VOR Checks for IFR: The 30-Day Rule, Tolerances, Logging, and How to Perform Each Check
- Nathan Hodell
- Aug 31, 2025
- 8 min read
Updated: Aug 5
The VOR check is one of those regulatory requirements that's simple in concept but full of details that show up on checkrides and in logbook reviews. How often is the check required? What's the tolerance for each method? What exactly must be logged? Who can perform it? Can you use GPS to do it? Getting these details right keeps you legal for IFR flight and demonstrates the kind of regulatory precision that examiners look for. Getting them wrong can mean an illegal IFR flight or a failed checkride question.
This post is the complete reference on VOR checks: the regulation behind them, each check method with step-by-step procedures, the exact tolerances, the required logbook entry, who can legally perform each check, and the common mistakes that trip pilots up.
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The Regulation: FAR 91.171
VOR checks are required by 14 CFR 91.171, which governs VOR equipment checks for IFR operations.
The core requirement:
No person may operate an aircraft under IFR using the VOR system
Unless the VOR equipment has been checked within the preceding 30 days
And found to be within the required tolerances
The 30-day window:
The check must be within 30 days before IFR flight using VOR
Not 30 days before any flight — specifically IFR flight using VOR
VFR flight doesn't require the check (though accuracy still matters)
The key point:
The check is required only for IFR operations using the VOR system. If you fly VFR, or fly IFR using only GPS (with VOR not used for navigation), the 30-day check isn't legally required. But for IFR flights where you'll use VOR, the check is mandatory.
The Required Tolerances
Each check method has a specific allowable error. This is heavily tested:
Check Method | Maximum Allowable Error |
VOT (VOR Test Facility) | ±4° |
Certified ground checkpoint | ±4° |
Certified airborne checkpoint | ±6° |
Dual VOR check | 4° between the two receivers |
Airway over known landmark | ±6° |
The memory pattern:
Ground-based checks (VOT, ground checkpoint): ±4°
Airborne checks (airborne checkpoint, airway): ±6°
Dual VOR: 4° between the two receivers
Why airborne checks allow more error:
Airborne checks (±6°) allow more error than ground checks (±4°) because of the additional variables in flight — position accuracy over the checkpoint, altitude, and signal conditions. Ground checks are more controlled and therefore held to a tighter tolerance.
Check Method 1: VOT (VOR Test Facility)
The VOT is a dedicated test signal — the most precise check method.
What a VOT is:
A special ground-based transmitter
Located at certain airports
Transmits a test signal (not a navigational radial)
Provides a known reference for checking
Finding a VOT:
Listed in the Chart Supplement (formerly A/FD)
VOT frequencies published
Not available at all airports
The procedure:
Tune the VOR receiver to the VOT frequency
Set the OBS to 360°
The CDI should center with a FROM indication
Alternatively, set OBS to 180° — CDI centers with TO indication
Check the error against ±4° tolerance
The memory aid for VOT:
"Cessna 182" — with the OBS at 360°, you should get FROM. Some pilots remember: "0° FROM, 180° TO." Another aid: the needle should center within 4° of these settings.
Why the VOT works this way:
The VOT transmits a signal that simulates being on the 360° radial from the station, regardless of where you are. So setting 360° should always center with FROM, and 180° should center with TO.
Tolerance: ±4°
Check Method 2: Ground Checkpoint
Designated ground checkpoints at airports provide a convenient pre-flight check.
What a ground checkpoint is:
A specific marked location at an airport (ramp or taxiway)
Marked by a sign
Has a published radial from a nearby VOR
Designed specifically for VOR checks
Finding ground checkpoints:
Listed in the Chart Supplement
Marked with a sign at the location
Specific to each airport that has one
The procedure:
Position the aircraft exactly at the marked checkpoint
Tune and identify the VOR
Set the OBS to the published radial for that checkpoint
The CDI should center
The TO/FROM should match the expected indication
Check error against ±4° tolerance
The precision requirement:
Position the aircraft accurately at the checkpoint
The published radial assumes you're at the exact spot
Being off the mark introduces error
Tolerance: ±4°
Check Method 3: Airborne Checkpoint
When ground facilities aren't available, airborne checkpoints allow in-flight verification.
What an airborne checkpoint is:
A published point in the air
Over a recognizable landmark
With a known radial from a VOR
At a specified altitude
Finding airborne checkpoints:
Listed in the Chart Supplement
Specific landmarks and altitudes
Published radials
The procedure:
Fly over the published checkpoint at the designated altitude
Tune and identify the VOR
Set the OBS to the published radial
Verify the CDI centers within tolerance
Check error against ±6° tolerance
The challenge:
Accurately flying over the checkpoint
Maintaining the specified altitude
Timing the check as you pass the point
Tolerance: ±6°
Check Method 4: Airway Checkpoint (Over a Known Landmark)
A variation when no published checkpoint is available.
The method:
Select a VOR airway radial
Find a prominent landmark on that airway centerline
Use a point along a published airway over a known landmark
The airway centerline is the known radial
The procedure:
Identify a published VOR airway with a prominent landmark on the centerline
The landmark should be along the airway (at least 20 NM from the VOR)
Fly over the landmark
Tune and identify the VOR
Set the OBS to the airway radial
Verify the CDI centers within ±6°
The 20 NM consideration:
The check should be done at least 20 NM from the VOR
This reduces the effect of position error
Closer to the station, small position errors cause large radial errors
Tolerance: ±6°
Check Method 5: Dual VOR Check
If the aircraft has two VOR receivers, they can be checked against each other.
