Required Aircraft Inspections: Timing Mechanics, Who Signs Them Off, ADs, and What a Pilot Can Legally Do
- Nathan Hodell
- Dec 17, 2025
- 14 min read
Updated: Aug 19
Aircraft maintenance inspections are the backbone of aviation safety, but the details behind them — the exact regulatory triggers, how the timing actually works, who's authorized to perform versus approve each one, and what a pilot is legally allowed to do without a mechanic — trip up a lot of pilots and owners. Understanding required inspections is not just about knowing the intervals; it's about knowing precisely when an aircraft becomes un-airworthy, how the calendar-month and hours-in-service clocks differ, and how Airworthiness Directives fit into the picture. This is guaranteed ramp-check and checkride territory, and it's genuinely important for anyone who owns or flies an aircraft.
This post covers required inspections in practical depth: each inspection with its regulatory basis, the calendar-month versus hours-in-service timing mechanics, who can perform and who can approve for return to service, the transponder and ADS-B airspace triggers, Airworthiness Directives and Service Bulletins, the progressive inspection alternative, ferry permits for overdue aircraft, and the preventive maintenance a pilot can legally do.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
How Inspection Timing Actually Works
Before the individual inspections, understand the two timing systems, because confusing them is a common error.
Calendar-month timing:
Measured in "calendar months"
Due at the END of the month, 12 (or 24) months later
Example: an annual completed on March 15, 2025, is due by March 31, 2026 (the end of the 12th calendar month)
You get the rest of the due month
Used for: annual, transponder, pitot-static, altimeter, ELT
Hours-in-service timing:
Measured in actual flight hours (time in service)
Due when the hour count is reached
Example: a 100-hour inspection is due at 100 hours of time in service since the last one
No "grace" based on the calendar
Used for: the 100-hour inspection
The "time in service" definition:
Time in service is from the moment the aircraft leaves the ground until it touches down
Not tach time or Hobbs time exactly (though often approximated)
The regulatory definition (wheels-up to wheels-down)
What counts for the 100-hour
Why the distinction matters:
Some inspections are calendar (annual), some are hours (100-hour)
The annual is due regardless of hours flown (even if the aircraft never flew)
The 100-hour is due regardless of the calendar (based on use)
Mixing them up leads to errors
The overlap:
An aircraft used for hire needs BOTH annual (calendar) AND 100-hour (hours)
Whichever comes first triggers that inspection
They run on separate clocks
Track both
The Annual Inspection
The annual is the most comprehensive required inspection, mandated by 14 CFR 91.409(a).
Who needs it:
ALL FAA-registered aircraft
Regardless of how often (or whether) they fly
Even an aircraft that sat all year needs an annual to fly
No exceptions for private aircraft
Frequency:
Every 12 calendar months
Due at the end of the 12th calendar month
Calendar-based (not hours)
Who can perform it:
An A&P mechanic (Airframe and Powerplant) with Inspection Authorization (IA)
The IA is the key credential for the annual
A regular A&P (without IA) cannot sign off an annual
The IA is a higher qualification
What it covers:
A thorough inspection of the entire aircraft
Airframe, engine, propeller, systems, components
Using a checklist (FAA Part 43 Appendix D scope, plus manufacturer's checklist)
The most complete routine inspection
The airworthiness connection:
An aircraft that fails an annual is NOT airworthy until discrepancies are corrected
The IA signs off the annual as airworthy (or provides a list of discrepancies)
Without a current annual, the aircraft can't legally fly
The annual is fundamental to airworthiness
The signoff:
The IA makes a logbook entry certifying the annual
The entry includes the date, aircraft time, and certification statement
"I certify that this aircraft has been inspected in accordance with an annual inspection and was determined to be in airworthy condition"
The logbook entry is the record (kept on the ground)
The 100-Hour Inspection
The 100-hour applies to aircraft used for hire, per 14 CFR 91.409(b).
Who needs it:
Aircraft used for flight instruction for hire (e.g., a flight school's rental/instruction aircraft)
Aircraft carrying passengers for hire
NOT required for aircraft used purely privately (not for hire)
The "for hire" trigger is key
The instruction nuance:
If an instructor provides instruction in an aircraft, and the aircraft is provided (rented), it needs 100-hour inspections
If the student owns the aircraft and hires only the instructor (not the aircraft), the 100-hour may not apply
The distinction: is the AIRCRAFT provided for hire?
