M20K 231 High Altitude Cruise

[jlunseth]

I have been on a mission to figure out how to operate my 231 in all the parts of its envelope.  The certified ceiling is 24,000.  We tried FL230 last year and had some excitement with "high altitude miss" where improper pressurization of the magneto leads to arcing across the cap in the thin air in the flight levels.  Got that fixed at annual over the winter, but just have not had the right circumstances to go that high again.  This weekend my copilot and I went to Kalispell from Flying Cloud in Minneapolis.  We had the usual share of adventures.  The vacuum pump pooped out on the first leg of our trip to KGPI, fortunately in VMC.  I made the mistake of leaving the plane on autopilot for a little bit while we decided what to do and then told ATC we needed to divert to KBIS (Bismarck).  As the AI wound down, the plane started bucking and gained a little altitude, which concerned ATC (because they knew our circumstances by then), but I disconnected the AP and made an uneventful landing.  Lots of flooding at Bismarck and throughout ND.  Bismarck Aero found a mechanic and the pump was changed in short order, but it was too late to continue so we stayed the night in the FBO at BIS.  For future information, they have a sleeping room with two single beds, worked out really well.  The trip out was at FL220, the winds aloft were not too bad, but another 2,000 feet to the service ceiling and they were screaming and in the wrong direction, so we did not have a chance to try FL240.

On the way back we got our chance.  ATC gave us FL240 for the leg from KGPI to our refueling stop at BIS.  As those of you know who have been on this site for awhile, my 231 has the Merlyn wastegate and Turboplus intercooler.  The Merlyn increases the critical altitude (the altitude at which the aircraft can no longer generate 100% HP), but I had never tested out where this happened with the Merlyn.  The critical altitude on the 231 as it came from the factory with the fixed wastegate was 15,000 +/- 500.  We found it at 22,500 with the Merlyn, using the numbers generated by the JPI's %HP algorithm.  It was quite a bit warmer than a standard day, -24C at FL240 which is around 10 deg. warm, so I suspect we would get better on a standard day or a cold winter one.  It was not until the last 1,000 that the climb rate reduced from my standard 500 fpm cruise climb.  We hit about 300 fpm from 23,000 to 24,000.

At altitude the plane quickly accelerated to about 182 TAS before I pulled it back to cruise.  We cruised at 27.4 MP and 13.7 gph.  The 13.7 seems to be the magic figure for my aircraft in a cruise range of 75-80%.  I am at the point where it does not seem worth it to go through the lean procedure with the JPI, I just pull it back to 13.7 and all the cylinder temps fall in line in the 370' and 380's at altitude.  Bear in mind that in the high thin air, cooling is not as good as lower down, so those are good working temps.

We made the trip from KGPI to KBIS in a flight time of 3:00.  We hit a ground speed at one point at cruise of 234 knots.  My copilot got a picture of a 233 or maybe the 234 that I will post when I get it.  Not too shabby for a fuel mileage of close to 20 mpg. 

I have a couple of goals to work on with the plane's envelope this summer.  One is to run a tank dry, or nearly dry.  At landing, if my gauges show zero on one tank or the other, there is still somewhere around 6-10 gallons in the tank.  Normally we land nowhere near dry in either tank, even for a long trip, but I would like to know how far my plane really will go on full fuel.  I have done some testing on the fuel flow meter with the JPI, and it is very accurate, much more so than the tanks.  I know within a tenth of a gallon what I have used.  Just need the right circumstances, I don't want to try this at FL240.  If I blow it and the engine stops, so does the turbo, and it will be quite awhile and several thousand feet before the engine will want to start again.

The other is to try LOP.  I flew some LOP last year, and have taken the Advanced Pilot Seminar, but the plane really did not like LOP, just ran rough.  I had it tweaked some at annual, so am anxious to try that again.  Wouldn't it be great if I could cruise at FL240 and 225 GS, at 11 or so gph?

