The L-13 Blaník is a two seater trainer glider produced by Let Kunovice since 1956. It is the most numerous and widely used glider in the world. In United States Air Force Academy service, it is designated TG-10C and is used for basic flight training.

Development

The L-13 Blaník was designed by Karel Dlouhý of VZLÚ Letňany ca. 1956, building upon the experience gained with the Letov XLF-207 Laminar , the first Czech glider to employ laminar flow wing profiles. The L-13 was developed as a practical glider suitable for basic flight instruction, aerobatic instruction and cross-country training. This design concept was combined with true and tested technology: metal construction, NACA laminar profiles and many standard-issue components of the Soviet aerospace industry .

The Blaník entered production in 1958 and quickly gained popularity as an inexpensive, rugged and durable type, which was easy to fly and operate. It was widely adopted in the Soviet bloc and was exported in large numbers to Western Europe and North America. Total production was in excess of 2650, or more than 3000 if variants are included. More than half a century after its first flight it is still the most common glider in the World.

In the cross-country role the Blaník achieved many two-seater World distance records during the 1960s in spite of having only fair performance.

The Blaník inspired other designs, notably the Démant and L-21 Spartak single-seaters developed to equip the Czechoslovak team in the 1956 and 1958 World Championships.

Characteristics

The effectiveness of the Blaník as a primary trainer is due to a blend of characteristics that facilitate progress of ''ab initio'' students towards solo flight, namely: slow landing speed, ample control deflections and an effective rudder. These are in effect typical of wood-and-fabric primary trainers such as the ASK 13 , which the Blaník resembles in handling, though not in materials, construction and aerodynamics.

For this reason, pilots trained in the Blaník require ''differences training'' in a modern two-seater before transitioning to high performance plastic single seaters.

The Blaník was originally stressed for simple aerobatics, including inverted flight where the aircraft has a single occupant. As a result of this latter requirement, intermediate level aerobatic training in the Blaník was done in solo flight with the instructor on the ground or in another aircraft. Following a manufacturer airworthiness directive in June 2010, all aerobatic manoevres were forbidden.

There are two design issues that pilots should be aware of. The wheel retracts counter-intuitively, with the gear handle moving forward for wheel up and back for down (this is reversed on the L-23). More importantly, the spoiler and flap handles have the same shape and are close to each other. This has led to a number of incidents where pilots have mistakenly operated the wrong handle, particularly in the landing pattern. Instructors recommend that a positive visual check of the spoiler operation be made during the prelanding checklist, and that the pilot's hand stay on the handle right through full stop.

The Blaník empennages are vulnerable. The horizontal stabiliser is low enough to be damaged when landing in brush, and one must not push on the vertical fin when ground handling as it is not stressed to carry loads fore-and-aft.

Construction

* Fuselage of semi-monocoque construction employing longerons and bulkheads, with an ovoid cross-section. The cockpit is covered with a two-part acrylic glass canopy.

* Trapezoidal single-taper wings with forward (negative) sweep, single-spar, all-metal construction. Metal ‘torpedo’ tips. Flaps and ailerons have a metal frame and are covered in fabric. Metal DFS type spoilers on the upper and lower wing surfaces.

* The horizontal tail surfaces fold up parallel to the fin for transportation and storage.

* The elevator and rudder are metal frames covered in fabric.

* The landing gear is semi-retractable and sprung with an effective oleo-pneumatic shock absorber, excellent features which assure landings with little or no damage even if the wheel is left (forgotten) in the raised position.

Main spar fatigue

A Blaník was involved in a fatal accident in Austria on 12 June 2010 when a wing spar failed at height, leading to separation of the wing and loss of control of the aircraft. The cause of the failure was attributed to fatigue. As a result, the manufacturer issued an emergency bulletin on 18 June 2010 mandating that each aircraft was to be grounded pending a full inspection of wing spars and compilation of usage patterns from logbook records. Following inspection, the aircraft was permitted to fly on a non-aerobatic basis only. Following further discoveries from the accident investigation, this method of investigating for fatigue has not proved conclusive and so the type remains grounded by the EASA and the FAA .

According to Let company, the current situation may change in spring 2011 when new processes of controll will be developed - http://www.let.cz/files//File/AI_opinion_gliders_092010_ENG.pdf

Europe

Following the accident, EASA released a number of directives regarding all Blaník variants. Initially, the directives mandated the aircraft be grounded unless logbook records show that dual flying hours comprise less than 50% of the aircraft's total flying time. Further investigation into the original accident has shown that these limits and analysis of flight records (such data also not being guaranteed to exist) will not be sufficient to guarantee safe operation of the type.

United States

Emergency airworthiness directives were published by the FAA. A subsequent Airworthiness Directive disallowed use of inspection solely by 10X magnification because of the possibility of metal fatigue that might not be observed by this method. As a result, on August 30, 2010, so inspected L-13 Blaníks were again grounded pending further consideration. This AD covered all L-13s without regard to serial number or category..

Australia

During the 1970s & 1980s, the Gliding Federation of Australia recognized the potential fatigue-life limitations of the Blaník and in conjunction with the Ansett Airlines NDT laboratory and the Civil Aviation Authority of Australia developed an inspection programme culminating in the issue of GFA AD-369, designed to monitor the condition of the fatigue-critical components. Most of these fatigue-critical components are called into question by the accident on 12 June 2010.GFA AD-369 gave 3 options for continued operation beyond 5000 hours or 18,000 launches.

*Option A. total wing replacement

*Option B. major spar modification

*Option C. periodic eddy-current inspection.

In 1984 Dafydd LLewellyn and Riley Aeronautics received Department of Aviation certification for a modification of the wing to extend its fatigue life. Nine Blaníks in Australia were modified and re-certificated as L-13A1 (option B in AD-369)..

GFA AD 663 was issued on 25 June 2010 imposing the operational limitations specified by the Type Certificate holder. GFA AD 663 does not apply to Blaníks which have been modified to L-13 A1 (Llewellyn Modification)..

Variants

* The L-13 AC Blaník is primarily intended for aerobatic training with a wider flight envelope enabling dual training up to intermediate-level. It combines the wings and cockpit of the L-23 Super Blaník with the single-piece canopy and conventional empennage of the L-13. This model is considered stronger and different enough from a conventional L-13 not to be affected by the FAA grounding.

* The Vivat is a touring motorglider derivative. The wings, fuselage and tail surfaces of the L-13 are mated to a cockpit featuring side-by-side seats and a conventional firewall -forward engine installation with either a Mikron M III AE four-cylinder inverted inline engine or a Limbach L 2000.

* An auxiliary-powered Blaník was also developed, with an external engine permanently mounted on a pylon above the rear fuselage. The L-13 J was Blaník with Jawa motor.

* The SL-2P twin fuselage Blaník was developed by Sportinë Aviacija in Lithuania as a flying laboratory for testing of laminar airfoils . The specimen profiles are fixed to a supporting frame erected between the fuselages. This variant is similar in concept to the modified Janus once operated by the DFVLR (today the DLR, or German Aerospace Center ) for the same purpose.

* The L-13 TJ (OK-3801) single-seat experimental motor glider fitted with a jet engine TJ100C with take-off thrust 1,0 kN from První brněnská strojírna Velká Bíteš.

* The L-13 B Bačostroj (OK-8902) single-seat experimental motor glider with Walter Mikron IIIA, 48 kW

* The L-13 A1 (Llewellyn Modification) to extend the fatigue life to nominally three times the basic Blanik L-13 life.