Tach Time vs. Hobbs Time — What Pilots Get Wrong

Ask ten GA pilots to explain the difference between tach time and Hobbs time, and you’ll get ten answers — at least three of which will be wrong. It’s one of the most common sources of confusion in general aviation, and it matters a lot more than most people think.

If you track oil changes by tach hours but accidentally record Hobbs hours, you could be running your engine well past the service interval. If you quote Hobbs time when your engine’s TBO is measured in tach time, you might panic about overhaul when you still have hundreds of hours of useful life left.

Let’s clear it up once and for all.

What each meter actually measures

Your aircraft likely has two hour meters on the panel (or at least one, with the other tracked in your logbook). They look similar and both count “hours,” but they work in fundamentally different ways.

Tachometer (tach time): measures engine RPM-adjusted time. The tach runs proportional to engine RPM. At cruise RPM (typically around 2,400), it roughly tracks real time. At lower RPM — taxi, runup, descent — it runs slower than real time. The tach only accumulates when the engine is running, and it’s the typical reference for engine maintenance intervals and TBO tracking.

Hobbs meter (Hobbs time): measures real elapsed wall-clock time. One Hobbs hour equals one real hour, regardless of RPM. It typically activates when oil pressure rises or the master switch is on (varies by aircraft). Hobbs is the common reference for rental billing, flight time logging, and some calendar- or clock-driven maintenance items.

For a typical piston single, you’ll see roughly 0.8–0.9 tach hours per 1.0 Hobbs hour during normal operations. The key insight: a tach hour and a Hobbs hour are not the same length of time. Because the tachometer runs slower during taxi, runup, and low-RPM operations, you’ll always accumulate tach time more slowly than Hobbs time in normal operations. Over the life of an engine, the difference between the two can be substantial.

Real-world example: You fly a 1.5-hour cross-country in your Cessna 172. Between taxi, runup, cruise, and the pattern at your destination, the Hobbs meter advances 1.5 hours. But your tachometer, which ran slower during the 10 minutes of taxi and the descent at reduced RPM, only advances 1.2 tach hours. Over 100 such flights, that’s a difference of 30 hours — nearly two oil change intervals on some aircraft.

Why it matters for maintenance tracking

Different maintenance items use different time references, and mixing them up has real consequences. Here’s how the common ones break down.

Items typically tracked by tach time

Engine-related maintenance intervals are almost always measured in tach hours, because the tachometer is a better proxy for actual engine wear. The engine works harder at higher RPM, and the tach reflects that proportionally.

Oil changes are the most common example. Many manufacturers and mechanics recommend oil changes every 50 tach hours (or every 4–6 months, whichever comes first). The real danger with mixed references is the reverse direction: if your maintenance item is due at a certain number of Hobbs hours and you track by tach, you’ll fly past the interval, because your tach reads lower than actual elapsed time.

The critical rule: Always know which time reference applies to each maintenance item, and track it using that reference. Never assume “hours” means the same thing on every line of your maintenance schedule. When in doubt, ask your A&P which meter they’re referencing.

Engine TBO (Time Between Overhaul) is measured in tach hours for most piston engines. Lycoming and Continental publish TBO figures in tach time. This is actually good news for owners: because tach hours accumulate slower than Hobbs, your engine’s “life” measured in real flying time is longer than the TBO number suggests.

Items typically tracked by calendar date

Some required inspections don’t care about hours at all. They’re on a fixed calendar schedule regardless of how much you fly.

Your annual inspection is due every 12 calendar months — not after a certain number of hours. The same goes for your transponder check (24 calendar months), pitot-static/altimeter check for IFR flight (24 calendar months), and ELT inspection (12 calendar months, with battery replacement on a separate schedule).

These are straightforward to track, but they still trip people up when they’re mixed in with hour-based items on the same spreadsheet with no clear labels.

Items tracked by “whichever comes first”

This is where it gets tricky. Many maintenance items have both a time-based and a calendar-based limit, and you must comply with whichever one arrives first.

For example, a common oil change recommendation might be: every 50 tach hours or every 4 months, whichever comes first. If you fly a lot, you’ll hit the hour limit first. If you fly infrequently (which is common for partnership aircraft), the calendar limit might arrive with only 20 tach hours since the last change — and you still need to change the oil because time degrades the lubricant regardless of use.

Common mistakes pilots make

These are the errors that come up again and again in hangar conversations, annual inspections, and forum threads. Each one is avoidable if you’re clear about which meter you’re reading.

Common mistakes:

• Recording Hobbs time in a “tach hours” column (or vice versa)
• Using Hobbs to estimate engine TBO remaining
• Ignoring calendar limits on items with dual triggers
• Quoting “total time” at pre-buy without specifying tach or Hobbs
• Mixing references when comparing service intervals across aircraft

Better practices:

• Label every entry: “tach” or “Hobbs” — never just “hours”
• Use tach time for engine TBO and engine-driven maintenance items
• Set reminders for calendar limits separately from hour limits
• At pre-buy, provide both tach and Hobbs totals with clear labels
• Use a tracking system that knows which reference applies to each item

What about aircraft without a tach meter?

Some older aircraft have only a Hobbs meter, or only a mechanical tachometer without a recording tach. Some have had one or the other replaced or zeroed at some point. Glass-cockpit aircraft may display engine time differently depending on the avionics manufacturer.

In these cases, talk to your A&P about how they track engine time for your specific aircraft. Many mechanics will establish a tach/Hobbs ratio based on your typical flying profile and use that ratio to estimate when your logbook references one versus the other. The key is to document the convention you’re using and be consistent.

How this applies to your maintenance records

Every line in your maintenance schedule should have a clear answer to three questions: What’s due? When is it due? And what time reference does “when” use?

If a maintenance item says “every 500 hours,” you need to know: 500 tach hours or 500 Hobbs hours? If it says “every 12 months or 100 hours,” you need to track both the calendar date and the hour meter — and you need to know which hour meter.

This is exactly the kind of problem that gets worse with scale. One aircraft with five maintenance items is manageable. One aircraft with forty recurring items, each tracked by a different combination of tach, Hobbs, and calendar — that’s where spreadsheets start to crack.

The bottom line: Tach time and Hobbs time are different measurements that serve different purposes. Neither one is “right” or “wrong” — but using the wrong one to track a maintenance item is always wrong. The single most important thing you can do is label every time entry clearly and make sure your tracking system (digital or paper) knows the difference.

A better way to track it all

This is one of the core problems TachMinder was built to solve. When you enter a maintenance item, TachMinder asks which time reference it uses — tach hours, Hobbs hours, calendar months, or a “whichever comes first” combination. It then calculates the next due date or hour mark based on the correct reference, and sends you reminders before you get there.

Your aircraft has two clocks running. Your maintenance tracker should know the difference.