High Altitude Operations

[fluffysheap]

I received this PM a couple of days ago...

Sir, I have a 1979 231 with intercooler, Merlyn wastegate, and GAMIs on a 700 hr LB engine. I'm just starting to fly it higher and wonder what fuel/engine settings or procedures you use at those altitudes? I do have a JPI 830 so I can monitor the engine pretty well.

 

Now, I'm not exactly Don Kaye here, but I guess I do have the this kind of plane, except with a JPI 700 instead of 830.  I guess I have maybe 150 hours or so above FL180, sometimes as high as FL250.  (And you don't have to call me sir, unless you're my waiter!)

 

Physiology

There are several good references on this topic, but I'll like a few that I like...

http://www.avweb.com/news/aeromed/181893-1.html?redirected=1

http://www.avweb.com/news/pelican/182156-1.html?redirected=1

http://www.altitude.org

 

The pilot is the most important system on the plane, so physiology is the most important part of flying at high altitude.  I agree with the majority of what John Deakin says.  One place I would disagree is on the use of the cannula, and the minimum oxygen levels.  Both of these seem to be highly susceptible to individual variance.  I have found that, for me, the cannula is capable of maintaining 90% or better at any altitude I have flown at.  Others may find, like Mr. Deakin, that even at low altitudes the cannula does not work for them.  The important thing is to carry an oximeter and use it.  Whatever altitude you're at, monitor your O2 and descend if you can't find a way to keep it up.  The other thing I might take slight exception to is the statement "Any time they do not use oxygen at those altitudes, brain cells are dying."  There's some truth to this, in that climbers returning from mountaineering expeditions have been found to have some brain injuries, even if they didn't experience serious altitude sickness.  But these expeditions take place over long periods of time, with heavy physical exertion.  Most of these climbers probably have oxygen levels in the 70s, possibly even 60s, for several hours a day over several days.  I know of no evidence that this is a risk for the short-duration hypoxia that pilots can experience.

 

Some perspective on cannulae and oxygen levels.  There are far more people using medical cannulae than oxygen cannulae.  Doctors use cannulae for medical oxygen supply at basically any flow rate.  Oxygen masks are used mostly in special cases, such as breathing pure oxygen, mixing with other gas (anesthesia), etc.  The typical medical cannula is rated up to about 5 LPM although there are some high flow ones that can go higher (usually 12, sometimes more).  However, even the standard-rate 5 LPM cannula is more than capable of going higher.  Aviation cannulae, at least the ones I have, are the size of the larger high-flow medical cannulae.

 

The FAA shows that a flow rate of about 4 LPM would be necessary at 30,000 feet.  So there is no question that a cannula can deliver the required amount of flow.  Researching this post, I found an FAA document stating that masks are required above FL180 is in case the pilot is a mouth-breather, or talks too much.  But the thing about talking is that it *always* lowers your oxygen level - you can observe it on the ground.  It is easy to breathe through the mouth when talking a lot, though, and you should be careful about that when using a cannula - again, regardless of altitude.

 

Although "normal" oxygen levels are anything over 93, with 95-98 being typical for someone in good health, doctors will not usually prescribe medical oxygen for a patient unless their resting O2 level is below 90.  Certainly, having a lower level than normal might cause you to feel a little tired, similar to how riding in an airliner (where the pressure altitude is typically 6000-8000') will leave you feeling tired after a long flight.  I believe that there is individual variation here too, but if you're over 90, you're unlikely to be impaired, with the possible partial exception of night vision.

 

In any case, here are some situations in which a cannula is better than a mask, regardless of the altitude:

* If you have facial hair

* If your mask doesn't seal well

* If you cough

* If you eat or drink.  A mask lifted away from the face to make room for food or water is basically useless.

 

I've experienced two oxygen-related failures.  One, a tank (actually valve) failure at 17K that left me with no O2, and another where an instructor took off his cannula to sneeze and forgot to put it back on.  My oxygen failure experience was noticed when I saw that I had no flow.  I checked my O2 level and found it about 86.  I descended and it got better.  It was no big deal, but it might have been worse if I'd been higher.  The problem with the instructor was more interesting.  He started tuning random frequencies into the radio.  I told him to put his nose hose back on and he got better.

 

My oxygen flow meter is built into the tubing, and needs to hang vertically.  I hang it from the pencil holder.  I also have a mask handy.  Like most turbo Mooneys, mine has built-in four-place oxygen, but there's basically no situation where I would carry three passengers to oxygen altitudes.  So, I keep my usual cannula plugged into one of the passenger stations, and I keep a mask plugged into the pilot station, but not twisted and locked.  Then I hang the mask from the pencil holder too.  In case something happens, I have the poor man's quick-don O2 mask.  I just have to twist the fitting and hold it up to my face.

