Reading Aircraft Logbooks: Complete Guide for Buyers

Understanding Aircraft Logbooks

The Three Primary Logbooks

Most aircraft have three main logbooks:

• Airframe Logbook: Documents the aircraft structure, systems, and airframe-related maintenance. Records total aircraft time, annual inspections, major repairs, modifications, and AD compliance.
• Engine Logbook: Documents all engine-related maintenance, time since major overhaul (SMOH), total engine time, cylinder work, and engine-specific ADs.
• Propeller Logbook: Documents propeller maintenance, overhauls, and time since overhaul. Also records strike inspections if the propeller contacted objects.

Some aircraft also maintain:

• Separate avionics logbooks for complex panel installations
• Component logs for specific systems (landing gear, instruments)
• Weight and balance records
• Supplemental Type Certificate (STC) documentation

What Logbooks Must Contain

FAA regulations require specific information in logbooks:

• Description of work performed
• Date work was completed
• Signature of person performing work
• Certificate number of the mechanic or repair station
• Return-to-service statement for inspections and major work
• Aircraft total time and cycles (for required inspections)

For annual inspections and major maintenance, entries must include specific language approving the aircraft for return to service.

How to Review Logbooks Systematically

Step 1: Verify Completeness

Start by confirming all logbooks are present and appear complete:

• Airframe, engine, and propeller logs all present
• Continuous history from aircraft manufacture or previous overhaul
• No missing pages or removed entries
• All logs properly labeled with N-number and aircraft details
• Original logs (not copies, unless original loss is explained)

Missing logbooks are serious issues. Ask why logs are missing and whether reconstruction is possible. Many lenders won't finance aircraft without complete logs.

Step 2: Verify Total Times

Confirm times match advertising and make sense:

• Total Airframe Time: Total hours the aircraft has flown since manufacture
• Total Engine Time: Hours since engine was new
• Time Since Major Overhaul (SMOH): Hours since last major engine overhaul
• Propeller Time: Time since propeller overhaul

Compare times to what's advertised. Discrepancies require explanation. Calculate average annual use—very low use (under 50 hours/year) or very high use (over 300 hours/year) have different implications.

Step 3: Review Annual Inspections

Annual inspections must be completed within 12 calendar months for the aircraft to remain airworthy. Review:

• Annual inspections completed every year without gaps
• Proper sign-off language: "I certify that this aircraft has been inspected in accordance with an annual inspection and was determined to be in airworthy condition"
• Signature and certificate number of authorized inspector
• List of discrepancies found and their resolution
• Consistent inspector or shop (frequent changes may indicate problems)

Any gap in annual inspections means the aircraft is not airworthy and requires inspection before legal flight.

Step 4: Check Airworthiness Directive (AD) Compliance

ADs are mandatory safety improvements issued by the FAA. Aircraft can't fly legally without AD compliance. Look for:

• AD compliance list showing all applicable ADs and compliance dates
• One-time ADs completed and documented
• Recurring ADs with proper interval compliance documented
• Recent AD research confirming no new ADs require attention

Cross-reference the aircraft's AD list with current AD listings for the make and model. Missing or non-compliant ADs are serious problems requiring immediate attention and expense.

Step 5: Look for Damage History

Review entries for any accidents, incidents, or substantial damage:

• Gear-up landings or hard landings
• Prop strikes requiring engine teardown
• Wing or fuselage damage from hangar rash or incidents
• Engine failures or forced landings
• Weather damage (hail, storms, floods)

Major damage requires FAA Form 337 (Major Repair and Alteration) documentation showing proper repair. Undisclosed damage history discovered during pre-buy is a major red flag suggesting dishonest sellers.

Not all damage history disqualifies aircraft, but it affects value and must be properly repaired and documented.

Step 6: Review Modifications and Upgrades

Modifications must be properly documented:

• Supplemental Type Certificates (STCs) for major modifications
• FAA Form 337 for major alterations
• Installation documentation for avionics upgrades
• Weight and balance recalculation after modifications
• Proper approval and return-to-service for all modifications

Valuable upgrades like modern avionics, engine monitors, or autopilots should be properly documented. Undocumented modifications can create insurance and airworthiness issues.

Step 7: Assess Maintenance Quality and Frequency

Quality maintenance shows through logbook patterns:

• Regular maintenance: Consistent annual inspections, oil changes at appropriate intervals, routine component servicing
• Detailed entries: Thorough descriptions of work performed, not vague or minimal entries
• Reputable shops: Work performed by known, reputable facilities rather than unknown or questionable sources
• Preventive maintenance: Evidence of proactive work (hose replacements, component overhauls) before failure
• Timely repairs: Issues discovered in annuals promptly addressed, not deferred repeatedly

Conversely, red flags include:

• Minimal maintenance between annuals
• Vague entries like "inspected and returned to service"
• Many deferred items from annuals
• Frequent shop changes suggesting problem-shopping
• Long periods of inactivity followed by rushed maintenance

Key Logbook Terms and Abbreviations

Understanding common logbook abbreviations helps interpretation:

• SMOH: Since Major Overhaul Hours (engine time since last overhaul)
• TBO: Time Between Overhaul (manufacturer recommended overhaul interval)
• STOH: Since Top Overhaul Hours
• SNEW: Since New (total time since manufacture)
• TSN: Time Since New
• TSO: Time Since Overhaul
• RTS: Return To Service
• IAW: In Accordance With
• AD: Airworthiness Directive
• SB: Service Bulletin
• STC: Supplemental Type Certificate
• 337: FAA Form 337 (Major Repair and Alteration)
• CRS: Certified Repair Station
• IA: Inspection Authorization

Understanding Engine Times and TBO

What Is Time Before Overhaul (TBO)?