Only a couple of other notes on the trip.  I filed for FL240 leaving BIS to FCM and they wanted me at an odd altitude.  The controller offered me FL210 and 230 and I took 210, but he questioned that altitude.  His exact statement was, "We don't see many Mooneys at that altitude."  So I don't know, maybe we need to organized a turbo Mooney group and buzz Salt Lake Center's facility at FL240 so they understand turbo Mooneys better. 

The other was the departure from KGPI.  For those who don't know the place, there are Rockies on both sides, and the ridge on the east is around 10,000.  Normally, we have plenty of time to climb over it, but this time my friendly winds aloft were pushing us pretty good.  I had to go Vx at 9,000 and at that, had to fly through a "notch" in the crest ridge.  Made for a pretty view though.  Copilot noted the "TERRAIN ADVISORY" on the GPS.  At least  now she knows what that is for.  She got some spectacular pictures.

[Parker_Woodruff]

Can't say nuthin but how excited I am to own a turbo Mooney here in a few weeks.

Do you have GAMIs?  Don Maxwell said my soon-to-be 252 doesn't wanna run LOP at 65% power.  No GAMIs on it, but that will be my first upgrade.

[FBCK]

I have the same configuration, the wastegate and the Turboplus intercooler and have been wondering how people operate these.  I have had the plane for 3 months now and haven't had much time to experiment.  Do you ever look at the guage for the intercooler or do you just use some know configuration (ie: 38inches for takeoff, 28 in cruise). I usually see a 40 difference gauge at takeoff which works out to a little under 38 if you follow the formula.  Any information would be appreciated.

[FAST FLIGHT OPTIONS LLC]

I have gotten my 231 up to FL 180.  At about 28" and 2500 RPM I was buring 10 GPH @ about 50 degrees LoP T.I.T.  It was warm out so I had the cowl flaps in trail. I dont have a wastegate or intercooler by the way and run with stock injectors.  I would think a 252 would do even better not sure why you could not run LoP?

[jlunseth]

I have GAMI's.  Your (almost your's) 252's induction system is balanced better than the 231's.  One of the issues with my aircraft I think, is that the MP, fuel flow, and consequent %HP and airspeed are constantly moving around according to my JPI.  Not much, but the tolerance is smaller on the lean side than on the ROP side.  ROP, I follow the APS recommendation of somewhere between -100 and -125 degrees, so I have a pretty good cushion if the engine settings move around.  On the lean side I would be setting for 60 LOP (APS recommendation), and it is not nearly as far back to the "red box" from there.  Also, the throttle linkage is maybe telling me something.  It takes quite a bit of throttle movement to go from "LEANEST" to -125 ROP.  It takes not much throttle movement to go from "LAST TO LEAN" to -60 LOP on the lean side. 

With the tweaks at annual, I am going to experiment with LOP again. But I learned the hard way with the "high altitude miss" to experiment when I am alone in the plane.  The copilot got pretty excited when the engine started bucking and spitting and thinking about quitting at FL230.

One comment I would make on the 252 (and I have to add I have never flown one), is that I would question the value of taking it to its service ceiling of 28,000.  Maybe it is set up way better than the 231.  But the effects of lack of cooling because of the thinness of the air become rapidly apparent at about FL210, and when you hit critical altitude and the climb rate starts to drop, getting up higher just happens at a crawl.  If you have a long way to go and a great wind at the higher altitude, it may be worth it.  But I think you would have to be looking at a trip of at least three hours, or you would have to start down as soon as you hit FL280.  That said, if I owned the plane, we would be up there giving it a try.

One thing to bear in mind, if you have not flown a nonpressurized aircraft in the flight levels, is that you need to start down alot sooner than you are used to with a normally aspirated aircraft.  If you are cruising along at FL240 at a GS of 240 thanks to a tailwind, you will have quite a bit less than 30 minutes to get down to landing altitude from 90 miles out.  If you just tip the nose over and throttle back a little to maintain MP at your cruise setting, you will be coming in even faster at, say, 260.  You will slow some in the lower altitudes, but you will still make your airport really fast. Unpressurized, you need to descend at about 500 fpm, and not more than 700 for the sake of passengers ears.   Thirty minutes at 500 fpm only gets you 15,000.  So you need to give the whole descent process some thought ahead of time.  This is when the approach controller picks you up and refers to you as "Mooney jet," assumes you are pressurized, and wants you to dump altitude at 2-3,000 fpm.  The plane can do it.  Your passengers' ears may not be so happy. 

