Part 45 Manual of Standards Instrument 2016

The MOS organises aircraft marking requirements around a small number of core concepts: sets of markings, location and number of sets by aircraft type, legibility and minimum dimensions, character construction, exceptions and approvals, and identification plate standards. These concepts are expressed as prescriptive technical rules rather than principles.

Sets of markings: The MOS treats a registration or nationality marking as a "set" (see Table 1 and Table 2). The instrument prescribes how many sets are required and where they must be displayed for each listed kind of aircraft (s 6.1-6.4). For fixed‑wing aircraft the rule matrix is more complex, splitting by tail configuration, multiple tails, maximum take‑off weight and whether the aircraft operates outside Australia (Table 1). Rotorcraft (s 6.2.1), airships (s 6.3.1-6.3.2) and balloons (s 6.4.1-6.4.2) have tailored location rules reflecting their structure.

Legibility and dimensional linkage: Legibility is linked to specified viewing distances in Table 3 and to minimum character heights in Table 4. The MOS makes legibility a measurable requirement: a set is "legible" if clearly distinguishable without optical aid from specific distances in daylight hours (s 7.3 and Table 3). Table 4 ties minimum character heights to aircraft categories and maximum take‑off weight thresholds (s 9.1). Where a surface is too small to meet the heights, the MOS requires the largest practicable characters while keeping proportions (s 9.2).

Character design rules: Section 8 sets precise typographical rules. Letters must be capital Roman characters, numbers Arabic, characters unornamented and consistently sized (s 8.1(a)-(d)). Width is 2/3 of height except "I" and "1" (s 8.1(e)). Stroke thickness is 1/6 of height (s 8.1(g)). Hyphen length is 2/3 of height (s 8.1(f)). Spacing requires at least 1/6 of height between characters (s 8.1(h)). Slant is allowed up to 35 degrees if legible (s 8.3). Vertical stacking is permitted only in limited situations (s 8.2).

Obscuration and conflicting marks: A set must not be obscured by a movable surface and must contrast with its background (s 7.1(c)-(d)). Designs, marks or symbols must not modify, obscure or create confusion about registration marks (s 7.4). Two carve‑outs are expressed: Defence Force symbols or call signs on Defence Force aircraft, and CASA‑approved designs (s 7.5).

Identification plate standards and entrance words: The MOS prescribes material and marking methods for the aircraft registration identification plate (s 11.1(a)-(b)), location requirements (s 11.1(c)-(d) and s 11.2), and accessibility standards (s 11.3). It also requires specific words to be displayed near aircraft entrances for limited, restricted, provisional and experimental certificated aircraft, with minimum height and contrast requirements (s 10).

Relationship to CASR 1998: The MOS repeatedly cross‑references Part 45 and various Part 21 certification provisions (s 4.2-4.3, Tables and notes). The MOS functions as the technical elaboration of the regulatory obligations under CASR 1998, making its scope contingent on the definitions and categories in the Regulations.

The MOS directly affects owners and operators of Australian‑registered aircraft, designers and manufacturers who supply aircraft or apply markings, maintenance organisations that apply or repair markings and identification plates, the Civil Aviation Safety Authority in issuing approvals, and regulatory compliance staff responsible for aircraft certification and continued airworthiness. The instrument’s audience is technical and operational.

Owners and operators: The MOS prescribes precise marking requirements that owners and operators must meet to comply with CASR 1998 obligations referenced in the MOS (s 5.1). Owners bear the cost and operational consequence of ensuring the correct number and placement of sets of markings (Tables 1 and 2). For aircraft operating outside Australia there are additional requirements (Table 1, item 4). Where aircraft are certificated under particular Part 21 provisions or eligible for specific certification pathways, different marking rules apply (s 6.1.1 and Table 2, item 8 of Table 4 cross‑references).

Manufacturers and designers: Aircraft manufacturers and completion centres must ensure that the airframe has suitable uninterrupted surfaces and sufficient area to accommodate required character sizes (s 9.1-9.3). Character proportions and placements (s 8.1, s 7.2) will affect livery design and structural layout. Where the lower wing surface is used for markings, the MOS sets positional and alignment constraints (Table 1, item 4).

