Definition
A specific rule within Title 14 of the Code of Federal Regulations (the federal aviation rules) that sets requirements for testing and inspecting the VOR equipment used for IFR (instrument flight rules) operations. It specifies how often a VOR check must be performed, the acceptable methods of checking it, the allowable error tolerances, and the record-keeping required to document each check.
Plain English
It is the federal rule that says: if you want to use your VOR navigation receiver for instrument flying, you have to test it on a regular schedule, the test has to be done in one of several approved ways, the result has to fall within set accuracy limits, and you have to write down the results.
Context Anchor
Seen during preflight planning, aircraft record review, and IFR readiness checks when a pilot verifies that required navigation equipment checks are current.
Derivation
CFR' stands for Code of Federal Regulations, the published collection of U.S. federal rules. '14 CFR' is the volume covering aeronautics and space. 'Part 91' is the section governing general operating and flight rules. 'Section 91.171' is the specific rule within Part 91 dealing with VOR equipment checks for IFR operations. Reading the citation as a path -- volume, part, section -- helps it feel less like a code number and more like an address.
Why Pilots Care
Accurate VOR checks prevent navigation errors that could lead to deviations from assigned routes or unsafe approaches in instrument conditions.
Intuition Check
“Part” does not mean an airplane component here. In this citation, “part” means a numbered group of federal rules, and “section” means one specific rule inside that group.
Example Sentence 1
Before the IFR cross-country, the pilot reviewed 14 CFR part 91, section 91.171 to confirm the VOR check was still within the 30-day window.
Example Sentence 2
The aircraft logbook showed the most recent 91.171 check had been completed within the last thirty days, allowing continued IFR operations.