What to Look for in a Pre-Buy Inspection

Buying a used general aviation aircraft is one of the most exciting — and potentially expensive — decisions a pilot can make. The right airplane at the right price can give you years of reliable flying. The wrong one can ground your bank account before it ever leaves the hangar.

The pre-buy inspection is your best defense. It’s the due diligence step that separates informed buyers from the ones who end up posting “well, I learned an expensive lesson” on aviation forums six months later.

Here’s what to focus on — and where surprises tend to hide.

Start with the logbooks, not the airplane

Before you even look under the cowling, the aircraft’s maintenance logbooks tell you most of what you need to know. A complete, well-maintained logbook history is often more valuable than the condition of any single component, because it tells you how the airplane has been cared for over its lifetime.

You’re looking for several things in the logbook review:

Logbook Review Checklist:

• Complete and unbroken logbook history from new (or as far back as possible)
• All annual inspections documented with IA signoff and date
• AD compliance entries for every applicable Airworthiness Directive
• Engine times that progress logically (no unexplained gaps or resets)
• Major repairs or alterations documented on FAA Form 337
• Consistent maintenance intervals (oil changes, filter replacements)
• Any damage history entries — and the repair documentation that follows

Missing logbooks are a serious red flag. An aircraft with an incomplete maintenance history is nearly impossible to value accurately, and you’ll face an uphill battle confirming AD compliance. Some buyers walk away from otherwise clean airplanes simply because a logbook volume is missing.

Watch for this: If the seller says “the logbooks were lost but the airplane has always been well-maintained,” proceed with extreme caution. Without documentation, “well-maintained” is just an opinion. Your A&P and IA need records they can verify.

AD compliance: the non-negotiable

Airworthiness Directives are legally mandatory under 14 CFR 39. Every applicable AD must be complied with, and that compliance must be documented. During a pre-buy, your mechanic should verify compliance with every AD that applies to the airframe, engine, propeller, and installed appliances.

This is one of the most time-consuming parts of a thorough pre-buy — and one of the most important. A single uncomplied AD means the aircraft is technically not airworthy. Depending on the AD, compliance could be a quick inspection or a major (and expensive) modification.

What to look for specifically:

• One-time ADs — Verify the compliance entry exists in the logbook with the correct AD number, date, method of compliance, and the mechanic’s signature.
• Recurring ADs — Confirm the most recent compliance and check that the next due date or tach time hasn’t been exceeded.
• Superseded ADs — Some older ADs have been replaced by newer ones. Make sure the current version is what’s been complied with.

Pro tip: Cross-reference the logbook AD entries against the FAA’s AD database for your specific aircraft make, model, engine, and propeller. The FAA’s AD search at drs.faa.gov is the authoritative source. Don’t rely solely on what the seller tells you is applicable.

Engine health: beyond the total time

Every listing touts the engine’s total time and time since major overhaul (SMOH). Those numbers matter, but they don’t tell the whole story. An engine with 800 hours SMOH that was flown regularly and had oil changed on schedule is in a very different place than one with 800 hours SMOH that sat in a hangar for three years between flights.

Compression check

A differential compression test is standard in any pre-buy. Your mechanic will check each cylinder with the piston at top dead center. The results are expressed as a ratio — for example, 76/80 means 76 PSI retained out of 80 PSI applied.

There’s no single “pass/fail” number that applies universally, but generally speaking, readings in the 70s/80 range on a well-broken-in engine are considered serviceable. What matters more than any single reading is the trend across all cylinders and where the air is leaking (past the rings, through the exhaust valve, through the intake valve, or past the cylinder base).

Oil analysis history

If the seller has been doing regular oil analysis through a lab like Blackstone Laboratories, ask for the reports. Oil analysis tracks the concentration of wear metals (iron, copper, chromium, aluminum, and others) in the oil over time. A single report is useful, but a series of reports showing trends is far more valuable.

Rising iron levels, for example, might indicate accelerating cylinder or cam wear. Elevated silicon can suggest a leaking air filter. The lab reports usually include commentary interpreting the results in context.

Real-world example: A buyer found a Cessna 182 with “only 400 hours SMOH” at a great price. The compression check came back fine. But the oil analysis history showed steadily rising iron and chromium over the last 200 hours — a pattern the lab flagged as concerning. The mechanic pulled a cylinder and found heavy cam lobe wear. The “great deal” would have needed a top overhaul within 200 hours. Without oil analysis history, that trend would have been invisible until something failed.