Instruction given in a student's own airplane doesn't trigger it
Frequency:
Every 100 hours of time in service
Hours-based (not calendar)
Resets after each 100-hour inspection
The 10-hour overfly provision:
The 100-hour limit may be exceeded by up to 10 hours
ONLY if necessary to reach a place where the inspection can be done
The overflown time is counted against the NEXT 100-hour interval
This is a ferry allowance, not a routine extension
Who can perform it:
A certified A&P mechanic (IA NOT required)
This is a key difference from the annual
Any A&P can do and sign off a 100-hour
The 100-hour is a lower credential requirement than the annual
The annual/100-hour relationship:
The scope of a 100-hour is essentially identical to an annual
BUT a 100-hour does NOT substitute for an annual
An annual CAN substitute for a 100-hour (an annual satisfies the 100-hour requirement too)
Only an IA signing it as an annual makes it an annual
Why the distinction:
The annual requires an IA; the 100-hour requires only an A&P
The annual is calendar; the 100-hour is hours
An annual counts as a 100-hour; a 100-hour doesn't count as an annual
Aircraft for hire need both clocks tracked
The Transponder Inspection
The transponder inspection, per 14 CFR 91.413, ensures accurate identification and altitude reporting.
Who needs it:
ANY aircraft with a transponder that is used
Specifically required for operations where a transponder is required
Transponders are required in Class A, B, and C airspace, above Class C, above 10,000 feet MSL (with exceptions below 2,500 AGL), and within the Mode C veil around Class B
If you have a transponder and use it, it needs the inspection
The broader trigger:
The inspection applies to ALL transponders, not just those in specific airspace
If the transponder is used (in any airspace requiring one), it must be inspected
Every 24 calendar months regardless
Even a transponder used only occasionally needs the current inspection to be used
Frequency:
Every 24 calendar months
Calendar-based
What's checked:
Transponder reply accuracy
Mode C altitude reporting
System integrity and performance
Per Part 43 Appendix F
The ADS-B connection:
ADS-B Out is now required in the same airspace where transponders are required (the "ADS-B rule airspace")
ADS-B equipment also has performance requirements
The transponder inspection and ADS-B are related (many transponders are ADS-B capable)
ADS-B Out is required in most controlled airspace since 2020
Why it matters:
A failed or out-of-date transponder inspection restricts where the aircraft can fly
Can't operate in airspace requiring a transponder without a current inspection
ATC relies on accurate transponder/Mode C data
Essential for the modern airspace system
The Pitot-Static / Altimeter Inspection
The pitot-static system and altimeter inspection, per 14 CFR 91.411, ensures accurate altitude reporting for IFR.
Who needs it:
Aircraft operating under IFR in controlled airspace
Required for IFR flight
Not required for VFR-only operations
The IFR trigger
Frequency:
Every 24 calendar months
Calendar-based
What's included:
Altimeter accuracy and calibration
Static system integrity (leak testing)
The automatic altitude reporting (encoder) accuracy
Compliance with tolerances (Part 43 Appendices E)
Why altimeter and static are together:
The altimeter reads static pressure
The static system feeds the altimeter (and other instruments)
They're tested together for altitude accuracy
The inspection covers the whole altitude-reporting chain
The combined inspection:
Often the transponder (91.413) and pitot-static (91.411) are done together
Both are 24-calendar-month inspections
Both relate to altitude reporting
Commonly scheduled together
Why it matters:
IFR flight relies on accurate altitude
ATC separation depends on accurate altitude reporting
Multiple instruments use static pressure
Essential for safe IFR operations
The VFR exception:
For VFR-only aircraft, the pitot-static inspection isn't required
But if the transponder is used (VFR in transponder airspace), the transponder inspection IS required
The static/altimeter inspection is specifically for IFR
Know which applies to your operations
The VOR Check
The VOR check, per 14 CFR 91.171, ensures VOR navigation accuracy for IFR — and it's different from the others.
