CS-23 twin engine requirements are defined within the EASA Certification Specifications for small aeroplanes and represent a fundamental shift in how twin-engine aircraft are designed, certified, and ultimately operated. The modern interpretation of CS-23 twin engine requirements moves away from rigid, prescriptive design rules and instead focuses on demonstrable safety outcomes, particularly in asymmetric and one-engine-inoperative conditions.

The latest amendment to CS-23 is widely regarded as one of the most significant changes in light aircraft certification in recent decades. For manufacturers, pilots, and regulators alike, CS-23 twin engine requirements fundamentally reshape how performance, controllability, and system reliability are evaluated in twin-engine aeroplanes operating close to their aerodynamic and performance limits.

Rather than prescribing exactly how an aircraft must be built, the modern CS-23 framework defines what safety objectives must be achieved. Under this objectives-based approach, CS-23 twin engine requirements emphasise real-world safety performance over legacy design assumptions. This certification philosophy has far-reaching implications for twin-engine aircraft, particularly in critical areas such as one-engine-inoperative (OEI) handling, minimum control speed (VMC), asymmetric thrust management, and system redundancy.

For twin-engine aeroplanes, CS-23 twin engine requirements are no longer satisfied by simple compliance with historical design formulas. Instead, manufacturers must demonstrate that the aircraft remains controllable, predictable, and operationally safe following engine failure across the certified envelope. This shift aligns certification more closely with the realities faced by pilots during high-workload phases such as take-off, initial climb, and abnormal operations.

From Prescriptive Rules to Design Objectives

Earlier versions of CS-23 relied heavily on prescriptive, technology-bound rules. Certification requirements often specified exact design features, test configurations, and assumed technologies based on conventional piston aircraft. While this approach worked for legacy designs, it gradually became a barrier as propulsion systems, avionics, automation, and safety concepts evolved.

The amended CS-23 deliberately moves away from this rigidity. Instead of mandating specific technical solutions, it establishes high-level safety and performance objectives. Manufacturers are no longer told how to design an aircraft, but rather what level of safety and controllability must be demonstrated.

For twin-engine aircraft, this transition is particularly important. Asymmetric thrust, OEI controllability, and marginal climb performance cannot be safely addressed through simplistic prescriptive rules. They require a certification framework capable of evaluating real operational behaviour, not just compliance with legacy assumptions.

The Role of AMC: Separating Objectives from Implementation

A cornerstone of the modern CS-23 structure is the clear separation between regulatory objectives and technical implementation, a distinction that directly shapes CS-23 twin engine requirements in practice.

Design-specific guidance has been removed from the core CS-23 regulation and relocated into the Acceptable Means of Compliance (AMC). CS-23 defines the safety intent, while AMC describes one acceptable way of meeting that intent.

This distinction is particularly critical for twin-engine aircraft certification. AMC material can be updated far more rapidly than primary regulations, allowing certification guidance to evolve alongside advances in propulsion control, autofeather systems, digital engine management, and flight envelope protection. Importantly, AMC are not mandatory rules—they are flexible compliance pathways that manufacturers may follow or replace with alternative solutions, provided an equivalent level of safety can be demonstrated.

Continuous Cooperation and Consensus Standards

To further support innovation without compromising safety, CS-23 introduces a model of continuous cooperation between industry, operators, EASA, and other authorities. Within the scope of CS-23 twin engine requirements, certification guidance is no longer frozen in long regulatory cycles, but can evolve through collaboration and shared technical standards that reflect real operational needs.

A key mechanism enabling this approach is the use of internationally recognised consensus standards developed through transparent and accessible processes. When such standards demonstrate compliance with CS-23 safety objectives, EASA may credit their use through a shortened rulemaking process, significantly accelerating certification without lowering safety margins.

The first major set of consensus standards supporting CS-23 was developed through international cooperation with ASTM International. This milestone substantially improved global harmonisation and reduced duplication between European and US certification efforts, particularly for twin-engine aircraft programmes.

Acceptable Means of Compliance (AMC) to CS-23 were published in December 2017, completing the transition to this modern, objectives-based certification framework.

Global Harmonisation: CS-23 and FAA Part 23

The reform of CS-23 did not occur in isolation. A parallel transformation of FAA Part 23 took place in the United States, entering into force shortly after the European amendment. EASA intentionally delayed the issuance of CS-23 to achieve the highest possible level of harmonisation, particularly for complex aircraft categories governed by CS-23 twin engine requirements.

As a result, CS-23 and FAA Part 23 now share a common certification philosophy. For twin-engine aircraft manufacturers, this alignment significantly simplifies certification strategies and supports aircraft designs intended for both European and US markets, reducing duplication of testing and regulatory interpretation.

CS-23 entered into force in Europe on 15 August 2017, marking the formal transition away from prescriptive certification rules toward performance-driven, objectives-based safety requirements—especially relevant for twin-engine aircraft operating near controllability and OEI performance limits.

