2. Regulatory and consumer rating requirements

Curtain airbag (CAB) performance is not judged by a single global standard. Instead, a CAB must satisfy a layered set of requirements: mandatory federal regulations, regional type-approval regulations, and voluntary consumer rating programs. Each framework defines its own impact configuration, barrier type, impact speed, occupant size, and injury criteria. For an engineer correlating CAE with physical testing, the practical consequence is that one CAB design may need to pass several distinct load cases, and the CAE model must be validated against each of them separately. This section maps the five most relevant frameworks — FMVSS 226, ECE R135, C-NCAP, Euro NCAP, and IIHS — and explains what each one actually demands from the CAB.

Mandatory Regulations: FMVSS 226 and ECE R135

FMVSS 226 (Ejection Mitigation) is the only U.S. federal standard written specifically around side curtain airbags. It does not measure occupant injury; it measures whether an occupant can be ejected through a side window opening during a rollover or side impact event. The test uses a linear impactor — a 18 kg (approximately 40 lb) headform — fired at the window opening at a controlled velocity, with the CAB deployed. The pass/fail metric is whether the headform is retained inside the occupant compartment. Compliance is evaluated across a grid of impactor target points covering the side window aperture, and the CAB must cover the opening sufficiently at each point. For CAE, this means the model must accurately predict coverage geometry and bag retention, not just head acceleration. Typical CAE outputs are the headform center-of-gravity trajectory and whether it crosses the window plane.

ECE R135 is the UN type-approval regulation for side impact protection, adopted in many markets outside North America. It defines a moving deformable barrier (MDB) side impact test and, in later revisions, a pole side impact test. The CAB's role is to protect the head against contact with the barrier, the pole, or the vehicle interior. Injury criteria are based on Head Injury Criterion (HIC) and, for the pole test, head acceleration limits. Unlike FMVSS 226, ECE R135 is fundamentally an injury regulation, so CAE correlation must reproduce head acceleration time histories, not merely containment.

Consumer Rating Programs: C-NCAP, Euro NCAP, and IIHS

Consumer programs are voluntary but carry strong market pressure, and they often lead regulation rather than follow it. Their protocols change on a defined cadence, so CAE teams must track the current rating year.

C-NCAP (China New Car Assessment Program) includes a side impact MDB test and, in recent versions, a side pole test. The side pole test is particularly demanding for CABs because the pole concentrates load over a narrow area and the head path is short. C-NCAP scoring combines HIC, head acceleration, and chest criteria; a CAB that deploys late or has insufficient coverage will show elevated head values in the pole test.

Euro NCAP has progressively tightened side impact. Its current side impact MDB test uses a heavier and higher barrier than older versions, and the side pole test remains a core element. Euro NCAP also evaluates far-side occupant protection, which is directly relevant to CABs because a far-side occupant can move toward the struck side and interact with the deploying or deployed curtain. Far-side assessment introduces occupant kinematics that a near-side-only CAE model will not capture.

IIHS (Insurance Institute for Highway Safety) uses a side impact test with a heavy MDB and, separately, a roof strength (rollover) test. The IIHS side rating is based on injury measures from a SID-IIs dummy, and the roof strength test evaluates roof crush resistance, which is related to the rollover environment in which a CAB must remain inflated. IIHS also conducts a driver-side small overlap frontal test, but that is outside the CAB scope. For CAB correlation, the IIHS side test is the primary target, with the roof test informing the rollover deployment strategy.

Comparative Overview

What This Means for CAE Correlation

Because the frameworks differ in barrier, speed, dummy, and metric, a single CAE model cannot be "correlated" in the abstract. The correlation must be scoped to a specific load case. In practice, the workflow is:

1. Identify the target framework and rating year for the vehicle program.
2. Build the CAE load case to match the protocol exactly — barrier geometry, impact angle, velocity, dummy model, and seating position.
3. Run the simulation and extract the same channels the physical test measures (head acceleration, HIC, head trajectory, bag pressure).
4. Compare against physical test data from the same configuration, and quantify the error in each channel.
5. Adjust the CAB model — fabric permeability, inflator mass flow, seam stiffness, tether geometry — until the error is within the program's correlation tolerance.

A common failure mode is applying a CAB model correlated for FMVSS 226 containment to an ECE R135 injury assessment. The containment model may have a coarse fabric representation that is adequate for trajectory prediction but too stiff or too permeable for accurate head acceleration. The symptom is a CAE head acceleration peak that is significantly higher or lower than the physical test, even though the containment result looks correct. The fix is to refine the fabric and inflator sub-models and re-correlate against the injury-based test.

Rollover and Deployment Timing

Rollover protocols — IIHS roof strength and the rollover portions of FMVSS 226 — impose a different requirement: the CAB must remain inflated long enough to provide containment during a multi-event rollover. This is a deployment duration requirement, not just a peak-pressure requirement. CAE must therefore model the inflator's pressure-time curve and the fabric leakage over a longer window than a side impact. A model tuned only for a 100 ms side impact may under-predict leakage at 3–5 s and show a bag that deflates too early. The observable symptom in CAE is a containment failure at later impactor points that does not appear at early points.

The table and diagram above are not a substitute for reading the current protocol documents. Each framework publishes detailed test procedures, and the rating year matters: a Euro NCAP 2023 side impact is not the same as a Euro NCAP 2020 side impact. Before starting any CAE correlation, confirm the exact protocol revision, the barrier specification, the impact velocity, and the dummy model version. These four parameters define the load case, and any mismatch will invalidate the correlation regardless of how well the CAB model is built.