Maintenance and modification organisations: The MOS requires markings to be securely applied, with secure defined as painting or attachment at least as permanent as painting (s 7.2). Organisations performing re‑marking, repainting or plate attachment must meet the physical and permanence standards and ensure legibility distances and contrasts are preserved following repairs or repainting. The accessibility requirement for identification plates requires maintenance planning where plates might otherwise be enclosed (s 11.3).

Regulator and approvals function: CASA is the decision‑maker for approvals referenced in the MOS. The MOS anticipates written CASA approvals for designs that would otherwise obscure markings (s 7.5(b)), and CASA approval as an alternative marking method for identification plates under regulation 45.135 of CASR 1998 (s 11.1(b)(ii)). The Director of Aviation Safety made this MOS provisionally under regulation 45.025 (preamble).

Special categories and the Defence Force: Aircraft in special certification categories (limited, restricted, provisional, experimental) must display specific words near each entrance and cockpit with set minimum size and contrast (s 10). Defence Force aircraft are carved out from the prohibition on designs that modify or obscure sets of markings (s 7.5(a)).

Third parties and international operators: Providers of aircraft livery paintwork, decal manufacturers and operators conducting international flights must account for additional marking requirements for aircraft operating outside Australia (Table 1, item 4). The MOS’s cross‑reference to Part 21 certification categories means that airworthiness and certification staff must coordinate marking compliance with the certification status of the aircraft.

Who pays and who decides: Practically, owners and operators pay the direct costs of marking work, replacements and potential downtime. CASA decides on approvals and interprets when deviations require approval under the referenced regulatory framework (s 7.5(b), s 11.1(b)(ii)). The instrument allocates technical obligations to the regulated community and procedural discretion to CASA.

The MOS imposes precise duties on aircraft owners, operators and maintainers as technical prescriptions to give effect to regulatory obligations in CASR 1998. It also sets out limited rights to obtain CASA approvals and exceptions.

Duties to display markings: Aircraft must display the prescribed number and location of sets of nationality and registration marks depending on aircraft type and certification status (s 6.1-6.4, Table 1, Table 2). For example, fixed‑wing aircraft generally must have one set on each side of the fuselage between specific reference points (Table 1, item 1). Rotorcraft must have one set on each side of the cabin, fuselage, boom or tail (s 6.2.1). Airships and balloons have distinct location rules (s 6.3, s 6.4). Where required, an additional set is to be displayed on the lower wing surface with alignment and equidistance conditions (Table 1, item 4).

Duties on appearance and legibility: Sets must be securely applied, legible, not obscured by moveable surfaces, and of contrasting colour to the background (s 7.1). Secure application is defined as painting or attachment at least as permanent as painting (s 7.2). Legibility is objectively tied to character height and specified viewing distances (s 7.3 and Table 3). Designers must ensure proportions of characters (stroke thickness 1/6 of height; width 2/3 of height; hyphen 2/3 of height; spacing at least 1/6 of height) and consistent height for letters and numbers (s 8.1).

Duties on character formation and arrangement: Characters must be unornamented Roman capital letters and Arabic numbers (s 8.1(a)-(b)). Vertical stacking is limited to airship centre‑lines, balloons and rotorcraft (s 8.2). Slanted characters are permitted up to 35 degrees from the perpendicular if legible (s 8.3). Minimum character heights depend on aircraft type and maximum take‑off weight, with distinct thresholds for wing versus vertical surfaces (s 9.1 and Table 4). Where the surface is too small, characters must be as large as possible while maintaining proportions (s 9.2). Symmetrical parts must have identical character heights (s 9.3).

Duties for certain certificated aircraft: The MOS mandates words to be conspicuously displayed near each entrance for limited, restricted, provisional and experimental aircraft classes (s 10). These words must be in capital letters, no less than 50 mm high, and of a contrasting colour (s 10(b)-(c)).

Identification plate duties: The registration identification plate must be made of steel or an equivalent fire‑resistant material, be marked by etching, stamping, engraving or a CASA‑approved method, and be affixed near the entrance where visible and accessible (s 11.1(a)-(c)). For manned free balloons the plate must be on the envelope and visible before inflation (s 11.1(d)). Accessibility is defined such that the plate must be revealable without tools or removing aircraft components when covered or enclosed (s 11.3).

