Volume I, Section 12
12 DESIGN FOR MAINTAINABILITY
{A} For a description of the notations, see Acceleration
Regimes.
This section contains the following topics:
12.1 Introduction
12.2 Design for Maintainability Design
Considerations
12.3 Design for Maintainability Design
Requirements
See the video clips
associated with this section.
12.1
INTRODUCTION
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This section contains considerations and requirements for designing
equipment and systems to facilitate maintenance.
Areas covered in this chapter include general equipment design requirements;
physical access; visual access; removal, replacement, and modularity
requirements; fault detection and isolation requirements; test point
design; and requirements for a maintenance data management system.
12.2
DESIGN FOR MAINTAINABILITY DESIGN CONSIDERATIONS
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Factors that should be considered when designing for maintainability
are provided below.
a. Non-Interference of Preventive Maintenance - Preventive maintenance
should be minimized and require as little crew time as feasible.
b. Flexible Preventive Maintenance Schedule - Preventive maintenance
schedules should be sufficiently flexible to accommodate changes in
the schedule of other mission activities.
c. Redundancy - If maintenance is necessary and system operations will
be interrupted, redundant installations should be considered in order
to permit maintenance without interrupting system operation.
d. Goals of Designing for Maintainability - The following are goals
for optimizing crew involvement in both preventive and corrective maintenance.
1. Reduce training requirements of crew.
2. Reduce certain skill requirements of crew.
3. Reduce time spent on preventive and corrective maintenance.
4. Increase maintenance capabilities during mission (especially corrective
maintenance).
e. Corrective Maintenance - The following factors should be considered
when designing for corrective maintenance tasks.
1. The benefit gained from repair should be worth the time and effort
expended on repair.
2. The time and effort involved in corrective maintenance should be
weighed against the cost and feasibility of carrying replacement units.
3. Required calibration, alignment, or adjustment should be easily
and accurately accomplished.
4. Automate fault detection and isolation tasks whenever possible.
12.3
DESIGN FOR MAINTAINABILITY DESIGN REQUIREMENTS
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12.3.1 Equipment Design
Requirements
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All flight hardware and software shall be designed to facilitate on-orbit
maintenance, check-out and shall be compatible with ground maintenance
capabilities.
Equipment design shall minimize both complexity and time requirements
for maintenance.
Equipment design for maintenance shall consider IVA as the prime resource;
maintenance by EVA shall be contingency only.
12.3.1.1 General
Maintainability Design Requirements
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General requirements to be followed when designing for maintainability
are presented below.
a. Growth and Update - Facilities, equipment, and software design shall
allow reconfiguration and growth during the mission.
b. Independence - Systems and subsystems shall be as functionally,
mechanically, electrically, and electronically independent as practical
to facilitate maintenance.
c. Maintenance Support Services - Maintenance support services ( e.g.,
electrical outlets) shall be accessible at potential problem locations
or at a designated maintenance location.
d. Reliability - Equipment design shall reduce to a minimum the incidence
of preventive and corrective maintenance.
e. Simplicity - Equipment design shall minimize maintenance complexity.
f. Time Requirements - Equipment design shall minimize the time requirements
for maintenance.
g. Equipment - Maintenance equipment and tools shall be kept to a minimum.
h. Hazardous Conditions - System design shall preclude the introduction
of hazardous conditions during maintenance procedures.
i. Critical Operations - Critical systems shall be capable of undergoing
maintenance without the interruption of critical services and shall
be maintained.
j. Non-Critical Operations - Non-critical systems shall be designed
to operate in degraded modes while awaiting maintenance. Degraded mode
operation shall not cause additional damage to the system or aggravate
the original fault.
k. Redundancy Loss - Notification of loss of operational redundancy
shall be provided immediately to the crew.
l. Connectors - Quick-disconnect connectors shall be used.
(Refer to Paragraph 11.10.3, Connector
Design Requirements, for specific requirements.)
m. Plug-In Installation - Plug-in type hardware installation and mounting
techniques shall be employed.
