I had the great pleasure of watching 2 AERO techs do annuals on 3 L-39s and do quite a bit of work on a 4th that is still being put back together and it's an interesting process, but as was said before, not a lot of parts are replaced during the annual. Most of the cost (other than getting them over here from the Czech Republic) was Jet-A to test & tune the engines. They also pulled all the tires (and replaced several of them due to wear, but that was needed anyway), repack the bearings, check the brakes, service the hydraulics, and air conditioner, and that's about it. The majority of the time they spent was tuning the engines for the new Ultra-Low Sulfur Jet-A as it was causing them to run a bit "hot" but other than that, it's amazing how little is actually involved as far as work. Most everything else is simply cleaned up or cleaned out and it's good to go for another year.
The thing that was stressed to me about the L-39 (and actually, just about all Russian-built jets and helicopters) while I was watching and learning is that they're sealed systems. For example, if everything is working right, the airplane can hold the hydraulic system pressure for a WEEK. Everything is on-demand instead of continuous, so many parts which get replaced on western aircraft annually last for most of the airplane's life. If you see something leaking from an L-29 or L-39, it's broke.
Still, 160 gallons/hr+ of Jet-A is expensive, so any perceived savings in maintenance quickly dissapear when you're having fun.