Time in service vs. calendar time

An engine that flies 100+ hours per year is generally healthier than one that flies 20 hours per year. Low-utilization engines are prone to internal corrosion from moisture accumulation and stagnant oil. If the engine has low total time but the aircraft is 30 years old, ask pointed questions about how often it actually flew.

The airframe: what your eyes can’t see

A walkaround can reveal obvious problems — dents, corrosion, tired paint, cracked fairings. But the real airframe concerns are the ones you can’t see without pulling panels and inspection plates.

Your pre-buy mechanic should be looking for:

• Corrosion — Especially in coastal or humid climates. Check wing spar areas, belly skin, battery box, and around fuel tank fittings.
• Previous damage history — Look for signs of repair: mismatched rivets, paint inconsistencies, wrinkled skin. Cross-reference with FAA Form 337 filings (available through the FAA Aircraft Registry).
• Landing gear condition — Retractable gear gets special attention. Check actuators, squat switches, up-locks, down-locks, and the gear doors for wear and proper rigging.
• Flight control surfaces — Check for proper travel, correct rigging, hinge wear, and any signs of hard landings or over-stress.

Don’t skip the belly. The underside of the fuselage often tells the truth about how the airplane has been treated. Oil stains, exhaust residue, and corrosion around drain holes and antenna mounts can indicate deferred maintenance.

Avionics and electrical

Modern avionics can represent a significant portion of an aircraft’s value. During the pre-buy, every installed piece of avionics should be powered up and tested. Your mechanic or an avionics shop should verify:

• All radios transmit and receive properly
• The transponder responds correctly (check the most recent transponder/pitot-static certification — required every 24 calendar months under 14 CFR 91.411 and 91.413)
• GPS databases are current (or factor in the cost to update)
• Autopilot tracks and holds heading, altitude, and nav sources
• ELT battery expiration date and last inspection
• No loose wiring bundles, corroded connectors, or jury-rigged repairs behind the panel

Outdated avionics aren’t necessarily a deal-breaker, but they are a negotiating point. Upgrading a panel is expensive, and knowing exactly what you’re working with helps you budget realistically.

Choosing your pre-buy mechanic

This might be the most important decision in the entire process. Your pre-buy mechanic should be:

• Independent from the seller. Never use the seller’s mechanic for your pre-buy. It’s a conflict of interest, even if the mechanic is perfectly honest.
• Familiar with the type. A mechanic who works on Bonanzas every week will spot Bonanza-specific issues that a generalist might miss. Type-specific knowledge matters.
• Willing to be thorough. A pre-buy is not an annual inspection, but it should be close to one in scope. Expect it to take a full day or more for a thorough evaluation. If a mechanic quotes you two hours, find someone else.

Expect to pay for a good pre-buy. Typical costs range from a few hundred dollars for a basic evaluation to over a thousand for a comprehensive inspection including a borescope exam and oil analysis. Whatever it costs, it’s a fraction of what a hidden problem will cost you after the sale.

What comes after the inspection

A pre-buy inspection almost always turns up something. That’s normal. The question is whether the findings are routine maintenance items or deal-breaking problems.

Your mechanic should give you a written list of findings, categorized by severity. Use this to negotiate. Sellers often expect some negotiation after a pre-buy, and a documented list of squawks gives you concrete numbers to work with — not opinions, but specific items with estimated repair costs.

Some findings are walk-away items: significant corrosion, a cracked spar, an engine with internal damage, or a long list of uncomplied ADs. Others are normal for an aircraft of that age and type, and the cost of addressing them should be factored into your purchase price.

How organized records change everything

The smoothest pre-buy inspections happen when the seller has well-organized maintenance records. A complete AD compliance list, a clear maintenance history with dates and times, and oil analysis reports all signal that the airplane has been cared for by someone who pays attention.

As a buyer, the absence of organized records should give you pause. As a seller, having your maintenance history organized and accessible can speed up the sale and support your asking price. Either way, the maintenance record is the aircraft’s biography — and buyers read it closely.

That’s one of the reasons we’re building TachMinder. Having your maintenance status, AD compliance, oil analysis trends, and inspection history in one place doesn’t just make ownership easier day-to-day — it makes the aircraft more transparent and more valuable when it’s time to sell.