Hobbs vs Tach Time: What Every Aircraft Owner Should Track

If you fly or own a light aircraft, you have probably come across two different ways of recording aircraft time: Hobbs time and Tach time.

They may both be displayed in hours, but they do not necessarily measure the same thing — and they can increase at very different rates.

Understanding the difference matters for aircraft owners, flying clubs and flight schools because different counters may be used for maintenance planning, aircraft rental, operating-cost calculations and record keeping.

So what exactly is the difference between Hobbs and Tach time, and which one should you track?

What Is Hobbs Time?

A Hobbs meter is essentially an elapsed-time counter.

Depending on how the aircraft is wired, it may begin counting when the:

• master switch is turned on,
• engine oil pressure rises,
• engine starts running, or
• another configured condition is met.

Once activated, a traditional Hobbs meter normally records time at a constant rate.

If it runs for one real-world hour, it records approximately 1.0 Hobbs hour, regardless of whether the engine spends that hour idling on the ground or operating at cruise power.

This makes Hobbs time straightforward to understand and one reason it is commonly used in aircraft rental operations.

However, the exact activation logic is aircraft-specific. You should not assume that every Hobbs meter starts and stops under the same conditions.

What Is Tach Time?

Tach time — often simply called Tach — is recorded by a tachometer or an associated recording mechanism.

Unlike a typical Hobbs meter, Tach time is generally related to engine speed.

At lower RPM, such as during taxi or extended ground operations, Tach time usually accumulates more slowly. At an appropriate reference RPM, it accumulates closer to real elapsed time.

This means a flight might show, for example:

but only:

The exact relationship is not universal. It depends on the aircraft, tachometer and how the system is calibrated or configured.

Hobbs vs Tach Time: The Main Difference

The easiest way to think about the two is:

The important point is that Hobbs and Tach are separate counters.

Neither is automatically the aircraft's official flight time, block time, airframe time or time in service for every regulatory or maintenance purpose.

Those concepts need to be treated separately.

Why Is Tach Time Often Lower Than Hobbs Time?

Consider a simple flight:

You start the engine, taxi to the runway, complete the run-up, depart, fly for an hour, land and taxi back to parking.

If the Hobbs meter is activated by engine oil pressure, it may run for essentially that entire period.

The Tach, however, is affected by engine RPM.

During taxi, warm-up and other low-power operations, it can accumulate more slowly than the Hobbs meter.

Over many flights, that difference can become significant.

There is therefore no universal conversion factor such as:

You may observe a relatively consistent relationship on a particular aircraft under similar operating conditions, but it should not be treated as a general rule.

How Do You Read Tach Time?

In many light aircraft, Tach time is displayed directly on or alongside the tachometer.

For example, if the counter shows:

that represents a recorded Tach value of 1,234.7 hours.

For operational tracking, the useful information is usually the difference between the start and end readings.

If you begin with:

and finish with:

the counter increased by:

Keeping the actual start and end readings can be particularly useful because it creates a continuous history rather than relying only on manually entered flight duration.

Which Time Should Be Used for Aircraft Maintenance?

This is where it is important not to oversimplify.

A maintenance requirement should be tracked against the counter or time basis that actually applies to that requirement.

Depending on the aircraft, component, maintenance programme and applicable instructions, a requirement may be based on:

• a relevant aircraft or engine time,
• Tach or another installed counter where applicable,
• landings or cycles,
• a calendar date,
• or more than one limit.

For example, a maintenance item might become due after a specified number of hours or after a specified calendar period, whichever occurs first.

That is why aircraft owners should not simply choose one favourite counter and use it for everything.

The underlying maintenance requirement determines what needs to be tracked.

Hobbs, Tach and Flight Time Are Not Necessarily the Same

This distinction is especially important when aircraft records start moving from paper or spreadsheets into software.

An aircraft may have several useful time values, including:

• Hobbs,
• Tach,
• airframe or total-time counters,
• engine-related counters,
• block time,
• flight time,
• takeoff and landing times,
• landings or cycles.

Trying to force all of these into a single number can create confusion.

A better approach is to record each relevant counter independently and use the appropriate one for the task it supports.

What Should Aircraft Owners Track?

For a simple privately operated aircraft, you may not need dozens of counters.

But if the aircraft actually uses both Hobbs and Tach, recording both readings consistently can be useful.

A practical flight record might contain:

Before flight

• Hobbs start
• Tach start

After flight

• Hobbs end
• Tach end
• relevant flight details
• landings, where needed

The system can then calculate the increase in each counter.

This becomes particularly valuable when the aircraft is shared between several pilots. Instead of reconstructing counter readings later from handwritten notes, each flight contributes to a continuous aircraft history.

Why Spreadsheets Become Difficult

A spreadsheet can work perfectly well for a simple aircraft operation.

The problem usually appears as the number of things being tracked grows.

You may eventually have:

• one sheet for aircraft hours,
• another for maintenance,
• calendar reminders for due dates,
• documents in a separate folder,
• bookings in a shared calendar,
• and expenses somewhere else.

None of these tools is necessarily bad. The difficulty is that the information is disconnected.

A flight changes the aircraft's counters. Those counters can affect maintenance remaining. Maintenance may have both hour-based and calendar-based limits.

Once those relationships matter, manually keeping several separate records synchronized becomes increasingly inconvenient.

Tracking Multiple Aircraft Counters in ReadyFlyLog

ReadyFlyLog is designed to keep those pieces connected without forcing every aircraft into the same setup.

An aircraft can have relevant counters such as Hobbs, Tach and other aircraft-specific counters, while individual flights record the corresponding readings.

Maintenance requirements can then be associated with the counter or calendar limits that actually matter.

For requirements with multiple limits, they can be tracked together — for example:

This makes it possible to see the aircraft's current status without manually comparing several spreadsheets after every flight.

The goal is not to replace approved aircraft records or determine which maintenance requirement legally applies to an aircraft.

It is to make the operational information you already need to track easier to keep organised and easier to understand.

Hobbs vs Tach: Which One Is Better?

Neither.

They measure different things.

Hobbs time is useful when you want a straightforward elapsed-time counter while a defined aircraft condition is active.

Tach time reflects an engine-speed-related counter and often accumulates more slowly during low-RPM operation.

For an aircraft owner, the better question is not:

"Should I use Hobbs or Tach?"

but:

"Which counters does my aircraft use, and which one applies to each thing I need to track?"

In many cases, the most useful answer is simply to track both.

Final Thoughts

Hobbs and Tach time look similar because both are expressed in hours, but they should not be treated as interchangeable.

Understanding the difference makes aircraft records easier to interpret and helps owners keep flight activity, counters and maintenance planning connected.

For a single aircraft with simple requirements, a notebook or spreadsheet may be enough.

As the operation grows — more pilots, more flights, more maintenance requirements or more aircraft — keeping those counters connected to the rest of the aircraft's records becomes increasingly valuable.

ReadyFlyLog brings aircraft counters, flight records, maintenance, bookings, documents and operating costs together in one simple workspace.