 

John Deakin carries a separate O2 bottle that he can use in case of emergency.  I don't have this, but it's better because it's totally independent.  Less plumbing hanging around, too.  On the downside, he's got to put it somewhere, and look around for it if his regular oxygen quits.

 

There's one more thing that I'd like to do, which is 231-specific.  Because of the funky wastegate (either the fixed one, or the Merlyn one - I think it would work with either), I bet there is some sort of throttle+trim setting that will descend in the flight levels, but naturally level off around 8-10K feet.  If you have some sort of O2 failure, just set the magic throttle setting and put the autopilot in heading/wing-leveler mode.  Now even if you pass out, you'll wake up safe and sound in an hour, assuming you don't run out of fuel.  I don't know what this setting is, but I might go look for it next time I go fly, and see if this actually works.

 

Engine Management

This is the most fun thing about flying a 231 at high altitudes.  I tend to agree with Mr. Deakin - you spend your time switching back and forth between checking the oxygen and checking the engine.  I'll share my experiences and power settings.

 

First, the critical altitude seems to be around FL220 in the summer, higher, perhaps FL240, in the winter.  I don't remember what the factory critical altitude is, but I think it's around FL170.  So that is a big upgrade.  But unfortunately, this works only at full power.  The turbo seems to only be able to make about 30"MP LOP in cruise around FL230, and something like 28" at FL250.  At FL220 and below, I use 2270 as cruise RPM.  I have experimented and find that it tends to give the best fuel economy.  This is in keeping with some other article I read somewhere that said 2300 is the most efficient speed for the prop.

 

The wastegate & intercooler don't change the certificated ceiling, but they definitely change the real-world ceiling.  I don't think a stock 231 would be very happy at FL240, but mine is happy enough up to 250, which is as high as I've gone.  It still climbs quite well at that altitude, 700FPM up to about 230 and 500FPM up to 250, although it only makes about 32" of climb MP.  I don't know how high it would go, although the 252 goes to 28000 and that's probably also possible.  But you wouldn't be able to make any decent cruise power up there, maybe 22" LOP.  Mount Everest would still be another thousand feet up.

 

I climb full power all the way up to whatever I'm cruising at, and I think this is the only real choice.  John Deakin says to do a cruise climb.  I usually climb at or near Vy unless CHTs go over 400, then I pitch down as needed to keep them at or below that number.  He argues that the cruise climb doesn't affect your actual enroute time that much.  It's probably true, but it costs a lot of fuel.  My plane burns 230% as much fuel climbing as it does in cruise.  Every extra five minutes in the climb costs a gallon of fuel.

 

LOP cruise, always, for me.  I use the Big Mixture Pull approach to leaning.  I am a little more cautious than Mr. Deakin.  I reduce power to 25", set cruise RPM on the prop, and then pull off the mixture.  Reducing the mixture will tend to drop the MP a little more, perhaps 22".  I then gradually add throttle back, watching the TIT on the JPI and finely adjusting mixture as needed.  When I'm really in practice, I can get the mixture pretty close to perfect, but if I don't fly in a couple weeks, it's not going to be quite right.  The first time I did this at high altitude, I think FL230, I put the fire out.  This is not good!  The prop doesn't stop, even at those altitudes, but if you manage to get combustion to stop, it only takes a moment for the turbo to stop too.  Now, especially at part throttle, you don't have any air in the engine.  To get it fired back up again you have to enrich and apply full throttle.  Then you have to throttle back right away to keep the engine from overspeed/overboost.  This whole business might cost you 500 feet, irritate ATC, and terrify your passengers.  Losing 500 feet off of 23,000 is a shameful display of poor airmanship, not to mention you might shock cool your cylinders by, like, eight degrees.

 

Keeping the mixture set just right is kind of an interesting balancing act, especially at the highest altitudes (240+).  Leaner mixture improves temperatures, both CHT and TIT, but reduces maximum boost - so your power is reduced even more than the leaning normally would.  The intake temperatures go up at the high altitude, because the turbo is working harder, and the intercooler loses effectiveness due to the thin air.  This means you need yet more boost to keep the same air intake density.  Hotter, richer mixture makes the TIT go up more.  At these altitudes, to maintain power, everything's at redline.  And to complicate matters, my engine seems to have a narrower margin for the onset of roughness/power loss due to excessively lean mixture.  I'm not sure if this is for real or just an artifact of everyting working harder up high.  But at 15000' I can burn 10.5GPH, blow 34" of MP and keep TIT in the low 1500s, and it runs smooth all the way down to what amounts to idle cutoff.  At FL250, burning 9.5GPH and 28" of MP, the TIT is right up at 1650 and if I lean it out it starts to misfire.