Who needs it:
Aircraft using VOR for IFR navigation
VFR VOR use doesn't require the check
Only for IFR VOR navigation
Frequency:
Every 30 days (the shortest interval)
Much more frequent than the others
Not calendar months — 30 days
The key difference — the pilot can do it:
Unlike the other inspections (which require a mechanic), the VOR check can be done by the PILOT
The pilot can perform and log the VOR check
No mechanic required
This is unique among the required checks
How the VOR check is done:
VOT (VOR test facility): at certain airports, tune the VOT; the check has a specific tolerance (±4°)
Ground checkpoint: a designated point on the airport with a published radial (±4°)
Airborne checkpoint: a designated airborne point with a published radial (±6°)
Dual VOR check: compare two VORs against each other (±4° between them)
Selected radial over a landmark: ±6° (for areas without designated checkpoints)
The tolerances:
VOT and ground checkpoint: ±4°
Airborne checkpoint and dual check: ±6° (dual is ±4° between the two)
The check verifies the VOR is within tolerance
Outside tolerance means the VOR can't be used for IFR
The documentation:
Must be logged: signature, location, error, and date (the acronym some use is "SLED" or just remember the four items)
Logged by the pilot (or whoever does it)
Kept as a record
Without a current, logged VOR check, no IFR VOR navigation
Why it's different:
Shortest interval (30 days)
Pilot can perform it (no mechanic)
Specific tolerances by method
The only "inspection" a pilot routinely does themselves
The ELT Inspection
The ELT (Emergency Locator Transmitter) inspection, per 14 CFR 91.207, supports post-accident search and rescue.
Who needs it:
Most civil aircraft (with some exceptions)
The ELT is required equipment for most operations
Broad applicability
Frequency:
Inspected every 12 calendar months
Calendar-based for the inspection
The battery replacement triggers (separate from the inspection):
After 1 cumulative hour of use
When 50% of the battery's useful life has expired
The battery has its own replacement schedule
These are distinct from the 12-month inspection
What's checked:
Proper operation
Mounting security
Antenna condition
Battery status and expiration
Per the 12-month inspection (91.207(d))
The 406 MHz standard:
Modern ELTs transmit on 406 MHz (digital, satellite-detected)
Older 121.5 MHz-only ELTs are no longer satellite-monitored (since 2009)
406 MHz ELTs provide better location and identification
Many aircraft have upgraded (though 121.5-only ELTs may still meet the U.S. requirement in some cases)
Why it matters:
A functioning ELT dramatically reduces search-and-rescue time
Activates on impact (g-switch) or manually
Transmits a distress signal
Can be life-saving after an accident
Airworthiness Directives (ADs): The Ongoing Requirement
Beyond the scheduled inspections, Airworthiness Directives are a critical and continuous compliance requirement the basic treatment doesn't cover.
What ADs are:
Legally mandatory directives issued by the FAA
Address a known unsafe condition in a type of aircraft, engine, propeller, or component
Compliance is REQUIRED (not optional)
Issued when a safety problem is identified
The two types:
One-time ADs: a single required action (inspect, repair, replace once)
Recurring ADs: a repeated action at specified intervals (inspect every X hours, etc.)
Both must be complied with
Recurring ADs add to the ongoing inspection burden
Why ADs matter for airworthiness:
An aircraft with an overdue AD is NOT airworthy
AD compliance is part of conforming to the type design
ADs are checked during the annual (and must be tracked continuously)
Non-compliance grounds the aircraft
How ADs work:
The FAA issues an AD for a specific model/component
Owners must comply by the specified time/action
Compliance is recorded in the logbooks
The annual verifies AD compliance
Service Bulletins (the related but different item):
Service Bulletins (SBs) are issued by the MANUFACTURER (not the FAA)
Generally NOT mandatory for Part 91 operators (unless referenced by an AD)
Recommend inspections, modifications, or actions
Become mandatory if an AD references them
SBs are recommendations; ADs are law
The distinction:
AD (FAA): mandatory
SB (manufacturer): usually recommended (mandatory if an AD adopts it)
Both address safety
Know the difference
Who Can Perform vs. Who Can Approve for Return to Service
An important distinction the basic treatment doesn't make: performing maintenance vs. approving it.
The two roles:
Performing the work (the physical maintenance/inspection)
Approving for return to service (certifying it's done correctly and the aircraft is airworthy)
These can be different people
The approval is the legal signoff
Who can approve for return to service:
A&P mechanic: most maintenance and the 100-hour inspection
IA (Inspection Authorization): the annual inspection, major repairs/alterations
Repair stations: per their certificate
The pilot: preventive maintenance (limited, below) and the VOR check
The IA's special role:
The annual inspection
Approving major repairs and major alterations (Form 337)
The highest level of maintenance approval for most GA
Required for the annual specifically
Why this matters:
Knowing who can sign off what
The annual needs an IA; the 100-hour needs only an A&P
Major alterations need an IA
The signoff makes the work legal
Preventive Maintenance: What a Pilot Can Legally Do
A practical topic the basic treatment omits: pilots can legally perform certain maintenance themselves.
