Wed Aug 14, 2019 9:12 am
Like so many things, it depends on your "mission" profile, and which airframe/engine combination you end up with!
Many NA (Normally Aspirated) travelling aircraft (eg TB20, Ovation, Bonanza) can easily climb into the mid teens without a problem, and generally have a service ceiling around FL200, limited by rate of climb. Adding a turbo does keep the climb going at a better rate for a somewhat higher ceiling, normally limited by certification HPL to FL250 (although for a period some were certified to FL280) and allows a higher TAS, which can be useful in some situations: eg Terrain avoidance, weather avoidance, strong tailwinds etc. However, almost certainly you will burn more fuel - you can maintain high power settings (and hence higher fuel flows) for the whole climb, (and you don't get all of this back in the descent) and while in the cruise: a higher TAS demands more power and thus fuel - you can for sure turn down the power and fly at NA speeds, but in that case why carry a turbo? If you do a lot of low level stuff, you could well find that your fuel burn is actually *more* than the corresponding NA airframe - turbo engines typically have a lower compression ratio than their NA equivalent, the expected use being that the turbo then raises this back to near NA equivalence by doing some of the compression outside of the cylinder - if the turbo isn't doing much work then the effective CR is low, and consequently the fuel efficiency drops off.
If your trips are sub 2 hours/300nm ish, unless you have significant terrain, I would suggest you're not going to get enough to make a turbo option worthwhile - even at 400nm it will be marginal (of course, just ballparks, and very type specific) - a climb and descent to FL200 while keeping engine temperature under control will probably be in the region of 150-200nm. That said, there can be a few places where >FL200 is useful (eg overflying Paris, or when there are much more suitable upper airways than lower), and the amount of traffic in level flight between FL200 and 250 is relatively small compared to above and below
The turbo itself doesn't normally add to much to maintenance - if you budget for a new turbo(2-5k) at mid-life of the engine that seems to be about normal from my experience and conversations, but it is very type specific and also depends on how you operate the engine. Remember with a NA you are hard pushed to do engine damage over 5000' or so - with a turbo you can catastrophically damage the engine all the way to critical altitude plus a few thousand feet. You also get a bunch of additional dependencies - if any one of the turbo, waste gate, controller(s) or hoses fail, then you get an un-flyable aeroplane. Depending on the type, there may be expensive requirements: on mine the exhaust clamps can only be used twice, but the exhaust doesn't need to come off at a scheduled inspection - if it had to be removed eg at annual then it would add another 600bucks for parts, plus the labour, every year. You really don't want to skimp in this area - the turbo can literally become a blowtorch under the cowling if things come adrift, and that sort of failure has killed aircraft and occupants in the past.
Where the costs start to ramp up is that with a turbo and its capabilities, you start to need more 'stuff' to use it effectively - eg oxygen consumption approximately doubles from FL130 to FL200, so one small bottle is no longer enough, without a de-ice or anti-ice system you may get trapped by a layer, you start to want a sferic/radar detector to avoid build ups without massive climbs and descents, another horizon with a different power source (which is likely a battery with a replacement schedule), and then a second alternator, and so the list goes on. It's the maintenance of all these that adds up, and likely to more than the cost of just the additional turbo maintenance. With the change in fuel burn your cost index will probably not change a great deal, just both factors are a bit more and you get to choose from a wider range!
To answer the specific questions:
1) assume it’ll never make it past TBO? Is this true? Not necessarily, I did a factory reman at TBO+10%
2) assume overhaul costs more? Compared to NA, almost certainly
3) what other regular maintenance costs should be factored into static or hourly rate? Depends on the extra 'stuff' you go for
You can sleep in an aeroplane, but you can't fly a house