7.3 Certification Specifications (CS-23/25/27/29/E) & Airworthiness Documentation

7.3 Certification Specifications (CS-23/25/27/29/E) & Airworthiness Documentation

Quick Answer: Certification Specifications (CS) are non-binding "Soft Law" technical standards issued by EASA under Article 76 of Regulation (EU) 2018/1139. When an applicant applies for a Type Certificate, EASA designates the applicable CS codes as the product's "Certification Basis," making compliance legally mandatory for that aircraft type. Key codes include CS-23 (normal aeroplanes ≤ 19 seats / ≤ 8 618 kg MTOM), CS-25 (large transport aeroplanes > 5 700 kg), CS-27 (small rotorcraft ≤ 3 175 kg / ≤ 9 seats), and CS-29 (large rotorcraft). Statutory airworthiness documents include the Certificate of Airworthiness (CofA - EASA Form 25, unlimited validity conditioned on an ARC), Restricted CofA (Form 24), and Permit to Fly (Form 20a/20b under Subpart P for non-conforming aircraft capable of safe flight under approved flight conditions).

Every civil aircraft operating in European airspace is the physical manifestation of thousands of precise engineering rules. Under the European regulatory architecture, these engineering rules are codified in Certification Specifications (CS). Furthermore, once an individual airframe is manufactured, it must be equipped with statutory documentation certifying its airworthiness, registration, and environmental conformity.

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The Legal Nature of Certification Specifications (EASA Soft Law)

Under Article 76 of Regulation (EU) 2018/1139 (the EASA Basic Regulation), the Executive Director of EASA is empowered to issue Certification Specifications (CS), Acceptable Means of Compliance (AMC), and Guidance Material (GM).

• Soft Law Status: In European jurisprudence, Certification Specifications are technically "Soft Law." They are non-binding advisory standards rather than legislative acts passed by the European Parliament or Commission;
• The Binding Mechanism: While a CS is non-binding in isolation, it becomes legally binding upon a specific aircraft model when EASA formally incorporates it into the Certification Basis of the Type Certificate (TC) under Part-21.A.17A. Once frozen into the certification basis, the applicant must demonstrate 100% compliance with those specifications to obtain and maintain the TC;
• Special Conditions & Equivalent Safety Findings: If an aircraft features innovative technology not contemplated when the CS was drafted (e.g. sidestick fly-by-wire controls, distributed electric propulsion, carbon-fiber passenger cabins), EASA issues Special Conditions (SC) pursuant to 21.A.16B. Conversely, where literal compliance is impossible but compensating factors maintain safety, EASA accepts an Equivalent Safety Finding (ESF).

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Core EASA Certification Codes: CS-23, CS-25, CS-27, and CS-29

Module 10 examinations require precise familiarity with the primary aircraft certification specifications:

+-----------------------------------------------------------------------------------+
|                         EASA CERTIFICATION SPECIFICATIONS                         |
+-----------------------------------------------------------------------------------+
| CS-23: Normal-Category Aeroplanes       CS-25: Large Aeroplanes                   |
| - Up to 19 passenger seats              - Transport category jetliners & turboprops|
| - MTOM ≤ 8 618 kg (19 000 lbs)          - Multi-engine aeroplanes > 5 700 kg MTOM |
| - Performance-based safety levels 1 to 4- Airbus A320/A350, Boeing 737/777/787    |
|                                                                                   |
| CS-27: Small Rotorcraft                 CS-29: Large Rotorcraft                   |
| - MTOM ≤ 3 175 kg (7 000 lbs)           - MTOM > 3 175 kg OR multi-engine         |
| - Certified for ≤ 9 passenger seats     - Transport Category A and B rotorcraft   |
| - Robinson R44/R66, Airbus H125/H130    - Sikorsky S-92, Airbus H175/H225         |
|                                                                                   |
| COMPONENT CODES:                                                                  |
| CS-E (Engines) | CS-P (Propellers) | CS-APU (Auxiliary Power Units) | CS-ETSO     |
+-----------------------------------------------------------------------------------+

1. CS-23: Normal-Category Aeroplanes

Following the landmark Amendment 5 modernization (harmonized globally with FAA FAR Part 23 Amendment 64), CS-23 transitioned from rigid prescriptive rules to objective, performance-based standards:

• Scope: Covers aeroplanes with a passenger seating configuration of 19 or fewer and a Maximum Take-Off Mass (MTOM) of up to 8 618 kg (19 000 lbs);
• Certification Levels: Divided into four safety tiers based on risk exposure and passenger capacity:
  • Level 1: 0 to 1 passengers;
  • Level 2: 2 to 6 passengers;
  • Level 3: 7 to 9 passengers;
  • Level 4: 10 to 19 passengers;
• Speed Categories: Classified as Low Speed (maximum operating airspeed V_{NO}/V_{MO} ≤ 250 knots CAS or M_{MO} ≤ 0.6) or High Speed (V_{NO}/V_{MO} > 250 knots CAS or M_{MO} > 0.6).

