Part 2 of 2 · Double M Aviation LLC · doublemaviation.com
The critical — and largely undocumented — transition from mechanical to digital time tracking, and what it means for your aircraft’s records.
In Part 1, we walked through how digital engine monitors track time and how the counter names and trigger logic differ across Garmin, JPI, and Electronics International. If you haven’t read it, that context matters here.
But here’s the thing: the fact that these instruments track time differently isn’t, by itself, the problem. Instruments have always had quirks. A mechanical Hobbs with a 10 PSI switch and one with a 15 PSI switch don’t produce identical records either.
The real problem is what happens at the moment a digital engine monitor is installed in an aircraft that has been flying — and tracking maintenance time — with mechanical instruments for years. That transition moment is where records can break down, quietly and permanently, if nobody handles it deliberately.
There are three questions every aircraft owner should be able to answer about their digital engine monitor:
- How was time being tracked in this aircraft before the monitor was installed?
- Which counter in the new monitor corresponds to that tracking method, and what is it called?
- Was the correct historical time documented and carried forward at installation?
Most owners can’t answer all three. Many can’t answer any of them.
What “Time” Means Before and after the Digital Engine Monitor
Before digital engine monitors became common, most GA aircraft tracked time one of two ways: Hobbs Time via a mechanical elapsed-time meter, or Tach Time via a cable-driven tachometer. When a digital engine monitor is installed, it comes with its own set of counters. Part 1 covered all of this in detail, but the key point here is that the counter names don’t always match what you’re used to — and the trigger logic isn’t always identical to what the mechanical instrument was doing.
The Transition Moment Nobody Documents
Here is where the real gap lives.
When a mechanical Hobbs meter was replaced, the new one typically started at zero. That physical constraint forced a logbook entry: the old meter’s last reading went in the record, the new meter’s starting point was established at zero, and the math was clear. Not because anyone required it — but because there was no other option.
Digital engine monitors don’t work that way. Their time counters can be set to any value at installation — or left at zero. The JPI EDM-960 provides digit-by-digit edit fields for Airframe HOBBS and engine hours in its Factory programming mode. The EI CGR-30P lets the installer set Tach Time via a Maintenance Password. The Garmin TXi EIS doesn’t address timer initialization as an installation task at all in either its maintenance manual or its AFMS.
What none of them provide is guidance on:
- What value to enter — which logbook entry is the authoritative source?
- Who is responsible — the installing A&P? The shop owner? The aircraft owner?
- How to document it — what logbook entry should be made to record what values were set and from what source?
This isn’t a criticism of the instruments. They’re well-engineered products. It’s an observation that the industry has not developed a standard practice for this transition, and no manufacturer has filled that void in their documentation.
The result: an aircraft that has been flying for 20 years arrives at a shop for an engine monitor installation. The new monitor is installed and powered up. Someone enters a Hobbs Time — or doesn’t. The aircraft flies away. And nobody may ever know whether the number on that screen reflects the aircraft’s actual history, a guess, or zero.
Field Adjustable vs. Not
Mechanical Hobbs meters and tachometers shared one characteristic that is easy to overlook until it’s gone: they could not be adjusted by the people who used them. If an analog Hobbs meter or tach needed to be recalibrated or reset, that work required a certified instrument repair shop. An A&P mechanic could remove the instrument and reinstall it, but adjusting the reading itself was outside their certificate authority. A pilot had no role in the matter whatsoever. The number on the instrument was, for all practical purposes, immutable in the field.
Digital engine monitors work differently, and the contrast is significant. Most allow time values to be set directly by the person configuring the instrument — no instrument repair station required. As covered in Part 1, the JPI EDM-960 exposes its Airframe HOBBS and Tach Time fields in a Factory programming mode accessible via a button sequence described in the pilot’s guide. The Garmin TXi EIS addresses timer initialization in its AFMS but does not describe it as a controlled or documented maintenance task. The EI CGR-30P gates its Tach Time behind a Maintenance Password — which represents a meaningful attempt to restrict access to the installer level — but that password is available from Electronics International upon request. None of these require the involvement of a certified instrument repair shop.
What this means in practice is that the integrity of the time record in a digital engine monitor depends almost entirely on the individual who sets it. That individual might be a knowledgeable A&P who carefully transferred the correct values from the aircraft logbook and documented the action. Or it might be someone who entered a round number because no guidance was available, or who left the counter at zero because they weren’t sure what it should be. There is currently no regulatory requirement, no manufacturer guidance, and no industry standard that specifies otherwise.
This is not an argument against digital engine monitors. They offer capabilities that mechanical instruments never could. It is simply an honest accounting of what changed: a system that once relied on physical constraints to protect the integrity of time records now relies on individual knowledge and diligence. For aircraft owners, that means the question of whether their engine monitor’s time values are accurate is no longer answered by the instrument itself — it’s answered by what happened the day it was installed, and whether anyone documented it.
What You Should Do
The good news is that this is a solvable problem — it just requires deliberate action that the industry doesn’t currently mandate.
- Find out how your aircraft tracked time before the monitor was installed. Check your logbook and maintenance records. Was time tracked on a Hobbs meter? A tach? Both? What was the last recorded value before the monitor was installed? This is your baseline.
- Identify the correct counter for your instrument and confirm what it’s called. Knowing what each timer in your aircraft is named — and what its triggers are — is vital here.
- Verify that the correct time was entered at installation. Find the maintenance logbook entry for the engine monitor installation. Does it document what values were entered into the time counters, and from what source? If it doesn’t — or if there’s no entry at all — that’s a gap worth closing now, while the history is still reconstructible from your records.
- Document the transition explicitly at your next annual. Ask your IA (Inspection Authorization) to note in the maintenance record: the current time counter readings from the digital monitor, the corresponding total time in the logbook, and a confirmation that the two reconcile. This creates a checkpoint that future owners, mechanics, and buyers can rely on.
The Bigger Picture
The aviation maintenance system is built on continuous, accurate records. The transition from mechanical to digital time tracking happened gradually and informally — aircraft by aircraft, installation by installation — without any industry-wide standard for managing the handoff.
The instruments themselves are not the problem. The problem is the moment between the old instrument coming out and the new one being set up: who sets the numbers, from what source, and how is that documented. Right now, the answer to all three of those questions is “whoever happened to be in the shop that day, using whatever they had on hand, with no required documentation.”
As an aircraft owner, you are the one person in that chain who has both the authority and the incentive to make sure it’s done right. Your logbook is the record. Your maintenance intervals depend on it. And unlike the instrument, the record doesn’t reset itself.