AMC1 ACNS.C.PBN.605 System performance demonstration

ED Decision 2019/011/R

The applicant should demonstrate the aircraft capability in terms of performance under design limit operational conditions (e.g. tailwinds and crosswinds, centre-of-gravity (CG) limits, temperature limits), and on representative procedures that include RF legs of varying radii. The applicant should also assess the effects of configuration changes (e.g. gear and flap extension and retraction).

The applicant should conduct a safety impact assessment based on the aircraft’s system safety assessments (SSAs) and identify any failure conditions that could potentially impact on performance. The functional hazard assessment (FHA) and SSA of all the aircraft’s systems that support RNP AR operations (RNAV systems, flight controls systems, flight guidance systems, displays, etc.) should, therefore, be revisited to identify these failures. System failures should include latent failures (‘integrity’) and detected failures (‘continuity’). For the detected failures, the monitor limit of the alert, the time to alert, the flight crew reaction time, and the aircraft response should all be taken into account and verified to ensure that the aircraft does not exit the obstacle clearance volume.

The intent of this requirement is to ensure robustness of the aircraft and its systems to failure conditions. Consequently, performing a safe extraction contingency procedure, i.e. initiating a missed approach, is not an acceptable means of demonstrating compliance against the criteria of CS ACNS.C.PBN.605(a), (b), and (c). These demonstrations may rely on crew action to intervene and place the aircraft back on the target track even if, in an operational environment, the crew is expected to initiate a missed approach procedure when the lateral or vertical criteria are exceeded. For compliance demonstration purposes, however, executing a safe extraction is not considered appropriate for demonstration of compliance with these criteria.

(a)     With reference to CS ACNS.C.PBN.605(a), any failure that is classified as ‘probable’ and supports the RNP AR operation should be assessed. Those failures that would require the flight crew to act or intervene should be assessed in a representative environment and design limit operational conditions by the applicant’s flight test pilots. The impact of the failure and the flight crew intervention should be such that the aircraft can be maintained within the 1 × RNP value and within – 75 ft altitude deviation.

(b)     With reference to CS ACNS.C.PBN.605(b), the same requirements apply for the case of an engine failure. If the engine failure would require the flight crew to act or intervene, then this should be assessed in a representative environment and design limit operational conditions by the applicant’s flight test pilots. The impact of the failure and the flight crew intervention should be such that the aircraft can be maintained within the 1 × RNP value and within – 75 ft altitude deviation.

(c)      With reference to CS ACNS.C.PBN.605(c), the same requirements apply, except that, for the case of failures classified as ‘remote’ but not ‘extremely remote’, the impact of the failure and the flight crew intervention should be such that the aircraft can be maintained within the 2 × RNP value and within – 75 ft altitude deviation.

(d)     With reference to CS ACNS.C.PBN.605(d), the applicant should demonstrate that no ‘extremely remote’ failure limits the flight crew’s ability to:

—                   intervene and place the aircraft back on the target track contained within the alert threshold; or

—                   safely extract the aircraft through manual intervention.

Safe extraction is defined as within 2 × RNP for the applicable approach and missed approach procedure, until such time that the aircraft is stabilised and reaches a safe altitude. The RNP for the missed approach procedure is usually higher than the RNP for the continued approach. For extremely remote navigational failure conditions (e.g. all flight management computers (FMCs) failed), the flight crew must be able to navigate the aircraft free of obstacles.

For departure procedures with close-in RF legs at or just beyond the departure end of the runway, and for missed approach procedures with close-in RF legs, the retraction of the landing gear and flaps and subsequent rapid acceleration may affect the RNP system’s ability to conduct accurate turn anticipation. An inaccurate turn anticipation calculation may result in an overshoot of a close-in RF turn. When this performance characteristic is present, the applicant should consider including a limiting airspeed for the initial phase of the departure or the missed approach in the AFM. The airspeed limit should not be lower than the best-climb airspeed with one-engine-inoperative.

The severity level of the above demonstrations (failure conditions in combination with the RNP approach containment requirements), as assessed by the test pilot, must still match the probability of the applicable failure condition (ref.: CS 25.1309).

Specific evaluations should be conducted to assess path excursions upon failures and the resulting RNP levels. Results should be documented in the AFM, AFM Supplement, or any appropriate aircraft operational support document which is approved by EASA and made available to the operator. In other words: If, for example, the worst-case result of the assessments that have been conducted to demonstrate compliance for ‘remote’ failures shows that the aeroplane diverts 0.40 NM from the published track, then the applicant should limit the authorised RNP to 0.20 NM.

[Issue: CS-ACNS/2]