The authority:
Under 14 CFR Part 43 Appendix A, holders of a pilot certificate (at least private) may perform preventive maintenance
On aircraft they own or operate (not for hire)
A specific list of allowed tasks
Must be logged
Examples of allowed preventive maintenance:
Changing the oil and oil filter
Replacing/servicing landing gear tires
Lubricating items not requiring disassembly
Replenishing hydraulic fluid
Replacing bulbs, reflectors, and lenses of position/landing lights
Replacing seats or seat parts (with approved parts)
Replacing batteries
(A specific list — not general maintenance)
What pilots CANNOT do:
Anything not on the preventive maintenance list
Major repairs or alterations
Complex engine work
The inspections (annual, 100-hour, etc.)
Work requiring an A&P or IA
The logbook requirement:
Preventive maintenance must be recorded in the logbook
Include the work done, date, aircraft time, and the pilot's certificate number and signature
The pilot approves their own preventive maintenance for return to service
Proper documentation is required
Why it matters:
Owners can legally do simple maintenance (saving cost)
But only the specific allowed tasks
Everything else requires a mechanic
Know the boundaries
When an Aircraft Is Overdue: Ferry Permits
What happens when an aircraft can't meet an inspection requirement, and how to legally move it.
The overdue aircraft:
An aircraft past a required inspection is not airworthy
It can't be legally flown normally
But sometimes it must be moved (e.g., to a maintenance facility)
A special provision exists
The special flight permit (ferry permit):
A Special Flight Permit (per 14 CFR 21.197) allows flying an aircraft that doesn't meet airworthiness requirements
For specific purposes: flying to a place for repairs/inspection, delivery, evacuating from danger, etc.
Issued by the FAA (FSDO) or a DAR
The aircraft must still be safe for the intended flight
When a ferry permit is used:
An aircraft overdue for an annual (to fly to the shop)
Moving an aircraft with a known discrepancy (that's safe to ferry)
Delivering a new aircraft
Specific, limited purposes
The conditions:
The permit specifies the conditions and limitations
The aircraft must be safe for the specific flight
Often specific route, no passengers, day VFR, etc.
Not a way around inspections — a way to move the aircraft to get them done
The 100-hour ferry provision (revisited):
Recall the 100-hour can be exceeded by up to 10 hours to reach an inspection facility
This is a built-in allowance (no separate permit needed for that 10 hours)
The annual has no such automatic overfly (needs a ferry permit)
Different provisions for different inspections
Staying Compliant: The Owner's Responsibility
Tracking inspections is a continuous responsibility.
The owner/operator's duties (91.403):
The owner/operator is primarily responsible for maintaining the aircraft in airworthy condition
Ensuring inspections are current
Ensuring AD compliance
Keeping the required records
The pilot's preflight responsibility (91.7):
The PIC must determine the aircraft is airworthy before flight
This includes verifying inspection currency
Checking the logbooks (during preflight planning) for current inspections and ADs
The pilot can't fly an aircraft they know is un-airworthy
Tracking methods:
Maintenance tracking software
Detailed logbook reviews
Inspection due-date lists
Staying ahead of deadlines
The consequences of lapses:
An overdue inspection grounds the aircraft (for the relevant operations)
Flying un-airworthy is a violation
Can result in enforcement action
Compromises safety
Common Misconceptions
"A 100-hour inspection counts as an annual."
No — a 100-hour does NOT substitute for an annual. But an annual DOES count as a 100-hour. Only an IA signing as an annual makes it an annual.
"The annual is due on the exact date 12 months later."
It's due at the END of the 12th calendar month. An annual done March 15 is due by March 31 the next year.
"Only aircraft in busy airspace need transponder inspections."
Any used transponder needs the 24-month inspection. The inspection applies broadly, not just in specific airspace.
"A pilot can't perform any maintenance."
Pilots can perform specific preventive maintenance (oil changes, tires, bulbs, etc.) on aircraft they own/operate, and can do the VOR check.
"Service Bulletins are mandatory."
