Your charts are on your iPad. Your weather is on your iPad. Your flight plan is on your iPad. Your VOR check log is on paper.
FAR 91.171 requires a current VOR check before operating under IFR, logged with four specific elements: date, place, bearing error, and the identity of the person making the check. Not a mental note. Not a general sense that the needles looked right. A record.
The regulation has been there since the beginning. Most pilots are either scribbling entries on whatever paper is nearby.
There’s no good reason for this to still be a paper problem in 2026. VOR checks take thirty seconds. Logging them should too — in a format that’s searchable, exportable, and actually meets the FAR.
That’s what SlingologyVOT is for.
The logging requirement, verbatim:
“Each person making the VOR operational check shall enter the date, place, bearing error, and sign the aircraft log or other record.”
— 14 CFR 91.171(d)
Four check methods are acceptable, each with its own error tolerance:
| Method | Where | Max Error |
|---|---|---|
| VOT (VOR Test Facility) | Airport with published VOT | ±4° |
| Ground checkpoint | Published ground checkpoint | ±4° |
| Airborne checkpoint | Published airborne checkpoint | ±6° |
| Dual VOR check | Compare two independent receivers | ±4° between them |
No format is mandated — electronic records are acceptable.
The Regulatory Case for Going Electronic
FAR 91.171 doesn’t prescribe a format. A napkin works. So does a spiral notebook, a sticky note, or a notes app.
The problem isn’t compliance. It’s durability.
Handwritten entries get lost when logbooks change. Sticky notes survive one flight. A notes app captures words but none of the structure — no PASS/FAIL determination, no locked signature, nothing to demonstrate the record wasn’t edited after the fact.
How long do you need to keep it?
FAR 91.171 specifies no retention period beyond currency. The regulation requires one thing: a valid check within the preceding 30 days. Everything older has already served its legal purpose.
There’s still a practical argument for keeping your full history. A VOR receiver that’s drifting doesn’t usually fail all at once — it drifts gradually, check by check. A log with history is the evidence an avionics shop needs to confirm a problem and trace when it started. Without it, you’re describing a symptom. With it, you’re handing them a diagnostic timeline.
FAR 91.171 requires a current check within the preceding 30 days. No retention period is specified beyond that. SlingologyVOT keeps your full history by default — zero cost, and potentially useful data if your VOR receiver ever needs a shop visit.
The FAA addressed electronic recordkeeping directly in Advisory Circular 120-78B. While written primarily for Part 121 and 135 operators, the guidance establishes that electronic records are acceptable provided the required data elements are captured and the record is protected from alteration after signing. Part 91 operators aren’t excluded — and the standard it describes is exactly what a well-designed electronic log should meet.
Electronic records are acceptable when they capture all required data elements and include a mechanism that prevents alteration after the record is signed. SlingologyVOT locks each entry after you sign it — the record cannot be edited once saved.
Electronic isn’t just acceptable. For a Part 91 pilot who wants a log that will actually hold up — and one that might save you a diagnostic bill someday — it’s the better choice.
What SlingologyVOT Does
The app is built around one workflow: open a check, enter your deviation, sign it, and move on. No account setup, no loading screen, no friction between you and a compliant log entry.
Logging a Check
When you open a new check the timestamp is already filled in — date and time captured the moment you open the entry. If you need to adjust it you can, but you’ll confirm the change. The deviation field is the centerpiece: as you type your bearing error a live PASS/FAIL indicator tells you immediately whether you’re within the tolerance for your check method. When you’re satisfied with the entry you sign it with your full name and save. The record locks. It cannot be edited after that point.
Your Check History
Every saved entry lives in your history, with a pass/fail summary at a glance. You can see immediately when your last check was, whether it passed, and whether you’re still current. The log is always on your device, always available, no cell service required.
Exporting Your Log
When you need a copy — for your records, your CFI, or an inspector — SlingologyVOT exports in three formats. Excel gives you a logbook you can print or archive. Plain text produces a clean human-readable log in the style of a traditional logbook entry. JSON is your full backup, importable onto any other device running the app.
Your VOT Sites
Finding a VOT station isn’t always obvious. The locations are published in the Chart Supplement — which most pilots have open approximately never when they’re on the ramp trying to get airborne. The Sites database lets you build your own reference list of known VOT and VOR checkpoint locations with airport identifiers, so the information is always one tap away inside the app.
Installing the App & Staying Current
SlingologyVOT is a Progressive Web App — it installs directly from your browser, no App Store or Google Play required. It means no account, no store approval delay, and no waiting for an update to clear review. The app is always current the moment you open it.
iPhone & iPad (Safari or Chrome)
Both Safari and Chrome support PWA installation on iOS and iPadOS.
Safari:
- Open the app URL in Safari
- Tap the Share button (the box with the arrow pointing up)
- Scroll down and tap Add to Home Screen
- Tap Add to confirm
Chrome:
- Open the app URL in Chrome
- Tap the three-dot menu in the top right
- Tap Add to Home Screen
- Tap Add to confirm
Android (Chrome)
- Open the app URL in Chrome
- Tap the three-dot menu in the top right
- Tap Add to Home Screen or Install App
- Tap Install to confirm
On some Android devices Chrome will show an automatic install banner at the bottom of the screen — tap that if it appears.
Desktop (Chrome or Edge)
- Open the app URL
- Look for the install icon in the address bar — a monitor with a down arrow
- Click it and confirm
Once installed the app runs in its own window, separate from your browser.
Staying Current
When a new version of SlingologyVOT is available the app detects it automatically in the background the next time you are connected to the internet. You’ll see a prompt asking you to update — one tap refreshes to the latest version.
After installing, go to Settings and enter your full name. This is the signature that will be locked into every log entry you save — get it right before you log your first check.
One Plane, Multiple Pilots
VOR checks are logged by the person making the check — not the aircraft, not the owner. If three pilots fly the same aircraft IFR in a given month, each one is responsible for their own current check. That’s fine. What gets messy is keeping a shared record that reflects all of them.
The classic scenario: a flying club aircraft, a CFI and student pair, or two co-owners splitting time on the same tail number. Everyone needs the log current. Nobody wants to maintain three separate records on three separate devices with no way to reconcile them.
SlingologyVOT handles this with two complementary approaches.
QR Sync — pushing a complete log to another device
When one pilot has been maintaining the log and needs to share the full history with another, QR sync does it in seconds. The source device generates a QR code containing the complete log payload. The destination device scans it and merges the incoming entries with whatever is already there. Existing entries are never overwritten — the merge is additive, by entry ID. The QR payload is one-time use and expires in ten minutes.
JSON Export & Import — the durable backup approach
For a more permanent shared record, one pilot exports a full JSON backup and sends it to another pilot via text or email. The recipient imports it via the file picker in the Settings app section. Same merge behavior — additive, no overwrites. This approach also doubles as your off-device backup strategy. Store the JSON in iCloud, Google Drive, or wherever your documents live.
SlingologyVOT deliberately has no cloud backend. There’s no account to create, no server storing your data, and no dependency on a third-party service that could change its pricing, its terms, or simply disappear. Your log lives on your device. You control it entirely.
Links & Resources
SlingologyVOT is free, open source, and runs entirely in your browser. No account, no subscription, no strings.
The source code is publicly available on GitHub under the MIT license. If you want to inspect it, fork it, or host your own instance, it’s all there.
Found a bug or have a feature request? The feedback form goes directly to the developer. Include your device and browser if you’re reporting a bug — it makes a real difference in how quickly it can be tracked down.
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