Re: A,B,C,D-checks

From:         ehahn@wren.mitre.org (Ed Hahn)
Organization: The MITRE Corporation, McLean, Va.
Date:         16 Feb 96 04:33:31 
References:   1
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pigpen@pacificnet.net writes:
> Just curious as specific differences between the different types of
> checks done on commercial aircraft.

Well, first off there are substantial (if not exactly major)
differences between the various aircraft types as to what goes on
during each of these checks, or what they are even called, for that
matter.

That said, at one airline that I worked at, the periodic maintenance
services were (from most to least frequent) for narrowbodies:

Preflight Check - before every flight
First Flight of the Day Check - before the First Flight of the Day
Periodic Service checks (PS Check) - about every 10 flight hours
A Checks - about every 30-40 flight hours (e.g. 1 every 3 days or so)
B Checks - about every 100-200 flight hours (e.g. 1 every 2-3 weeks)
C Checks - about every 3000 flight hours (depending on type)
           (e.g. about 1 per year)
Heavy C Checks - about every 15000 flight hours (e.g. about 1 every
                 three to four years).

The D Check was not a standard check at this airline.

The frequent checks (i.e. everything before B Check on the above list)
are "check the fluid levels"-"check the tire pressure"-"fill the potable
water" kind of checks - i.e. nothing that couldn't be done at a
majority of airports.

B Checks had more inspections performed, and generally take the
aircraft out of service for a day or so.  These are generally done at
a smaller number of airports.

C Checks are where major items are inspected and replaced, and take
the aircraft out of service for about 5 days or so.  Overhaul and
engineering modifications to aircraft are also performed here (i.e.
approved changes to the aircraft configuration - not just topping
off/replacing fluids or inspections).  Depending on the aircraft, a
flight test must occur after a "C" check, which performs specific
routine and non-routine in-flight tests to ensure that systems
operate (e.g. ensure cabin pressure relief valves work as operated,
ensure that the emergency masks drop from overhead when the cabin
pressure is decreased, fuel dumping, etc.)  Aircraft painting may also
happen here as needed.  This check usually only occurs at one or maybe
two places for a given airframe in an airline's system.

Heavy Cs are where major overhaul work is performed, such as control
surface refurbishing, major structural modifications.  These can take
from 4 to 6 weeks to accomplish.

Now then, the above checks are only a skeleton.  Many of the newer
aircraft are coming out with different kinds of checks, such as Major
Base Visits rather than C Checks.  Also, certain aircraft (such as
widebodies) may have more thorough C checks, without having Heavy Cs.

Finally, each airline's maintenance program is individually approved
by the FAA (based on the airframe manufacturers recommendations), so
significant differences can and do occur between different airline's
maintenance program layout (although they are all approved to meet a
minimum level).

This is all of my recollection from a couple of years ago, and only
reflects one person at one airline's perspective (I was an avionics
engineer, so some of the more structures / hydraulics kinds of things
were not my area of specialty).  Any errors in the above are mine.

ed

--------   Ed Hahn | ehahn@mitre.org | (703) 883-5988   --------
The above comment reflects the opinions of the author, and does not
constitute endorsement or implied warranty by the MITRE Corporation.
Really, I wouldn't kid you about a thing like this.