With happenings on the Croatian GA scene once again grinding to a halt as the winter fogs set in – and several in-progress articles remaining stalled for a stubborn lack of information – I had once again decided to dip into my airliner photo bag and pull out a small Photo File to make Achtung, Skyhawk! seem actually alive 😀 . Thankfully, my travels of late had frequently taken me among the region’s turboprops both small and large, many of which tended to have a couple of interesting stories behind them. Naturally, with my camera being a permanent traveling companion, very few of these had managed to go escape being documented, allowing for enough material to take a quick trip through Turboprop World… 🙂
Doing its best not to go amphibian, the first daily flight to Osijek (OSI/LDOS) – operated by AIS Airlines on behalf of local carrier Trade Air – trudges through the rain towards Zagreb’s RWY 05. Nowadays quite a rare sight – even though it is one of the more successful British passenger designs – the Jetstream family can trace its roots back to the 1960s Handley-Page HP.137, a light (but loud!) 18-seater that had made its name with a long and distinguished career with the Royal Navy. Equipped with Garrett TPE331 engines instead of the visually- and aurally-distinctive Turbomeca Astzaous of the original, the Jetstream is a real hot-rod, and can even keep pace the odd large transport turboprop. An interesting detail is the baggage pod; while the original design had included enough space for a reasonable amount of baggage (stowed in the rear fuselage), the addition of a toilet later in the production run had drastically reduced that capacity – requiring a solution most often seen on piston and turboprop singles…
A peek into the nose of DCI’s sister ship, NCI. Quite an anachronism in an age when even the smallest piston single has digital avionics, the cockpit of the Jetstream leaves little doubt that this is a 60s design. Alongside a somewhat unusual instrument layout on the center console, interesting details include a lone, basic IFR GPS, and the absence of even a cursory autopilot – a feature that some pilots despise, while others laud for the nowadays rare chance of experiencing “manual” airline operations. A big thank you for this shot goes to the crew Ed and J.J., who had – with typical Dutch openness – warmly greeted me and answered a ton of my Achtung, Skyhawk-y questions!
A welcome (pun intended) splash of color at Zagreb as one of Welcome Air’s Dorniers lights up for departure following an unscheduled stop. Apart from its unusual elegance, the Do-328 can also boast an advanced wing design, which – when coupled with the type’s abundance of power – places it among the fastest passenger turboprops in the air today. With typical “high speed” cruise figures of around 620 km/h, only the Q400 (650 km/h) and the SAAB 2000 (670 km/h) are able show it their tails in level flight…
Even though it is – rarity-wise – the aviation equivalent of the light delivery van, the King Air nevertheless rarely fails to attract attention out on the ramp. Looking mighty and regal in the crisp afternoon sun of Munich Airport (MUC/EDDM), D-IICE was manufactured back in 1977 (not that you can tell from the outside!), and despite being a native of the airport can often be found all over Europe.
An eye-catching train of ATR-42-300s waiting out their fate on Zagreb’s maintenance apron. Latterly owned by South American carrier Aviateca – operating mostly out of Honduras and Guatemala – all three have over the months become well-known residents of the airport, though the leading machine will soon become the first to fly the nest. Originally known as HR-AXN, it had recently been re-registered G-ISLJ and will – if the Internet is to be believed – imminently join the fleet of UK operator Blue Islands. Somewhat more worse for wear, HR-AUX and TG-TRB in trail have a more uncertain future ahead however… but, they at least have a past to compensate, having previously been two of the three 42s operated by Croatia Airlines in the 90s and 2000s, known as 9A-CTU and 9A-CTT respectively.