TBO is the manufacturer's recommended interval for engine overhaul, typically:

• 1,200-1,500 hours for smaller engines (like Lycoming O-320, O-360)
• 1,500-2,000 hours for larger engines (like Continental IO-520, IO-550)
• Some engines have 2,000+ hour TBOs

TBO is a recommendation, not a legal requirement for Part 91 operations. Well-maintained engines can safely exceed TBO if compression and other parameters remain good. However, engines approaching or beyond TBO require careful evaluation.

Evaluating Engine Condition

When reviewing engine logs, look for:

• Recent compression test results: Should be 70/80 or better on all cylinders
• Oil consumption: Entries noting oil added (high consumption suggests wear)
• Cylinder work: Top overhauls, cylinder replacements, valve work
• Borescope inspections: Internal cylinder examination results
• Oil analysis trends: Lab reports showing metal content in oil
• Regular oil changes: Every 25-50 hours or as recommended

An engine at 1,100 SMOH with excellent compression and oil analysis might be more valuable than an engine at 500 SMOH with marginal compression and cylinder work.

Major Overhaul Documentation

Major overhauls should be thoroughly documented:

• Detailed overhaul work order from repair station
• List of parts replaced (cylinders, pistons, bearings, etc.)
• Return-to-service statement
• Test run data
• Zero-time or near-zero-time designation
• Cost of overhaul (provides value context)

Quality overhauls from reputable shops are valuable. "Owner-assisted" or questionable overhauls may not meet standards. Learn more about engine overhaul costs and financing.

Identifying Logbook Red Flags

Missing or Incomplete Entries

• Gaps in dates or time suggesting missing pages
• Incomplete annual inspection write-ups
• Missing AD compliance documentation
• No mention of known required maintenance

Evidence of Altered or False Entries

• Erasures, white-out, or obviously changed entries
• Different handwriting for same supposed entry
• Entries that don't match timeframes (work supposedly done when aircraft was stored)
• Multiple different certificate numbers for same signature name

Damage History Not Previously Disclosed

• References to "gear-up" or "hard landing" repairs
• Prop strike entries or sudden prop overhauls
• Major structural work not mentioned by seller
• 337 forms for substantial damage repairs

Deferred Maintenance Patterns

• Same discrepancies listed year after year in annuals
• Many items listed as "deferred" or "to be corrected"
• Minimal routine maintenance between required inspections
• Evidence aircraft was flown with known deficiencies

Questionable Repairs or Maintenance

• Work performed by uncertified individuals
• Entries without proper mechanic certificate numbers
• Major work with minimal documentation
• Repairs using non-approved parts or methods

What to Do When Logbook Issues Are Found

Minor Issues

For minor logbook discrepancies:

• Request clarification from seller
• Ask seller's mechanic to provide additional documentation
• Consider whether issues affect value or airworthiness
• Factor into negotiation but don't necessarily walk away

Major Issues

For serious logbook problems:

• Consult with your pre-buy mechanic and attorney
• Request seller address issues before closing
• Negotiate significant price reduction to compensate
• Consider whether financing or insurance will be affected
• Be prepared to walk away if problems are too severe

Issues like missing logbooks, undocumented major damage, or non-compliant ADs often make aircraft unfinanceable or dramatically reduce value.

Digital Logbooks and Electronic Records

Some aircraft now use digital logbook systems:

• Benefits: Backup protection, searchability, organization, remote access
• Acceptance: FAA approves electronic signatures and records with proper procedures
• Verification: Ensure digital systems include proper authentication and backups
• Transition: Many owners maintain both paper and digital logs during transition

While digital logbooks are gaining acceptance, paper logs remain the standard. If purchasing aircraft with digital logs, verify they meet FAA requirements and request complete backup copies.

Logbook Review Best Practices

• Take your time: Thorough logbook review takes hours, not minutes
• Bring your mechanic: Experienced mechanics spot issues you might miss
• Take notes: Document questions, concerns, and items requiring clarification
• Cross-reference: Verify consistency between airframe, engine, and prop logs
• Research the type: Know common issues with the specific aircraft model
• Check 337s: Request copies of all FAA Form 337s and verify they match logbook entries
• Don't rush: Sellers pressuring rapid logbook review are suspicious

The Value of Complete, Well-Maintained Logbooks

Aircraft with complete, detailed logbooks commanding premium prices because they:

• Prove the aircraft history and condition
• Demonstrate responsible ownership
• Are easier to finance and insure
• Reduce buyer risk and uncertainty
• Maintain better resale value

Conversely, missing or poor logbooks can reduce aircraft value 20-50%. A $150,000 aircraft with missing engine logs might only be worth $100,000-$120,000 due to uncertainty and financing difficulty.

Understanding logbook review is essential for making informed aircraft purchase decisions. Invest the time to thoroughly review logs or hire professionals to assist. The few hours spent reviewing logbooks can prevent expensive mistakes and help you purchase aircraft with confidence.