[[Template blog/front/global/commentTableHeader is throwing an error. This theme may be out of date. Run the support tool in the AdminCP to restore the default theme.]] Posted March 11 · Report reply Form 337 is a field approval for changes to an aircraft certified under Part 23. S-LSA are certified under ASTM consensus standards, not part 23, so a form 337 does nothing for an S-LSA. You need and LoA for MRA from the manufacturer, which serves the same purpose as a 337 for a Part 23 airplane. Most FAA docs you read will assume certified airplanes, and not give much regard to LSA or experimental airplanes, so you can't always follow the guidance in those publications.