Somewhat of a stereotypical way of knowing you’re at a former East German airport… one of the many remains of Interflug scattered all the way from the North Sea down to the Czech border, DDR-STG is not the only preserved Il-18 out there; but it likely is the only one still fulfilling a useful function. Produced back in 1962 (and originally known as DM-STG), it would serve with Interflug all the way into 1988, when it would be withdrawn from service and stored at Erfurt Airport (ERF/EDDE). In modern times however, it would take on the role of an airport personnel training aid – as a consequence of which it does get some occasional care, and can even be towed around for pushback practice…
The two schools of commuter turboprop design: the slow, unpressurized, but tough and STOL-capable Turbolet – and the fast, refined, delicate and complicated Metroliner. Worlds apart, both serve as a fascinating glimpse into what made the Eastern and Western markets so different in the 60s and 70s – and why have their designs survived as long as they did. Ironically, both are now more commonly found shuffling freight than people…
A little show of force at Zurich Airport (ZRH/LSZH). Even though these stands are usually dominated by Austrian Airlines’ Q400s, today’s rendezvous of Croatia Airlines flights from Dubrovnik (DBV/LDDU) and Zagreb (ZAG/LDZA) had slightly tipped the scales on the apron…
And finally, Quebec Alpha revving up for an evening departure out of a rainy and gloomy Zagreb. A visually curious aircraft from any angle, the Q400’s long fuselage makes it look relatively compact – even though it is quite a large and heavy aircraft. Those seemingly-normal R408 propellers are in fact 4.11 meters in diameter – a couple of centimeters more than the huge prop of the Vought F4U Corsair!
In one of those instances in life where innocent scrolling through aircraft photos leads to several hours’ worth of research, I am once again able to present one of my (unplanned 😀 ) historical works, borrowing now on a theme set by my previous PC-6 engine piece. The unsuspecting party this time is the pudgy Let L-410 Turbolet, one of the Czech Republic’s most notable and successful aircraft – and a machine that many in Eastern and Southeastern Europe associate with durability, practicality and robust versatility (alongside the An-2 naturally).
The train of thought that had derailed this time was the realization of the sheer number of L-410 variants out there – 39 by my count so far! – all identified by cryptic and complex shorthand such as “UVP”, “E-17”, “E-S”, “AB” and so on. Coming to terms that I’m (at best) a bit hazy on what they all actually mean, I’ve decided I might as well dig a little deeper and see if this mass of numbers and letters can be forced to make some sense… 🙂
All aboard! One of the several Turbolets to have visited Lučko over the years, OM-PGD is by far the rarest, being a member of the “early” L-410M family – nowadays quite an unusual sight.
Despite its apparent complexity though, the classic L-410 line can actually be split into only eight easily-manageable families. Given that various online sources give various specs and details for each generation, I’ve decided to go back to the absolute basics, and concentrate solely on the one definitive source – the Turbolet’s Type Certification Data Sheet (or TCDS), available both from the Czech Civil Aviation Authority and EASA.
But, since fiddling with the sort of detailed specs that can be found in such a document would defeat the purpose of a “clarification”, the aim of this article is simply to construct an overview of the general changes from family to family – and not a thorough analysis of all 39+ versions. To keep the comparison clean, tidy and understandable, only the bits that were changed between models are noted – with an exact side-by-side comparison of key performance specs and characteristics (also my own work) provided at the end.
So, starting from the top…
L-410 “originals” (6 aircraft): despite the Turbolet being most closely associated with the Walter M601 series engine, the type’s beginnings were actually tied to another turboprop great, the Pratt & Whitney Canada PT6A. While the L-410 had been designed from the outset with the M601 in mind, the engine was still not usable by the time the Turbolet was nearing the detail design stage in the mid-60s. In order not to hold up development, the Let works had soon decided to slot in a replacement engine, the PT6A-27, developing 680 SHP. Given that engine-propeller combinations are not really open to experimentation, the choice of a Western engine had necessitated the use of a Western propeller, in this case the Hamilton 23LF-343 three-blade unit with a 2.6 meter span.
Following flight and ground testing of the four XL-410 prototypes, the design was finally “frozen” at the beginning of the 70s, subsequently entering production in 1971 as the L-410. However, only six aircraft of this series were ever produced – with all having subsequently been converted to the L-410A standard.
