How TachMinder computes next-due dates

Almost everything TachMinder does — reminders, due-soon warnings, the maintenance table itself — comes down to one question: when is this item due next? This guide explains how the app computes that answer, which clocks it watches, and what makes the countdown move.

Two clocks, and whichever runs out first wins

A recurring maintenance item in TachMinder counts down on two clocks at the same time: engine hours and calendar months. That mirrors how most real-world intervals are written — "every N hours or N months, whichever comes first." The app tracks both countdowns independently, and the one that runs out first is the one that governs. An aircraft that flies a lot tends to hit hour limits first; an aircraft that sits tends to hit calendar limits first.

The "fires first" hint is a planning aid — not the clock

Next to items with both an hour limit and a calendar limit, TachMinder shows a hint estimating which clock is likely to run out first. That estimate uses a rough assumption of about 10 flight hours per month to compare the two countdowns. Let's be plain about what that is: a planning aid, nothing more. If you fly 3 hours a month or 30, the estimate will be off — but the actual countdowns won't be, because they track your real recorded hours and real dates. The regulatory clock is always the real one; the hint just helps you see which limit deserves your attention when you're planning ahead.

Hours basis: tach hours and Hobbs hours are not interchangeable

Each hour-based item tracks which meter its interval counts against — tach time or Hobbs time — and it counts down against the matching meter reading only. That's why the app labels remaining time as "tach hrs left" or "hobbs hrs left" rather than a bare "hours left": an unlabeled hours figure invites exactly the kind of mix-up that corrupts a maintenance schedule. Recurring AD limits run on the explicit clock recorded for each AD — calendar months, engine hours (measured by tach), propeller hours, airframe hours, or cycles — never a bare, unlabeled "hours." If the difference between the two meters is new to you, start with Tach time vs. Hobbs time.

What makes the countdown move

Logging flights is what advances the hour-based clocks. When you log a flight, the change in the tach reading — the tach delta — flows into every hour-based item tracking that meter, and their remaining hours tick down accordingly. If flights aren't being logged, the hour-based countdowns sit still even though the airplane is flying, and your due dates quietly drift out of touch with reality. See Logging flights for the mechanics. Calendar clocks, of course, advance on their own.

What resets the countdown

When work gets done — a mechanic signs off completed work through TachMinder, or you mark an item complete yourself — the item's next-due values recompute from the completion point. The hour clock restarts from the hours at completion; the calendar clock restarts from the completion date. From there, both countdowns begin again.

Editing intervals: the app pushes back when it should

You can edit a recurring item's interval, but the app treats two situations differently:

• Beyond the manufacturer's recommendation: if you raise an interval past what the manufacturer recommends, TachMinder asks for explicit confirmation — it tells you the new interval is longer than the manufacturer recommends and makes you say yes on purpose. Once you confirm, the app remembers your choice rather than nagging you every time.
• Beyond a regulatory limit: if the interval would exceed a regulatory limit, the edit is blocked outright, and the app shows you the regulation involved. There is no override — a confirmation dialog can't make a regulatory limit longer.

A note on TBO

Engine time between overhaul (TBO) deserves its own mention, because it's widely misunderstood. Under Part 91, TBO is a manufacturer's recommendation — it is not a legal requirement to overhaul the engine at that number. Because TBO varies by engine and because it's the owner's call how to treat it, TachMinder asks you to enter your engine's TBO value rather than assuming one for you. The app then tracks progress toward the number you entered.

The takeaway

Log your flights, and the clocks take care of themselves: hour-based items count down against the right meter, calendar items count down on the wall clock, and whichever runs out first drives the due date. The estimates are labeled as estimates, the real countdowns run on real recorded values, and when the app pushes back on an interval edit, it tells you why.