 

Higher RPM is a way of making back some of this loss.  Instead of 2250-2300, 2400 or even 2500 can help basically everything.  Unfortunately, the faster prop is less efficient, so your fuel economy will go down.

 

Above FL230, sometimes it's necessary to open cowl flaps to keep the CHTs under 380.  The air is cold, but it's thin.  Opening the cowl flaps also seems to take about 15 degrees off the TIT.  I have no idea why this is, but others have reported the same thing.  I don't know if it's for real, or if it's a measurement artifact.  Fortunately, because of the thin air the added drag is relatively low.  It costs maybe 5 KTS for half cowl flaps.

 

Switching to ROP gives more MP (more energy in the exhaust), and lower TITs.  But it increases CHTs.  I don't think you can actually get any more usable power ROP, you just burn more fuel.

 

Overall, going above about FL220 doesn't really help matters much, unless (as sometimes happens) you need to go really high to get over ice, or to pick up winds that outweigh the loss of engine efficiency.  A third reason is in case there is mountain wave.  A 4000' high block clearance from FL210 to FL250 is a lot less scary over the Rockies than one that goes from 16K to FL200.

 

Descending isn't much different than at lower altitudes, it just takes longer.  You'll gain speed as you descend, so you might overshoot if you aren't careful.  I find that ATC has basically no idea of when I need to start descending and will always do it too late if I don't request it.  It takes almost an hour to come down from FL250 to sea level.  Remember to keep pulling the throttle out as you descend, because MP builds up fast as you come down.  Non-231s probably don't have to do this.  The main thing is that since the descent is so long, the engine is going to get cold no matter what you do.  As long as the temperature decline is gradual, cooling won't really be a big deal.  The main thing is to remember that you need to gradually warm it back up again - you want 250 before you apply climb, or even full cruise, power.  Prop at minimum speed and MP in the 20-22" range will help with that in the last few thousand feet of the descent.

 

EDIT: I forgot to mention issues with IAT (induction air temperature), which is measured after the intercooler, and CDT (compressor discharge temperature), which is measured between the turbo and the intercooler.  I have found with the intercooler installed that basically nothing I do will make the IAT go above the original CDT redline of, I believe, 280.  So I have no concerns about preignition/detonation as a result of overly hot intake air.  But, I'm slightly concerned about turbo overspeed and/or excessive turbo heat.  So I don't let the actual CDT get over 300... which still isn't something that tends to be the limiting factor compared to TIT.  My JPI usually shows CDTs about 30 degrees hotter than the factory gauge.  (There's a similar effect with TITs, but it evens out by the time the TIT gets near the 1650 redline).

 

Airframe Issues

 

Controls really are lighter and less responsive in the thin air.  Pitch control is about the same force, but roll is lighter.  IAS may drop, maybe all the way down to 115KTS or so (which is still 170KTAS at FL250!).  The autopilot makes it go away, but hand-flying at these altitudes is sort of... weird-feeling.  It's not hard, it's just different.

 

Weather

 

I don't think I ever really felt weird about high altitude flying until I looked down on a cirrus (the cloud, not the plane!  I'm always looking down on the planes.  Sometimes the owners, too.)

 

FL250 will not clear a thunderstorm, but it will clear virtually everything else.  One advantage of being up high is that in a summer weather system that has both layered clouds and storms, you can usually get above the layers and see the storms.  Takes the guesswork out of it.  Only do this during the day, and only if the storms are isolated or widely scattered!  And you must be up at altitude before you get to the storms, not climbing up between them.  Remember that a building CB can develop faster than you can fly away from it, and having one come up from under you might have unfortunate consequences.

 

In winter weather, you sometimes will see that a frontal system will push cloud layers up to FL300 or so.  This is tricky.  MOST of the time, there is no ice in these clouds, or not enough to matter.  I've never seen an ice airmet going above FL270, and most of them only go up to FL240 or less.  But I've seen plenty of pireps for ice above the vertical limit of an ice airmet, usually during a frontal passage.  It happened yesterday, actually, in Oklahoma/North Texas.  This is not to say that you shouldn't fly over an airmet (I fly over airmets all the time), or even that you shouldn't fly over a frontal system.  But regardless of the vertical limits of an airmet, you should overfly it only with caution, and solicit pireps before you get there.  If you have an engine failure while over ice, the only way down is through it.  If your personal minimums say not to fly over ice, that would be a totally reasonable thing.

 

One advantage to dealing with ice at very high altitude - if anything about ice can be called an "advantage" - is that you might fly into ice, but find you've got thousands and thousands of feet of potentially warmer air below you.

 

Dress warmly.  There's another thread about this.  It's usually 30 or 40 below.

 

ATC

 

ATC says funny things.

"Say again type aircraft"

"Say again altitude"

"Are you pressurized?"