Rights to approvals and exceptions: The MOS recognises two limited rights to deviate from the banned modification or obscuration of markings. Defence Force symbols and call signs on Defence Force aircraft are excepted from the prohibition on modifying or obscuring markings (s 7.5(a)). CASA may give written approval for a design, mark or symbol that would otherwise obstruct or confuse markings (s 7.5(b)). CASA also may approve an alternate method of marking the identification plate under regulation 45.135 of CASR 1998 (s 11.1(b)(ii)). These approvals are the procedural avenue for industry to seek departures from the MOS’s prescriptive technical requirements.

Scope and interplay with CASR 1998: The MOS repeatedly ties its definitions and application to Part 45 and Part 21 of CASR 1998 (s 4.2-4.3, multiple cross references). Therefore compliance with these duties is framed as part of meeting regulatory obligations under CASR 1998. The MOS does not itself include enforcement penalties; it provides technical standards that work with the broader CASR regulatory framework (preamble and s 5.1).

The MOS is a Manual of Standards that prescribes technical specifications; the instrument text as provided sets technical duties but does not itself state penalties or enforcement procedures. The MOS is made under regulation 45.025 of CASR 1998 and is intended to give effect to the regulatory requirements in Part 45 of CASR 1998 (preamble and s 5.1). Enforcement and sanctions for non‑compliance with marking and identification obligations are matters for the CASR 1998 regulatory regime rather than for the MOS text itself.

What the MOS contains: The instrument establishes mandatory‑sounding technical requirements (e.g., the number and location of sets, character heights and proportions, plate materials and placement) and gives CASA the role of approving exceptions in specified circumstances (s 7.5(b), s 11.1(b)(ii)). It therefore creates objective standards by reference that enforcement authorities can apply. The MOS also defines concepts like "securely applied", "legible", and "accessible" for the purpose of compliance measurement (s 7.2, s 7.3, s 11.3).

What the MOS does not contain: The MOS text provided does not specify offences, fines, suspension powers, or procedural enforcement steps. There is no schedule of penalties or explicit administrative enforcement mechanisms within the instrument itself. The MOS does not set out inspection, rectification notice or removal procedures.

Where to find penalties and enforcement mechanisms: Since the MOS is made under CASR 1998, those seeking the enforcement regime, offence provisions, and penalty schedules must look to CASR 1998 and the enabling Act for the full compliance and enforcement framework. The MOS references regulation numbers in CASR 1998 (for example, regulations 45.025, 45.090, 45.125 and 45.135 are expressly referenced in the MOS: see the preamble and ss 5.1, 10, 11.1(b), 11.1(b)(ii)). Any legal consequences for failing to meet the MOS standards will therefore be framed and executed under the relevant provisions of CASR 1998 and any enforcement or administrative powers in the enabling statute, not in the MOS text provided.

Role of CASA approvals in enforcement: The MOS creates discretionary approval points where CASA's written approval alters the strict application of a prohibition (s 7.5(b)) or approves alternate marking methods for identification plates (s 11.1(b)(ii)). Those approvals function as compliance‑authorising acts. Failure to obtain required approvals where the MOS requires them may be treated as non‑compliance under the regulations incorporated by reference. The MOS’s definitions of "legible", "securely applied" and "accessible" provide objective criteria that inspectors or assessors can apply when determining conformity.

Practical implication for regulated entities: Aircraft owners and operators should treat the MOS as providing mandatory technical standards to which they must conform for marking and identification. While the MOS itself does not list penalties, it establishes the technical baseline against which compliance will be judged under CASR 1998 and subject to CASA oversight and discretion as provided by the regulatory framework. Entities planning deviations should obtain CASA written approvals where the MOS allows them (s 7.5(b); s 11.1(b)(ii)) to avoid enforcement exposure.

The MOS is explicitly framed as the technical standard to give effect to provisions in Part 45 of CASR 1998 and interacts with various parts of CASR 1998, particularly Part 21 certification provisions. The instrument contains recurrent cross‑references and conditional applications tied to the regulatory categories and certification statuses defined in CASR 1998 (s 4.2-4.3 and throughout Chapters 2-4).