(Refer to Paragraph 11.5.3, Mounting
Hardware Design Requirements, for specific requirements.)
n. Quick Release Fasteners - Quick release fasteners shall be used
where consistent with other requirements (e.g., strength, sealing).
(Refer to Paragraph 11.9.3, Fastener
Design Requirements, for specific requirements.)
o. Replacement Capabilities - Capacity of replaceable or reserviceable
items (filters, screens, desiccant units, battery power supplies, etc.)
shall be higher than the minimum functional requirements of the system.
p. Automation - Fault isolation, inspection, and checkout tasks shall
be automated to the extent practical.
q. Restraints - Personnel and equipment mobility aids and restraints
shall be provided to support maintenance.
(Refer to Paragraph 11.7.2.3,
Personnel Restraints Design Requirements, and
Paragraph 11.7.3.3, Equipment
Restraints Design Requirements, for specific requirements.)
r. Special Skills - Maintenance requiring special skills shall be minimized.
s. EVA - Maintenance requiring EVA shall be minimized.
t. Soldering, Welding, and Brazing - Soldering, welding, brazing, and
similar operations during maintenance shall be minimized.
12.3.1.2 Physical
Accessibility Design Requirements
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Design requirements for physical access to equipment for the purpose
of maintainability are provided below.
a. Relative Accessibility - Items most critical to system operation
and which require rapid maintenance shall be most accessible. When relative
criticality is not a factor, items requiring most frequent access shall
be most accessible.
b. Access Dimensions - The minimum sizes for access openings for two
hands, one hand, and fingers are shown in
Figure 12.3.1.2-1.
c. Access - Access to inspect or replace an item (e.g., an ORU) shall
not require removal of more than one access cover.
(Refer to Paragraph 11.4.3, Closures
and Covers Design Requirements, for specific requirements.)
d. Mounted Components - When feasible, components shall be no more
than one deep in a bay or rack.
(Refer to Paragraph 11.5.3.1,
General Mounting Design Requirements, for specific requirements)
e. Environmental Control and Life Support Systems (ECLS) - Subsystem
equipment supporting ECLS for safe **IVA** environment shall be accessible,
removable, and repairable by an EVA suited crewmember.
(Refer to Paragraph 14.3.2.5,
EVA Working Envelope, for additional information.)
f. Shape - Accesses shall be designed to the shape that will enable
the crewmember to do his/her job and not be limited only to conventional
shapes.
g. Number of Accesses - Whenever possible, one large access shall be
provided rather than a number of small ones.
h. Protective Edges - Protective edges or fillets shall be provided
on accesses that might injure crewmembers or their equipment.
(Refer to Paragraph 6.3.3, Mechanical
Hazards, for specific requirements.)
i. Covers- - Where physical access is required, one of the following
practices shall be followed, with the order of preference as given.
1. Provide a sliding, translating, or hinged cap or door where debris,
moisture, or other foreign materials might otherwise create a problem.
2. Provide a quick-opening cover plate in a cap that will meet stress
requirements.
j. self-supporting Covers - All access covers that are not completely
removable shall be self-supporting in the open position.
k. Rear Access - Sliding, rotating, or hinged equipment to which rear
access is required shall be free to open, translate or rotate its full
distance.
l. Damage Inspection and Repair - Where feasible , the design of structures
and equipment, including their interfaces and all portions of the pressure
shell, bulkheads , and seals shall be accessible for damage inspection
and repair. This shall apply to exterior as well as to interior surfaces.
m. Use of Tools and Test Equipment- Check points, adjustment points,
test points, cables, connectors, and labels shall be accessible and
visible during maintenance. Sufficient space shall be provided for the
use of test equipment and other required tools without difficulty or
hazard.
n. Fold-Out/Pull Out Drawers and Cabinets - Fold-out/pull-out drawers
and cabinets shall be used where possible to provide ease of access.
o. Slide-Out Stops - Limit stops shall be provided on racks and drawers
which are required to be pulled out of their installed positions for
maintenance. The limit stop design shall permit convenient overriding
of stops of or unit removal.
p. Service Points for Fluid Systems - Service points for filling, draining,
and purging or bleeding shall be in accessible locations.
q. Plug Connectors - Full access shall be provided to plug connectors.
r. Cables:
1. Cable access - Cables shall be routed so as to be readily accessible
for inspection and repair.
