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Maintaining airborne auxiliary power systems on aircraft

URN: SEMAE3324
Business Sectors (Suites): Aeronautical Engineering Suite 3
Developed by: Enginuity
Approved on: 2021

Overview

This standard identifies the competences you need 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 winged aircraft, and covers the units and components of airborne auxiliary power units (APU) which are installed on the aircraft for the purpose of supplying a single type or combination of auxiliary electric, hydraulic, pneumatic or other power. It includes the power and drive section, fuel, ignition and control systems, wiring, indicators, plumbing, valves and ducts up to the power unit. It does not include generators, alternators, hydraulic pumps or their connecting systems, which supply and deliver power to their respective aircraft systems, unless directly associated with the APU. The maintenance activities will include the removal, fitting and testing of a range of airborne auxiliary power system components. You will remove the required components and fit approved replacements, as appropriate. You will then need to test and adjust the completed system to meet the aircraft maintenance manual, change documentation (service bulletin) and airworthiness requirements.

Your responsibilities will require you to comply with the specific practices and procedures identified in the aircraft manual, change/service bulletin documentation and airworthiness requirements for the maintenance activities undertaken, and to report any problems with these activities that you cannot personally resolve, or that are outside your permitted authority, to the relevant people. You must ensure that all tools, equipment and materials used are correctly accounted for on completion of the activities, and that all necessary job/task documentation is completed thoroughly, accurately and legibly. You will be expected to work with a minimum of supervision, taking personal responsibility for your own actions and for the quality and accuracy of the work that you carry out.

Your underpinning knowledge will provide a good understanding of your work, and will provide an informed approach to applying the appropriate maintenance techniques and procedures to aircraft airborne auxiliary power systems. You will understand the component removal, fitting and testing methods and procedures, and their application, along with the auxiliary power systems maintenance requirements. You will know how the equipment functions, the common problems that can occur, the purpose of the individual components and associated defects, in adequate depth to provide a sound basis for carrying out the maintenance activities, correcting faults and for ensuring that the airborne auxiliary power system is maintained to the required standard.

You will understand the safety precautions required when working on the airborne auxiliary power system, especially those for ensuring that the power system, and its fuel supply, is safely and correctly isolated. You will be required to demonstrate safe working practices throughout, and will understand your responsibility for taking the necessary safeguards to protect yourself and others in the workplace.

Notes:

  1. This standard is designed to cover the practical experience requirements of the Airline Transport Association (ATA) Chapter 49 Airborne Auxiliary Power.
  2. To display competence in this standard, it is necessary to both remove and fit airborne auxiliary power system components. You must remove components; however, you may fit a replacement component where the original was previously removed by another person. You should also be aware of how to leave a system in a safe condition if maintenance tasks cannot be completed. This covers both the physical systems and the job documentation.

Performance criteria

You must be able to:

  1. work safely at all times, complying with health and safety and other relevant regulations, directives and guidelines
  2. follow the relevant maintenance schedules to carry out the required work
  3. carry out the maintenance activities within the limits of your personal authority
  4. carry out the maintenance activities, and replace components in the specified sequence and in an agreed timescale
  5. report any instances where the maintenance activities cannot be fully met or where there are identified defects outside the planned schedule
  6. complete relevant documentation in accordance with organisational requirements
  7. dispose of waste materials in accordance with safe working practices and approved procedures
  8. leave the aircraft and system in a safe and appropriate condition, free from foreign object debris on completion of the activities

Knowledge and Understanding

You need to know and understand:

  1. the specific safety practices and procedures that you need to observe when working on aircraft airborne auxiliary power systems (including any specific legislation, regulations/codes of practice for the activities, equipment or materials)
  2. the need to check that cabin/cockpit switches, selectors and circuit breakers are in the correct position before applying any form of external power (such as electrical, hydraulic, air or vacuum)
  3. the importance of maintenance on aircraft auxiliary power systems, and impact upon (Extended Range Twin-Engine Operations Procedures) ETOPS systems, Electrical Wiring Interconnect Systems (EWIS), legislation and local procedures
  4. the requirements and importance of understanding and applying human factors as defined by the regulatory requirements and the potential impact if these are not adhered to
  5. the hazards associated with carrying out maintenance activities on aircraft airborne auxiliary power systems, and with the tools and equipment used (such as handling oils, aviation fuel, the safe release of fuel and other fluids, traps from moving parts, hot parts of engines, misuse of tools), and how to minimise them and reduce any risk

  6. the protective equipment that you need to use for both personal protection (PPE) and protection of the aircraft

  7. what constitutes a hazardous voltage and how to recognise victims of electric shock

  8. how to reduce the risks of a phase to earth shock (such as insulated tools, rubber matting and isolating transformers)

  9. the importance of aircraft husbandry and of ensuring that, throughout the maintenance activity, the aircraft and work area are maintained free from foreign objects, and the implications of FOD to the safety of the aircraft

