Turbo versus non turbo

BTW – countless accident reports of aircraft taking off or going around unable to climb due to density altitude – also in Europe…

Sure – but that’s because most pilots never do takeoff performance calcs IME it is never covered in the PPL. If you fly a fairly powerful plane then you get away with it… mostly…

I took-off the other day at an airfield elevation 170ft, QNH 1006 an OAT of 48C and a dew point of 17C. That presented a DA of a little under 4500ft. Under ISA the TODR would have been 833m whilst the calculated value for that specific scenario was 1230m. In itself, the increase TODR isn’t too much of a problem when operating from a 4000m runway. Indeed a blisteringly hot Shoreham would have looked like a realistic proposition. However, the next step is a bit more interesting.

Take the same scenario with an engine failure on climb-out (we’re in twin piston turbo-diesel aircraft). The book says that, in an ideal world at SL under ISA you should achieve 163ft/min. With our DA of 4500ft the number decreases to 87ft/min. That is a perilously small number when you consider AFM figures are normally optimistic and they assume we all fly like Sky Gods.

Dave, is 163/87fpm at 92% MCT power or 100% power?

95%. You can only exceed 95% for 5 minutes.

DA62?! Yes but I presume if the cows were getting bigger, you could exceed it awhile longer?

Indeed. One would keep the good lever as far forward as possible. It would be interesting to see how long it would go before the computer said no.

75% power output is 75% power output regardless if the MP comes from a dense atmosphere down low or a turbo up high

Not quite, if you look at the typical profile of a flight.

The typical (“educated”) NA pilot flies at 65% power when mucking about low down, say a few k feet. The air is dense, and the power is, ahem, 65%. Not 75% or 85%.

If this typical NA pilot (“educated” and, in Europe, with an IR) does a long flight, he/she climbs from sea level up to say FL100 (if the wx is nice you fly at ~FL100), and the 100% power rapidly reduces as you climb. At FL100 you are still full-throttle but the MP is something like 20" and (one could look it up) the HP is down to something like 65% again. Then you set peak EGT or LOP and you are flying, for the next X hours, at something like 50%.

Whereas the typical turbo pilot climbs at 100% power and this 100% does not reduce. It will be 100% for the whole 10 mins to FL100! This is your first massive difference from NA. The only way the NA pilot could approach this level of “abuse” would be by flying at 500ft with everything fully forward, and then leaning the mixture a bit to raise the CHT to match what happens at 100% power at FL100 due to the thinner air (about 700 millibars) Of course the NA pilot pays a price for the lack of a turbo and his climb to FL100 takes 15-20mins depending on how aggressive he is.

Then, in cruise, the turbo pilot who has paid for the turbo is going to be flying at at least 65% power. Those I know about fly everywhere at 75% power and the “educated” ones have matched injectors and fly LOP. A lot of Conti pilots (SR22T) cruise at 85%. Then you get a load of turbo owners who find they exceed TIT (another thing you don’t have on a NA engine, and it is a hard limit) unless they fly ROP, and these people will mostly be flying at 75% too. This is another level of “abuse” relative to NA.

Then you get the actual engine design. These engines can’t deliver 100% at peak EGT and not overheat. Well, with a 100kW air conditioner blowing into the cowling they could This is why we have this gradual worsening of reliability. There is nothing sudden. Perhaps if you fly at 550F CHT it might be relatively quick But if you throw in all these factors into the pot, you do find the greater level of “abuse” of the turbo engine does translate into a shorter top end life. It is nothing sudden or obvious which is why you can have unlimited forum bandwidth debating it.

The bottom end of the engine is fine – it knows nothing about what is going on, at a given RPM and torque and the temperature doesn’t change much because the oil is thermostatically controlled. This is why turbo engines have top end overhauls and not complete overhauls.

So, yeah, I am open to criticism because I am writing this and I don’t have a turbo, but hey loads of people here do have turbos so let’s see them write (more than a line or two) explaining why there is no reduction in reliability due to the turbo

I used to be in the Socata owners’ group (got the shove in 2008 for disagreeing with some people ) and we had this debate (TB20 v. TB21) there. Not a single person could produce evidence of any TB21 (or any other turbo plane) which got anywhere near TBO. And this is supported by private comms with a load of owners. They have all had top overhauls at something like 800-1000hrs.

A performance gain from flying higher of 17 knots using same amount of powe/fuel. Now TBs are not the fastest aircraft, a Cirrus and Mooney NA will probably outrun it at lower altitude but for the same airframe, I think it significant.

I agree – nobody is saying a turbo doesn’t work. I am just saying it comes at a cost.

However for some reason not everybody gets the better MPG. A TB20 seems to have some 20% more range than a TB21. We have done this one before but I am not sure it was ever settled why. I think it must be because most of the TB21 owners fly ROP to keep it below the TIT limit, and that is a ~20% hit on the range straight away.

The TIT limit is a big big thing in the turbo world, which a lot of pilots struggle with. The traditional solution has been to fly ROP and thus chuck some fuel in.

If you are happy with your TB20 I can recommend you getting your own hangar, its much better if you have to do repairs and so on. And surely a much better investment than upgrading the aircraft. :-)

I agree 100.000% but unfortunately The Universe is owned by properly owners. They control everything. This is why a lot of people escape to farm strips, etc, etc, but that had drawbacks too (waterlogging, a different kind of politics, etc). I nearly bought a hangar in 2005, 220k or so, but it slipped through my fingers after I paid out 5k for paperwork (a crooked estate agent etc).

Don’t baby your engine