Jul 18, 20223 min read

Aircraft Maintenance D Check

The C Check is heavy maintenance that happens about every year and a half and takes a few days to a week. However, the heaviest C Check is minor compared to a D Check overhaul requiring several weeks, a month, or more. For a D Check, the aircraft is essentially taken completely apart, every component checked, repaired, or replaced before the aircraft returns to service.

If you don’t think this is critically important to flight safety, consider 1988 when Aloha Airlines flight 243 had its fuselage ripped open at 24,000 feet due to improper or poor maintenance leading to metal fatigue/failure. The takeoff was uneventful into clear weather. Shortly after the B737 reached cruising altitude, a piece of the fuselage tore open to reveal Hawaiian blue sky where the first-class ceiling was supposed to be. A cabin attendant near the front was sucked out of the plane. At the time, the B737 was 19 years old, had 35,496 flight hours, and 89,690 flight cycles.

The plane was part of a program Boeing had established known as the Aging Fleet Program. The program monitored and surveyed aircraft changes/deterioration as they reached the end of their design life. Boeing had recommended that once planes exceeded 75% of the 737’s design life, these be totally stripped and the structures upgraded. This would require the aircraft being out of service for a month or two. At the time, this airline decided their aircraft were safe because of short flying distances at low altitudes that didn’t require full pressurization of the fuselage. Flight hours were low but flight cycles (pressurization) were high compared to the 75,000 FC the aircraft were designed to handle.

We now know that this was completely wrong. An aircraft design life has three parameters: calendar age, flight hours, and how many times it has taken off and landed: flight cycles. It was determine that repeated pressurization and depressurization caused high stress on the fuselage. Even more than the actual flight hours. Another major contributing factor was found to be the higher than normal salt air associated with island hopping.

The New D-Check

The Aloha was a first generation classic 737-200. Maintenance scheduling of newer aircraft such as the 737NG involve multiple intermediate C Checks leading up to the P48 check or D check. The D check happens at approximately 48 months or 24,000 FH. (Although there are aircraft maintenance cycles when the D Check is completed at 9 years or 40,000 FH). The typical six C Checks leading up to the D Check does reduce the amount of work performed during a D Check. Still, it typically requires 20,000 to 30,000 man hours and up to 2 months of down time to complete.

Aircraft Maintenance D Check

Along with the complete tear down, actual work varies based on incorporating Engineering Orders (EO), Airworthiness Directives (AD), Service Bulletins (SB), and modifications (MOD). The D Check is a time when airline operators often make major modifications that can include everything from avionic upgrades to entertainment upgrades to galley/seat/toilet reconfiguration, and more. The D Check is also the time when many leased aircraft are returned to the lease company to be reconfigured for a different customer.

D-Check: Take the Airplane Apart

The aircraft is stripped apart both inside and out. Seats, overhead storage, galleys, toilets, side and floor panels are all removed. Major components are also taken off such as air stairs, landing gear, flaps, electrical equipment, hydraulics, and pneumatic components, etc. Each aircraft system is taken apart, checked, repaired or replaced and reinstalled. The engines are removed but have separate maintenance and replacement routines.

The exterior of the plane is stripped of paint for inspection and to have corrosion maintenance performed. Equipment such as cherry pickers are used to access many parts of the exterior including the vertical fin and horizontal stabilizer.

Various types of non-destructive testing are the most used inspection techniques to detect structural deterioration from factors such as corrosion, fatigue, fabrication defects, operation and maintenance, and overloading. Common non-destructive testing methods are X-rays, eddy current probes, magnetic field checks, liquid penetrant testing, ultrasonic inspection, and thermographic inspection.

Of course, there is a post-maintenance flight test. This is called a “functional check flight,” or FCF. But most systems (especially overhauled systems) first go through rigorous ground operating cycles. But some tests must be done in flight such as APU start at altitude. The FCF begins with a very detailed flight plan based on the specific work performed on the aircraft. Sometimes, key technicians that have signed off on the D Check ride on the flight. It’s possible the flight will require restricted airspace. The ARTCC controller is advised of the route or altitude specific for the checks occurring for the flight. The FCF doesn’t happen until the aircraft is signed back into service as being airworthy.