An interesting detail – first seen on the final two prototypes – was the de-icing system, which had dispensed with standard pneumatic boots in favor of a TKS fluid system. Increasingly popular on today’s high-performance GA aircraft, this system works by bleeding a special glycol-based fluid through micro pores on the wing and tailplane leading edges, preventing the accumulation of ice. While the system is significantly more effective than classic boots, the protection provided is dependent on the quantity of fluid carried – a factor that had limited the aircraft’s exposure to icing conditions to only 40 minutes.
L-410A (25 aircraft): picking up where the original left off, the A model was introduced at the beginning of 1972, but had essentially differed only in the propeller fit – with the Hamiltons giving way to Hartzell HC-B3TN-3D units with the same three blades and 2.6 meter span. Interestingly, some sources state that this version had also introduced a Maximum Take Off Mass (MTOM) increase to 5,700 kg from the original’s 5,400 kg; however, the TCDS states that both the original and A models were certified for the higher mass.
Sub-versions include the L-410AB, L-410AF and L-410AS
L-410M (108 aircraft): by the mid-70s, the M601’s bugs had finally been ironed out, making it suitable for installation on the Turbolet. Debuting in 1975, the M would be fitted with the increased-diameter M601A unit, developing 740 SHP and whirling three-blade Avia V508 propellers with a 2.5 meter span. Despite the increased power, this new setup had actually had a slightly detrimental effect on performance, with cruise speed now reduced by 10 km/h / 5 kts to 350 km/h / 189 knots, and ceiling by 100 m / 300 ft to 6,000 m / 19,700 ft.
The switch to the M601 had also marked the end* of the TKS de-icing system and the return to standard pneumatic boots, resulting in the lifting of the 40 minute restriction for flight in icing conditions.
* however, I’ve been told that the TKS system was actually discontinued in 1974, so it may be possible that a handful of the final A models were also fitted with boots…
Sub-versions here include the L-410MA/MU (such as OM-PGD from the featured photo)
L-410UVP: as was the case with many Eastern European aircraft, the Turbolet’s main market was – by default – the USSR, especially in the hands of the state behemoth Aeroflot. Having originally instigated the development of the L-410 as a potential substitute for the vast fleets of An-2s still being used on commercial services, Aeroflot had naturally been interested in getting the most out of the design, having had some reservations about the M model’s ability to operate in the Soviet backwoods. Under the airline’s urging, Let had in 1979 introduced the improved UVP, whose primary claim to fame was full STOL performance (with UVP being short for ukorochennovo vzlyota i posadki, or “improved take-off and landing” in Russian).
While at a glance it looks just like its predecessors, this new model is in fact a somewhat larger machine, sporting a 0.8 meter fuselage stretch (for a total of 14.17 meters in length) – and, critically, a 2 meter increase in wingspan to 19.48 m. Fitted with a pair of ground spoilers to dump lift on landing, these changes were in theory all that was needed to achieve the required short-field performance.
However, all these mods had also resulted in a significant increase in empty weight, which – without an appropriate increase in MTOM – had led to a reduction in payload (despite the extra room on board). Further complicating matters was the revised powerplant, now taking the form of the 700 SHP M601B and its associated Avia V508B propeller – making for an 80 HP deficit at take-off. This problem would be mitigated somewhat in 1983 with the introduction of the 735 SHP M601D and Avia V508D – as well as Service Bulletins which, when implemented, allowed an MTOM increase from 5,500 to 5,800 kg for the M601B and 6,000 kg for the M601D.
Despite this, the added bulk (and re-certification to Soviet standards) had also resulted in a reduced ceiling of 4,250 m / 14,000 ft, and a reduction in passenger capacity from 19 to 15 (not even swapping the metal rudder for a fabric-covered one had helped bring the weight down).
