Aircraft Maintenance Checks

[Bazzo]

Can anyone please give me an idea of how the typical large passenger aircraft is checked structurally? e.g. when the check is made (FC / FH) and what is done at this check.

 

[davidjh]

Very generally; inspections are done in a "layered" fashion. The manufacturer creates a recomemneded maintenance plan which is blessed (approved)by his type certification office. The opertor can modify that plan somewhat, and gets approval from whatever regulatory authority governs his operations. As I recall (And I'm asking for verification) the most in-depth inspection takes place about every 4000 flight hours. On short haul airplanes the interval may be set by number of flights (ie; many more takeoff, pressurization, and landing cycles for any given number of flight hours). I believe you can find more specific info on the FAA website, and even on the manufacturer web sites. Others here probably have more specific info at their fingertips.

 

[sshenold]

What davidjh has is correct. Each manufacturer develops its own 'recommended' program, which is approved by that manufacturer's governmental agency. Each operator is free to develop their own program, and get it 'blessed' by their governmental agency.

Each operator uses its airplanes in a slightly different way from every other operator. For example, in Japan 747s are used on commuter routes. Obviously, those airplanes going to get a lot more cycles (landing/takeoff) than the more common use as a long-haul transcontinental airliner, and the maintenance program has to take this into account.

Furthermore, in the interests of economics, operators can (with their government's approval) 'phase' one part of the maintenance program into another. Usually, this involves breaking down the higher-level (more intensive) periodic maintenance checks into smaller sub-tasks, which are then incorporated into the lower-level (less intensive) checks. One of the more common ones is to break the D check (the highest level periodic maintenance check) into smaller sections and incorporate a different section into the C check (the next lower level). The complete D check is still accomplished in the same intervals it would have been had it not been broken up, and the airline doesn't have the airplane down for maintenance as long as it would have had it not 'phased' the D check into the C checks.

Also incorporated into these checks are other programs, such as the corrosion prevention and control programs and supplemental structural inspection programs.

Not every operator uses the letter check system; they all do similar tasks, however, they just call them by different names.

So, there's no standard answer as to when a certain check falls due. Each operator typically has its own maintenance program tailored to its operational patterns. And what's done on each check makes up a pretty sizable book, not something you'd want to post here.

Most structural inspections are done on the C or D checks (or the operator's equivalent of these). On a C check, typically the airplane is washed, brought into a hangar, opened up (most all access panels removed), and various types of inspections performed (mostly visual, but also a variety of NDT inspections). Various tests for wear and condition are also performed, and the filters on the airplane's systems are changed. The airplane is lubed. Any repairs or modifications that are needed are done; the interior may be updated or otherwise refurbished; the engines may be changed as well, depending on their own maintenance program. The airplane is put back together, and a wide variety of tests and operational checks performed. The airplane goes through a post-maintenance test flight (and any required corrections accomplished) prior to being returned to revenue service.

A D check is even more in-depth than a C check.

Operators also revise their maintenance programs from time to time. Usually this is based on some sort of failure analysis; it's much more cost-effective to change something before it breaks than to wait until it fails at the gate and strands your customers. So, some things may be added to the maintenance program. Other times, the same sort of analysis shows that some component is much more reliable than originally thought, or some sort of improved inspection technique has proved to be effective and so the item or area can be inspected less frequently. So the maintenance program for a given operator changes from time to time.

Maintenance programs and maintenance scheduling are both very complex subjects. How the airlines managed to keep all of it straight in the days before computers is beyond me.

Regards,
Sean Shenold

 

[Bazzo]

Thanks for both your answers, they were both a great help, by the way, can you please tell me on which type of aircraft you work on?