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Legal

Disclaimer

Twotter Revived is a non-commercial fan project, not affiliated with, endorsed by, supported by, or even acknowledged by Microsoft, Asobo, or Aerosoft. We don't accept monetary donations, but we're happy to accept code or assets for use in the mod.

Twotter Revived is not endorsed or certified by De Havilland Aircraft of Canada.

Twotter Revived is not approved by anyone in any way, shape, or form for training or any other real-world application. Any real-world references are provided for entertainment and educational simulation use only and may be incomplete, outdated, configuration-specific, or inaccurate.

The base aircraft

The base aircraft is not redistributed by this project. The mod ships as binary deltas that are applied to your own copy of the Aerosoft Twin Otter v1.1.1 on your own machine. The original add-on remains the property of its rights holder: Aerosoft or Microsoft; the ownership situation is genuinely unclear.

The practical consequence: the assembled mod cannot be redistributed. Once the installer has run, the folder on your disk contains reconstructed derivatives of files you licensed from Aerosoft. Only the unassembled form — the .xdelta patches plus our own new files — may be distributed.

Modified sim assets are used under Microsoft's Game Content Usage Rules and Working Title's MIT-like license.

Our own code

The complete project source cannot be published, because CFG and XML files still contain fragments of the base aircraft. What is published is the WASM module source, which is the heart of the mod.

The custom code written for this project, other than the bundled components listed below, is placed in the public domain. Take it apart, use it, modify it, redistribute it.

I only ask that you report bugs so I can fix them for Twotter Revived. This is just a polite request, not a legal requirement.

Licenses

Third-party code included in, or shipped alongside, the mod.

libradio

  • License: CDDL-1.0 — SPDX CDDL-1.0 — covering the parts of libradio we ship (see below), not the underlying ITM code
  • Attribution: the thread-safety modification present in these files is Saso Kiselkov's contribution
  • Obtained from: libradio by Saso Kiselkov, vendored byte-identical and unmodified
  • Underlying algorithm: created by the U.S. Department of Commerce NTIA/ITS, Institute for Telecommunication Sciences

The two files we ship carry an NTIA banner and no copyright notice or license block of any kind. That is deliberate upstream: libradio has no repository-level license.

Our own glue is separate and not derived from those files: the bridge wrapper is an independent reimplementation of the shim libradio ships as CDDL-headered code, and the surface-constants table was reimplemented from ITU-R P.527 rather than copied.

puff — DEFLATE inflate

Decompresses the terrain tiles, so the WASM module needs no zlib at runtime.

  • License: zlib — SPDX Zlib
  • Copyright: © 2002–2013 Mark Adler, all rights reserved (version 2.3, 21 Jan 2013)
  • Upstream: zlib, contrib/puff

Clause 2 of the license requires that altered versions be plainly marked, so: this copy is altered. The <setjmp.h> dependency was removed — the MSFS wasm sysroot ships no setjmp.h — and the original setjmp/longjmp "out of input" escape was replaced with an overrun flag and an error return. The inflate algorithm and its output are unchanged, and the changes are marked in the source.

Working Title GNS 430/530

The GNS 530 and GNS 430 units in the panel. Vendored and patched in-tree.

  • License: MIT with a field-of-use restriction — full text.
  • Copyright: Microsoft Corporation. Authored by Working Title Simulations.
  • Upstream: msfs-avionics-mirror

The restriction is the part that matters: it grants the usual MIT permissions — use, modify, distribute, sublicense, sell — but only for use within Microsoft Flight Simulator. Licensing for anything else is prohibited.

The GNS bundle also carries msfssdk.js and garminsdk.js, built from @microsoft/msfs-sdk and @microsoft/msfs-garminsdk, under the same license.

xdelta3

Reconstructs the modified aircraft files on your machine during installation. Shipped as xdelta3.exe in the mod folder — an unmodified upstream binary, distributed alongside our work rather than linked into it.

Bundled audio

FlyByWire Simulations — CC BY-NC-SA 4.0

The fbw_egpws_* callout files and fbw_flap_actuator.wav derive from FlyByWire Simulations' a32nx-wwise, under Creative Commons Attribution-NonCommercial-ShareAlike 4.0.

Most modifications are pure format conversions to 16-bit mono PCM; the terrain, glideslope and terrain-ahead clips were additionally cut by hand from a longer source clip. All remain CC BY-NC-SA 4.0.

Two obligations follow, and they attach to the mod as distributed, not merely to the files:

  • NonCommercial. This mod may not be sold or distributed commercially.
  • ShareAlike. Derivatives of those audio files carry the same license.

Black Square / Boris Audio Works

The optional Turbine Duke sound mod re-uses the Turbine Duke's Wwise soundbank with kind permission from Black Square and Boris Audio Works. You must own the Black Square Turbine Duke, no bank content is redistributed.

Project-owned callouts

The custom TAWS bank and the copilot checklist responses, were generated by a commercial TTS service for this project and since no person was involved in creating them, nobody can claim any rights for them. Whether it was ethical to base them on the voice of a person is beyond my paygrade. If anyone feels offended, let me know.

Bundled models

Ground power unit — CC BY 4.0

The GPU cart that appears beside the aircraft is "GPU (Aiport Ground Power Unit)" by omri_ha_muglob, under Creative Commons Attribution 4.0. Bagolu fitted it to the Twin Otter.

The wheel chocks are Bagolu's own model, donated to the project.

Data

Source License
ETOPO 2022 60″ global surface DEM (NOAA NCEI) — terrain elevation U.S. Government work, public domain
Natural Earth 1:10m ocean / lakes / glaciated areas — coastlines, water, ice public domain
ESA WorldCover 2021 v200 — land cover CC BY 4.0, attribution required
OurAirports navaids.csv — navaid coverage public domain
FlightGear nav.dat — navaid service ranges, historic NDB database GPL-2.0-or-later

ESA WorldCover: © ESA WorldCover project / contributors.

FlightGear's nav.dat originates in the X-Plane navdata compiled by Robin A. Peel. The shipped artifacts are derived data — service-range tables and the historic NDB database — baked at build time; the GPL source itself is not redistributed and the WASM build does not depend on it.

In practice

If you are redistributing or building on this project:

  • The mod as distributed is NonCommercial, because of the FlyByWire audio.
  • The assembled mod folder cannot be redistributed at all — it contains reconstructed Aerosoft-derived files.
  • Attribution is required for ESA WorldCover, for the FlyByWire audio, and for the ground power unit model.
  • CDDL and GPL components carry source-availability obligations, met by the published WASM source and by xdelta3 being an unmodified upstream binary.

Reviewed 2026-08-02.