The sub-versions of this family include the L-410FG and L-410T
L-410UVP-E: to cure the UVP’s ills, the Let works would in 1986 debut the further improved UVP-E (E for ekonomicheskiy, or “economic”), also known briefly under the tongue-twister L-410UVP-L-E. This would be fitted with the 760 SHP M601E or E-21 engine (the latter intended for hot-and-high conditions), spinning more substantial Avia V510 five-blade propellers with a 2.3 meter diameter and fully automatic autofeather system. Even though cruise speeds had remained the same as on the UVP, the new aircraft could boast significantly higher maximum weights, with an MTOM of 6,400 kg and a Maximum Landing Mass (MLM) of 6,200 kg.
Apart from regaining its 19 passenger capacity (in part due to the reconfiguration of the rear baggage compartment), this version had also introduced optional tip-tanks, which together added 315 kg of fuel to its basic 1,000 kg internal capacity. Other changes had also included a revised avionics setup, now fully compatible with both Western and Eastern ground navigation aids – as well as the reintroduction of limitations for icing conditions, with the E now restricted to low and moderate icing down to a minimum temperature of -20 degrees Centigrade. The reason for this turn of events is somewhat uncertain, but was suggested to be due to the design of the oil-to-fuel heat exchanged fitted to this model, which could lead to the overcooling of both the oil and fuel in extremely low temperatures.
Sub-versions were greater in number, including the L-410UVP-E1, E2, E3, E4, E6 and E8
L-410UVP-E9: a further update of the basic UVP-E, debuting in 1988. Generally identical, the E9 had only introduced MTOM and MLM increases to 6,600 and 6,400 kg respectively
Sub-versions include the L-410UVP-E9A, E9D, E13, E14, E15, E16, E17 and E19
L-410UVP-E20: what would eventually become the definitive classic Turbolet had appeared in 1990, sporting only a 40 cm fuselage shrink and a (once again) restored ability to continuously operate in icing conditions (likely due to the removal of the offending heat exchanger). While this had hardly set the world alight, the changes “on paper” were much more significant, with the E20 being the first of the family to be fully certified against both FAA FAR 23 and (today’s) EASA Part 23 regulations. Being “universally accepted” in the world had also meant that the E20 would remain in production until this day, with new builds still being delivered left and right at the time of writing.
Sub-versions include the L-410UVP-E20C, E20D, E20G and E27
L-410UVP-EPT (one aircraft): in what would become an interesting piece of circularity, 2016 would see the Turbolet’s return to PT6 power with the EPT, a third-party conversion penned by Aero Servis of the Czech Republic. Having become dissatisfied with General Electric’s handling of the M601 program – citing rising costs and dropping support quality – the company decided to replace the Walter with the more potent PT6A-42, flat rated to 800 HP and spinning a modern Avia AV-725 five-blade aluminium propeller. Though Aero Servis does not cite any performance figures, they do say that the installation reduces fuel consumption and noise, while increasing both hot-and-high performance and the maximum cruise speed.
As of SEP 2021 when this chapter was added, only one aircraft has so far been converted.
The Lone Ranger. Currently the world’s sole EPT, OK-LRA had started out in life as a regular UVP-E9, manufactured in 1989 and sporting the serial 892216. It would be converted to Pratt Power in 2017, making its first flight on 1 September the same year. For the past 12 or so months, it had been operating domestic Croatian PSO flights on behalf of local carrier Trade Air, replacing the previous L-410 OK-LAZ
L-420 (one aircraft): the last of the Turbolet classics, the L-420 also had the dubious distinction of creating confusion far greater than its “production run”. Introduced in 1998, the 420 was often labelled as just a re-branded E20; however, in reality, the model was actually another refresh of the E line, intended for certification and export to the US market. Sporting 790 HP M601F engines driving the same V510s, the 410 could boast a top speed of 375 km/h / 202 knots and an “A level” ceiling of 6,100 m / 19,700 ft. Interestingly, the aforementioned FAR regulations had necessitated a number of unusual changes to the basic design, including the removal of the engines’ Integrated Electronic Limiter Unit (IELU) – which would reduce engine power in case any of its parameters were exceeded – the addition of a passenger door on the right side of the fuselage (identical to the one on the left) and reinforced flight control linkage.