"Is that a turboprop conversion?"

"I've never seen a Mooney up that high before"

And once in a while someone just won't believe it.  "Did you say FL240?"  "Affirmative, FL240, that's two four thousand"  "Do you mean two thousand four hundred?"  "Negative, two four thousand"  ....... pause ........ "OK"

[John Pleisse]

A great post and an excellent read.

[jetdriven]

I think I just doubled my knowledge of turbo Mooneys right there. Great post, Sheap.

[Awful_Charlie]

Good post and great read - thanks fluffysheap

 

I agree with just about everything there, with a couple of small additions and notes.  For the climb, the (Bravo) book states 34"/2400 and I get about 21-22GPH for that running ROP. The cowl flaps might have to be opened a bit in the climb, and closing them when levelling out at ToC slows the cylinder cooling rate. I tend to use 30"/2200 in the cruise for 13-15GPH, and when established there, program the altitude pre-select for 8-10000' or MSA (whichever is the higher) with a 700'min descent - if I get an oxygen problem up there, I just have to hit two buttons, which I hope is going to be easier than fumbling round for backup oxygen (which I don't carry) Coming down certainly does take some preparation - I tend to use 600'/min, which around is half an hour from FL250 to 1000', but is easily more than 100nm out. I often use the Garmin vertical speed calculator to give me the descent point calculated at 500'/min, as tipping the nose over increases the airspeed and RoD required - the 500/600 ratio works out close enough.  Of course the winds change as you descend too, so it's never going to be an exact calculation, so 5nm per 1000' is a good rough and ready reckoner which is unlikely to be substantially out. In the descent, maintaining 20" of MP keeps everything in the green, but have to take a twist out of the MP every couple of thousand feet to avoid it creeping up, although you can work it to your advantage to adjust the point where you reach target altitude.

 

Love the ATC comment, I've had most of those too!

[chrisk]

Great post!   And so much of it mirrors my very limited experience. 

 

I've had my plane up to FL21 just once.  Your right, it does fly different at those altitudes.  I would describe it as mushy.   And my auto pilot had some issues with the altitude hold.  It would work fine for a while, then start to oscillate.  I've been told this is probably the "altitude can" and an easy fix.  Fortunately the air was very smooth and hand adjusting the altitude every 5 minutes or so was no big deal.  --Anyway, it was never an issue with any of my other flights up to 17K

 

And I was laughing a bit as I read about you putting out the fire.  My one attempt at this altitude was ROP, as I was in fear of putting out the fire.  The POH has some scary words about the subject (under reduction of power)  "If engine does not restart, .......repeat after descending below 12,000 feet.".  That day, I just didn't want to explain to ATC that I was screwing around with the mixture and lost the engine and NEEDED to descend.  Maybe on a nice day when I have some more time.

 

And at those altitudes, I consider an oximeter a necessary.  It's too easy for something to go wrong and they are inexpensive.   In my case, I sat on a section of the tubing before take off and it came un-connected.  The flow meter said yes, but the O2 never made it to the cannula.  The oximeter clued me in on the way up.

[Ron McBride]

Thank You for sharing the information.  I have been interested in how to use a turbo. I am dreaming of having one.  I learned a lot.

 

Ron

[Danb]

Ron, I have a Bravo and utilize its capabilities, that was a well thought out synopsis of high flying I agree with what-Chalie chimed in with I basically use his no. in my flying..great info. for Mooneyspace 

[RJBrown]

Great post.

When alone I always had the backup O2 bottle on the right front seat set up and strapped in ready to go.

Interesting write up of a Cessna 337 accident. http://aviation-safety.net/wikibase/wiki.php?id=38638

What put a healthy young woman to sleep killed an older man. Oxygen symptoms are different for each individual. Learn yours either in a chamber or with a saftey pilot.

In the Rocket at 24,000 eastbound at 300+ knots does get a few comments/questions. In a Rocket you can never fully close the cowl flaps. They are set up to close to what they call "in trail" at about 1" open. I would climb at full power as quickly as possible with cowl flaps fully open. Closing to trail just before leveling off or the mechanism got too hard to operate. At altitude she never ran hot. Normal cruise for me was, pre GAMI, 72% 20 gph. Run ROP. Cruise climb cost gas, shorter the time to climb the better. Solo the Rocket was still climbing over 1500fpm at 26K. Even fully loaded climb was always over 1000FPM. That plane could be at 24,000 quicker than the J could get to 12.

[jlunseth]

You sound like me.  A few thoughts.