Direct cross‑references: The MOS is made under regulation 45.025 of CASR 1998 (preamble and s 5.1). It explicitly implements the requirements referred to in regulation 45.090 (display of words on certain aircraft) at s 10 and implements plate requirements referenced in subregulation 45.125(2) at s 11.1. The identification plate marking method may alternatively be done by a method approved under regulation 45.135 of CASR 1998 (s 11.1(b)(ii)). Those direct references show the MOS is not freestanding: it is the detailed standard called up by the Regulations.

Interaction with Part 21 certification: The MOS conditions marking requirements on the certification category and eligibility under certain Part 21 provisions. For example, some marking requirements for aircraft operating inside Australia differ where an aircraft is certificated under regulation 21.189 or paragraph 21.191(d) or (e), or is eligible under subparagraph 21.189(1)(a)(ii) (s 6.1.1 and Table 2). Table 4 explicitly references aircraft certificated under regulations 21.184, 21.186, 21.190 and various paragraphs of 21.191, as well as the class of aircraft under paragraph 21.189 (s 9.1 items 7 and 8 and associated notes). In practice, the MOS ties marking technical obligations to an aircraft’s legal certification status and thereby links marking compliance to airworthiness certification and operating permissions governed by Part 21.

Defence Force carve‑out: The MOS allows Defence Force symbols or call signs on Defence Force aircraft to be exceptions to the prohibition on designs modifying, obscuring or creating confusion about registration marks (s 7.5(a)). This provision shows an interaction point between the MOS marking standards and Defence Force operational markings as a separate legal regime.

CASA approvals and regulatory discretion: The MOS provides that designs may be approved in writing by CASA (s 7.5(b)) and that alternative marking methods for identification plates may be approved under regulation 45.135 (s 11.1(b)(ii)). These clauses connect the MOS’s prescriptive technical content with CASA’s approval powers under CASR 1998, giving CASA a gatekeeper role where deviations are sought.

Relationship to enforcement and other regulatory obligations: While the MOS sets technical standards, enforcement, offences and penalties are determined under CASR 1998 and the enabling legislation. The MOS therefore supplements the Regulations by supplying objective technical criteria that enforcement personnel will apply. Compliance with the MOS will be relevant to certification, continuing airworthiness, and operations, given the cross‑referencing to Part 21 and to specific regulatory provisions for special categories of aircraft.

Practical regulatory sequencing: The MOS’s requirements can affect certification processes managed under Part 21. For example, classification as a limited, restricted, provisional or experimental aircraft triggers entrance‑word display obligations (s 10) and minimum character heights differ for aircraft certificated under particular Part 21 provisions (Table 4 and notes). Consequently, certification decisions on category can alter practical marking obligations, and marking design must be coordinated with certification and airworthiness planning.

No broader statutory conflicts or overrides in the text: The instrument does not attempt to override or amend CASR 1998; it is expressly made under regulation 45.025 to prescribe requirements. Any conflict between the MOS and the Regulations would require resolution within the legal framework governing delegated instruments and the enabling Act. The MOS also includes a definition clause that references Part 45 for meaning of words and phrases (s 4.3), limiting interpretive divergence.

The MOS itself contains minimal amendment history in the text provided. The preamble and initial sections state that the instrument is the Part 45 Manual of Standards Instrument 2016 and that it commences on 4 July 2016, the date on which amendments to remake Part 45 of CASR 1998 are given effect (preamble and s 2). The instrument is signed by Mark Alan Skidmore, Director of Aviation Safety, dated 23 May 2016 (preamble). Beyond these items, the instrument text supplied does not list subsequent amendments, revocations or replacement instruments.

What the text records and implies: The MOS is presented as a remake of Part 45 MOS, implying that there was a prior MOS for Part 45 that this instrument replaces on commencement (preamble and s 2). It does not contain a schedule of amendments or transitional provisions in the text provided. The formal signature line and date identify the issuing authority and the date the instrument was signed, which is relevant for legal provenance (preamble and s 1).

What to check for later amendments: Because the MOS is a delegated instrument implementing CASR 1998 obligations, it is normal for Manuals of Standards to be updated from time to time by CASA to reflect regulatory amendments, operational experience, or international standards changes. The text provided does not record any subsequent variation or revocation, so practitioners should check the current consolidated MOS and the CASA legislative instruments register for any later amending instruments, revision notes, or replacement instruments after 4 July 2016.

