Previously, the use of SET-aircraft has not been permitted in commercial air transport. This has been due to the increased risk in the event of engine failure, compared to multi-engine aircraft. Modern turbine engines have however become very reliable, and together with the requirements embedded in the provisions for the SET-IMC special approval, the residual risk is considered low enough to allow commercial air transport with SET-aircraft.

The provisions on SET-IMC approvals impose requirements on the aircraft operator's procedures for flight planning. According to the main rule, the route must be planned such that the aircraft will be able to glide to a safe emergency landing site in case of engine failure from any point along the route. However, the operator can to some extent plan with up to 15 minutes of the en-route flight to be carried out without a safe emergency landing site within reach. The operator's procedures for such planning shall be submitted to the Competent Authority for assessment as part of the application for a special approval for SET-IMC operations.

The predominantly mountainous terrain in Norway and the typical weather conditions, characterized by changing weather, often with low cloud base and poor visibility, can make it challenging to find suitable emergency landing sites and to perform safe landings at such sites.  These conditions create an increased safety risk for SET-IMC flights in Norway when they are conducted over mountainous terrain. These conditions will also make it challenging to operate scheduled SET-IMC routes in such areas, as the weather conditions will make it difficult for flights to follow the timetable. In order to ensure an acceptable level of safety, The Norwegian Civil Aviation Authority will therefore require operators applying for special approval for SET-IMC operations in order to fly passenger routes, to establish requirements for weather minima and visibility in their procedures for flight planning. The determination of such minima will have to be made on the basis of a specific risk assessment.

As a guideline, the Civil Aviation Authority believes that the following weather minimums should apply to ensure an acceptable risk level with SET-IMC passenger route flights in Norway:

Required Drift Down / Glide Minima

• En-route altitude and gliding profiles shall ensure minimum 2000 feet AGL to a point VMC 3000 feet AGL above landing site, to ensure a controlled landing.
• En-route and during glide down to the 3000 feet AGL point, IMC is acceptable.

Weather Minima at landing Site

• At landing site. Minimum cloud base shall be 3000 feet AGL and visibility minimum 5 km.

The Norwegian Civil Aviation Authority will also consider the necessity of establishing national regulatory requirements on weather minima for SET-IMC operations in Norway.