FBCK asked if I look at the gauge for the intercooler.  When I first got the plane I had the "differential temp" gauge, but I installed a JPI 930 and took the "diff. temp" gauge out.  I have readings for Compressor Discharge Temperature and Induction Air Temperature on the JPI, and the difference between the two is what would read out on the diff. temp gauge.  I could do the mental math.  But the JPI has an algorithm that produces %HP, and I generally just use that.  It is pretty close to the result I get if I do all the mental math and look at the Turboplus chart.

On the standard 231 without the intercooler, I would warn you that the CDT goes redline at WOT ( and you should be at WOT, meaning whatever setting gives you 100% HP)above about FL210.  It is not important in the intercooled aircraft, because the purpose of the original redline was to protect the engine from high induction air temps and the intercooler drops those temps quite a bit.  I was seeing a differential temp of around 135 F, or about 75 C, on this last trip at FL240.  You should not allow the CDT to go over redline in the nonintercooled 231.

[jlunseth]

PS on "standard configurations" for the intercooled 231, I just use one and that is on takeoff.  I generally use just a little over 36", say 36.3-36.5.  The JPI generally says I am over 100% HP if the MP is higher than that, and I find that 36 is really all I need anyway.  One footnote is that when I first got my plane the intercooler was old and dusty.  I was not getting much cooling from it, so I had it cleaned at annual two years ago by Willmar.  Turboplus can also do it.  Made quite a difference. 

[Parker_Woodruff]

Quote: jlunseth

 It takes quite a bit of throttle movement to go from "LEANEST" to -125 ROP.  It takes not much throttle movement to go from "LAST TO LEAN" to -60 LOP on the lean side. 

[thinwing]

Jlunseth...thanks for high altiude pirep....Parkers concern about flight above 23k a real concern...period of useful conscioness rapidly degrades after 23k in average adults.You are quite correct about cooling at 25k,the highest I have flown...temp control to about 380,i achieve with cowl flaps.(2900/2400)TIT 1600.....214 tas.Where they can be closed in cruise @ 19k,I have to somtimes open them up again.The Bravo I fly isnt equipped with inlet or outlet air temp guages so it just tit and cvl hd temps to watch.The thing you did not touch on but in my opinion is most important is condition of pilot and passengers...I check for mask fit,use a pulse oxyimeter and make sure everyone is in good health with no breathing issues.I also try to err on side of too much o2 when setting flowrate.Descents are made at 20in ,airbrakes out,800/900 ft per min.I teach all passengers the valsalva manuver (scuba divers clear)and remind atc when its time to start down(I program a vnav descent profile on the 530)...most of the time they have me cleared for lower before I have asked.Lastly,you have probably noticed how empty the high teens to middle twenties are...they belong to us turbo guys...the kerosene burners are 10k higher and the nonturbo traffic  10klower...all and all a greatride above the weather with a smooth ride...regards kp couch

[jlunseth]

Quote: Parker_Woodruff

 

[231Pilot]

Sorry guys, I didn't post this or the following message, but I can't delete them so I'll just edit in apologies.

Lew

[231Pilot]

 

Sorry guys, I didn't post this or the preceding message, but I can't delete them so I'll just edit in apologies.