  10. how to extract and use information from aircraft manuals, log books, flight logs, charts, system and physical layouts, specifications, symbols used in aircraft auxiliary power unit systems, and other documents needed in the maintenance activities

  11. how to carry out currency/issue checks on the specifications you are working with
  12. terminology used in aircraft airborne auxiliary power systems, and the use of system diagrams and associated symbols
  13. the principles of operation of the airborne auxiliary power system being worked on, and the function of the units that make up the system (such as power plant, starting and ignition, fuel and control, oil, exhaust, engine control, indicating) 
  14. the techniques used to remove components from aircraft auxiliary power system without damage to the components or surrounding structure (such as release of pressures/force, draining of fuel/fluids, removal of components, and the need to protect the system integrity by fitting blanking plugs and ensuring exposed components are correctly covered/protected)
  15. why electrical bonding is critical, and why it must be both mechanically and electrically secure
  16. the various mechanical fasteners that will need to be removed and replaced, and their methods of removal and replacement (such as threaded fasteners, special securing devices)
  17. the various types of electrical connector that are used, methods of unlocking, orientation indicators and locating and locking-in of the connections
  18. the need to label and store correctly components that require repair or overhaul, and to check that replacement components have the correct part/identification markings and accompanying release documentation
  19. how to fit auxiliary power unit components safely and correctly (such as use of lifting and handling equipment; ensuring the correct tightness of connections; eliminating stress on pipework/connections; carrying out visual checks of all components)
  20. how to make adjustments to components/assemblies to ensure that they function correctly (such as travel and working clearance)
  21. why securing devices need to be tightened to the correct torque, locked and labelled, and the methods to be used
  22. the tools and equipment used in the maintenance activities, and their calibration/care and control procedures
  23. why tool/equipment control is critical, and what to do if a tool or piece of equipment is unaccounted for on completion of the activities
  24. how to carry out routine checks and servicing of the aircraft auxiliary power system
  25. the types of test to be carried out on the aircraft auxiliary power system, and the test equipment to be used
  26. the methods and procedures to be used to carry out the various tests on the auxiliary power system
  27. the importance of carrying out the tests in the specified sequence, checking all readings and movements at each stage
  28. how to record the results of each individual test, and the documentation that must be used
  29. how to analyse the test results, and how to make valid decisions about the acceptability of the aircraft auxiliary power system
  30. the procedures to be followed if the equipment or system fails to meet the test specification
  31. the recording documentation to be completed for the activities undertaken and, where appropriate, the importance of marking and identifying specific pieces of work in relation to the documentation 
  32. the procedure for the safe disposal of waste materials, scrap components and fuel/fluids
  33. the extent of your own authority and to whom you should report if you have problems that you cannot resolve

Scope/range

Scope Performance

​

  1. Carry out all of the following during the maintenance of the aircraft airborne auxiliary power system:

    1. ensure that appropriate authorisation to work on the aircraft is obtained, and observe all relevant isolation and safety procedures
    2. obtain and use the correct documentation (such as job instructions, technical instructions, aircraft manuals and maintenance documentation)
    3. obtain the correct tools and equipment for the activity, and check that they are in a safe, tested and usable condition and within current calibration dates
    4. adhere to procedures or systems in place for risk assessment, COSHH, personal protective equipment and other relevant safety regulations and procedures to realise a safe system of work
    5. ensure the safe isolation and draining of fluid lines before breaking into the system
    6. ensure that the relevant safety devices and mechanical/physical locks are in place (where appropriate)
    7. use approved removal, fitting and testing techniques and procedures at all times
    8. return tools and equipment to the correct storage location on completion of the activities
    9. ensure that work carried out is correctly documented and recorded
    10. ensure that any outstanding tests are correctly documented

  2. Carry out maintenance on three of the following parts of the aircraft auxiliary power system:

    1. cowling/containment    
    2. starting and ignition
    3. indicating
    4. power plant       
    5. exhaust      
    6. bleed air system
    7. engine              
    8. oil system 
    9. APU hydraulic system
    10. fuel and control   
    11. engine controls   
    12. APU generating system and wiring
    13. fire detection

  3. Remove and fit four different aircraft auxiliary power system components (at least two must be from group A):

Group A

1.  air Intake            
2.  pumps (fuel, oil)  
3.  valves/valve mechanisms
4.  exhaust unit       
5.  fuel manifolds   
6.  actuators
7.  exciter ignition unit        
8.  fuel control unit     
9.  torque converters
10.  coolers (air, oil, fuel)     
11.  fuel flow divider  
12.  thermocouple
13.  starter motor             
14.  fuel metering units  
15.  tacho generator/speed sensor
16.  engine control units       
17.  fuel nozzle        
18.  data modules
19.  starter clutch assembly   
20.  ignition unit          
21.  electronic control boxes
22.  cooling fan                        
23.  hydraulic pump         
24.  fire seals and shrouds
25.  heat exchangers (oil, fuel)      
26.  generator

