Personally I think that once you past the idea of "1930's boots technology" on a state-of-the-art corporate jet, they still make a lot of sense. When I hear guys bad mouth Astras or G200s because of the boots I pretty much just assume that they've never flown the airplane. Having to screw around with minimum N1 settings and airbrakes in order to maintain sufficient pressure in the leading edges isn't a big deal, but it does add one more item to deal with at a high workload portion of the flight. It's all probably what you're used to. I had been away from it for quite a while while I was flying the IAI products and I'd just gotten lazy. I certainly don't mean to bad mouth the 50. It's probably the nicest flying airplane out there. I consider it more of a quirk than anything else - kind of like the mole on what's her name's face.
We have a straight 50 with the -3D engines. We had Duncan install the Proline 21 system. The ballpark cost for the conversion is $2.0 +/- a little one way or another depending upon some options that you go with.
We didn't mod the engines, they came wit the airplane. I don't know that I would bother with the -3D mod for now. You really don't get much for it other than DEECs - the performance numbers are the same for it and the -3Cs. I'd let Allied Signal do the mods when they core the engine, otherwise you'll have to pay for it. There are a couple of other engine mods out there that may or may not make sense. Allied Signal has a -40 mod that essentially turns it into a 50EX. I have a couple of friends that did that to their Proline 21 Falcon 50 and they ended up with better than 50EX airplanes - the 50EXs come with Proline 4 equipment. That engine mod hasn't been all that popular though. There is another engine mod that's just becoming available. I forget their name, but some group is installing the TFE731-4 engine in the airframe. It's supposed to give you most of the performance benefits of the -40 for a lot less cost. If it is successful in the marketplace it would be something that we would take a look at. The straight 50 isn't known for it's enthusiastic climb capabilities up at altitude and a little more power could be appreciated.
I'm not at liberty to divulge what we paid for the airplane, what I can say is that for somewhere between $10M and $12M you'll be able to find a clean DA-50, do a thorough C check, a complete interior and exterior refurb, and do the full Proline 21 install. You'll end up with an Falcon 50 that's arguably better than new.
As far as the Proline 21 mod for the Falcon 50 goes. Right now the number of straight 50s that have either had the mod completed or have contracted to have it done is right around 30 or a little more than 10% of the fleet. More operators are getting in line each month. It appears as though, a some point in the future, it's going to be one of those "must have" mods - kind of like the Garrett engine conversion was on the Falcon 20.
As far as the effect on maintenance costs; yes, it will help a little. In our case, the MSP rate on the -3Ds is slightly less than the original -3Cs. As far as airframe maintenance goes, we basically zeroed our our inspection cycles so we were able to start fresh on our inspection intervals. We have always participated in the Collins CASP program so we know what our avionics maintenance costs will be. It may not be for everyone, but it works for us.
If you're serious about going the DA-50 Proline 21 conversion PM me and we'll talk "off forum".
LS