2. Cable trays - Wire harness and fluid lines mounted in cable trays
shall be located for ready access.
3. Cable loops - Panel, console, and rack mounted components shall
have slack cable lengths or maintenance loops sufficient for removal
of the connectors after the component has been extracted from its installed
location, unless adequate internal access (physical and visual) is provided.
4. Cable Routing - Cables shall not be routed external to the face
of the equipment rack.
s. Fuses and Circuit Breakers - Fuses and circuit breakers shall be
readily accessible for removal, replacement, and resetting. The condition
of fuses (good or blown) shall be readily discernible without having
to remove the fuse.
t. Structural Members - Structural components of units or chassis shall
not prevent access to or removal of equipment.
u. Hazardous Conditions - If a hazardous condition exists behind an
access, a safety indictor shall be provided. The access shall be equipped
with an interlock that will de-energize the hazardous conditions when
the barrier is opened or removed, and a manual override shall be provided.
v. Structural Loads and Deformations - Compartment doors, access panels,
and structural attachments for equipment that is to be removed and reinstalled
shall be designed to be operated in both ground and orbit environments,
being insensitive to structural deformation caused by change in g-loading,
pressure differential, etc
Figure
12.3.1.2-1 Minimum Sizes for Access Openings for Two Hands, One
Hand and Fingers
Reference: 2, pg 184; NASA-STD-3000
297
12.3.1.3 Visual
Access Design Requirements
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Requirements for visual access are provided below.
a. Visual Access - Where visual access only is required, the following
practices shall be followed with the order of preference as given.
1. Provide an opening with no cover except where this might degrade
system performance.
2. Provide a transparent window if dirt, moisture, or other foreign
materials might create a problem.
3. Provide a quick-opening metal cover if a transparent cover will
not meet stress or other requirements.
b. Visual and Manual Access - If the crewmember has to be able to see
the task, design of the access shall be large enough to allow simultaneous
visual as well as physical access; otherwise a separate window shall
be provided for visual access to monitor task performance.
Refer to Paragraph 12.3.1.2, Physical Accessibility
Design Requirements, for additional requirements.)
c. Labeling:
(Refer to Paragraph 9.5,
Labeling and Coding, and Paragraph
9.4.4, Caution and Warning Displays, for related requirements.)
1. Access labeling - Each equipment access shall be labeled to indicate
items visible or accessible through it.
2. Visibility - Relevant labels and mounting instructions shall be
visible during all maintenance activities.
3. Identification labels - Each access shall be labeled with a number,
letter, or other symbol which is directly cross-referenced to the maintenance
procedures.
4. Plug configuration labels - When a plug-in device has to be inserted
through a hole with limited visual access, a label adjacent to the access
shall indicate how the pins on the device will align with the holes
in the socket.
5. Component identification labels - Electrical cables, fluid lines,
and other subsystem protective shields shall be labeled or otherwise
coded to allow for positive identification.
6. Hazard labels - Accesses shall be labeled with appropriate hazard
labels, advising of any hazard existing beyond the access and stating
necessary precautions.
7. Hinged cover labels - If instructions applying to a covered item
are lettered on a hinged door, the lettering shall be oriented to be
read by the crewmember performing maintenance when the door is opened.
(Refer to Paragraph 9.5.3, Labeling
and Coding Design Requirements, for additional requirements.)
d. Fluid and Gas Line Connectors - Where feasible, fluid and gas connectors
shall be located and configured so they can be inspected, and so that
any leakage is obvious
(Refer to Paragraph 11.10,
Connector Design Requirements, for other connector requirements.)
12.3.1.4 Removal,
Replacement and Modularity Design Requirements
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Design requirements for removal, replacement, and modularity are provided
below.
