What are the intervals at which a full structure inspection take place ?

How much time does a test commonly take for narrow and wide body aircraft ?

Is every part in the structure manually inspected by the operator in level C Check or some parts only depending on the Structure repair manual (SRM) of the aircraft?

  • $\begingroup$ Such elements are generally dependent on the certification authority, and can vary from country to country. Which country are you targeting? $\endgroup$ – mins Oct 27 '15 at 22:12
  • 2
    $\begingroup$ What do you mean by a "full fuselage and structure testing"? This can be anything from the manufacturer's initial structural testing ("proof test"), which usually only happens once on a prototype designates for that purpose which is cycled until it fails) to the fatigue inspections performed during the airframe's service life (which are typically dictated by manufacturers) $\endgroup$ – voretaq7 Oct 27 '15 at 22:14
  • $\begingroup$ i didn't put that in perspective !! lets say im targeting UAE ! $\endgroup$ – MTF_09 Oct 27 '15 at 22:15
  • $\begingroup$ the second one, during the airframe's service life $\endgroup$ – MTF_09 Oct 27 '15 at 22:16
  • $\begingroup$ There is a great video of a full D-Check for a 747 here $\endgroup$ – Dave Oct 28 '15 at 1:18

The FAA "Class D" maintenance/inspection of commercial airliners, which must occur every 6 years, is extremely thorough:

  • The cabin trim is completely removed to inspect the airliner's skin and superstructure from the inside. Seats, safety equipment and control system linkages are also inspected in detail with each individual cable or wire tested.
  • The plane's exterior paint is removed to inspect the exterior skin, including rivets and seams.
  • All critical moving parts on the exterior of the plane (engines, gear, ailerons, flaps, spoilers, rudder, elevators, shock booms, etc), are removed from the plane, disassembled and visually inspected for wear or damage, then re-assembled and tested.
  • All pressurized lines (hydraulic, water, fuel) are removed from the plane and each part individually visually inspected before being re-assembled and pressure-tested in the plane.
  • The cockpit is stripped, all moving parts in the control column and throttle assemblies visually inspected, and all flight instruments visually inspected and tested.

There are many more maintenance/inspection tasks than I list, I'm just going through the major ones. The plane is more or less stripped to its bare metal frame, every component visually inspected and tested, then the aircraft is reassembled with individual systems tested again on the airframe. The process can take up to two months for each aircraft, and can cost between 5 and 7 million US dollars. Most major airlines have either their own in-house maintenance crews or the manufacturer perform this maintenance, with a team of FAA-employed inspectors overseeing and signing off on all the work.

Most airliners receive 3 Class D inspections in their lifetime; at the fourth, occurring around 24 years of airframe age, the cost of a Class D is typically not considered worth it given the aircraft's remaining value, and the aircraft is usually retired and either sold to smaller non-US airlines operating in jurisdictions with less stringent standards, mothballed and stored for potential replacement parts value to the rest of the fleet, or sold for scrap. Some airframes in service with some airlines have been kept through four Class Ds in order for the airline to have sufficient time to procure replacements; American's remaining MD-80s are largely newer airframes acquired in the TWA merger, but there are a few of American's own deliveries left in the fleet which are past 24 years and are still around mainly to allow American sufficient time to work through the bankruptcy, United merger and procurement of replacements (Boeing's 737 production lines are going full-tilt and American is not the highest-priority customer, so American had to split its order in this class between the 737 and Airbus's A319 and A321).

Because of the level of detail and deconstruction required, including complete cabin trim and paint removal, airlines often use this maintenance interval to perform any desired upgrades or tweaks to their fleet configuration. Common upgrades in recent years have included:

  • adding winglets for vortex reduction (reducing drag and allowing closer spacing at busy airports),
  • upgrading avionics and even control systems (some older airframes can be upgraded from direct cable-driven hydraulics to electro-hydraulic controls allowing partial automation of flight with EFDs),
  • upgrades to cabin trim including re-spacing of passenger seats (more or less legroom or "slant", balancing between class seating),
  • replacing galley equipment and other secondary systems that may be a different standard from the rest of the fleet,
  • adding new passenger conveniences (such as satellite-based Wi-Fi, headrest IFE, etc) and
  • new exterior paint schemes for corporate rebranding (such as American's ongoing rebadging from their decades-old AA silver to the new American white, and Southwest's recent move to the "Heart" scheme from its older "Canyon Blue" and the original red/gold before that).