 

First, some people seem to fixate on the whole O2 thing.  In practical fact it is pretty easy.  There is not much point in flying a 231 above FL220 for several reasons.  One is that most of the time, you are not going to gain enough TAS and tailwind in 2000 feet (from 22000 to the service ceiling of 24000) to justify the additional climb.  Probably you will be above the critical altitude, which will make the climb rate anemic in the last couple of thousand feet.  The most critical factor in my personal assessment, is time of useful O2, which starts to drop sharply above FL220.  I don't have my charts where I am writing this, but from recollection the time of useful consciousness at FL240 is 3:30 to 4 minutes, and nearly double that at FL220.  If you allow for a minute or two to detect and diagnose an O2 system failure, the time at FL240 does not allow you a safety margin to get down.  And the aircraft is not really very happy up there, the engine cooling is poor, as you note the CDT is elevated.  My standard high cruising altitude, therefore, is FL200 to FL220.  The plane is plenty fast there.  The Bravos, Acclaims, and even the 252 have better intercoolers and engine cooling, and I leave the higher flying to them.

 

BTW, the service ceiling of the aircraft is 24,000, and since that is in the POH as a limit, it should not be exceeded.  I can't think of any reason why the 231 with the Merlyn and intercooler would suddenly fall out of the sky at 24,033 feet when it would not at 24,000, but since it is in the POH it in effect is a regulatory limit, so I do not exceed it. 

 

There is "lore" that the pilot is required to wear a mask above 18,000.  I have heard this from a number of instructors, including some of the Mooney PPP instructors.  I have looked into the regs., and 91.211 does not say anything like that.  Cruising around the regs., I eventually found a provision in Part 23 having to do with certification of aircraft, which essentially states that an O2 system certified for use above 18,000 must be equipped with a mask.  There is nothing that says that the pilot must wear a mask instead of a cannula at a certain altitude.  That said, I always do.  I have found that when I use a cannula above FL180 I use O2 faster out of the system, and I am marginally hypoxic.  Not enough to be hazardous, but my O2 level might be 88 or 90 %, and I noticed after a few flights that my speech would be a little hesitant when I spoke to ATC.  So I use a mask above FL180, yes it makes it hard to eat and drink but it makes me much richer in O2.  I have a miked mask for the purpose.  The passenger who flies with me most often uses a cannula at all altitudes, I have offered her a mask but she does not want one ans seems fine up high.

 

I carry an oximeter, but am accustomed to my personal symptoms of onset of hypoxia so I don't use the oximeter much anymore.  I also have a ball valve that I can use to set the flow, and the ball tells me if O2 is flowing (Precise Flight product).  That said, I set the O2 flow to achieve 92% or better.  If find that even at 90 I have minor symptoms of hypoxia, and I would bet that is true of most pilots once they learn how to recognize the symptoms.

 

The onset of hypoxia, to me, is the same physical feeling as anxiety - slightly elevated breathing rate and a feeling of breathing high up in the chest - but without the psychological component.  I am familiar enough with this system that if I feel it, I always react right away, checking and adjusting the O2 flow as needed.

 

There is no way in the 231 to set a "floor" altitude and cause the aircraft to autodescend to that altitude and level off.  You could probably do that with alt. preselect, but most 231's do not have that.  No matter, at FL220 and below there is plenty of time to react and descend if necessary.  I have just never found it necessary, the O2 system works well. 

 

I have done dozens of flights spending from 2-4 hours at 18-22,000, a few at FL230 when the winds were markedly better there than FL210, and one flight to FL240 just to say I did it, but there is really no good reason to go up there. 

 

I looked into the issue of a high CDT extensively, and asked some experts including asking the question at one of the GAMI/APS live seminars, and asking Mooney.  With the intercooler installed the CDT is irrelevant.  The temperature that protects the turbo is TIT, not CDT.  The CDT redline was there for the factory configuration to prevent detonation due to hot induction air entering the cylinders.  As long as you do not exceed the redline(s) for TIT (there are two of them), the turbo is perfectly happy.

 

The weather is much easier to see and avoid at those altitudes, in fact you fly over most of it.  The one thing worth mentioning is induction icing.  In the flight levels you will sometimes fly through clouds or large general areas of haze that are caused by ice crystals.  The ice crystals will eventually plug the induction air filter.  In recognition of this, Mooney long ago retrofitted automatic air door systems to the aircraft that did not have them, and they were installed in all turbo's after a certain point.  If, by chance, you have an aircraft that does not have an automatic door, you will have to pull the Alt Air yourself.  The symptom is a gradual decline in MP (because the engine does not have air to breath), and pulling the Alt Air will result in a slight lower MP that what you had set, because the air will be warmer, but the engine will be fine.  On the aircraft with automatic doors, the Alt Air light will come on, on the annunciator panel, indicating the door has opened. 

 

My highest sustained cruise speed was a trip from KPIA to KFDK at FL210, 275 kts. and under 2 hours from Ill. to Md.  Sipping O2 with the Love Channel on the Sat. radio (mellow music, I am not a hip hop fan).