(Refer to Paragraph 11.5.3.2,
Alignment Devices Design Requirements, and
Paragraph 11.5.3.1, General Mounting Design Requirements, for additional
requirements.)
a. Removal - Systems and subsystems shall be designed so that failed
Orbital Replacement Units (ORUs) can be removed without damaging or
disturbing other components.
b. Surface Removal - Replaceable units shall be designed for removal
through the surface facing the crewmember as he works on the equipment.
(Refer to Paragraph 11.5.3.1,
General Mounting Requirements, for other specific requirements)
c. Independence - Where feasible, it shall not be necessary to remove
or disable an operable unit to obtain access to a defective replaceable
unit.
d. Component Labeling - Each removable component and its position on
the unit shall be labeled with corresponding numbers or other identification.
(Refer to Paragraph 9.5.3, Labeling
and Coding Design Requirements, for specific requirements.)
e. Isolation Valves - Subsystems that contain liquids or high pressure
gases (pressures exceeding 125 psia) and require maintenance shall be
provided with isolation or disconnect valves to permit isolation and
servicing and to aid in leak detection.
f. Spillage control - Replaceable units shall be designed to control
spillage and the release of gases during removal or replacement.
g. Energized Units - Replaceable units and payloads which supply or
receive energy shall be designed so that the power can be removed before
repair, removal, or replacement is attempted. If stored energy can pose
a hazard, provisions shall be made for its dissipation prior to maintenance.
(Refer to Paragraph
6.4, Electrical Hazards Design Requirements, for specific requirements.)
h. Fastener Coatings - Paint and/or coatings shall not adversely affect
removal or installation of fasteners.
i. Short Life Components - Easy replacement shall be provided for components
that fail frequently (e.g., lamps and fuses).
j. Guide Pins - For mounting and replacement of replaceable units,
guides and guide pins shall be provided for alignment.
k. Replacement Specificity - All replaceable items shall be designed
so that it will be physically impossible to insert the unit incorrectly.
l. Related Items - Items of the same or similar form which have different
functional properties shall be readily identifiable and distinguishable,
and shall not be physically interchangeable. This indication shall be
readily discernible with the component in its installed position.
12.3.2 Testability
Design Requirements
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12.3.2.1 Fault Detection
and Isolation Design Requirements
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Design requirements for fault detection and isolation are provided
below
(Refer to Paragraph 9.4.4.3, Caution
and Warning Display Design Requirements,
Paragraph 9.4.2.3, Visual Displays Design Requirements, and
Paragraph 9.3.3, Control Design Requirements, for specific requirements.)
a. General - Equipment design shall facilitate rapid and positive fault
detection and isolation of defective items.
b. Checkout - On-board fault detection/isolation shall be automated
and pre-programmed for mission-critical and/or life support systems.
On-demand system checkout shall also be available.
c. Diagnostic Capability - Equipment shall have an integrated diagnostic
capability for all functional failures identified as known or expected
to occur, in mission-critical and life support systems.
d. Replacement Unit Status - When feasible, REPLACEMENT UNIT design
and configuration shall permit verification of operational status prior
to installation without the need for disassembly.
e. Sensors - The status of sensors on replacement units shall be verifiable
with respect to accuracy and proper operation.
f. Manual Override - A manual override capability for all automatic
control functions shall be provided.
g. Portable Equipment - When built-in test equipment is not available,
diagnostic tools and/or portable equipment shall be provided for fault
isolation to the replacement unit level.
h. Critical Malfunction Alarm - If critical equipment is not regularly
monitored an alarm (auditory, visual, or both) shall be designed to
ensure detection.
i. Power Failure Indication - An indication shall be provided to reveal
power failures.
j. Power Interrupt - A positive indication of an open circuit shall
be provided by a fuse or circuit breaker.
k. Out of Tolerance - A positive indication shall be provided when
equipment has failed or is not operating within tolerance limits.
