Definition
A minimum performance standard issued by the FAA for specified materials, parts, and appliances used on civil aircraft. When a piece of equipment is approved under a TSO, it has been shown to meet the design, manufacturing, and performance requirements set out in that standard. For GPS receivers used in IFR flight, TSO-C129, TSO-C145, and TSO-C146 are the most commonly referenced standards, each defining what the receiver must be able to do and how reliably it must do it.
Plain English
A TSO is an FAA-issued rulebook that says what a piece of aviation equipment must be able to do and how well it must perform. If a GPS or other unit is built and approved to a particular TSO, it has been tested and accepted as meeting that rulebook.
Context Anchor
Seen in avionics manuals, GPS equipment approvals, aircraft records, and discussions about whether a GPS unit may be used for instrument navigation.
Derivation
Technical' refers to the equipment's engineering and performance details. 'Standard Order' means an official, standing instruction issued by the FAA. Together it describes a fixed, published specification that manufacturers must build to.
Why Pilots Care
Only TSO-approved equipment meets the legal requirements for IFR navigation and installation on certificated aircraft.
Intuition Check
Do not read “Technical Standard Order” as a command given to the pilot. Here, “order” means an official FAA standard for equipment performance. Also, TSO approval of the equipment is not the same thing as approval to install or use it in every aircraft.
Example Sentence 1
The panel-mount GPS in this airplane is approved under TSO-C146, so it can be used for IFR en route, terminal, and approach navigation.
Example Sentence 2
Before buying new avionics, the pilot checked that each unit was listed under the appropriate TSO.