Despite the increased performance and the additional changes, the design had not fared well against Western competition, with reportedly only one aircraft made – and a test conversion from a stock E20 at that (though it is now normally flying in commercial operations).
L-410NG – the new kid on the block: even though this one doesn’t count towards the classic family tree, I’d decided it would be unfair to skip over it – especially since it picks up the baton of the L-420. Having flown for the first time in 2015, the NG is essentially a thoroughly updated current-production E20, marketed (ironically) primarily to operators in the former USSR. The highlights of its transformation include a three-screen Garmin G3000 glass cockpit, composite materials in non-critical areas (such as doors), an elongated nose for increased baggage space – and a significant power boost in the form of the 850 SHP General Electric H85-200 unit and its associated five-blade Avia AV-725 prop. While the latter sounds like quite the change, the engine in question is actually a modernized M601, born during GE’s purchase of Walter.
The extra grunt naturally promises increased performance and capability, with an MTOM of 7,000 kg, MLM of 6,800 kg – and cruise speeds now in the 420 km/h / 227 knot range. Interestingly, the update also includes a significant boost in fuel capacity, with 2,340 kg now available with tip tanks, versus 1,315 for an equivalent E20.
Another visiting Turbolet – but this time a basic UVP that had mistakenly ended up on the military helipad and had to be pushed back… manually
The Devil Is In The Details
Given that the above pretty much covers 75% of the Turbolet’s family tree, I thought it a shame not to list – at least in passing – some of the aforementioned sub-versions. However, as they are not contained or listed in the TCDS, this review is based on various human and online sources and includes just a short snipped of what made the version special or different… 🙂
L-410AB: the L-410A fitted with Hartzell HC-B4TN-3 four-blade propellers
L-410AF: a one-of aerial photography and cartography version with a glazed nose and fixed nose wheel
L-410AS: ten examples built for the Soviet market, with upgraded navigation and communication equipment. Used to test the design’s suitability for use in the USSR
L-410MA: a standard L-410M refitted with the UVP’s M601B engine and V508B propeller
L-410MU: an alternate designation for the L-410MA
L-410FG: identical in function and configuration to the L-410AF, but based on the first UVP; seven produced. Interestingly, to enable the camera operator to reach his station in the nose, the copilots control column had to be removed; despite this, the aircraft had remained a two-pilot machine. It is unknown however (though likely) that the AF had also “suffered” from the same issue
L-410T: a standard UVP modified to be able to better handle freight, with a larger cargo door and cabin modifications to be able to accommodate pallets, stretchers and bulky cargo
L-410UVP-E1: two aircraft for Bulgaria, reportedly for a mixed photography/transport role
L-410UVP-E2: a modified version for the Polish Maritime Office
L-410UVP-E3: version optimized for skydive operations
L-410UVP-E6: navaid calibration version
L-410UVP-E-S: a “salon” VIP version with integrated steps and plush interior
L-410UVP-E4 & E8: exact details unknown
L-410UVP-E9A: a version for the Swedish marked fitted with Bendix/King avionics
L-410UVP-E9D: a sole example modified with a Bendix/King EFIS cockpit system
L-410UVP-E14: another model with Bendix/King avionics, intended for the transport of military dignitaries
As always, I would like to extend my sincere thanks to former L-410 F/O Enes Handžar, who had shed a lot of light on the Turbolet’s history and quirks!