Group B
27. cowling/containment covers
28. cable harness/wiring/switches/plugs
29. engine mounts               
30. indicators and warning devices
31. vibration dampers       
32. solenoids
33. battery                             
34. fire detection units
35. rod assemblies/levers and linkages
36. fire wire
37. damper/connector arm       
38. transducers
39. tube assemblies        
40. fire bottle
41. attachment bolts       
42. seals                      
43. igniters 
44. filters (fuel, oil, air)
45. sensors        
46. pressure switches
47. magnetic chip detectors
48. cables          
49. pipes and hoses  
50. thermostat     
51. relays
52. other specific components

  1. Carry out fifteen of the following maintenance activities:

    1. removing the cowling and containment covers to expose components to be removed
    2. carrying out fault diagnosis and system checks
    3. preparing the system for maintenance (such as isolating, draining fluids)
    4. disconnecting electrical connections   
    5. refitting components in the correct position, orientation and alignment
    6. removal of bonding   
    7. disconnecting/removing hoses and pipes   
    8. setting and adjusting replaced components (such as travel, working clearance)
    9. removing securing devices and mechanical fasteners      
    10. supporting equipment to be removed     
    11. making mechanical connections
    12. dismantling equipment to an appropriate level
    13. making electrical connections
    14. covering (protecting) exposed components, wires, pipework or vents     
    15. carrying out bonding
    16. torque loading as required
    17. checking components for serviceability       
    18. replenishing fluid systems
    19. replacing damaged/defective components      
    20. replacing single use items such as seals, filters, gaskets
    21. ensuring that replacement components have the correct part numbers
    22. carrying out system functional checks
    23. fitting blanks, labelling (and storing in the correct location) components that require repair or overhaul
    24. applying bolt locking methods (such as split pins, wire locking, lock nuts)

  2. Service/check aircraft airborne auxiliary power systems, to include carrying out five of the following:

    1. visually checking the system for damage and leaks 
    2. visually inspect APU bay for condition and damage
    3. inspecting hot section        
    4. checking exhaust components for security of attachment
    5. inspecting magnetic chip indicators       
    6. changing filters (fuel, oil, air)               
    7. checking heat shields for damage and security
    8. visually checking cable harnesses, pipe work and fittings for correct gaps/clearances and signs of chafing 
    9. checking indicating and warning systems  
    10. replacing the battery
    11. servicing and replenishing the oil system
    12. checking power take-off shaft(s) for correct connection and alignment    
    13. inspecting the intake door system

  3. Carry out three of the following tests on the aircraft auxiliary power system:

    1. checking that ground start mechanisms operate correctly
    2. checking aux/APU battery condition prior to APU start up
    3. checking that the APU cuts out at correct time (emergency shutdown)
    4. fuel flow is operating correctly
    5. engine pressure ratios are within specification
    6. engine temperature is within specification
    7. the bleed air system functions correctly
    8. hydraulic pressures are attained
    9. electrical generation equipment functions correctly
    10. normal operating RPM is achieved and maintained under load
    11. fire detection and protection equipment is functioning

  4. Complete the relevant paperwork, to include one from the following and pass it to the appropriate people:

    1. job cards/work sheets         
    2. aircraft cabin log
    3. computer records                   
    4. aircraft log book
    5. aircraft technical log

  5. Carry out maintenance on aircraft airborne auxiliary power systems in compliance with one of the following:

    1. Civil Aviation Authority (CAA)/European Aviation Safety Agency (EASA)
    2. Extended Range Twin-Engine Operations Procedures (ETOPS) (where appropriate)
    3. Ministry of Defence (MoD)
    4. Military Aviation Authority (MAA)
    5. Aerospace Quality Management Standards (AS)
    6. Federal Aviation Authority (FAA)
    7. aircraft maintenance manual/approved change documentation (service bulletin)
    8. manufacturers standards and procedures

Scope Knowledge

Values

Behaviours

​

You will be able to apply the appropriate behaviours required in the workplace to meet the job profile and overall company objectives, such as:

  • strong work ethic
  • positive attitude
  • team player
  • dependability
  • responsibility
  • honesty
  • integrity
  • motivation
  • commitment

Skills

Glossary

Links To Other NOS

External Links

Version Number

3

Indicative Review Date

2024

Validity

Current

Status

Original

Originating Organisation

Enginuity

Original URN

SEMAE3324

Relevant Occupations

Engineer, Engineering, Engineering and Manufacturing Technologies, Engineering Technicians

SOC Code

5234

Keywords

engineering; aeronautical; aircraft auxiliary power systems; auxiliary electric; hydraulic; pneumatic; power and drive section; fuel; ignition and control systems; wiring; indicators; plumbing; valves and ducts