Cross‑reference to CASR 1998 amendment cycles: The MOS expressly ties its commencement to amendments to remake Part 45 of CASR 1998 (s 2). That suggests the MOS and the underlying Part 45 regulations may be updated in tandem. To understand amendment sequencing and whether any subsequent regulatory changes affect the MOS, consult the amendment history of CASR 1998 Part 45 and any later MOS instruments on CASA’s website or the Federal Register of Legislation.

Practical implications of the absence of amendment listings in the MOS text: Because the MOS text provided does not include later amendment entries, users relying on these standards should establish a practice of checking the Federal Register of Legislation, CASA’s legislative instruments page or consolidated versions of the MOS to ensure they are working from the current instrument. The instrument itself is dated 2016 and is the authoritative standard as of commencement; any post‑2016 changes will not appear in the printed 2016 text.

The MOS text provided does not contain or reference any litigation history. It is a technical Manual of Standards and does not record judicial interpretations, cases, or legal challenges. The instrument names no court decisions and includes no appendix of authorities.

Implication for practitioners: Because the instrument does not contain litigation history, there is no direct source evidence within the MOS about how courts or tribunals have interpreted its provisions. Practitioners seeking to understand legal disputes involving marking requirements, the application of CASA approvals under s 7.5(b) or s 11.1(b)(ii), or the enforcement of Part 45 obligations must search case law databases, tribunal decisions, or published administrative determinations external to the MOS. The MOS itself cannot be taken as evidence that any given wording has been litigated.

What can be inferred about dispute potential from the MOS: The MOS creates objective and measurable technical standards, for example definitive character proportions, legibility distances and plate accessibility criteria (ss 7-9, 11.3). Objective standards reduce some interpretive uncertainty. However, areas that invite discretionary decision‑making by CASA,such as written approvals for designs that would otherwise modify or obscure markings (s 7.5(b)) and approval of alternate plate‑marking methods (s 11.1(b)(ii)),are plausible focal points for dispute about the scope of CASA’s discretion, the form and conditions of approvals, and whether an approval is required. Such disputes would arise under the caselaw governing administrative discretion and the statutory framework of CASR 1998 rather than within the MOS text.

Recommended steps prior to litigation: Because the MOS leaves approvals and enforcement to CASA and the regulatory framework, entities considering a challenge should compile the MOS text referred to (including commencement and cross‑references), any CASA written approvals or refusals, and any enforcement notices issued under CASR 1998. The MOS’s detailed, technical wording will be central to any dispute about compliance and whether an aircraft’s markings meet the objective standards set in the MOS (for example, legibility distances in Table 3, character proportions in s 8.1, and plate accessibility in s 11.3).

The MOS is detailed and prescriptive. A number of practical "gotchas" arise from its precise phrasing and cross‑references that can create compliance traps if not anticipated.

1. Certification status changes marking obligations: Several marking requirements differ depending on whether an aircraft is certificated under particular Part 21 provisions or eligible for a Part 21 certification pathway (s 6.1.1 and Table 2, and Table 4 notes). If an aircraft’s certification status changes during its lifecycle, the marking requirements can change. Owners must confirm the applicable Part 21 category because the MOS makes minimum character heights and the number/location of sets contingent on certification categories (Table 4 items 7 and 8, Table 2).

2. Operating outside Australia imposes extra requirements: Table 1 item 4 requires a set on the left half of the lower surface of the wing structure, or equivalent, for fixed‑wing aircraft operating outside Australia or with MTOW greater than 5 700 kg. The MOS specifies alignment, parallelism to the leading edge and equidistance from leading and trailing edges (Table 1, item 4). This can be challenging for aircraft with non‑standard wing shapes or small lower surface areas.

3. Surface size and proportion requirements: Where a surface is too small to allow characters to be as high as required by Table 4, subsection 9.2 requires characters to be as high as possible while keeping proportions. This is a qualitative balancing act that requires judgement about "as high as possible" and proportion maintenance,practically, manufacturers and owners must document the constraint and how they preserved proportions to support compliance.

