Maintaining airborne auxiliary power systems on aircraft (Aviation Maintenance) Revision — Excellence, Achievement & Learning Limited Occupational Qualification

    This unit identifies the training and development required in order that the apprentice can demonstrate that they are competent inbeing able to carry out maintenance activities on aircraft auxiliary power systems, in accordance with the aircraft maintenance manual,approved change documentation (service bulletin) and airworthiness requirements. It covers both fixed wing and rotary wingedaircraft, and covers the units and components of airborne auxiliary power units (APU) which are installed on the aircraft for the purposeof supplying a single type or combination of auxiliary electric, hydraulic, pneumatic or other power. It includes the power and drivesection, fuel, ignition and control systems, wiring, indicators, plumbing, valves and ducts up to the power unit. It does not includegenerators, alternators, hydraulic pumps or their connecting systems, which supply and deliver power to their respective aircraftsystems, unless directly associated with the APU. The maintenance activities will include the removal, fitting and testing of a range ofairborne auxiliary power system components. They will remove the required components and fit approved replacements, asappropriate. They will then need to test and adjust the completed system to meet the aircraft maintenance manual, changedocumentation (service bulletin) and airworthiness requirements.

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    Key Marking Points

    Maintaining airborne auxiliary power systems on aircraft (Aviation Maintenance)

    EXCELLENCE-ACHIEVEMENT-AND-LEARNING-LIMITED
    vocational

    This subtopic equips learners with the practical skills and knowledge to perform maintenance on airborne auxiliary power units (APUs) on both fixed-wing and rotary-wing aircraft. It covers the systematic removal, replacement, and testing of APU components, including power drive sections, fuel systems, and control wiring, ensuring strict adherence to aircraft maintenance manuals and airworthiness regulations. Mastery of these tasks is critical for ensuring reliable auxiliary power supply for aircraft systems during ground operations and emergency in-flight scenarios.

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    Learning Outcomes
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    Assessment Guidance
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    Key Skills
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    Key Terms
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    Assessment Criteria

    Assessment criteria

    EAL Level 3 Diploma in Aviation Maintenance (Development Competence)

    Topic Overview

    The EAL Level 3 Diploma in Aviation Maintenance (Development Competence) is a vocational qualification designed for individuals pursuing a career as aircraft maintenance technicians or engineers. It covers the fundamental knowledge and practical skills required to maintain, repair, and certify aircraft in compliance with UK Civil Aviation Authority (CAA) and European Aviation Safety Agency (EASA) regulations. This diploma is part of the Apprenticeship Standard for Aviation Maintenance Technician and provides a pathway to achieving Part-66 Aircraft Maintenance Licence (AML) categories A or B1/B2.

    The qualification is structured around key modules including aerodynamics, aircraft structures, propulsion systems, electrical and electronic systems, and maintenance practices. Students learn to interpret technical documentation, use specialised tools, perform inspections, and troubleshoot faults. Emphasis is placed on health and safety, human factors, and regulatory compliance, ensuring graduates can work safely and effectively in a highly regulated environment. This diploma is essential for those seeking employment with airlines, MROs (Maintenance, Repair, and Overhaul organisations), or the military.

    Within the broader Design and Technology curriculum, this qualification applies engineering principles to real-world aviation contexts. It bridges theoretical knowledge with hands-on competence, preparing students for immediate employment or further study. Mastery of this diploma demonstrates a high level of technical proficiency and adherence to industry standards, making it a valuable asset for career progression in aerospace engineering.

    Key Concepts

    Core ideas you must understand for this topic

    • →Airworthiness and Regulatory Framework: Understanding CAA/EASA Part-145 (maintenance organisations) and Part-66 (licensing) requirements, including the role of the Continuing Airworthiness Management Organisation (CAMO).
    • →Aircraft Structures and Materials: Knowledge of airframe construction (monocoque, semi-monocoque), stress analysis, corrosion prevention, and non-destructive testing (NDT) methods like dye penetrant and eddy current.
    • →Powerplant Systems: Principles of gas turbine and piston engines, including fuel systems, ignition, lubrication, and thrust reversal. Students must be able to perform engine runs and troubleshoot performance issues.
    • →Electrical and Avionic Systems: Understanding AC/DC generation, distribution, and protection; avionics communication (VHF, HF, SATCOM); and navigation systems (VOR, ILS, GPS).
    • →Human Factors and Error Management: Applying SHELL model (Software, Hardware, Environment, Liveware) and understanding fatigue, stress, and communication errors to reduce maintenance mistakes.

    What You Need to Demonstrate

    Key skills and knowledge for this topic

    • Award credit for demonstrating correct interpretation of the aircraft maintenance manual when selecting removal and fitting procedures for APU components.
    • Award credit for evidence of proper use of safety practices, including lock-out/tag-out procedures and system depressurization before component removal.
    • Award credit for accurate testing and adjustment of the APU system post-maintenance, verifying performance against specified parameters in the service bulletin and AMM.
    • Award credit for meticulous documentation of all maintenance actions, including part numbers, serial numbers, and torque values, in accordance with airworthiness record-keeping standards.

