Leeham News and Analysis
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Leeham News and Analysis
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Bjorn’s Corner: Keeping airliners operational. Part 6
By Bjorn Fehrm
May 26, 2017, ©. Leeham Co: Last week we looked at a maintenance plan for a common aircraft, the Airbus A320. We learned how typical maintenance tasks were described in the aircraft’s Maintenance Planning Document (MPD). It’s the central document the aircraft OEM issues that describes the minimum recommended maintenance for the aircraft to keep it airworthy.
Each operator builds its own maintenance plan for the aircraft, in cooperation with its local Airworthiness authority. The MPD is then a base input for the plan. Before we go into how such a plan can look, we describe the principal MPD task types that such a plan contain.
Figure 1. The first modern maintenance program was formed around the Boeing 747. Source: Wikipedia.
Task types
The maintenance tasks in a Maintenance Planning Document (MPD) are divided into different groups:
Systems and Powerplant
This group includes many aircraft systems like flight controls, hydraulic system, electrical system, pneumatic system with air conditioning and the APU.
Where older maintenance concepts dismantled and changed parts in running systems (and with it, many times triggering faults) the modern ideas build on tests for correct function and wear detection.
Typical examples of tasks are given in Figure 2. It lists checks of the proper functioning of the Flight control system.
Figure 2. MPD task to check the proper function of the flight control system on the A320 series. Source; Airbus. Click to see better.
Once again, how to read the list:
- The task number. Each task has a unique task number where the first six position forms an ATA (Air Transport Association of America) identification number to what part of the aircraft is involved.
- The Zone where the task shall be performed. Each part of the aircraft has been given a Zone designation.
- Description on what shall be done, what skill is needed (EL= Electrical knowledge, AF=Airframe knowledge), the type of action (OP=Operational check/test, FC=Functional check/test) and what preparation and access is needed (the numbers are the doors/panels that need to be opened/removed). BITE is short for Built In Test Equipment, the system’s own test facility.
- The Threshold/Interval column describes the scheduling for the maintenance action. Here, the lowest Interval (I) is 500 Flight Hours (FH) or six months (MO); the highest is 18,000FH or 10 Years (YE).
- Source describes from where this task has come. CMR stand for Certification Maintenance Requirement. MRB stand for Maintenance Review Board. LUR identifies tasks that add calendar limits to flight hours or cycles to make sure the task is carried out even if the aircraft has low utilization.
- Reference give links to the task in the Aircraft Maintenance Manual and to the MRB report.
- Men says how many persons are need to do the task.
- M/H stand for Man Hour to complete the task.
- Applicability says if it’s applicable to FAA controlled aircraft or non-FAA jurisdiction aircraft and before/after which Airbus modification status (the PRE/POST 23051) or Service Bulletin (27-1057) this task is applicable.
System tasks have task limits from a few Flight Hours or Cycles (e.g. for brake wear control. A Flight Cycle (FC) is a flown sector.) up to many FH/FC complemented with calendar limits to guard against low utilization (Task 271400-02-2 in this example).
Structure
The example we gave last week was a structure task. These have often long Calendar limits. The tasks we described had 6 year intervals.
Figure 3. MPD task to inspect the structure around the passenger’s door. Source; Airbus. Click to see better.
Zonal
Zonal tasks are special instructions to do checks (often General Visual Inspection (GVI) for corrosion/deterioration/cracks) in different parts (Zones) of the aircraft, Figure 4. These task are broken out so they can be grouped with other task which require access to the same areas.
Often these tasks mean opening hatches or removing panels and insulation. It’s therefore important to group them carefully with other task so as not to tear down areas of the aircraft unnecessarily.
Figure 4. MPD Zonal tasks, inspecting areas of the aircraft. Source; Airbus. Click to see better.
Some notes about the tasks in the figure:
- The task number has ZL ahead of the unique task number, marking it’s a Zonal task
- The type of action is often GVI = General Visual Inspection.
- The Intervals (I) are often calendar time, here six years (YE) and then “20 MO or 6000 FH”.
- Applicability says its applicable to certain models in the A320 series with certain build standards. ACT stand for extra tanks in the cargo bay (Auxiliary Center Tanks).
In the next Corner, we will describe how these tasks are grouped into Checks in an operator maintenance plan.
Maybe you are just getting to the discussion of remanufactured parts, the “chain of custody,” the mechanic’s stamp, all of these procedures which are so important. I hope little is done to expedite these plans and processes in the desire for higher profits. I know a while back there were concerns associated with substandard LRUs making it on to passenger planes.