CS-23 Twin-Engine Requirements in Practice

Although CS-23 is written as a technology-neutral certification specification, its objectives-based philosophy has very concrete implications for the certification of twin-engine aircraft. In practice, CS-23 twin engine requirements fundamentally change how safety, controllability, and performance must be demonstrated under asymmetric thrust conditions.

Under the modern CS-23 framework, manufacturers are required to demonstrate that a twin-engine aeroplane remains controllable and predictable following the failure of one engine. Compliance with CS-23 twin engine requirements is based not on rigid design prescriptions, but on evidence showing that:

• directional control can be maintained after failure of the critical engine,

• controllability is preserved above VMC under worst-case, realistic certification conditions,

• one-engine-inoperative (OEI) performance limitations are accurately measured and clearly communicated,

• systems essential to asymmetric flight—such as propellers, fuel, ignition, and autofeather mechanisms—meet reliability objectives appropriate to multi-engine operation.

Rather than prescribing fixed climb gradients, control deflections, or legacy design solutions, CS-23 requires a coherent safety case demonstrating that the aircraft behaves predictably across its approved operating envelope. This allows compliance to be achieved using conventional piston twins, FADEC-controlled engines, diesel or Jet-A propulsion, and advanced avionics architectures—provided the certification evidence supports the stated safety objectives.

OEI Performance, VMC, and Control Priorities

One of the most significant changes introduced by CS-23 is how one-engine-inoperative performance is evaluated for twin-engine aircraft.

VMC as a Safety Boundary, Not a Target

CS-23 recognises that VMC is not a fixed number, but a condition-dependent boundary influenced by power, configuration, bank angle, centre-of-gravity position, and drag. Within CS-23 twin engine requirements, certification must therefore demonstrate controllability under the most adverse realistic combinations of these factors, rather than relying on simplified or idealised assumptions.

For pilots, this reinforces an essential principle: VMC is a hard limit of control, not a performance technique. Aircraft documentation, operating limitations, and training guidance must clearly reflect this reality to prevent misinterpretation during one-engine-inoperative scenarios.

Honest OEI Performance Representation

CS-23 also acknowledges an operational truth often misunderstood by pilots transitioning to twins: many light twin-engine aeroplanes cannot guarantee a positive rate of climb after engine failure.

Rather than masking this limitation, CS-23 requires OEI performance to be:

• honestly measured,

• operationally meaningful,

• clearly reflected in AFM/POH data and procedures.

This approach aligns certification with real-world pilot decision-making and discourages unsafe assumptions about twin-engine “performance margins”.

From Old CS-23 to New CS-23: What Changed for Twins

Prescriptive Era Limitations

Under older CS-23 versions, twin-engine certification often became a checklist exercise based on legacy designs. This approach limited innovation, slowed approval of new systems, and struggled to adapt to modern propulsion and automation.

Modern CS-23 Expectations

The revised CS-23 shifts the focus to fundamental safety questions that sit at the core of CS-23 twin engine requirements:

• Can the aircraft be safely controlled after critical engine failure?

• Are OEI procedures intuitive, realistic, and survivable?

• Does system design reduce pilot workload during asymmetric flight?

• Is safety maintained across the entire operating envelope?

For twin-engine aircraft, this represents a far more realistic and operationally meaningful certification philosophy—one that aligns regulatory compliance with how multi-engine aeroplanes are actually flown, managed, and survived in the real world.

Why CS-23 Matters for Twin-Engine Pilots and Designers

For designers, CS-23 provides freedom to innovate—paired with the responsibility to prove safety under demanding asymmetric conditions defined by CS-23 twin engine requirements. For pilots, it directly influences aircraft behaviour, operational limitations, and the structure of emergency procedures, particularly during one-engine-inoperative scenarios.

In the twin-engine domain, CS-23 reinforces a principle long understood by experienced multi-engine pilots:

Twin engines do not guarantee performance—only options.
Certification must reflect that reality.

Conclusion

The evolution of CS-23 represents far more than a regulatory update—it marks a fundamental change in how safety is defined, demonstrated, and maintained for modern twin-engine aircraft. By moving away from prescriptive design rules and toward clearly defined safety objectives, CS-23 aligns certification more closely with real-world operational risks, particularly those associated with asymmetric thrust and one-engine-inoperative conditions.

For twin-engine aeroplanes, this shift reinforces a critical truth: certification is no longer about satisfying isolated numerical thresholds, but about proving predictable behaviour, controllability, and honest performance across the full operating envelope. OEI handling, VMC margins, system reliability, and pilot workload are no longer abstract compliance items—they are central elements of the certification safety case.

From a pilot’s perspective, CS-23 supports clearer limitations, more realistic performance data, and aircraft designs that better reflect operational reality. For designers and manufacturers, it enables innovation without compromising safety, allowing new propulsion concepts, automation, and redundancy strategies to be introduced—provided they meet robust, demonstrable safety objectives.

To explore how these certification principles translate into real aircraft, including conventional piston twins and modern twin-engine designs, see the full overview here:
👉 https://melibrary.pro/article/twin-engine-aircraft-list/