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I've been looking at possibly buying a used Mooney M20 or a used Cessna 172. One of the big differences between the two is that the Cessna is a fixed prop, and the Mooney is a constant speed propellor. While CSPs sound really nifty and efficient...I can't help but wonder if they would be more expensive to maintain. Both in terms of regular maintenance and just the fact that it has more parts that can break.

Is a CSP more expensive to maintain than a fixed pitch prop? And why? If you can include some numbers that relate to the planes mentioned, that'd be cool too...

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  • $\begingroup$ Is it more expensive to maintain? Yes, if it needs maintenance. Just like anything else that's a little more complicated. For example if you have retractable gear they are more expensive than fixed if they need it. Regular inspections and routine maintenance can keep those costs down though. The other side is that if something does break, it may break more than the prop assembly, for example if the engine is overspeed because the prop governor fails then you have much much more cost. $\endgroup$ – Ron Beyer Aug 21 '16 at 1:25
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    $\begingroup$ @RonBeyer -- there's also the issue of scheduled maintenance... $\endgroup$ – UnrecognizedFallingObject Aug 21 '16 at 2:30
  • $\begingroup$ Having just had an annual done on my C-177B Cardinal, that portion was very minor (I don't think it was even called out on the hour rate sheet). The oil changes also don't seem to be much more frequent but I never maintained a fixed pitch plane myself. $\endgroup$ – Ron Beyer Aug 21 '16 at 2:41
  • $\begingroup$ @RonBeyer So you're saying maintenance cost on a CBP is more or less $0? And the only per hour cost I'll incur is an amortized hourly charge for an eventual inspection? $\endgroup$ – Jay Carr Aug 21 '16 at 22:25
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    $\begingroup$ A 172 an M20x are two totally different planes. What do you expect to do on your typical flight? $\endgroup$ – acpilot Aug 21 '16 at 23:36
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Yes, they are more expensive because they are more complex. I would not avoid buying a plane simply because it has a CS prop because, in the grand scheme of airplane ownership, CS prop maintenance has a small impact on your hourly cost. Maybe ~$1/hr for a piston single with repetitive ADs.

Now, the maintenance of a CS prop can range from a simple visual inspection of about \$100 USD to a complete replacement of the prop (up to \$15,000 USD, maybe more for large props). An owner will likely not do an "overhaul" when the prop reaches TBO but will instead opt for an IRAN (Inspect, Repair As Necessary). This is typically a little cheaper than a full blown overhaul and, from a practical perspective, extends the life of the propeller as an overhaul would. At some point you'll end up grinding blades and balancing the prop but that's not annual maintenance. I've seen the cost quoted at around \$1,500 USD for a high performance piston single IRAN which comes to less than $1/hr for a private owner.

All in, a CS prop only adds a small amount to the hourly operating cost. In my opinion, the performance gains are worth the cost in most cases.

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  • $\begingroup$ So, basically, if I perhaps put a little money into a "maintenance fund" for every hour I fly the plane, I can just put aside a few dollars for every hour flown and cover the cost of an IRAN? How often does an IRAN turn into an overhaul though? It seems like the cost could be variable... $\endgroup$ – Jay Carr Aug 21 '16 at 20:59
  • $\begingroup$ Short answer, yes on the fund. If there is corrosion in the hub or a crack in the hub the prop shop will probably recommend an OH. Shop around with some prop shops and see what they think. I'd check to see if the Mooney has a "no-AD" prop. Many have been switched to no-AD already. Not a dealbreaker but it's a good negotiating tool to beat down the price a little. The gear system maint on the Mooney will probably cost more than the prop maint over a given period (still not awful, just plan for it). Also, prop governors can be pricey to OH (~1500us @ 2000hrs if I recall) so keep that in mind. $\endgroup$ – acpilot Aug 21 '16 at 21:31
  • $\begingroup$ Also, for what it's worth, the manual gear and flap systems on the M20C-F are great. $\endgroup$ – acpilot Aug 23 '16 at 13:10
  • $\begingroup$ You'll have to forgive me, but what is a "no-AD" prop? $\endgroup$ – Jay Carr Aug 23 '16 at 13:33
  • $\begingroup$ A propeller without any aiworthiness directives tied to it. Some props have an AD requiring an inspection for cracks every 100hrs (or some short interval). Props without that AD do not require periodic inspections. I don't know the details of the inspection but it does require a mechanic to spend time on inspection and paperwork which translates into billable hours. If a crack is found you need to repair or replace the offending component which = even more expense. $\endgroup$ – acpilot Aug 23 '16 at 14:01
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One thing about constant speed props that nobody has mentioned is that there is also a cost savings. a fixed pitch prop is like a one speed transmission, where constant speed prop can be adjusted for the current need (climb vs cruise). This means that, at least in theory, you can either go faster at a given rate of fuel flow, or burn less fuel at a given speed.

Whether or not this makes constant speed props cheaper in the long run probably depends a lot on the plane and the frequency and style in which you fly.

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  • $\begingroup$ Perhaps, but this is all starting to remind me of the problem with early (current gen) hybrid petrol/electric cars. They cost so much more that by the end of the time you had the car, you still hadn't made the money back in supposed fuel savings. I think I'd almost rather a 172 with a mogas conversion if I was really keen on cutting fuel costs. $\endgroup$ – Jay Carr Aug 23 '16 at 13:35
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ACPilot hit it on the head in a general sense but ill add to that a bit to more directly cover the direct parts of the question as well as the aircraft in question. I don't know which Mooney you are looking at but if you are also looking at 172's I can assume you are looking at the M20C's D's and E's possibly J models (although the C's and D's are closest to the 172). The older variants of these planes along with being CS props are subject to an AD that requires 100 hour eddy current inspections. This makes these props more expensive to maintain than even their non AD counterparts.

As mentioned the CS props can be more expensive simply because they have moving parts. But for this particular case the AD can add quite a bit of cost. It should be noted that there is a Non-AD prop available for these airframes and many have been converted.

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A fixed prop is relatively low maintenance. Basically, all you do it file out cracks for maintenance.

A constant speed prop has moving parts. There is usually an oil linkage. Usually, a constant speed prop has to be overhauled at regular intervals like the engine.

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  • $\begingroup$ Filing out cracks is generally frowned upon. A cracked aluminum prop is a wall decoration. However, small knicks that have not led to cracking can be repaired by filing and polishing. Over time the prop does need to be rebalanced as well. $\endgroup$ – J Walters Aug 21 '16 at 14:26

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