Hobbs vs Tach Time & SMOH — Aircraft Engine Hours Explained

Mixing up hobsmeter time and tach time is one of the most common sources of confusion in aircraft maintenance records -- and when it goes unresolved, it costs owners money and obscures the real condition of the engine. John breaks down the difference between the two gauges, why they're not interchangeable, and what happens when a mechanic or owner has been flip-flopping between them for years without documenting which one was used. The tachometer measures engine use; the hobs measures how long the master switch has been on. For maintenance purposes, tach time is what drives decisions about overhaul intervals and AD compliance. Jeff explains what to do when a gauge is replaced, fails, or doesn't match prior entries -- and why "estimated" and "unknown" are legitimate and important entries when the true time can't be substantiated. Leaving a time gap unexplained is far worse than flagging it accurately. The conversation gets into time since major overhaul (SMOH) and total time since new (TSN) -- what both mean, how to read them on an overhauled engine versus a first-run engine, and what a remanufactured engine serial number actually tells you. John explains why knowing SMOH is the most critical number when deciding whether to repair or fully overhaul an engine, and what the Continental and Lycoming recommended intervals actually represent. In this episode, we cover: The functional difference between a tachometer and a hobsmeter -- and which one to use for engine maintenance Why flipping between tach and hobs time in logbook entries creates confusion and red flags What to document when a gauge is replaced or fails mid-ownership When to write "estimated" or "unknown" in a logbook entry -- and why that is the right call Time since major overhaul (SMOH) vs. total time since new (TSN) -- how to read both What a remanufactured engine serial number means for Continental and Lycoming engines Why high total time since new is not the same as a worn-out engine How SMOH drives repair vs. overhaul decisions at Signature Engines 00:00:41 Introduction -- why tach vs. hobs matters 00:02:41 How entries should note time and which gauge to use 00:05:27 What to do when you only have one gauge 00:06:50 Documenting gauge replacements and time gaps in the logbook 00:09:21 When "estimated" and "unknown" belong in a logbook entry 00:10:30 Time since major overhaul vs. total time since new 00:13:27 Remanufactured vs. overhauled -- what zero-time actually means 00:16:12 How many times can an engine be overhauled? Get in touch! Web - SignatureEngines.com Email - [email protected] YouTube -    / @signatureenginesinc