How to Read an Aircraft Maintenance Logbook
An aircraft's maintenance logbooks are its biography. Every annual inspection, engine repair, avionics installation, and AD compliance is supposed to be documented in the logs. For a pre-purchase inspection, the logbooks are often the first thing a savvy buyer reviews — and for good reason. The story they tell about an aircraft's past is essential to understanding its current condition and future costs.
But logbooks aren't always easy to read, especially if you're not an A&P mechanic. This guide explains what you'll find in aircraft logbooks, how to interpret the entries, and what should give you pause.
The Three Logbooks
Most GA aircraft have three separate logbooks:
Airframe logbook: Documents the aircraft's overall maintenance history — annual inspections, major repairs, alterations, damage history, and any modifications to the aircraft structure or systems.
Engine logbook: Tracks the engine's maintenance history, including total time since new (TTSN), time since major overhaul (SMOH), oil changes, cylinder work, and any engine-related repairs.
Propeller logbook: Records prop inspections, repairs, overhauls, and importantly, any prop strikes — even ground strikes during taxi.
Some older aircraft may have combined logs, or multiple logbooks if an engine has been replaced or overhauled multiple times.
What to Look for in the Airframe Log
Annual inspection entries: Every 12 calendar months, a certificated A&P mechanic with Inspection Authorization (IA) must perform an annual inspection and sign off the aircraft as airworthy. Each annual entry should include the date, aircraft total time, the A&P's name and certificate number, and a statement that the aircraft was found airworthy (or a list of items required for return to airworthiness).
Look for: consistent annual entries with no gaps. A gap of more than 13-14 months between annuals isn't automatically disqualifying, but it needs explanation.
AD compliance entries: When an Airworthiness Directive is complied with, it should be documented in the logbook with the AD number, compliance date, aircraft time, and who performed the work. Cross-reference these with the current list of applicable ADs for the aircraft make and model.
337 forms: Major repairs and alterations require FAA Form 337 to be filed. These should be referenced in the logbook and copies should be available. If an aircraft has had significant modifications (an avionics upgrade, a supplemental type certificate installation), 337s document that the work was done legally.
Damage history: Repairs following accidents or incidents should be documented. Look for entries referencing structural repairs, sheet metal work, or anything that might indicate previous damage.
What to Look for in the Engine Log
Time since overhaul (SMOH): Engine manufacturers publish a recommended time before overhaul (TBO) — typically 1,500 to 2,000 hours for most GA piston engines. An engine well past TBO isn't necessarily a problem (many engines run reliably past TBO under owner-produced maintenance), but it affects value and planning.
Compression check history: Good annual entries include cylinder compression check results. A healthy cylinder runs 70/80 or better. Declining compression over multiple annuals tells a story about engine wear.
Oil consumption: Some pilots note oil consumption in the engine log. An engine burning more than a quart per hour is burning more than ideal.
Top overhauls and cylinder replacements: Documentation of cylinder work, valve jobs, or top overhauls tells you what problems the engine has had.
Red Flags to Take Seriously
Missing logbooks: This is a serious problem. Logbooks can be replaced with a new book starting from the current date, but the loss of historical records removes the ability to verify AD compliance, previous damage, or the accuracy of time claims. Missing logs significantly affect aircraft value and insurability.
Alterations without 337s: If an aircraft has avionics, an STC'd modification, or other non-standard equipment without 337 documentation, those modifications may not be legal.
Prop strike without documented inspection: Any prop strike — even a minor one — requires an engine inspection per Lycoming and Continental service instructions. An undocumented prop strike that wasn't properly inspected is a liability.
Gaps in annual inspection dates: Gaps may indicate periods where the aircraft was operated without a current annual (illegal) or simply wasn't flown. Either way, understand the reason.
White-out, erasures, or altered entries: Logbooks are legal documents. Alterations are a serious red flag.
Keeping Your Own Logbook
Once you own an aircraft, maintaining complete and accurate logbooks is both a legal requirement and protection for your investment's resale value. Aeradex's Hangar feature lets pilots track maintenance history digitally, including receipt scanning that reads shop invoices and auto-populates maintenance entries. It doesn't replace the physical logbook requirement, but it gives you a searchable digital record of your aircraft's history.