Lew

 

[Bennett]

Some years ago, when I had my Trophy 261 Conversion, my 231 became in effect a 252 with a 12 volt electrical system, and the POH was that of the factory 252.  I'm sure that some forum members know of other differences, but during the 16 or 17 years I owned her, I flew her in accordence to the 252 POH.  I found that the critical altitude was just about 24,000',(Flight Level 240), and I made many cross country flights (West to East) between 23,000 and 27,000' (with a one time ATC approved) 29.000' run of about 500 miles. There were two reason for going so high.  One, to get above (most of the) weather, and two, for the fastest ground speed possible.  I had 115 gallons usable (why I ended up with more than the standard Monroy conversion is beyond me, but we tested it several times), and so once I was up there, I seldom flew lower until my destination. This is a hostile environment, and I used good quality (read expensive) masks with built in microphones, flow meters, and a hooked up second oxygen tank with its own duplicate masks.  Handling gets, in my opinion, marginal at 27,000', let alone 29.000'.  I always added Prist to the fuel, and changed to the "pressurized" magnetos. During the winter months, I could just about reach the lower edges of the jet stream, and this gave incredible ground speeds - 315 knots confirmed by ATC on one crazy run.  Truly a tiger by its tail.  So far as let downs to destination, I would plead with ATC to let me start down 125 miles from my destination, but mostly they held me until about 75 miles to go.  Of course coming back, I seldom flew above 14,000' or 16,000' because of the winds.  For what it is worth, at the high altitudes I flew WOT, and ROP of about 125 degrees.  CHTs were under 390 degrees, but I had to watch TIT closely, and there were times when I had to reduce throttle, and even open the cowl flaps a bit. Nowadays I fly a 201, and generally stay below 14,000' .  I'm not so much in a hurry now, and I like flying without oxygen. Sure I will use a cannula at 12,000' or above in daylight, and about 8,000' and above at night on a long trip, but flying is more fun (for me) now, and a naturally aspirated 201 is quick enough for my current purposes.

 

 

[KLRDMD]

Quote: Parker_Woodruff

 Don Maxwell said my soon-to-be 252 doesn't wanna run LOP at 65% power.  No GAMIs on it, but that w

ill be my first upgrade.

[donshapansky]

Hey Parker, glad to hear you're about to get a 252! I agree with your personal limit, anyone above Fl230 better be a jet jock with high altitude chamber experience because it's 2 minutes to lights out at FL250! A couple of flights back I climbed to FL200 from FL180 at 1000'/min LOP was 15.5 gph smooth at 214 KTAS, ground speed was up there at 265 KTS in light snow.

[dmevans]

Quote: jlunseth

I have been on a mission to figure out how to operate my 231 in all the parts of its envelope.  The certified ceiling is 24,000.  We tried FL230 last year and had some excitement with "high altitude miss" where improper pressurization of the magneto leads to arcing across the cap in the thin air in the flight levels.  Got that fixed at annual over the winter, but just have not had the right circumstances to go that high again.  This weekend my copilot and I went to Kalispell from Flying Cloud in Minneapolis.  We had the usual share of adventures.  The vacuum pump pooped out on the first leg of our trip to KGPI, fortunately in VMC.  I made the mistake of leaving the plane on autopilot for a little bit while we decided what to do and then told ATC we needed to divert to KBIS (Bismarck).  As the AI wound down, the plane started bucking and gained a little altitude, which concerned ATC (because they knew our circumstances by then), but I disconnected the AP and made an uneventful landing.  Lots of flooding at Bismarck and throughout ND.  Bismarck Aero found a mechanic and the pump was changed in short order, but it was too late to continue so we stayed the night in the FBO at BIS.  For future information, they have a sleeping room with two single beds, worked out really well.  The trip out was at FL220, the winds aloft were not too bad, but another 2,000 feet to the service ceiling and they were screaming and in the wrong direction, so we did not have a chance to try FL240.

On the way back we got our chance.  ATC gave us FL240 for the leg from KGPI to our refueling stop at BIS.  As those of you know who have been on this site for awhile, my 231 has the Merlyn wastegate and Turboplus intercooler.  The Merlyn increases the critical altitude (the altitude at which the aircraft can no longer generate 100% HP), but I had never tested out where this happened with the Merlyn.  The critical altitude on the 231 as it came from the factory with the fixed wastegate was 15,000 +/- 500.  We found it at 22,500 with the Merlyn, using the numbers generated by the JPI's %HP algorithm.  It was quite a bit warmer than a standard day, -24C at FL240 which is around 10 deg. warm, so I suspect we would get better on a standard day or a cold winter one.  It was not until the last 1,000 that the climb rate reduced from my standard 500 fpm cruise climb.  We hit about 300 fpm from 23,000 to 24,000.

