The Differences between Unscheduled and Scheduled Aircraft Maintenance

Aviation is an industry that never rests. With 8,000 to 12,000 aircraft in flight at any moment and 4.3 billion passengers flying a year, aircraft are constantly in operation. Despite being very advanced, just like all other machines, they face eventual wear, tear, and failure. Because of this, it is vital that they are constantly maintained to verify the airworthiness and integrity of each and every component. Within the services of aircraft maintenance, there is both scheduled and unscheduled maintenance. In this blog, we will discuss what both scheduled and unscheduled maintenance is, various types of each, and what they entail.

Scheduled Maitanenance :-

Scheduled maintenance includes any type of maintenance that is pre-planned and is known to both the pilot and the aircraft technician. These types of maintenance are conducted on set intervals, and they are done as a preventative measure to protect systems and components. Scheduled maintenance is a mixture of manufacturer recommended, federally mandated, and generally advised intervals. Types of scheduled maintenance include 50 & 100 hour inspections, annual inspections, and the ABC check system.

For any aircraft that is flown commercially, for hire, or for training, it is typical to have inspections conducted after every 50 and 100 hours of flight time. The 50 flight hour maintenance check is not federally mandated by the FAA, but it is highly recommended and falls in line with other standardly scheduled services, such as 50 hour oil changes. During the 50 hour inspection, parts and components such as the spark plug are examined, cleaned, and gapped. The aircraft engine is also checked during this time for any signs of damage, wear, or issue.  Any presented problem is then addressed with either repair or replacement. 100 hour inspections are required by the FAA and are written in Title 14 91.409 of the Code of Federal Regulations (CFR). The 100 hour inspection and maintenance procedure is much more comprehensive, and many components are inspected including the aircraft seats and seatbelts, flight control surfaceslanding gear, wheel brakes, and more. Damaged or worn components are then repaired and replaced as necessary to maintain airworthiness of the aircraft. Annual inspections are done every 12 months as required by the FAA, and much of the maintenance and inspection is similar to the 100 hour maintenance interval. At this time, the aircraft logbooks are also reviewed, as well as many systems and components tested.

ABC System of Schedule Maintainance :-

The ABC system is scheduled maintenance that many airlines and airworthiness authorities use when referring to 4 checks. These include the A, B, C, and D check, in which A & B are lighter checks and C & D heavier ones. These checks are also done either at one’s own facility or at a maintenance, repair, and overhaul (MRO) site.

A check system :-

The A check is conducted every 400-600 flight hours, or 8 to 10 weeks, and is typically conducted in a hangar. Though the checks that are done during this time vary depending on the type of aircraft or hours flown, typical procedures include the changing of filters, lubrication of hydraulics and flight control surfaces, and inspection of all emergency components.

B check system :-

The B check is very similar to the A check, and by conducting a check of A-1 through A-10, all B checks are then complete.

C check system :-

C checks are conducted every 18 to 24 months or on a set interval as described by the manufacturer. These checks are much more comprehensive than the B checks, requiring a great amount of the components to be inspected and putting the aircraft out of service for a week or two. 

D check system :-

The final check, the D check, is considered one of the most heavy and expensive aircraft maintenance procedures, and is only conducted every 6 to 10 years. During this check, nearly the entire aircraft is disassembled, sometimes even the paint is removed as well to check the fuselage metals. D checks can take upwards of 2 months time and upwards of million of dollars in expenditures. by the end of a D check, the aircraft could be considered almost like a brand new plane with every component being inspected, maintenanced, and replaced as necessary.

Unscheduled aircraft maintenance :-

Unscheduled aircraft maintenance, on the other hand, is only performed when there is a malfunction or breakdown of a system or component. As with its name, unscheduled maintenance is unknown or unforseen and may occur during the pre-flight inspection, scheduled inspection, or while the aircraft is in operation. There are many types of issues that could cause unscheduled maintenance, such as a worn tire, sheared vacuum pump, or any issue in which a system or component is not considered airworthy as per the FAA. Unscheduled maintenance can be very costly, as they both require a part to be repaired or replaced, as well as force an aircraft to remain out of service, possibly causing cancellations or delays of scheduled flights. When unscheduled maintenance is needed, the pilot would have to report the issue and then have a maintenance request made.

Because unscheduled maintenance can prove to be very time consuming and expensive, it is very important to ensure that it does not occur by taking the proper steps to avoid it. Although some types of inspection and maintenance for aircraft are not required, they should always be conducted so problems are caught much earlier, or avoided altogether. This can also prevent undetected problems from worsening if left alone. Altogether, normal routine inspections and maintenance such as pre-flight inspections, 50 & 100 hour inspections, yearly maintenance, and the ABC system should always be followed to save time and money.

            At Fulfillment by ASAP, owned and operated by ASAP Semiconductor, we can help you find parts and components that you need for you aircraft maintenance, new or obsolete. As a premier supplier of parts for the aerospace, civil aviation, and defense industries, we're always available and ready to help you find all the parts and equipment you need, 24/7x365. ASAP Semiconductor is an FAA AC 00-56B accredited and ISO 9001:2015 certified enterprise, and the only independent distributor with a no China sourcing pledge. For a quick and competitive quote, email us at or call us at +1-714-705-4780.


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