2. CS-25: Large Aeroplanes

CS-25 governs large transport aeroplanes (multi-engine turbine-powered aircraft with an MTOM exceeding 5 700 kg or seating configurations greater than 19 passengers):

• Represents the most rigorous structural and systems airworthiness standard in aviation;
• Mandates comprehensive Damage Tolerance analyses for airframe structures (ensuring cracks are detectable before structural failure);
• Requires multi-redundant system architectures (fail-safe principles) where no single failure can prevent continued safe flight and landing;
• Enforces strict cabin flammability criteria (heat release and toxic gas limits) and the famous 90-second emergency evacuation rule using only half the available exits.

3. CS-27: Small Rotorcraft

CS-27 specifies airworthiness standards for small helicopters and gyrocopters:

• Mass and Seating Limits: Maximum Take-Off Mass ≤ 3 175 kg (7 000 lbs) and passenger seating configuration of 9 or fewer;
• Covers both single-engine and light twin-engine rotorcraft (e.g. Bell 206/407, Robinson R44, Airbus H125/H135).

4. CS-29: Large Rotorcraft

CS-29 governs large, multi-engine transport rotorcraft:

• Mass and Seating Limits: MTOM > 3 175 kg or rotorcraft certified for multi-engine transport categories;
• Divided into Category A (rigorous engine isolation and multi-engine performance ensuring safe fly-away or rejected landing following an engine failure at any flight phase) and Category B (single-engine or light multi-engine rotorcraft with non-guaranteed engine-out climb capability).

5. Component Certification Codes

• CS-E (Engines): Standards for piston and turbine aircraft engines, addressing bird strikes, blade containment, turbine burst protection, and icing;
• CS-P (Propellers): Structural integrity, feathering mechanisms, overspeed protection, and blade pitch control;
• CS-APU: Auxiliary power units providing electrical and pneumatic power;
• CS-ETSO: European Technical Standard Orders defining minimum performance standards for specified parts and appliances (e.g. flight data recorders, seat belts, emergency locator transmitters).

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Statutory Airworthiness Documentation

Every individual aircraft must carry specific airworthiness documents issued under Part-21 or international law before taking flight:

1. Certificate of Airworthiness (CofA — EASA Form 25)

Issued under Part-21 Subpart H by the competent authority of the Member State of registry. (21.A.174 is the application rule within Subpart H, so avoid quoting it as the issuing rule; 21.A.173 is the classification of standard versus restricted certificates.)

• Prerequisites: Proof of conformity to the approved Type Certificate (demonstrated via EASA Form 52 for new aircraft or an Export Certificate of Airworthiness for imported aircraft) and verification of airworthy condition;
• Indefinite Validity: Under European regulations, the standard CofA has an unlimited duration;
• The ARC Prerequisite: While the CofA itself does not expire, it is legally invalid unless accompanied by a currently valid Airworthiness Review Certificate (ARC — EASA Form 15a, 15b, or 15c) issued under Part-M or Part-ML. If the ARC expires, the CofA becomes dormant, and the aircraft cannot fly.

2. Restricted Certificate of Airworthiness (Restricted CofA — EASA Form 24)

Issued under Part-21 Subpart H — classified as a restricted certificate of airworthiness by 21.A.173(b) — by the State of registry for aircraft conforming to a Restricted Type Certificate (RTC) or to specific airworthiness specifications ensuring adequate safety. It explicitly restricts operational privileges (such as prohibiting commercial air transport of passengers).

3. Permit to Fly (EASA Form 20a / Form 20b — Subpart P)

A Permit to Fly is issued under Part-21 Subpart P (21.A.701) when an aircraft does not meet, or has not been shown to meet, applicable airworthiness requirements, but is capable of safe flight under defined operating conditions. Typical applications include:

• Development flight testing of new prototypes or major modifications before TC/STC approval;
• Ferry flights of an aircraft with an expired ARC, overdue maintenance, or minor structural damage to a maintenance facility;
• Aircraft flight testing following extensive base maintenance checks;
• Customer demonstration flights prior to formal commercial delivery.

