EASA has been developing new rules for ‘All Weather Operations’ (‘AWO’) such as low visibility approach and landing. A notice of proposed amendment (NPA) has been published and comments are invited [NPA 2018-06]. Whereas rules for the different aviation domains have previously been developed separately for this project EASA took a ‘cross-domain’ approach.
A Complex System
Aircraft operations in the 21st century are remarkably safe. We take for granted that aircraft land and take-off in conditions that make driving on the highway very dangerous (for example thick fog). In order for these ‘low visibility operations’ to proceed safely many different components of the aviation system need to work together. Ground-based radio-aids radiate signals to guide aircraft towards a runway; avionics on board the aircraft decode those signals and feed instrument displays on the flight deck; pilots interpret those displays and make control inputs to determine the aircraft trajectory and so on. Accidents can be caused not just by individual failures or errors but also by the interactions of the different components of the system such as between air traffic control and the pilot, the pilot and the aircraft or ground-based navigation systems and the on-board avionics.
Before drafting new rules, the AWO working group conducted a comprehensive hazard analysis using ‘STPA’, or Systems-Theoretic Process Analysis. STPA is a relatively new analysis technique that attempts to identify scenarios that could produce accidents in complex systems. Unlike traditional hazard analysis techniques, it considers not only the failures of individual components within a system but also the interactions between different components within a system. The working group for this task (RMT.0379) included about 80 experts from different aviation domains including airworthiness, aerodromes, air operations, air traffic control, air navigation services and pilot-training. The group also included a leading academic expert on STPA, Dr Nektarios Karinikas of the Amsterdam University of Applied Sciences. Dr Karinikas guided the working group on the use of the STPA methodology, which was new to many. The hazard analysis involved many hours of teleconference as the experts involved were spread all over the globe.
How safe is LTS Cat I?
One of the conclusions of the hazard analysis is that that “there is a latent safety risk in relation to lower-than-standard CAT I” (‘LTS CAT I’). The current regulations allow an operator to fly an automatic approach and landing in visibilities as low as 400m runway visual range using a flight control system certified for CAT II and a runway and instrument landing system (ILS) designed for CAT I operations. The identified safety risk comes from the interaction between the aircraft avionics, the ILS and the airport surroundings. While a CAT II flight control system must be capable of controlling the aeroplane down to a decision height as low as 100 feet the certification specifications do not require a demonstration that this is possible with an ILS classified as ‘I/T/1’ as is currently allowed; also autoland systems are tested using airports approved for CAT III operations where the ground underneath the approach to the runway needs to be clear, level and free from obstructions. This may not be the case at a CAT I airport.
None of this is to suggest that current LTS CAT I operations are unsafe, only that there could be a combination of circumstances in which an operator conducting LTS CAT I operations to the minimum standard without conducting their own risk assessment could find that the system does not perform as expected.
Special Authorisation Category I (SA CAT I)
At the same time as removing LTS CAT I the NPA proposes a new operational credit ‘SA CAT I’. Like LTS CAT I, SA CAT I will allow operation in lower visibility than CAT I without satisfying all the requirements for CAT II. Regulations for SA CAT I will cover all the different aviation domains so there will be specific certification requirements for aircraft and avionics, aerodromes and the air operator. The benefit of SA CAT I will be a lower decision height, potentially down to 150ft.
Other benefits of the cross-domain approach
The cross-domain approach to rulemaking will also bring other benefits, in particular the terminology used will be consistent across all domains and the NPA proposes introduce new standards for the use of ‘Enhanced Vision Systems’ for approach and landing and simpler approval processed for CAT II/III operations
Who wrote this?
Andrew McKechnie is an expert in Air Operations and Director of McKechnie Aviation LTD. Since 2015 Andrew has been working as a consultant to EASA RMT.0379 developing new regulations for All Weather Operations.