The method:
Compare two independent VOR receivers
Both tuned to the same VOR
The difference between them is the check
The procedure:
Tune both VOR receivers to the same VOR station
Identify the station
Set both OBS knobs to center the needles (or to the same radial)
Compare the two radial indications
The difference must not exceed 4°
The convenience:
Can be done anywhere (no special checkpoint needed)
Can be done in flight or on the ground
Quick and practical
Common in IFR-equipped aircraft
The limitation:
Requires two independent VOR receivers
Checks them against each other, not against a known reference
Both could theoretically be off in the same direction (unlikely but possible)
Tolerance: 4° between the two receivers
The Required Logbook Entry
Every VOR check must be logged. The required elements form a mnemonic:
The four required items:
Date of the check
Place where the check was performed
Bearing error (the error observed)
Signature of the person performing the check
The mnemonic: "SLED" or simpler approaches
A common mnemonic for the required elements is to remember "signature, location, error, date" or use memory aids. One approach:
Signature
Location
Error
Date
Where to log it:
A separate VOR check log
The aircraft logbook
Any reliable record
Must be retained and available
Example log entry:
Date: 06/23/2026
Place: KBJC ground checkpoint
VOR Error: +2°
Signature: [Pilot signature]The retention requirement:
The log must be kept
Available for the IFR flight
Demonstrates compliance with the 30-day requirement
Who Can Perform Each Check
An important regulatory nuance: who is authorized to perform VOR checks?
VOT, ground, airborne, dual, airway checks:
The pilot can perform these checks
No special certification required
The pilot logs and signs
The repair station check (a fifth method):
A certified repair station can perform a VOR check
Using a radiated test signal
Tolerance: ±4°
Performed by certified personnel
This is in addition to the pilot-performed checks
The practical point:
For most pilots, the VOT, ground, airborne, and dual checks are pilot-performed. The repair station check is an option when equipment is being serviced.
Can You Use GPS for a VOR Check?
A common modern question: can GPS substitute for a VOR check?
The answer: No.
The VOR check verifies the VOR receiver's accuracy
GPS is a different system
GPS cannot check the VOR receiver
The 30-day VOR check must use one of the approved methods
Why this matters:
Even with GPS, if you'll use VOR for IFR, the VOR check is required
GPS accuracy doesn't validate VOR equipment
The approved methods (VOT, ground, airborne, dual) are specific
The exception:
If you fly IFR using only GPS (and don't use VOR for navigation), the VOR check isn't required for that flight. But if VOR is part of your IFR navigation, the check applies.
Common Mistakes Pilots Make
1. Forgetting the 30-day requirement:
Letting the check expire
Flying IFR with an expired check
A regulatory violation
2. Incomplete log entries:
Missing one of the four required items
Not signing
Insufficient record
3. Confusing the tolerances:
Using ±4° for airborne (should be ±6°)
Using ±6° for ground (should be ±4°)
Mixing up the dual VOR tolerance (4° between)
4. Imprecise positioning:
Not accurately at the ground checkpoint
Not over the airborne checkpoint
Introducing position error
5. Not identifying the VOR:
Checking against an unidentified station
The station could be unreliable
Always identify first
6. Thinking GPS substitutes:
Believing GPS validates VOR
Skipping the VOR check
Still required for IFR VOR use
The Practical Workflow
Before an IFR flight using VOR:
Check the log: Is there a valid VOR check within 30 days?
If expired, perform a check: Choose an available method
Identify the VOR or VOT: Confirm the signal
Perform the check: Follow the procedure for your method
Verify tolerance: Within ±4° (ground) or ±6° (airborne)
Log it: Date, place, error, signature
Now legal for IFR VOR use
The convenient methods:
Dual VOR check: Easiest if you have two receivers (anywhere)
Ground checkpoint: Convenient before departure
VOT: Most precise if available
The reality:
Many IFR pilots perform a dual VOR check before flight (if equipped) because it's quick and requires no special checkpoint. Others use ground checkpoints or VOTs when available.
On the Written Test and Checkride
VOR checks appear consistently on instrument tests and orals. The most commonly tested topics:
The 30-day requirement (FAR 91.171)
The tolerances (±4° ground, ±6° airborne, 4° dual)
The required log entries (date, place, error, signature)
The check methods (VOT, ground, airborne, dual)
Who can perform checks
GPS doesn't substitute for VOR checks
Quick Reference
The Regulation:
FAR 91.171
VOR check within 30 days for IFR use
Required only for IFR using VOR
Tolerances:
Method | Tolerance |
VOT | ±4° |
Ground checkpoint | ±4° |
Airborne checkpoint | ±6° |
Airway over landmark | ±6° |
Dual VOR | 4° between |
Repair station | ±4° |
Memory Pattern:
Ground-based: ±4°
Airborne: ±6°
Dual: 4° between
VOT Procedure:
Tune VOT frequency
OBS 360° = FROM (or 180° = TO)
Within ±4°
Required Log Entry:
Signature
Location
Error
Date
Check Methods:
VOT: Test facility signal (±4°)
Ground checkpoint: Marked airport location (±4°)
Airborne checkpoint: Published in-flight point (±6°)
Airway: Over landmark, 20+ NM from VOR (±6°)
Dual VOR: Two receivers compared (4° between)
Repair station: Certified test (±4°)
Key Facts:
Required only for IFR using VOR
30-day window
GPS does NOT substitute
Always identify the VOR first
Pilot can perform most checks
Must be logged with all four items
Common Mistakes:
Expired check
Incomplete log entry
Confusing tolerances
Imprecise positioning
Not identifying the station
Key Principle:
For IFR flight using VOR, check within 30 days, meet the tolerance for your method, and log date/place/error/signature.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.