+-----------------------------------------------------------------------------------+
|                         PERMIT TO FLY ISSUANCE ARCHITECTURE                       |
+-----------------------------------------------------------------------------------+
| STEP 1: APPROVE FLIGHT CONDITIONS (21.A.708)                                      |
| - Defines flight envelope, crew qualifications, deactivated systems, airspace.    |
| - Approved by: EASA (mandatory if flight safety of design is affected); OR        |
|                Approved DOA under privilege 21.A.263(c)(6).                       |
|                                                                                   |
| STEP 2: ISSUE PERMIT TO FLY                                                       |
| - EASA Form 20a: Issued by the Competent Authority (National Aviation Authority). |
| - EASA Form 20b: Issued by an approved DOA (21.A.263(c)(7)) or an approved        |
|                  Part-CAMO / Part-CAO organisation with specific privileges.      |
+-----------------------------------------------------------------------------------+

4. Noise Certificate (EASA Form 45)

Issued under Part-21 Subpart I by the State of registry (21.A.204 is the application rule within Subpart I). It certifies that the individual aircraft complies with the noise standards established in ICAO Annex 16, Volume I (e.g. Chapter 3, 4, or 14 noise certification levels). Mandatory for all motor-powered civil aircraft operating in Europe.

5. Certificate of Registration (CofR)

Issued by the National Aviation Authority of the State of Registry pursuant to Articles 17 to 21 of the Chicago Convention and national legislation. It establishes:

• The nationality mark and registration mark assigned to the aircraft (e.g. F-GZCP, D-AIBA);
• The serial number, manufacturer, and registered owner;
• Prohibits dual nationality (an aircraft cannot be legally registered in more than one state simultaneously).

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Comparison of European Airworthiness & Operating Certificates

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Practical Maintenance Scenario: Ferry Flight Under a Permit to Fly

Operational Context

A Boeing 737-800 operated by an EU carrier sustains minor ground equipment damage to a trailing edge flap fairing at an outstation where no sheet-metal repair capability exists. The structural damage prevents the issuance of a standard Certificate of Release to Service (CRS), and the aircraft cannot fly commercially under its standard Certificate of Airworthiness.

Regulatory Steps to Execute Ferry Flight under Subpart P:

1. Engineering Evaluation: The operator's continuing airworthiness organisation (CAMO) coordinates with an approved Part-21 DOA (the aircraft manufacturer or airline DOA).
2. Flight Conditions Approval (21.A.708): The DOA establishes Flight Conditions: the damaged flap fairing is temporarily removed, flight is restricted to non-commercial operations, airspeed is capped at 250 knots CAS, flight into known icing is prohibited, and flight crew is limited to essential flight crew only (two pilots, no passengers). The DOA approves the flight conditions under privilege 21.A.263(c)(6).
3. Permit to Fly Issuance: Under privilege CAMO.A.125, the operator's approved Part-CAMO issues EASA Form 20b (Permit to Fly) referencing the approved flight conditions.
4. Release to Service: A Part-145 licensed engineer performs the fairing removal, completes the required pre-flight servicing, and signs a Maintenance Release / CRS referencing the Permit to Fly and approved flight conditions.
5. Flight Execution: The flight crew safely ferries the aircraft to the airline's main maintenance base for permanent structural repair.

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EASA Module 10 Examination Tips & Regulatory Traps

• CS-23 Thresholds: Memorize the modern CS-23 scope: ≤ 19 passenger seats and ≤ 8 618 kg (19 000 lbs) MTOM.
• CS-27 vs CS-29 Dividing Line: The dividing line for rotorcraft is exactly 3 175 kg (7 000 lbs) MTOM and 9 passenger seats. Helicopters above 3 175 kg fall under CS-29.
• CofA Validity Trap: A common exam question asks: "What is the validity of an EASA Certificate of Airworthiness (Form 25)?" The correct answer is Unlimited duration, provided the aircraft is maintained in accordance with rules and holds a valid Airworthiness Review Certificate (ARC). Do NOT choose 12 months—12 months is the validity of the ARC!
• Permit to Fly Issuers: Remember that an Authority issues Form 20a, while an approved DOA, CAMO, or Part-CAO issues Form 20b.
• Flight Conditions Approval: A Permit to Fly can never be issued without approved Flight Conditions under 21.A.708.