VOR Equipment Check Under 14 CFR 91.171: The 30-Day Pilot-Logged Item Every Flight School IFR Trainer Quietly Misses

A CFII files IFR for a 1500 instrument lesson, picks up the clearance, gets vectors toward the localizer, and forty minutes into the flight the student asks when the airplane's VOR check was last done. The CFII does not know. They land, pull the airframe log on the ramp, and find the last VOR equipment check signed thirty-three days ago. The airplane is mechanically fine. The flight was not legal under 91.171 from engine start. The lesson gets re-flown, the student loses the IFR time, and the chief instructor adds a line to the dispatch checklist that should have been there the day the airplane was added to the IFR pool.

14 CFR 91.171 is the rule that catches a flight school's IFR trainer in a different way than the 24-month inspections do. It is a 30-day check, it is performed by the pilot rather than a mechanic, and it is logged in a place a maintenance binder will not see. A school that tracks 91.411 and 91.413 cleanly can still walk an instrument student into an illegal flight if 91.171 is not someone's named job at dispatch.

What 91.171 Actually Requires

14 CFR 91.171 says that no person may operate a civil aircraft under IFR using the VOR system of radio navigation unless the VOR equipment has been maintained, checked, and inspected under an approved procedure, or operationally checked within the preceding 30 days and found to be within the permissible bearing error limits the section sets out.

The qualifying language matters. The rule is triggered by operation under IFR using the VOR system. A VFR cross-country that homes on a VOR is not bound by 91.171. An IFR flight that uses only GPS for navigation, on a route where every fix is a waypoint and no airway centerline is being tracked off a VOR, is a softer case the school's chief instructor needs to take a position on, because most school IFR lessons still tune and identify a VOR somewhere on the route and the cleanest policy is to treat every IFR launch as a 91.171 flight.

The clock is 30 days, not 30 calendar days and not a calendar month. A check performed on 12 May is good through 11 June. The day-count is the trap, because instructors who learned the 24-month language of the pitot-static and transponder rules reach for a month boundary by reflex, and the airplane is one day past legal before anyone notices.

The Four Legal Check Methods

91.171(b) and (c) name the methods. Each one has a tolerance, and the tolerance is part of the legal entry.

A check against an FAA-operated VOT, the ground-based test signal at a tower-equipped airport, is good to within plus or minus 4 degrees. The pilot tunes the VOT frequency, centers the OBS, and confirms a TO indication on a 180 setting or a FROM indication on a 360. A VOT check on a quiet ramp at a home field is the cleanest entry a school's chief instructor can ask for, because the test signal is a known calibration.

A check against a published ground checkpoint, the painted dot on the ramp at a designated airport, is also good to within plus or minus 4 degrees. The locations are published in the Chart Supplement. The pilot taxis the airplane onto the designated spot, tunes the published VOR and bearing, and confirms the indication is within tolerance.

A check against a published airborne checkpoint is good to within plus or minus 6 degrees. The wider tolerance is the rule writing in the airborne fix variability. The published checkpoints, again from the Chart Supplement, are over identifiable terrain or a published radial and altitude.

A dual VOR check, available only on an airplane with two independent VOR receivers, requires both units to be tuned to the same VOR, the OBSs centered on the same TO or FROM indication, and the two bearing readings to agree within 4 degrees of each other. The dual check is the one a school's IFR trainer can do en route on a Tuesday morning if the airplane is dual VOR equipped, and it is the method that most often shows up in a clean school's logbook.

The Aeronautical Information Manual section on VOR receiver checks walks the procedure in operational language. The rule itself is the four lines above. The AIM language gives the school a printable reference for the dispatch desk.

What the Logbook Entry Has to Contain

91.171(d) names the four pieces of the entry: the date, the place, the bearing error, and the signature of the person making the check. The location of the entry is permitted to be the aircraft log or other reliable record. Many schools keep a dedicated VOR check log in the airplane, a clipboard near the avionics stack with a running ledger, because the maintenance binder is not where a pilot will reach during a preflight.

The bearing error is not optional and not the same as a pass-or-fail box. A check that recorded plus 2 degrees on a VOT is a different entry than a check that recorded zero, and a check that recorded plus 5 degrees on a ground checkpoint is a check that is out of tolerance and the airplane is not legal IFR on the VOR until the equipment is investigated. The number on the page is what gives the chief instructor a trend. An airplane whose bearing error has crept from plus 1 to plus 3 across two checks is an airplane whose next maintenance write-up is going to happen sooner than the avionics shop expects.

The 30-Day Clock as a Dispatch Item

A school that tracks 91.171 on a dispatch board is a school whose IFR trainer does not launch without a current check. The same calendar discipline that runs through the AD tracking pattern applies, with one important difference: 91.171 is a pilot-performed check, so the soft-flag and the hard-block belong on the dispatch screen instead of the maintenance summary. The next-due date is the date of the last check plus 29 days, the soft flag goes up at day 25, and the hard block sits on day 30.

The school whose dispatcher releases the airplane only after seeing a current VOR check entry is the school whose IFR students do not waste a Hobbs hour on a lesson the FAA would not count. A school whose squawk discipline is already clean can extend the same write-it-down-before-it-flies rule to the VOR entry, because the failure mode is the same: a piece of paperwork that should have happened on the ramp, and did not.

Where the Check Fits the Instrument Student

The VOR check is also a teaching moment. An instrument student who has watched their CFII perform a dual VOR check on a Saturday morning, write the entry into the logbook, and brief the bearing-error number against the trend has learned the operational discipline behind the instrument proficiency check before the IPC ever comes due. The 91.171 entry is small enough to feel administrative and large enough that an instrument-rated pilot who skips it has set themselves up for an enforcement conversation the day a VOR-tracked approach goes wrong.

The Quiet Failure Mode

91.411 and 91.413 expire on a date the maintenance shop already knows. 91.171 expires on a date the pilot is supposed to know, in a log the pilot is supposed to read, and most flight school IFR trainers fly four or five times in a 30-day window without anyone consciously checking the entry. The trainer that gets caught is not the one whose CFII forgot the rule. It is the one whose school never decided whose desk the check lives on. A chief instructor who writes the answer to that question once, prints it at dispatch, and trains the CFII pool to look for the entry before engine start is a chief instructor whose IFR fleet stays legal across a year of lessons. The rule is short, the consequence is small in money and large in reputation, and the only way to miss it is to have never named the person responsible for the 30-day clock.