Reading the aircraft maintenance manual

What the AMM covers

The aircraft maintenance manual (AMM) is the primary technical document used by maintenance personnel to service, troubleshoot, inspect and repair an aircraft, whether on the flight line or during heavy maintenance in the hangar.

• In scope: servicing, line troubleshooting, functional testing, and on-aircraft component removal and replacement.
• Out of scope: work performed off-aircraft on removed components belongs in component maintenance manuals (CMMs). Structural damage assessment and repair belong in the structural repair manual (SRM).

Customization and effectivity

An AMM is not a generic manual; it is tailored specifically to an operator's fleet. Every task and page carries effectivity data that defines which specific tail numbers or line numbers the instructions apply to.

Effectivity reflects the aircraft's current configuration, including all service bulletins (SBs) and major modifications embodied by that operator.

The golden rule

Always verify that the aircraft tail or line number and its active modification state match the manual's effectivity before performing any task.

Boeing: the page block system

Boeing organizes the AMM using the standard ATA chapter-section-subject numbering. Within each subject, pages are grouped into standardized page blocks based on the nature of the maintenance task:

• 001 to 099, description and operation: system architecture, signal flow and unit locations.
• 101 to 199, fault isolation: diagnostic flowcharts and fault codes, often held in the separate fault isolation manual (FIM).
• 201 to 299, maintenance practices: combined procedures, used for small or consolidated tasks.
• 301 to 399, servicing: fluids, pressures and lubrication points.
• 401 to 499, removal and installation: component swap-out procedures.
• 501 to 599, adjustment and test: operational checks, rigging and calibration.
• 601 to 699, inspection and check: visual, NDT and dimensional checks.
• 701 to 799, cleaning and painting: surface preparation and protective coatings.
• 801 to 899, approved repairs: minor on-aircraft repairs.

Airbus: task numbers

Airbus uses a task numbering convention designed for digital parsing and precise cross-referencing with the minimum equipment list (MEL). A typical task number has 12 digits, for example 29-11-00-200-001:

• ATA reference (29-11-00): chapter (here hydraulic power), section and subject.
• Function code (200): the task category. For example, 000 is removal, 400 installation, the 200s inspection and check, 040 deactivation, 440 reactivation, and 710, 720 and 730 the operational, functional and system tests.
• Unique identifier (001): differentiates similar tasks within the same function code.

Airbus: subtasks and modular layout

Every Airbus task is modular and broken down into numbered subtasks, for example SUBTASK 29-11-00-010-001. Each task follows the same layout:

• 1. Reason for the job: context and objective of the work.
• 2. Job set-up information: required tooling, consumable materials and safety precautions.
• 3. Job set-up: access panel removal, zone isolation, circuit breaker tagging and safety flagging.
• 4. Procedure: step-by-step technical instructions.
• 5. Close-up: panel reinstallation, circuit breaker resets, clean-up and logbook entries.

MEL integration

Deactivation and reactivation procedures carried out for deferred defects have their own function codes (040 and 440), allowing the MEL to reference the maintenance actions directly.

Three levels of maintenance testing

Following a component replacement or repair, a post-maintenance test is required to verify system integrity. The three levels increase in depth and in the equipment required, from a basic go/no-go operational test, through a functional test against design tolerances, to a full system test. Technicians perform only the specific level of testing required by the task procedure.

If a test fails, document the exact failure indications and transition to the fault isolation manual (FIM).

1. Operational test

• Objective: confirms basic operation of a unit or system.
• Scope: equivalent to a flight crew check. It verifies go/no-go capability without measuring detailed outputs.
• Tooling: minimal to no ground support equipment required.

2. Functional test

• Objective: verifies that a system performs within its minimum design specifications.
• Scope: quantitative measurement of voltages, pressures, flow rates or response times under simulated conditions.
• Tooling: may require external ground power, hydraulic test carts or specialized test sets.

3. System test

• Objective: comprehensive testing to verify and restore the complete design performance, alignment and internal tolerances of the system.
• Scope: covers interconnected sub-systems, backup logic and fail-safe modes.
• Timing: carried out when the task calls for it, typically after major work on the system or during heavy maintenance checks.

Key takeaways

• Check effectivity first: never execute a procedure without verifying the aircraft line number and active modification state.
• Boeing identifies tasks by page blocks: memorize key ranges such as 401 for removal and installation and 501 for adjustment and test.
• Airbus identifies tasks by function codes: 12-digit task numbers that the MEL can reference directly.
• Match test depth to the work done: use the test level (operational, functional or system) the task close-out specifies.