now to get a 337 authorization to install a backup alternator but the FSDO has been sitting on the request for a year because of COVID. There is an alternator available by B&G. However, one of our members recently reported the failure of a dual reversionary G275 system, and the consensus conclusion seemed to be that the safest routes is to have attitude indicators of two different technologies, which means keeping the vacuum system and the King AI, and then installing a 275 (or another electric backup that is legal for primary as a turn coordinator). That is what I have right now. The other route would be to install the dual 275 system and add an RC Allen AI with a backup battery. It is also electric, but different method of operation. It is a moot point though, until the FSDO’s are actually back and running.

We just had a fatal incident that may have been caused by disorientation in IMC, see the Minnesota Crash thread. Going with a system such as the dual reversionary 275 that has a single point of failure is out of the question for me.

[RJBrown]

The 1979 231 did not have on board oxygen standard. The avionics package was much worse than any later models, analog with 170B radios and such. Back seat was changed to separate and removable in 84 I think. The 79s has cute little curtains on the rear windows.         
The LB with wastegate and intercooler make a big difference. Handled properly TBO is now probable. Flown per the original operating handbook they averaged 1200 hours.

The airframe was the same. Lots of speed mods were available. Long range tanks are a plus as are speed brakes.
with an avionics upgrade the originals panel becomes moot. Oxygen is a must have. Both onboard and a portable back up.

[jlunseth]

Even the later avionics were not that good by today’s standards. The vacuum instruments are good. The pump needs to be replaced every 500 hours though. The KFC 200 autopilot is really good, certainly the more modern APs are better, but for the analog era the 200 is robust. The King Silver avionics are pretty much outdated now. If it has the original avionics yiou will probably need to spend quite a bit avionics, you need a good WAAS GPS at a minimum.

[dlthig]

I think split rear seats started in 82.

[c1tice]

I installed 2 GI 275 and a GTN 750 after a vacuum pump failure in my 1981 231. The 275’s have 30 minutes of self contained battery back up. So if your alternator fails you have your battery power then your 30 minutes of 275 backup time. I have not tested and I have to believe you would have at least 45 minutes of 275 time after an alternator failure. I hope I could find a place to land in 45 minutes. With an iPad  an iPhone and a iPad mini and the backup batteries I don’t see the need for a backup alternator. I don’t do a lot of hard IMC. Just my opinion. 

[jlunseth]

It is a myth that removing the vacuum system saves weight. Every panel change I have made towards more glass and less old-style instrumentation has gone the other way, not big changes, but you lose a few pounds of useful load every time.

The 231 has a single 12 volt battery. You should not think of it as any kind of a backup at all. If you switch the master off and look for a place to land you should drop the gear while you still have current, and fly gear down, you likely will not have enough juice to drop the gear at your point of intended landing. You might have enough juice to use radios when you land and you might not. Been there done that.

I personally would not rely on a half hour internal backup to get me home either. There are many places I have flown, mostly out west and over mountains, where a half hour would not be sufficient, especially if you happen to be in the flight levels when the problem starts. It takes awhile just to get down if you mean to avoid Vne. Been there done that also. I had to do an emergency landing from 19k in Canada, from about 20 miles out from an emergency airport once upon a time. We had not been in IMC for the previous two hours, but got pushed by tailwinds into the back side of a system and 7000 feet of the descent was VMC. I would not have succeeded without fully functioning instruments.

In the 231, with its ability to travel high and far, you will find yourself dealing with some kind of weather issue on every long trip. Personally, I decline to have any single point of failure instrument systems in my aircraft. If you want to get rid of the vacuum you need at a minimum, a backup alternator for the electrical, and because of the virtual shutdown of the FSDO’s that is really hard to get right now. If you install a dual reversionary 275 system you need a third AI of a different technology, say an RC Allen electrical gyro. One of the members of the forum had a failure of some kind put red X’s on both 275’s just a few months ago. Garmin was looking into it. But I have not read that there is a fail safe resolution. The speculation is that loss of airspeed input caused the entire system to shut down. Don’t expect to gain useful load with the necessary instrumentation. I have one 275 as a backup to the vacuum instruments. If the FSDO ever approves the 337 for the backup alternator (it has been about a year now), I will remove the vacuum, put in the backup alternator, install the dual reversionary 275, and put an RC Allen  AI over on the co-pilot side. I figure that all will run north of 12-13 AMUs and cost me another 10 pounds at least, but I am worth it.  I fly many Angel Flight passengers and they are worth it. This may sound overly conservative, but as I have said, I have lost the alternator and all power, I have had the pleasure of descending from 19k through IMC over the Great Lakes. The 231 is a great machine, but you are helpless without robust and well backed up instruments. 

Just remember, lose your instruments and your remaining lifespan will be about 173 seconds. 

[ArtVandelay]

It is a myth that removing the vacuum system saves weight. Every panel change I have made towards more glass and less old-style instrumentation has gone the other way, not big changes, but you lose a few pounds of useful load every time.

You’re doing the something wrong. My J was as delivered 1744.
When I bought it it was up to 1796. Now it’s 1743, and after next update (G3X) I figure it will end up at 1728.

[David Lloyd]

21 hours ago, c1tice said:

I installed 2 GI 275 and a GTN 750 after a vacuum pump failure in my 1981 231. The 275’s have 30 minutes of self contained battery back up. So if your alternator fails you have your battery power then your 30 minutes of 275 backup time. I have not tested and I have to believe you would have at least 45 minutes of 275 time after an alternator failure. I hope I could find a place to land in 45 minutes. With an iPad  an iPhone and a iPad mini and the backup batteries I don’t see the need for a backup alternator. I don’t do a lot of hard IMC. Just my opinion. 

Either in the pilot guide or AFMS for the GI 275 it describes the minimum allowable battery reserve as 30 minutes below 25000' service ceiling and 60 minutes above that.  When mine were installed, the shop as part of the documentation tested each, one 65 minutes, the other 85 minutes.  Don't know if those numbers reflected time till depleted or was it just some time longer than 60 minutes.

If your only alternator dies, the GI 275 will continue to run off the aircraft battery unless either you cut aircraft power to them or run the aircraft battery down to some point where the 275's internal battery takes over.  Another alternative is to turn one of the 275's off, and keep the other as a backup with a fresh battery.  So, if your only alternator dies, it is up to you to make a decision how to make best use of the aircraft battery or instrument batteries.  Lots of options.  I need to look up, don't remember reading at what voltage the 275 will switch to internal battery.  Does anyone know?  edit:  Garmin Chat looked it up, internal docs only, 11.0 to 30.3 volts.  So if your alternator died, aircraft battery voltage would begin going down, eventually to 11 volts at which the 275 would switch to it's own internal battery.  Unless you switched it sooner.

I did have an alternator fail early one morning within a few miles of Pittsburgh.  IFR weather at AGC, good weather at home.  I made a couple calculations and advised ATC alternator out, change destination to Charlotte.  Turned off one Dynon, the strobe and nav radio.  Battery volts an hour and a half later was 11.2, never used the backup battery for the other Dynon.

As jlunseth points out, sometimes all that is not enough.  Sometimes you might need two aircraft batteries, or two alternators, or two engines, etc.