Service Bulletins (from the manufacturer) are generally recommendations for Part 91. Airworthiness Directives (from the FAA) are mandatory.
"An overdue aircraft can never be flown."
A Special Flight Permit (ferry permit) can allow flying an overdue/non-conforming aircraft for specific purposes (like flying to the maintenance shop).
Final Thoughts
Aircraft maintenance inspections are more than regulatory checkboxes — they're proven safety measures with precise rules behind them. The annual (every 12 calendar months, IA required) and 100-hour (every 100 hours, for hire, A&P) run on separate clocks; the transponder and pitot-static (24 calendar months) support the airspace system and IFR; the ELT (12 months) supports search and rescue; and the VOR check (30 days, pilot-performed) keeps IFR VOR navigation accurate. Layered on top are Airworthiness Directives — mandatory, continuous, and part of what keeps an aircraft conforming to its approved design.
Understanding the timing mechanics, who can perform versus approve each inspection, the preventive maintenance a pilot may legally do, and how ferry permits work turns inspection knowledge from memorized intervals into real operational competence. Keeping track of it all is a continuous responsibility — one that protects pilots, passengers, and aircraft alike.
On the Written Test and Checkride
Required inspections appear consistently on tests and checkride orals. The most commonly tested topics:
The inspection intervals (annual 12 months, 100-hour, transponder/pitot-static 24 months, ELT 12 months, VOR 30 days)
Annual vs. 100-hour (IA vs. A&P, calendar vs. hours, substitution)
The 100-hour 10-hour overfly provision
VOR check methods and tolerances (and that the pilot can do it)
Airworthiness Directives (mandatory) vs. Service Bulletins
Preventive maintenance a pilot can perform
Quick Reference
Timing Systems:
Calendar months: due at END of the month (annual, transponder, pitot-static, ELT)
Hours in service: due at the hour count (100-hour)
Time in service: wheels-up to wheels-down
The Inspections:
Inspection | Interval | Trigger | Who Signs |
Annual (91.409a) | 12 calendar months | All aircraft | IA |
100-hour (91.409b) | 100 hours | For hire | A&P |
Transponder (91.413) | 24 calendar months | Transponder used | A&P/repair station |
Pitot-static (91.411) | 24 calendar months | IFR | A&P/repair station |
VOR check (91.171) | 30 days | IFR VOR nav | Pilot can do |
ELT (91.207) | 12 calendar months | Most aircraft | A&P |
Annual vs. 100-Hour:
Annual: 12 months, IA, all aircraft
100-hour: 100 hours, A&P, for hire
Annual counts as 100-hour; 100-hour does NOT count as annual
100-hour: +10 hour overfly to reach a shop (counted against next interval)
VOR Check Methods/Tolerances:
VOT: ±4°
Ground checkpoint: ±4°
Airborne checkpoint: ±6°
Dual VOR: ±4° between them
Log: Date, Place, Bearing error, Signature
ELT Battery Replacement:
After 1 cumulative hour of use
At 50% of useful life
(Separate from the 12-month inspection)
Airworthiness Directives (ADs):
FAA-issued, MANDATORY
One-time or recurring
Overdue AD = not airworthy
Checked at annual, tracked continuously
Service Bulletins (SBs):
Manufacturer-issued
Generally recommended (Part 91)
Mandatory only if an AD references them
Preventive Maintenance (pilot can do):
Oil changes, tires, wheel bearings
Bulbs, spark plugs, batteries
Safety wire, lubrication, hydraulic fluid
On owned/operated aircraft (not for hire)
Must be logged with certificate number
Ferry Permit (Special Flight Permit, 21.197):
Allows flying a non-conforming/overdue aircraft
For specific purposes (to the shop, delivery, etc.)
Must be safe for the flight
Issued by FSDO or DAR
Who Approves Return to Service:
A&P: most maintenance, 100-hour
IA: annual, major repairs/alterations
Pilot: preventive maintenance, VOR check
Key Principle:
Required inspections run on two clocks: calendar months (annual 12, transponder/pitot-static 24, ELT 12) and hours (100-hour). The annual needs an IA; the 100-hour needs an A&P and only applies for hire. The VOR check (30 days) is the one a pilot can do. Airworthiness Directives are mandatory and continuous. A pilot can legally do listed preventive maintenance, and a ferry permit can move an overdue aircraft to the shop.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.