 

PS I don't do a cruise climb.  I climb full rich at 100% power to altitude, then level off and set for cruise.  Best way to fly a turbo, and what the GAMI/APS people recommend.

[Joe Zuffoletto]

Excellent post, thank you for taking the time to write it.

 

My own minimal $0.02: I wear a mask at any O2 altitude because cannulae dry out my nostrils and give me nosebleeds. Even though I have a goatee I've never had a problem with the seal, which I confirm on a regular basis with my pulse oxymeter.

[jlunseth]

Yes, the cannulas can be a little irritating to your nasal passages.

 

I looked up a table on time of useful consciousness.  At FL250 it is 3-5 minutes, at FL220 it is 5-10 minutes.  So to reiterate what I said in my earlier post, why go to 24 or 25 when there is little to gain, but the time to detect, diagnose, and get lower in the event of a problem is cut in half.  I leave that to the Acclaim guys, whose aircraft have better AP's than mine.

[Joe Zuffoletto]

Why go to 24 or 25 when there is little to gain, but the time to detect, diagnose, and get lower in the event of a problem is cut in half.  I leave that to the Acclaim guys, whose aircraft have better AP's than mine.

 

This Acclaim guy agrees; I've been above 220 a couple times to outclimb some enroute weather, but I never flight plan above 220.

[aaronk25]

What cruise speeds are you seeing out of your LB engine at fl220 or so? I ask because I know that even with the LB engine in the 231 cruise speed is down from a 252 with the same engine but when I asked the question in another thread the operators were not keeping the throttle all the way in like you do......I like your driving technique !

[rainman]

Thanks for the post. Very good. Let me know if you get down San Antonio way so I can get you up in my 231 and learn your technique better.!

[jlunseth]

 

Descending isn't much different than at lower altitudes, it just takes longer.  You'll gain speed as you descend, so you might overshoot if you aren't careful.  I find that ATC has basically no idea of when I need to start descending and will always do it too late if I don't request it.  It takes almost an hour to come down from FL250 to sea level.  Remember to keep pulling the throttle out as you descend, because MP builds up fast as you come down.  Non-231s probably don't have to do this.  The main thing is that since the descent is so long, the engine is going to get cold no matter what you do.  As long as the temperature decline is gradual, cooling won't really be a big deal.  The main thing is to remember that you need to gradually warm it back up again - you want 250 before you apply climb, or even full cruise, power.  Prop at minimum speed and MP in the 20-22" range will help with that in the last few thousand feet of the descent.

 

 

ATC

 

ATC says funny things.

"Say again type aircraft"

"Say again altitude"

"Are you pressurized?"

"Is that a turboprop conversion?"

"I've never seen a Mooney up that high before"

And once in a while someone just won't believe it.  "Did you say FL240?"  "Affirmative, FL240, that's two four thousand"  "Do you mean two thousand four hundred?"  "Negative, two four thousand"  ....... pause ........ "OK"

 

 

I don't have the same problem with engine cooling in the descent that you do.  I agree that engine cooling is a serious issue.  I bet my technique is different.  I just tip the nose over, disconnect Alt Hold, adjust the trim on the AP for about 500 fpm, and pull of just one inch of MP.  I pull of the inch of MP because if I don't, the ram air effect is going to boost the MP to something higher than what I was cruising at, and I just want to descend at the same cruise settings I was using before the descent.  Typical ground speed will be in the 230-270 kts range. I agree you will need to adjust MP as you descend, or it will go up.  If you do it this way, the CHT's stay nice and warm, the engine is happy, and you are home in jig time.  You do need to plan the descent though.  I start mine anywhere from 75 to 125 nm out, depending on the winds.  If the wind at cruise altitude is high, say 75 knots, and it does not reduce much until much lower (which I can see on the MX20 with Sat WX), then I know I will maintain a very high ground speed throughout most of the descent and will need 100-125 nm, If the winds are markedly slower low down, my TAS is going to go down also, and it does not pay to start the descent until closer in, say the 75 mile mark. 

 

My favorite is "Mooney jet, join the Twolf arrival direct Twolf" (turbine arrival).  I just said ok. They caught me after awhile, but it sure was fun while it lasted!

[carusoam]

Thread drift, but you mentioned it!

Anoxia (low O2) caused traumatic brain injury (TBI) is to be avoided at all cost...

Useful consciousness of a few minutes is serious business. Especially if you don't know how long the problem has been going on already and logic isn't your strong suit at that time.

If you are fortunate enough to survive it, the recovery can take years, and it may not be a complete recovery.

Just saying...

Keep the discussion going, it could save some bacon!

Questions for the really high flyers...

