Equivalent Safety Finding UK.ESF.F.0001 (Degraded Flight Instrument External Probe Heating System Consultation: CS-25 Large Aeroplanes

For comments on the proposal that the CAA introduces an equivalent safety finding concerned with degraded flight instrument external probe heating systems on a large transport aeroplane.

The UK CAA is consulting on an Equivalent Safety Finding that is applicable to CS-25 Large Aeroplanes.

This Equivalent Safety Finding provides the compensating factors to reach an equivalent level of safety to direct compliance with requirement CS 25.1326(b)(2) based on the detection of degraded anti-icing performance, through monitoring of the possible erroneous air data - potentially caused by a malfunction of a probe heating system - from one or more probes.

The Consultation Paper Equivalent Safety Finding UK.ESF.F.0001 Issue 1 provides the full detail of the identified issue, and the associated Equivalent Safety Finding.

We received 2 responses.
The first response highlighted the relevance of this proposed ESF to an ongoing investigation into partial probe heater failures on a large aeroplane type. In the referred partial failure, the probe tip could become iced up while the probe body remained ice free. Further, the method of probe heater failure detection during maintenance was not able to reliably detect this partial failure condition. The comment highlighted the relevance to the fundamental issue addressed by the ESF, and the need for the careful consideration and assessment of the scheduled maintenance actions for reliability.

The second response highlighted the need to address common mode faults and the potential for erroneous readings to occur from multiple primary air data systems. The commend further highlighted potential alleviations for common mode failures through system redundancy, dissimilarity, and robust comparator logic within the ADS.

We acknowledge the responses and thank the responders for their suggestion.
For response 1, the comment is not incorporated. Whilst we acknowledge the issue raised is a point of concern, our position is that this is currently covered by the Equivalent Safety Finding by incorporating as a compensation factor the use of DFCS monitoring system and its redundancies, as expressed in points 2.a.1, 2.a.2 and 2.a.3 of the ESF.

For response 2, the comment is not incorporated. The objective of this ESF is to identify compensation factors for the lack of strict compliance with 25.1326(b)(2) considering specific failure modes in which there is a degraded anti-icing performance of these sensors, as expressed in Section 2 of this ESF. To achieve this, the ESF identifies means where the aircraft systems trigger messages to the crew informing about the unreliability of these sensor, triggering suitable operational response in accordance with AFM procedures.

The concerns raised in the comment are part of the technical investigation that is carried out in the systems referred in this ESF in context of other points of CS 25 (eg. 25.1309).