Fuel management

[A64Pilot]

1 hour ago, jlunseth said:

 

There are a combination of things going on. Remember that the turbocharger is what produces MP above ambient, and it is not mechanically connected to anything in the engine, so it is not driven by the prop. The prop can be windmilling and the engine turning, but unless there is exhaust the turbo winds down. Another complicating factor is that the mags are pressurized and pressurization will fail without your pressurization source (the turbo) in operation. If pressurization fails the spark can arc across the mag cap to a cylinder the spark was not intended for. The resulting phenomenon, called “high altitude miss,” is very hard on the engine. Or the spark may simply be insufficient to cause combustion. See e.g. https://www.aviationpros.com/home/article/10388584/magnetos-under-pressure

At 20k the standard ambient pressure is less than 14”. From experience I can tell you that the engine cools rapidly when you are at best glide and the engine is not firing. So now you are trying to start a cold to very cold, low compression engine, without enough spark. The POH for the 231 warns that you may need to descend below 12,000 for restart to happen. Even then, there are warnings throughout the POH against operating the engine at greater than 16” or 20”, depending on how low the oil temp has fallen. The POH warns that if the OT falls below 100, an attempt to run the engine at power may result in a sudden engine stoppage. What is going on, is that the oil is too cold to properly lubricate the turbo bearings, and the OT must be warmed before making power. Add in that you are crossing the Alps, or the Rockies, and you can’t descend to 12k and don’t have time anyway to warm the engine if it gets cold. It is not a wise move.

These recommendations in the POH are conservative. And sure, if you jump on the fuel switch the instant the engine stumbles it should fire right away. I have done it. But I have done it over the mid-section of the US where I have all day to restart or find an airport if things don’t go perfectly. I would not do it, say, on a jump from Great Falls to Kalispell. Having done it, and having more than 6 hours of fuel on board, I no longer feel the need to  give it a try. Much safer, is to test your fuel flow meter so you know its readings are accurate, fill the tanks correctly, know to a few tenths what is in each tank, and then not run one dry.

Turbo ops are different from NA ops in many ways. For one thing, the NA is unlikely to fly at an altitude where unpressurized mags will not function properly. Turbo ops are very safe, in some ways safer than NA because you have an improved ability to get above weather. But you cannot just blythely apply NA ops to turbo ops or vice versa.

PS I want to say that I admire curiosity and people who are willing to ask questions and seek information. I wish, when I first started flying my 231, that there had been more pilots savvy about turbo ops out there. I am not a CFI nor an engineer, just a pilot with some hours. I learned about “high altitude miss” the hard way, by having the “joy” of experiencing it firsthand, and I also learned the hard way not to “climb to target EGT,” which means leaning in a climb, an NA technique. Turbo ops are not complicated, when I fly, I have reduced my power settings to just some simple techniques. But to do that, one must understand the “why.” If anyone has more to teach about turbo ops I would love to hear it. 

You may already know all of this and or have seen it before =, but if not it may be worth reading

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_61-107B.pdf.

I know about pressurized mags, but bought that unpressurized ones would work marginally at least up into the low 20’s or so. I know they will up to 20K  anyway as my Maule’s service ceiling was 20K and it was a NA airplane. I had to look that up, I didn’t remember 

‘How high do you guys fly? 

[jlunseth]

2 hours ago, A64Pilot said:

You may already know all of this and or have seen it before =, but if not it may be worth reading

https://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_61-107B.pdf.

I know about pressurized mags, but bought that unpressurized ones would work marginally at least up into the low 20’s or so. I know they will up to 20K  anyway as my Maule’s service ceiling was 20K and it was a NA airplane. I had to look that up, I didn’t remember 

‘How high do you guys fly? 

I visited the service ceiling once, just to say I did it. That’s 24,000. The climb rate from 22,5 up to 24 is really anemic. I generally do not fly above 21k though. The time of useful consciousness starts to fall quickly, so you have just single digit minutes, and if you consider that it may take some time to discover you have an O2 problem, and then to troubleshoot it, and then to get down to a better altitude, well, I just don’t go above 21k unless there is a really good reason. It is also directional and depends on trip length. Generally, going up to the flight levels is counterproductive on an east-west leg, and on the other hand, gives you a great speed boost on a west-east leg. It takes quite a bit of time to get up to, say, 20k. I use 500 fpm, so that would be 38 minutes from a 1,000 foot airport. Therefore, the trip has to be long enough to justify it and you have to plan the descent as far as 125 miles before your destination. 

I used to fly in the flight levels all the time just to say I did it. Now it is probably a half dozen times a year, whenever it is of real benefit. Much more common is just getting above the everyday stratocumulus, which is usually in the 14-18 range, I do that often. 

I was not aware of the FAA Circular but I am aware of most or all of the issues it addresses. Quite a bit of it, like explosive decompression or the coffin corner, are for pressurized jets and turboprops and not very relevant to us. There is training when you get to the High Altitude endorsement and for the ME ATP, for the jet type issues. I am not aware of any really good, comprehensive training aimed at turbocharged piston aircraft. Most of it you learn by doing, or going to individual courses here and there like the APS course in Ada, OK. Unfortunately, most instructors are either NA pilots or jet pilots in real life, so even at the Mooney PPPs I have not heard a lot about the issues that affect turbo piston aircraft. There are a bunch of unique things, high altitude miss, for example, or induction icing, or getting yourself trapped above an ice cloud deck, or hypoxia, or general engine management. Parvez Dara gave a talk at a Mooney PPP on hypoxia that was good. Everything else is taught in bits and pieces in other courses of much more general application.