The sudden appearance of An-2 9A-DIZ at Lučko – usually a sign of impending parachute activity – should have warned me that a foreign visitor, hired for the same purpose, would probably follow suit. It didn’t, so I was pleasantly surprised this morning to come across a sharp-looking Turbolet visiting the field for the first time, just waiting – begging 😀 – to be photographed :D. Despite being a relatively common aircraft in these parts, I was naturally at it within minutes and soon discovered that it was not as ordinary as I had first thought…
Registered in Slovakia, OM-PGD was – like OK-SAS, the last Turbolet to visit – in town for some parachute ops, substituting for AK Zagreb’s poor old Cessna 185 which, more than a year after its unfortunate propstrike, is still nowhere near airworthy status. And while externally identical at a glance to any other Turbolet out there, PGD is in fact a rather rare early MA model, one of the first Turbolet marks produced in any significant quantity – and one of the few non-STOL models still flying today…
Designed from the outset to be powered by the Czech Walter M601 turboprop – the PT6 of the Eastern Block – that was in development at the time, the first production L-410s got off to a slightly more ironic start when M601 development delays and problems forced the temporary selection of another engine. The only one suitable and available was the PT6 itself, so to avoid any further disruptions, the first production series – the 28-strong L-410A – was fitted with two PT6A-27s and sent on its way.
In the mean time, the basic M601 design had matured into the slightly larger and more powerful 700 HP M601A which – coupled with an Avia V508 three-blade propeller – was deemed ready for the L-410. In this form the aircraft became the slightly-less rare L-410M, of which about 108 were produced.
What can today be considered as the “bog standard” Turbolet – the UVP – differed in some respects from the M model from which it was developed, including a slightly longer fuselage, larger wings, a taller vertical stabilizer and 730 HP M601B engines. And while the M didn’t have what you’d describe as a long take-off run, the UVP was the first model to introduce the STOL capability for which the design is now famous (500 m with a 1800 kg payload!).
The MA though was a mix of the two worlds, being the basic L-410M powered by the UVP’s M601B engines. Where it fits into the design lineage I’m not sure, but given that almost all M versions flying today are MAs, it is safe to assume that this version is a retrofit. Be that as it may, it represents the last of the L-410 “originals” and was sufficiently rare to get my full attention :D.
Even when looking at it for awhile, it's hard to distinguish it from the "normal" UVP... only the slightly shorter fuselage is a giveaway...
That's a pretty large behind! 😀 Despite its questionable aesthetics when viewed from this angle, the fuselage is commodious and very practical - and I'm told well suited for and liked by parachutists
Blending well with the dull overcast... like on many parachute versions, the standard doors had been removed and fitted with a much more practical "garage door"
Ready to go off road :). The unusual landing gear bay arrangement frees up space within the cabin - making it simpler and structurally sounder - while also allowing the main wheels to have a wider track, which is very useful on uneven terrain
To handle the rough stuff, L-410s of all marks are fitted with large low pressure tires and very, very powerful "packet" disk brakes. Unlike brakes on smaller aircraft - which usually have only one small brake pad and caliper - the "packet" brake consists of a full-size circular pad, providing friction across the whole disk. To press it in, the L-410 uses seven pistons, which make the brake forces very strong and enable a lot of hard breaking before the brakes lock up. Note also the red line painted across the wheel and tire: this is a handy way of checking whether the tire is properly inflated when you don't have a manometer handy. Should the tire become significantly under-inflated, on takeoff and landing it would start to rotate around the wheel itself, moving the mark out of alignment
Though not as extensive as on other STOL aircraft, the windscreen provides a very good field of vision for both crew. Windscreen wipers are a must for all-weather operations
Two of the L-410's unusual features are the copilot side door, just below the side window, and the confusing black fuselage stripe. This thin layer of rubber is necessary protection of the fuselage - and especially the joint between two fuselage sheets - from bits of ice shot off the prop when its deicer is in operation. On many light aircraft - I've especially noted this on Piper twins - this protection is a metal sheet and is usually painted over so it is not easily noticeable
A generation apart... one of the L-410's design aims was to replace the An-2 in many of its transport roles; but it ended up just supplementing it, as the venerable Anushka almost outlasted the L-410 in production...