    Assessment Criteria

    Key criteria assessors look for in your portfolio

    • Award credit for demonstrating correct interpretation of the aircraft maintenance manual when selecting removal and fitting procedures for APU components.
    • Award credit for evidence of proper use of safety practices, including lock-out/tag-out procedures and system depressurization before component removal.
    • Award credit for accurate testing and adjustment of the APU system post-maintenance, verifying performance against specified parameters in the service bulletin and AMM.
    • Award credit for meticulous documentation of all maintenance actions, including part numbers, serial numbers, and torque values, in accordance with airworthiness record-keeping standards.

    Assessment Guidance

    Guidance for achieving higher grades

    • 💡During practical observations, verbalize your actions clearly, referencing relevant manual sections to demonstrate understanding and not just mechanical skill.
    • 💡When compiling your portfolio, include annotated photographs and work logs that explicitly cross-reference each step to the maintenance manual to show evidence of compliance.
    • 💡Prepare for professional discussions by revising common troubleshooting scenarios for APU start failures, such as low fuel pressure or ignition unit faults, and link them to system schematic diagrams.
    • 💡Always perform a final inspection using the aircraft’s minimum equipment list (MEL) considerations to confirm that no deferred defects remain that could affect airworthiness.
    • 💡Always reference the specific regulation or manual section in your answers. For example, when discussing a repair, cite the relevant CAA CAP 553 or EASA Part-145 clause. Examiners reward precise referencing.
    • 💡In practical assessments, demonstrate your thought process aloud. Explain why you are performing each step (e.g., 'I am applying a torque seal to indicate the bolt has been torqued correctly'). This shows understanding, not just rote procedure.
    • 💡For written exams, use diagrams to illustrate systems (e.g., hydraulic circuit, electrical bus). Label components clearly and show flow direction. A well-drawn diagram can earn marks even if the written explanation is brief.

    Common Mistakes

    Common errors to avoid in your coursework

    • Confusing the APU's power output interfaces (electric, pneumatic, hydraulic) with the main aircraft power generation systems, leading to incorrect isolation of the unit.
    • Failing to consult the latest revision of the service bulletin before commencing work, resulting in use of outdated component specifications or procedures.
    • Neglecting to perform a full functional check of the APU after maintenance, including monitoring start-up sequence, exhaust gas temperature, and load acceptance, which may hide latent faults.
    • Overlooking secondary locking mechanisms on electrical connectors or fluid couplings during re-installation, which can cause in-flight disconnections.
    • Misconception: 'If a component looks fine, it doesn't need testing.' Correction: Visual inspection alone is insufficient. Many defects (e.g., internal cracks, electrical shorts) require functional testing or NDT. Always follow the maintenance manual's inspection criteria.
    • Misconception: 'Torque values are just guidelines.' Correction: Torque specifications are critical for safety. Under-torquing can cause loosening; over-torquing can damage threads or cause stress fractures. Use a calibrated torque wrench and apply the correct value from the manual.
    • Misconception: 'I can skip logbook entries if the work is minor.' Correction: All maintenance actions must be recorded in the aircraft technical log or maintenance record. Failure to document can lead to airworthiness violations and safety risks.

    Frequently Asked Questions

    Common questions students ask about this topic

    Before You Start

    Prior knowledge that will help with this topic

    • •Basic Engineering Principles: Understanding of physics (forces, pressure, electricity) and mathematics (algebra, trigonometry) at GCSE level or equivalent.
    • •Health and Safety Awareness: Familiarity with COSHH, manual handling, and risk assessment procedures, as covered in a Level 2 Engineering qualification.
    • •Manual Dexterity and Tool Use: Prior experience with hand tools (spanners, screwdrivers) and measuring instruments (micrometers, multimeters) is beneficial.

    Key Terminology

    Essential terms to know

    • This unit identifies the training and development required in order that the apprentice can demonstrate that they are competent inbeing able to carry out maintenance activities on aircraft auxiliary power systems, in accordance with the aircraft maintenance manual,approved change documentation (service bulletin) and airworthiness requirements. It covers both fixed wing and rotary wingedaircraft, and covers the units and components of airborne auxiliary power units (APU) which are installed on the aircraft for the purposeof supplying a single type or combination of auxiliary electric, hydraulic, pneumatic or other power. It includes the power and drivesection, fuel, ignition and control systems, wiring, indicators, plumbing, valves and ducts up to the power unit. It does not includegenerators, alternators, hydraulic pumps or their connecting systems, which supply and deliver power to their respective aircraftsystems, unless directly associated with the APU. The maintenance activities will include the removal, fitting and testing of a range ofairborne auxiliary power system components. They will remove the required components and fit approved replacements, asappropriate. They will then need to test and adjust the completed system to meet the aircraft maintenance manual, changedocumentation (service bulletin) and airworthiness requirements.

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