4. Legibility distances are precise and daylight‑only: s 7.3 ties legibility to specific distances in Table 3 and requires legibility "in daylight hours" without optical aid. Markings adequate under artificial or low light may still fail to meet the MOS. The specified distances (for example, 500 mm height legible at 150 m; 50 mm at 15 m) are strict and must be demonstrably met.

5. Character geometry rules affect livery and design: The width, stroke thickness and spacing rules (s 8.1(e)-(g) and (h)) are precise and will interact with aircraft livery and branding. Operators that overlay logos near registration marks must ensure characters remain unornamented and visually distinct. A decorative font or stylised number that departs from the proportions may be non‑compliant even if readable.

6. No allowance for temporary coverage by some equipment: s 7.1(c) forbids markings being obscured by a movable surface. If the aircraft has removable panels or external equipment that might obscure marks in some configurations, owners must ensure required marks remain unobscured in normal configurations or secure CASA approval.

7. CASA approval requirement and evidence: The MOS allows CASA written approval to authorise designs that might otherwise obscure or confuse markings (s 7.5(b)). In practice, absence of documented approval creates exposure. Operators should obtain written CASA approvals in advance and retain documentary evidence.

8. Identification plate accessibility standard: s 11.3 permits plates to be internal or external but requires that if covered, they be revealed without tools or removing aircraft components. Embedding plates behind panels requiring screw removal would fail the accessibility test. For manned free balloons the plate must be visible before inflation (s 11.1(d)), a timing and placement constraint that might be overlooked during manufacture.

9. Defence Force exceptions narrowly drawn: The MOS excepts Defence Force symbols or call signs on Defence Force aircraft from the general prohibition on modifications to markings (s 7.5(a)). That carve‑out is limited to Defence Force aircraft and does not give commercial operators latitude to adopt similar solutions without CASA approval.

10. Vertical stacking limitations: Vertical stacking of characters is allowed only on airship centre‑line, balloons and rotorcraft (s 8.2). Operators of other types must use horizontal arrangements or seek approvals where physical constraints might suggest stacking.

11. Slant angle cap: Characters may be slanted up to 35 degrees from the perpendicular (s 8.3). Livery designs that use larger slants or italicised fonts risk non‑compliance.

12. Multiple prescriptive rules multiply complexity: The MOS’s many interlocking technical rules mean compliance requires a systems approach,each marking must satisfy location, material or attachment, character geometry, legibility distance, contrast, and non‑obscuration simultaneously. Failure in any one dimension risks non‑conformity.

Mitigation steps: Owners and operators should cross‑check aircraft against the MOS checklists in Chapters 2-4, document constraints where Table 4 size requirements cannot be met and obtain CASA written approvals where the MOS permits (s 7.5(b), s 11.1(b)(ii)). Where marking design is integrated with livery, liaise with manufacturers early and ensure maintenance manuals include marking requirements to avoid regression on repaint or repair. Keep records demonstrating legibility distances and character sizes as part of the aircraft’s records.

Complying with the MOS is a matter of translating the instrument’s technical prescriptions into factory or maintenance‑shop practices, documentation and, where necessary, CASA approvals. Below is a practical compliance checklist with steps tied to the MOS provisions.

1. Establish the aircraft’s legal category and status. Record the aircraft’s certification under Part 21 provisions referenced in the MOS because Table 2, Table 4 and s 6.1.1 link marking obligations to certification status (s 4.2-4.3, s 6.1.1, Table 4 notes). Ensure records show whether the aircraft is certificated under 21.189, 21.191(d)-(e), or eligible under 21.189(1)(a)(ii), and whether it is limited, restricted, provisional or experimental.

2. Determine the required number and locations of sets. Use Table 1 for fixed‑wing aircraft operating outside Australia or with MTOW thresholds and Table 2 for fixed‑wing aircraft operating inside Australia that meet specified certification criteria (ss 6.1.2-6.1.4, Tables 1-2). For rotorcraft, airships and balloons apply the specific subsections (ss 6.2-6.4). Document precise panel locations and reference points (for example, between trailing edge of wing and leading edge of tail).

3. Verify marking surface sizes. Measure vertical and wing surface areas where marks will be placed and compare against minimum character heights in Table 4 (s 9.1). If a surface cannot accommodate required heights, prepare a documented rationale showing characters as large as possible and that proportions are maintained, per s 9.2.

