L39 GXT

Ultimate L39 L39GXT-58 and -62

Email Questions or Comments Last Updated 09/10/20

The results of my work with the L-39 have turned out better than I could have hoped for. The project is entering a new phase where, at a minimum, we are endeavoring to make fully converted aircraft available on a limited basis. One of my oldest friends and fellow engineer, Per Hogdahl, has taken up the project of building planes moving forward. It was never my intent to build but one or two planes and it is good to see Per picking up the ball and taking the project to the next level.

You can find current information about these wonderful airplanes along with contact information for Per and his crew at East 2 West Aero (link to web site).

I'll continue to work to further develop these aircraft and the production techniques surrounding their builds. This includes my curiosity surrounding the viability of the TFE731-5BR in an L-39. I may even update these pages from time to time as a continuing web blog of my work. However, all commercial activity should be directed to East 2 West Aero.

The Ultimate L39 or CarbonL39 project has moved beyond the initial creation phase so I am taking some time to clean up this page to focus it more on what is happening now and less of the history of the project. The original work can be found here and the follow on approach here.

I built the first plane for me. The moment I rolled out on the runway for the first Phase 1 flight, powered up to max continuous cruise power (885 degC) and let go of the brake I knew the plane was no longer an L39. Getting off the runway in one third of the 4000 foot runway as opposed to the stock plane’s taking two thirds was my second data point. Climbing to 6500 feet over the field in very short order and stalling at 68 KIAS sealed the deal for me.

The question ever since that first flight was what to do with the work that had been done.

The first answer was to put the first plane, call it a development mule, in the hands of someone that would put a ton of time on it and fly it hard. If there were going to be issues, I wanted to find them quickly. The plane went to a Reno Racer/Airshow type who put (best guess) about 125 hours on the plane in 8 months. We also had an independent third party Professional Engineer (PE) confirm my engineering and issue a report on same. That write up was needed for the plane to race Reno and a serialized version of it is used for support in airworthiness issuance for every conversion.

The next thing we tackled was who would want it and how would we respond. The web site links at the top of this page depict a lot of that thought but several things have become clear over the last year.

First, there are but three of us and we are not experts in L39 systems. The idea that we could magically come up with the twenty years of experience that the major players in L39 community have in supporting the complex L39 systems was just nuts. There is no way for us to support pushing TFE731s into the existing L39 environment. There is no way for us to support the installation of a motor only with just a few Hawker gauges for N1/N2 and TIT while simultaneously leaving in place all the wiring, inverters and like original L39 elements. Something would stop working in the process and we would have no idea how to address it. It became clear that there was only one path for us; we needed to remove most of the L39 from the L39.

What we could do is design and produce complete replacement airframe wiring harnesses based on properly marked Tefzel wire and modern Mill Spec connectors. We standardize on Garmin G3X avionics. Like most any business jet motor conversion, the motor and avionics suit are a set and we go the extra step to remove the remainder of the original L39 electrical system. I have kept all the L39 gear, flaps and trim logic by transferring the required diodes and relays to a single printed circuit board using modern surface mount diodes and aircraft Leach relays. The logic, labels and wiring are identical to the L39 schematics but the implementation is modern and supportable. All the inverters are replaced with a single 115 Vac unit that supports the three temperature mixing motors in the ACM system along with the Pitot/Static heat on the 58s where we use Challenger 600 series Pitot/Static components (with AC heaters). The "58" refers to one of two build variants for our planes. The 58 is a 5800 pound (target) empty weight and the 62 is 6200 pounds (target) empty weight. The difference between the two builds are principally hydraulic pump/reservoir/accumlators repositioning and the use of Challenger 600 series pitot static components on the 58. There are a number of smaller differences but these are the two big items. The labor to convert is about double for a 58 as compared to a 62. We are also building a TFE731-5BR development mule to see if using the larger engine is an option.

On the business side, it became clear that there were two types of need. First there would be the existing owners looking for a cost effective motor swap. The write up I did on cost to convert was based on an owner doing as I did and finding their own engine and doing their own conversion. The idea was that we would offer a “kit of parts” that a craftsman could use to convert their aircraft. These components would be the ones simply too difficult to do a one off of for a single conversion. This includes things like complex composites parts that required large complicated molds. It turned out that most (basically all) L39 owners had no interest in this. It would appear as though Code1 has the correct receipt for doing existing customer conversions as they have had the most success short of our work.

The second need for what we do is the person that wants a high performance fully supportable two place jet based on a fighter/trainer airframe for exhibition and air racing. We have become experts in the airframe, gear, and other systems we retain while removing just about everything from the plane. We have yet to touch an airframe (up to five now) where we have not found significant issues with basic systems and/or flight controls. We have generated all of the required tooling to properly clean and pressure grease flight control bearings and, when needed, remove and replace worn bearings. We keep gear, flaps, trim and ACM. The rest goes. I am completing a 58 now for our demo plane (5800 lb target empty weight target) and I have supplied a friend who wants to do conversion planes moving forward with several airframes and a hangar on the field for building those planes for enthusiasts that want them for their own exhibition/air race needs. He is about half way through building the first one now. His planes will be “62s” (6200 lb target empty weight).

On the parts building side, I began working with a retired Flight Safety composites instructor about six months ago to move from a one off hobbyist approach to a more uniform proper airframe component approach. The current method of generating parts is far too labor intensive and requires a craftsman’s approach to insuring quality. Short of a two part mold, pre-preg and an autoclave, resin infusion molding is the best approach. I’m generating high temperature tooling and build techniques that should allow us to support more than just our internal airframe builds. The modeling for things like core hangars, bleed air mixing and shut off valves, rear motor mounts and the like is being done and released to machine shops for fabrication. We moved from using the original rear engine mounts from the AI-25 to a bespoke one designed to more properly transfer loads to the fan duct without the need for bonding secondary brackets. Rear loads are transferred directly to multiple surfaces. Our TI mounts are also lighter than the original units. Lastly, we can source them in tens of pairs which is difficult to do if we stayed with the AI-25 mounts. When we get to the position that we have a process that can generate repeatable parts of very high quality in a reasonable amount of time, we will seek out a partner with deep roots in the L39 community to provide conversion support. I do not anticipate selling components directly to end users. More likely than not, this project will end up simply providing completed airframes to a limited number of people.