Glider instrument panel
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tso instruments form 337 field approval faa

When you are considering replacing or adding new equipment and avionics in your glider, it is important to first consider the category of glider you are working with. Experimental gliders offer a wider and normally cheaper range of options to choose from.  Alternatively, Standard Type Certificated aircraft are held to stricter guidelines for their installed equipment. 

Title 14 of the Code of Federal Regulations requires that certain avionics, instruments, and equipment be approved even when operated under part 91, general aviation. The certification process for 14 CFR parts 91 must include verification that the required instruments and equipment are approved. Required instruments are listed in the Type Certificate Data Sheet (TCDS) and often also found in the Aircraft Flight Manual. Although not often listed in TCDS's or Aircraft Flight Manuals, radios and transponders can be considered required equipment for flying in certain airspaces and airports which must also be approved.

Approval can be obtained in accordance with one of the following:

  •  Use of a Technical Standard Order (TSO). TSO is a minimum performance standard for specified materials, parts, and appliances used on aircraft.
  •  Acceptance as part of the aircraft on the original Type Certificate (TC) or Supplemental Type Certificate (STC)
  •  Field approval (FAA Form 337, Major Repair and Alteration). Editors note: What is a FAA form 337?  The person who performs or supervises a major repair or major alteration must prepare FAA Form 337. The form is executed at least in duplicate and is used to record major repairs and major alterations made to an aircraft, airframe, powerplant, propeller, appliance, or a component part thereof.  The purpose is To provide aircraft owners and operators with a record of major repairs and major alterations indicating the details and approvals; To provide aircraft owners and operators with a record of major repairs and major alterations indicating the details and approvals.

Field Approval can be obtained through form 337, a form must be filed with the FAA.

It is important to note that you can have non-required, secondary equipment such as collision avoidance systems, flight computers and secondary electronic vario’s installed in addition to your primary/required equipment. The installation can be done by a certificated mechanic as a minor alteration with a simple logbook endorsement as long as they adhere to the following: The installation must not appreciably affect weight, balance, structural strength, performance, power plant operation, flight characteristics, or other qualities affecting airworthiness and must be completed according to accepted practices.

The language used in the CFR’s in this case leaves room for interpretation as to what can be considered a minor alteration. In my opinion there is no one single modern instrument you can have installed in your glider that will have an appreciable effect on your weight and balance. 

It is best that you do not replace or alter your required equipment with non-approved equipment. The simplest route is to install your secondary avionics/equipment as a minor alteration. Make sure that your secondary equipment is properly installed according to accepted practices. Communicate with your installer/IA to ensure that you are on the same page. At the end of the day, your IA will have to sign off your glider with your installed equipment at the annual inspection.

Danny Schaefer
Schaefer Aviation
www.schaeferaviation.com

Banner Photo by Petr Kolmann

danny schaefer  Danny Schaefer is the owner and operator of Schaefer Aviation. Schaefer Aviation specializes in glider repair, refinish, and inspection. He is a licensed glider pilot, has over 12 years of experience in the field, has refinished more than 20 gliders, repaired and inspected many more. His shop is located at the Williams Soaring Center Airport in California. He prides himself on his attention to detail and ability to complete complicated composite repairs. To find out more information about Schaefer Aviation, check out www.schaeferaviation.com.