At altitude the plane quickly accelerated to about 182 TAS before I pulled it back to cruise.  We cruised at 27.4 MP and 13.7 gph.  The 13.7 seems to be the magic figure for my aircraft in a cruise range of 75-80%.  I am at the point where it does not seem worth it to go through the lean procedure with the JPI, I just pull it back to 13.7 and all the cylinder temps fall in line in the 370' and 380's at altitude.  Bear in mind that in the high thin air, cooling is not as good as lower down, so those are good working temps.

We made the trip from KGPI to KBIS in a flight time of 3:00.  We hit a ground speed at one point at cruise of 234 knots.  My copilot got a picture of a 233 or maybe the 234 that I will post when I get it.  Not too shabby for a fuel mileage of close to 20 mpg. 

I have a couple of goals to work on with the plane's envelope this summer.  One is to run a tank dry, or nearly dry.  At landing, if my gauges show zero on one tank or the other, there is still somewhere around 6-10 gallons in the tank.  Normally we land nowhere near dry in either tank, even for a long trip, but I would like to know how far my plane really will go on full fuel.  I have done some testing on the fuel flow meter with the JPI, and it is very accurate, much more so than the tanks.  I know within a tenth of a gallon what I have used.  Just need the right circumstances, I don't want to try this at FL240.  If I blow it and the engine stops, so does the turbo, and it will be quite awhile and several thousand feet before the engine will want to start again.

The other is to try LOP.  I flew some LOP last year, and have taken the Advanced Pilot Seminar, but the plane really did not like LOP, just ran rough.  I had it tweaked some at annual, so am anxious to try that again.  Wouldn't it be great if I could cruise at FL240 and 225 GS, at 11 or so gph?

Only a couple of other notes on the trip.  I filed for FL240 leaving BIS to FCM and they wanted me at an odd altitude.  The controller offered me FL210 and 230 and I took 210, but he questioned that altitude.  His exact statement was, "We don't see many Mooneys at that altitude."  So I don't know, maybe we need to organized a turbo Mooney group and buzz Salt Lake Center's facility at FL240 so they understand turbo Mooneys better. 

The other was the departure from KGPI.  For those who don't know the place, there are Rockies on both sides, and the ridge on the east is around 10,000.  Normally, we have plenty of time to climb over it, but this time my friendly winds aloft were pushing us pretty good.  I had to go Vx at 9,000 and at that, had to fly through a "notch" in the crest ridge.  Made for a pretty view though.  Copilot noted the "TERRAIN ADVISORY" on the GPS.  At least  now she knows what that is for.  She got some spectacular pictures.

[mooniac58]

Thanks for the post dmevans!  I never really thought of that angle and I guess I don't have to with a non-turbo bird...but seems valid enough!

When the day finally comes that I have the resources/articles section done we might all benefit if you prepared a feature outlining things from the persective of a ATC and Mooney pilot.

[thinwing]

dmevans...thanks for the controllers pov.I see you state your location as Hayward so I assume you work oakland center or one of the norcals and you might have even worked me .I frequently file for high teens low twenties and my climb is typically 500 ft per min in a cruise climb to altitude.These altitudes are chosen (as you are no doubt aware)to climb on top of weather or for terrain avoidance/smooth ride.I have noted that I am always cleared as filed at these altitudes but your mention of transitioning of turbine a/c from enroute to terminal through them is interesting.I assume you could always turn a merging target left or right if you ended up with a conflict??Anyway sorry if us little guys increase your workload but us with turbocharged equipment run best at these altitudes and as another controller pointed out to me ,no body but turbocharged piston aircraft are using...thanks for the input..kp couch