Aircraft acquired in airline mergers are often the highest priority as the airline seeks to rebadge and standardize the acquired fleet to operate alongside its existing planes (and ensure no corners were cut by a struggling airline that would create liabilities going forward).

  • $\begingroup$ thanks for the detailed answer, there are more issues i would like to know about since you seem expert on the subject. 1-regarding aircraft exterior skin (different types of composites) how often do they need to be checked (without dis-assembly) using conventional methods like (ultrasonic and active thermal imaging) for all structure faults (like delamination, disbonding, cracks, weak bonds or rivets ... etc) how much does it cost to inspect in terms of money and working hours ? thanks again :) $\endgroup$ – MTF_09 Oct 28 '15 at 18:47
  • $\begingroup$ A comprehensive visual inspection of the airframe's exterior skin is done at every "A-check" which is performed about every 250 flight hours and can be done overnight. Inspection of the most likely places to show stress (wing-body joint, engine nacelle mount, on/around control surfaces etc) is done at least daily if not between flights. All higher levels of maintenance include items checked in these two levels, much like your car's 30k service interval typically includes an oil change and tire rotation/inspection. More in-depth inspections are performed at C-check intervals. $\endgroup$ – KeithS Oct 28 '15 at 19:09
  • $\begingroup$ do u have more information about what i exactly the type of structure inspection (airframe, composite structures) in B and C checks regarding type, time, and labor cost ? truly thankful :) $\endgroup$ – MTF_09 Oct 28 '15 at 19:31
  • $\begingroup$ No I do not; exactly what is done at every tier of maintenance interval, and exactly what those time intervals are and thus what it costs to maintain a particular aircraft, depends on the model of aircraft. Additionally, airlines are allowed to stretch out the inspections normally performed in one tier across several other maintenance stops, which is called "progressive maintenance" and keeps the aircraft closer to a serviceable state at any given time in its life. $\endgroup$ – KeithS Oct 28 '15 at 19:36

The inspection schedule of each aircraft is decide by the manufacturer in coordination with the regulatory authorities and released to the customer and varies from aircraft to aircraft.

  • In case of civil aircraft, 'full' inspection, usually called the D-level checks, is carried out every 4-5 years.

  • It depends on the aircraft condition and factors like aircraft age, how it was maintained etc. it is time consuming, usually taking a few months depending on the replacement requirements and resources (like no. of technicians available).

  • The structure is thoroughly inspected- Every single part is subjected to checks in addition to any checks mandated by regulatory authorities/manufacturer or required by customer.

  • $\begingroup$ Aren't most airliners on a progressive inspection now rather than putting the aircraft down for such a long period of time? $\endgroup$ – Lnafziger Oct 28 '15 at 16:52
  • $\begingroup$ @Lnafziger - As I understand it, there's so much to be done on a major airliner for a Class D that just doing it all at once is the most efficient way; the total time out of service involved in doing all the Class D inspections over more smaller maintenance stops would be greater than just biting the bullet. At the very least, no lesser level of maintenance involves stripping the cabin trim and exterior paint for a full fuselage stress inspection which by itself would be a lengthy and costly process. $\endgroup$ – KeithS Oct 28 '15 at 18:46
  • $\begingroup$ However, you seem to be right that smaller, more frequent inspections incorporating a rotating schedule of D-check items are the norm especially for VLAs (of which there tend to be few airframes in service and fewer spares capable of running the route). The first D-check of an A380, however, was a complete all-in-one teardown, so it varies by model, airline and airframe age. $\endgroup$ – KeithS Oct 28 '15 at 19:07

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