Release mechanisms are the glider (and towplane) component that retain the towline [[Rings]] during launch, and allow the glider pilot to safely release the [[Aerotow Rope]] or cable at the termination of an aerotow or winch launch. ## Location Release mechanisms may be located in the nose and/or the belly ("CG") of the glider. For most gliders, aerotow launches are typically accomplished using the release in the nose; winch launches are typically accomplished using the release in the belly. ![[PW-6U aerotow and winch limitations.png]] ### Nose release Use of the nose release is preferred (or may be **required** by the glider manufacturer) for aerotow launches, as it situates the towline force close to the glider's longitudinal axis, well ahead of the CG. This maximizes glider stability, and minimizes changes in pitching moment as towline tension varies throughout the launch. ![[GFH nose hook aerotow.png]] Use of the nose release should be avoided (or may be **prohibited** by the glider manufacturer) for winch launches, as cable tension produces an increasing nose-down pitching moment as the glider flies closer to the winch. ![[Piggott nose hook winch launch.png]] ### Belly or CG Release Use of the belly/CG release is preferred (or may be **required** by the glider manufacturer) for winch launches, as it minimizes changes in pitching moment as the glider flies closer to the winch. ![[Piggott CG hook winch launch.png]] Use of the belly/CG release should be avoided (or may be **prohibited** by the glider manufacturer) for aerotow launches, as it situates the towline force well below the glider's longitudinal axis, and closer to the CG. This reduces glider stability, and increases changes in pitching moment as towline tension varies throughout the launch. ![[GFH CG hook aerotow.png]] ### JAR-22 Gliders certificated under JAR-22 include the following requirements, per JAR 22.711 and 22.713: - Release mechanisms used for winch launching must automatically release ("back-release") if the glider overflies the winch - The release control must actuate each release at the same time, if more than one release is installed ## Schweizer Release Mechanism The first Schweizer glider design, the SGP 1-1 (1930), featured a simple fixed hook attachment at the nose of the glider, for shock cord and auto-tow launches. Subsequent Schweizer designs have featured the two-piece release mechanism still common on many gliders flying today. ![[SGP 1-1.jpg]] \[SGP 1-1 featured in the National Soaring Museum] ![[GFH Schweizer release mechanism.png]] ![[2-33 release rigging 1.png]] ![[2-33 release rigging 2.png]] \[from Schweizer 2-33 and 2-33A Parts & Maintenance Manual] The tow hook is a free-swinging L-shaped piece of metal, with the pivot situated at the upper front portion. The rear of the tow hook features a step, that allows the tow hook to be retained by a cutout in the release arm. The release arm is connected to the release control by a cable, and is held in position by spring tension. When the pilot pulls the release control, cable tension pulls the upper portion of the release arm forward against the spring tension, releasing the tow hook and allowing it to pivot forward under towline tension, releasing the ring. Additionally, the tow hook will automatically release ("back-release") if the ring applies rearward pressure to the release arm. Pilots and ground crew must use caution to ensure that the tow hook is properly seated in the release arm, as incorrect seating can lead to premature release or [[Towhook Release Failure]]. ![[SA-001_6 tow hook engagement.png]] Schweizer gliders are affected by two airworthiness directives (87-02-01 and 87-17-01) relating to the specific parts, condition, and operation of the tow release mechanism; both reference *Service Bulletin No. SA-001*. [[Rings#Schweizer Rings]] **must** be used with a Schweizer release mechanism. Tost rings can **mistakenly** be used, with a high risk of the rings becoming stuck and causing a [[Towhook Release Failure]]. ## Tost Release Mechanism Tost has been manufacturing glider release mechanisms since 1952. E series release mechanisms are designed as nose-mounted aerotow releases, while G series release mechanisms are designed as CG-mounted winch releases. Both use [[Rings#Tost Rings]]. ![[PW-6U Tost release mechanism.png]] \[Tost nose release mechanism on a PW-6U] The glider's release control is rigged to the release lever, or in some gliders, rigged directly to the segment. The segment is a semi-circular pivoting plate, connected to the release lever and hook. To actuate the release, the pilot must overcome the force of a spring in the release mechanism, rotating the segment and opening the hook. The smaller circular ring fits through the ring cage, and is retained by the hook until the pilot pulls the release control. G series release mechanisms feature a pivoting ring cage that automatically release ("back-release") when a predetermined angle is reached, typically 83°. ![[TOST Anatomy of Tow Release.jpg]] ![[TOST disassembled tow release.jpg]] In addition to wear on the external release mechanism components (especially the hook), the release spring is considered a life-limited component. As such, Tost stipulates that releases **must** be overhauled at 2,000 launches, equivalent to 10,000 actuations. Release springs are always replaced, along with any excessively worn components, and the release mechanism is tested prior to being returned to the customer. ## References - FAA, *Glider Flying Handbook* - Politechnika Warszawska *PW-6U Sailplane Flight Manual* - Piggott, Derek *Ground Launches* (1996) - JAR-22 *Sailplanes and Powered Sailplanes* - retroplane.net *Schweizer SGP 1-1* [National Soaring Museum](http://www.retroplane.net/soaring_museum/schweizer_sgp_1-1/accueil.htm) - Schweizer Aircraft Corp. *Schweizer 2-33 and 2-33A Parts & Maintenance Manual* (1984) - K&L Soaring, LLC *Service Bulletin No. SA-001.6* (2009) - TOST, *Product Catalog* (2022) - Wings & Wheels *Tost Tow Release Overhaul* [Tost Tow Release Overhaul](https://wingsandwheels.com/blog/post/tost-tow-release-overhaul) ### by Kevin Sakson