l. Trouble-shooting Sequence - A sequence of trouble-shooting checks
shall be specified to maximize trouble-shooting efficiency.
m. Test Equipment Verification - All electronic test equipment shall
have built-in test capability.
n. Test Equipment Accuracy - The accuracy of all test equipment shall
exceed that of the equipment being tested.
o. Adjustment Controls - Appropriate feedback shall be provided for
all adjustment controls and shall be readily discernible to the person
making the adjustment while making the adjustment. Adjustment controls
shall be reversible without dead band, slop, hysteresis, or striction
as reversal.
p. Calibration Damage - Calibration or adjustment controls shall be
provided with appropriate stops to prevent damage to the system. Calibration
controls shall provide an indication (visual or audible) when stops
are reached.
12.3.2.2 Test Point
Design Requirements
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Design requirements for test points are provided below.
(Refer to Paragraph 9.2.3.2, C/D
Placement and Integration - Design Requirements,
Paragraph 9.4.4.3, Caution and Warning Design Requirements, and
Paragraph 9.5.3, Labeling and Coding
Design Requirements, for additional requirements.)
a. Self-Checking - Appropriate test points shall be provided where
a unit is not completely self-checking.
b. Proximity - Test points shall be provided at or near maintenance
locations.
c. Adjustment - Test points used in adjusting a unit shall be in physical
and visual proximity of the controls and displays used in the instrument.
d. Labeling - Each test point shall be clearly labeled with a description
of its function, or, at a minimum, with a code number keyed to the maintenance
manual.
e. Warning Labels - Test points shall be marked with appropriate warning
labels when the application of conventional test probes could cause
damage to internal circuits (e.g., integrated circuits) or injury to
personnel.
f. Troubleshooting - Sufficient test points shall be provided so that
it will not be necessary to remove subassemblies to accomplish troubleshooting/fault
diagnosis.
g. Test Cable Termination - If it is essential that test cables terminate
on control and display panels, the panel test receptacles shall be located
so that the test cables will not interfere with controls and displays.
h. Layout - Primary test points shall be grouped in a line or matrix
that reflects the sequence of tests to be performed.
i. Grouping - A control panel or a series of functionally autonomous
panels shall be used to group test points whenever possible.
j. Testing and Servicing - Rear plug connectors shall be accessible
for testing and servicing except where precluded by potting, sealing,
or other requirements
12.3.3 Maintenance
Information Management Systems Design Requirements
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Design requirements for maintenance information management systems
are provided below.
(Refer to Paragraph 13.4.3, Information
Management Design Requirements, for other specific requirements.)
a. System Capabilities - As a minimum, the on-board information systems
shall provide:
1. Command and status indications to/from all subsystems for the purpose
of system maintenance and trouble-shooting procedures.
2. Trend data acquisition and analysis.
3. Status of consumables.
4. Fault detection/isolation.
5. Scheduled maintenance data.
6. Repair/replacement information.
7. Replacement unit maintenance history and maintenance checklists.
b. Recording and Retrieval - The system shall provide for the recording
and retrieving of maintenance information in near real-time.
c. Fail Operational Systems - All systems that incorporate an automated
fail-operational capability shall be designed to provide crew notification
and data management system cognizance of malfunctions until the faults
have been corrected.
d. Replacement Unit Characteristics - A characteristic matrix of all
replacement units shall be included in the data base containing such
information as:
1. Replacement unit ID number.
2. Bite (replacement units containing built-in-test-equipment).
3. Hazardous system factors.
4. Critical system status.
5. Availability.
6. Shelf-life limits.
7. Serial number traceable to manufacturer.
8. Batch data.
9. Date of manufacture.
10. Storage constraints.
e. Sparing Status - Replacement unit sparing status shall be provided
to ensure that procedures and on-board repair materials are adequate
for each mission.
f. Spares Inventory - The automated information management system shall
contain an on-orbit spares inventory to identify the numbers and locations
of replacement units stowed in the space module.
(Refer to Paragraph 13.3.3, Inventory
Control Design Requirements, for specific requirements.)
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