4. Specify character geometry and construction. Adopt character dimensions consistent with s 8.1: height per Table 4 and s 9.1; width 2/3 of height except "I" and "1" (s 8.1(e)); stroke thickness 1/6 of height (s 8.1(g)); spacing at least 1/6 of height (s 8.1(h)); hyphen length 2/3 of height (s 8.1(f)). Use unornamented Roman capitals and Arabic numerals (s 8.1(a)-(b)). Where slanted characters are used ensure slant does not exceed 35 degrees and remains legible (s 8.3).

5. Ensure legibility requirements are met. Verify that characters meet the legibility distances in Table 3 for the chosen character heights (s 7.3 and Table 3). Test in daylight conditions without optical aid and retain photographic or measurement evidence where appropriate.

6. Secure application and permanence. Apply markings by painting or affixing materials that are at least as permanent as painting (s 7.2). For identification plates, use steel or an alternative material that withstands fire at least as well as steel (s 11.1(a)). Mark plates by etching, stamping, engraving or a CASA‑approved method (s 11.1(b)).

7. Position and protect marking visibility. Ensure marks are not obscured by moveable surfaces (s 7.1(c)), are of a colour contrasting with the background (s 7.1(d)), and are placed so that symmetry requirements are met for paired sets (s 9.3). For fixed‑wing lower wing markings, align sets as nearly as possible parallel to the leading edge and equidistant from leading and trailing edges, with the top of the set toward the leading edge (Table 1, item 4).

8. Display entrance words where required. For limited, restricted, provisional and experimental aircraft, display the relevant word near each cabin and cockpit entrance (s 10(a)). The word must be in capital letters, no less than 50 mm high, and contrast with the background (s 10(b)-(c)).

9. Fit identification plate correctly. Attach the registration identification plate near the aircraft entrance where it is clearly visible and accessible (s 11.1(c)). For manned free balloons place the plate on the envelope where clearly visible before inflation (s 11.1(d)). If the plate is covered or enclosed, ensure it can be revealed without tools or removing aircraft components (s 11.3).

10. Plan for maintenance and repainting. Incorporate marking requirements into maintenance schedules and repainting specifications. Ensure that repaint processes preserve character dimensions, spacing and contrast. Where maintenance might temporarily affect legibility, schedule it to avoid periods where markings are required for operations.

11. Document and retain approvals. If a marking or design would modify, obscure or create potential confusion, obtain written CASA approval under s 7.5(b) before applying the design. For alternative plate marking methods, obtain the CASA approval provided for under regulation 45.135 and referenced in s 11.1(b)(ii). Keep approvals with the aircraft records to evidence compliance.

12. Audit and evidence compliance. Periodically inspect aircraft against the MOS checklist. Maintain measurement records, photographs showing legibility distances, production specifications for plate manufacture and any CASA approvals. Where an aircraft operates outside Australia, verify that additional lower‑wing marking requirements are present and correctly aligned (Table 1, item 4).

13. Coordinate with certification and operations teams. Because marking obligations interact with Part 21 certification categories, coordinate marking decisions with the certificate holder and with any modifications or changes of category. Update records when certification status changes.

14. Seek regulatory guidance where uncertain. If an objective application of the MOS is unclear, engage CASA for interpretive guidance or to seek written approval where the MOS contemplates deviations (s 7.5(b), s 11.1(b)(ii)). Where compliance would require structural alterations, consult an appropriately authorised maintenance organisation and document the technical justification for any alternative approach.

Concrete examples of likely tasks: preparing a fixed‑wing commuter aircraft for international operations will typically require adding a lower wing marking set meeting Table 1, item 4 alignment rules and checking wing lower surface area for the required 500 mm wing character height if MTOW >5 700 kg (Table 4, items 1-2 and Table 1, item 4). Repainting a limited category aircraft must ensure the entrance word "LIMITED" is in capitals at least 50 mm high and remains clearly visible at each cabin and cockpit entrance (s 10).

By following the checklist and keeping documentary evidence of measurements, material choices, marking permanence and CASA approvals where required, operators and owners can convert the MOS’s prescriptive technical standards into verifiable compliance practices.