(1) Do you have a back up O2 system in the event something limits the flow of the primary O2 system?

(2) How do you handle the pre-flight aspects of the back-up system?

(3) Do you constantly monitor O2 level while flying?

Thank you and best regards,

-a-

(Experience: I have the good fortune to work with people recovering from brain injuries of various types, I'm not an expert)

[jlunseth]

People who haven't flown up high much, focus on what might happen if......  It is not that difficult.  If you are hypoxic you know quickly.  Then you do something about it quickly.  You are not going to be in a position where you have been hypoxic to a point where you are seriously impaired, and then you get more hypoxic and it is too late.  I will grant that it is useful to know your individual symptoms so if they happen you know what is happening and react properly.  The one thing to guard against is hazardous attitudes.  Just like antiauthority, invulnerability, etc., "tough-it-out-itis."  Everyone needs air and there is no such thing as being immune or less in need of breathing than others. 

 

I have an oximeter but never use it anymore.  I know my settings on the Precise Flight valve and all I need to do if I want to be sure my O2 is working is to look at the ball in the valve and if it is up, O2 is flowing and I am most likely still breathing.  The oximeter does help at first until you are familiar with the process.  The one caveat is that some people think high 80's or even a 90 is fine.  It is not, you should be 92% at least, and higher is better.  I do hang the oximeter from the prop or mixture control so my passenger can use it if they want.

 

It is a different problem in jets, they are at a level where the time of useful consciousness is in seconds and if the system fails and you have no backup,  you are not going to survive.  The Payne Stewart aircraft was at 39k.  Time of useful consciousness at 40,000 is 15 to 20 seconds.  In recognition of that, there are regulations that require one pilot to be on a mask even if flying a pressurized aircraft for truly high altitude flight, both Part 91 and Part 135.  We can't get that high. 

 

In our Mooneys, if you fly at FL220 you have 5-10 minutes to react and get down, and you gain useful consciousness time relatively quickly in the descent.  At FL200 which is two minutes in a conservative descent, you have 30 minutes of useful consciousness. 

 

I do think that one key thing is to understand that the time of useful consciousness is changing rapidly above FL200, and if you go above FL220 you should know what you are doing and be able to detect and react very quickly.

 

I have a Spare Air (very small scuba tank with installed mouth regulator), but I generally don't carry it unless I am doing an ocean crossing (in case the aircraft submerges), although it would do the job as a backup at altitude.  Just too much stuff.  There is no pre-flight to do on the built-in O2 system in the aircraft except check the gauge.  You can also plug in the users' O2 lines, but you can do that in the air also it is very simple.  If you are smart you put the equipment on before you get to 12k and just check the flow, then forget it. 

[Jeff_S]

I have a Spare Air (very small scuba tank with installed mouth regulator), but I generally don't carry it unless I am doing an ocean crossing (in case the aircraft submerges), although it would do the job as a backup at altitude.  

 

That's interesting about the Spare Air. I was curious if such a thing would be helpful. But does the output of the Spare Air maintain the sea level (or wherever it was filled) density of oxygen or does the regulator normalize it to local density? I ask because when breathing regular air at depth the regulator automatically increases the partial pressure of all gasses which of course can lead to the bends. I'm curious if the opposite effect is true as well, and breathing from your Spare Air is not really helping much. This is a question, not a criticism...have you confirmed this?  (The scuba dive master in me wants to know.)

[RJBrown]

Spare Air is not oxygen. It is just compressed air. Read the link I posted above. Compressed air will not help at altitude. Spare air would be useful in the water not at altitude.

[chrisk]

There is a nitrox version o f spare air, which can be up to 40% O2.  It's in the $300 range + a fill at a dive shop + maintance + ....   If your really worried about this, maybe the more cost effective solution would be "Boost Oxygen" for $12.50, which is 95% O2. --Good for ~120 inhalations.

 

On the other hand, I recall some one here was selling low cost O2 systems.  And that might be better as a backup.

 

As for me, I do not have a backup O2 system.  I'd consider one, but weight, cost and hassle factor  seem high for the potential benefit.  Seriously, how long is it going to take with screwing around with a second O2 system vs loosing a few 1000 feet.  Speed breaks, landing gear, spiral down at 2000 fpm.

[fluffysheap]

I found a controller's perspective on things:

http://www.avweb.com/news/sayagain/192511-1.html?redirected=1

 

What cruise speeds are you seeing out of your LB engine at fl220 or so? I ask because I know that even with the LB engine in the 231 cruise speed is down from a 252 with the same engine but when I asked the question in another thread the operators were not keeping the throttle all the way in like you do......I like your driving technique !

 

170-175 TAS.  But I'm not sure how accurate that is.  The 231 is a little slower than the 252 because it's slightly draggier, but drag matters less at those altitudes.  The main thing I see is that 252s usually seem to be able to maintain 11GPH or more LOP up there, whereas I can't due to TIT limits, even though I have all the engine upgrades.

 

There is "lore" that the pilot is required to wear a mask above 18,000...  I have looked into the regs., and 91.211 does not say anything like that.

I don't think it's in Part 91, but you can't find a cannula certified above 18,000.  I'm not sure if that means that it's legal for part 91 but not 121/135, legal only for experimentals, or not legal for anybody.  If for some reason you go on a checkride above FL180 (maybe you are getting a twin rating in a MiG-25), wear a mask

 

Going above FL220 doesn't really improve performance most of the time.  The only time I do it is to go over weather, or if it's windy over the Rockies.  Performance at FL230 is slightly better, but engine temperatures are higher.  At 240 and 250, performance is a net loss in my 231 (typically need cowl flaps).  Now, if you have a Rocket with massive spare performance at all altitudes...

 

As for hypoxic brain injury (HACE).  Certainly it's bad news, but I've seen no evidence that it's a real risk for us. The only cases of it I could find were mountain climbers, who spend days doing heavy physical activity while simultaneously at high altitude and usually also ignoring early hypoxia symptoms.  People exposed to short duration hypoxia (for instance, airliner decompression) don't seem to get it.  If there's a doctor or a case study who can contradict me, I'm listening (but I think doctors all fly Bonanzas).

 

You can do your own tests.  Go do some hard aerobic exercise.  Then immediately check your oxygen level with your oximeter.  It's going to be in the 80s.  Maybe the low 80s.  Anyone who exercises for half an hour is going to have oxygen levels similar to high-altitude hypoxia, and over a longer period of time.

 

I tend to feel that a secondary O2 system like Spare Air is not worth the trouble.  Realistically problems all seem to be with the in-cabin components, and I carry spares of those.  You would need the pure O2 version, the ordinary compressed air would do nothing - someone posted an NTSB report of a plane that crashed after the onboard oxygen tank got filled with ordinary compressed air somehow, and the pilot believed it was working because of the flow indicator.

 

If going into the oxygen altitudes, I, and all passengers, put on and test all the oxygen equipment while on the ground.  The passenger connections (and I usually use a passenger connection as well) are in the back seat and not very accessible, and anyway the passengers usually need help getting it set up.

 

What is funny is that, while passengers who are not used to GA flight are often reluctant to fly at all, I've never had one balk at flying around at 23000 feet or whatever.  To them, it seems low, compared to the 35000 they are used to!

[jlunseth]

The Spare Air can be filled with 40% nitrox (40% O2).  I agree, carrying regular air would be unavailing.  I also have a pony with 75% O2 I used to use for technical diving, but I agree that a backup is not needed and just too much stuff in the cockpit.  I did check through the local dive shop, and a 100% O2 Spare Aire is not available. 

[fantom]

How would you check, after the fact, if your tank was somehow accidentally filled with compressed air?

[jlunseth]

Facilities don't generally mess with using compressed air out of tanks, they use compressors because it is cheaper and there is an endless supply so long as their is electricity, so there is not anything to accidentally substitute.    The O2 tanks must be filled by an A&P or someone checked out to fill O2, a lineman without training can't just do it.  When they roll the big green tanks out to your aircraft, it can only be one thing.  There would be a slight chance of that if you take a portable tank to a scuba shop, where they have both compressed air in tanks at high pressure for filling scuba, and compressed O2, but there again, the person doing the filling is certified.  It is possible to test for O2 level, I have a tester for technical diving, but not necessary with aircraft.  Any scuba shop that fills O2 will have a tester, if you are concerned about it you could ask them to test the O2.  They normally would not test pure O2, they have the tester to test percent of O2 in various breathing gas mixes.  They would assume that if the gas came out of the O2 tank, it is pure O2.

 

There is also only one type of fitting that attaches to the port on the aircraft.  In ingenious A&P could probably stick that fitting on an incorrect tank, but it would be intentional and obvious because the tank would not be green.

[chrisk]

What cruise speeds are you seeing out of your LB engine at fl220 or so? I ask because I know that even with the LB engine in the 231 cruise speed is down from a 252 with the same engine but when I asked the question in another thread the operators were not keeping the throttle all the way in like you do......I like your driving technique !

 

Attached are pictures from my last flight at FL210.   The TAS was 168KTS, and fuel flow was 10.7 gph (well ROP, around 60% power).  I had close to a 45 kt tail wind, giving a ground speed of 210 kts, which is the reason I went that high.

 

CHT, CDT, and TIT were all middle green. There was certainly more speed to be had.

 

LB engine with a Merlyn waste gate controller.