General :

For the safety requirement of low flying military Aircrafts, all the transmission lines and transmission line structures falling within the safety zone of air field and air to ground firing ranges shall meet the requirements of Directorate of flight Safety, Air Headquarters as per code of practice for Design, Installation and Maintenance of Overhead lines as per IS:5613 (Part-III) Section-1: 1989.

Requirement of Directorate of Flight Safety :

The extracts of requirement of Directorate of Flight Safety with regard to visual aids for denoting transmission line & transmission line structures are given below:-

• All the power utilities shall comply with the following:-
  • The transmission lines and transmission line structures of height 45 m and above shall be notified to the Directorate of Flight Safety (DFS), Air Headquarters (Air HQ), New Delhi.
  • For construction of any transmission line/structure or a portion thereof, falling within a radius of 20 Kms around the Defence aerodromes and air to ground firing range, provisions of the aircraft Act 1934, Sec. 9A as amplified by the associated Gazette notification 50988 Part II Section 3 Subsection (ii) dated 1988-03-26 shall be complied with. Towards this, a No Objection Certificate (NOC) shall be obtained from the concerned aerodromes authorities.
  • Within a radius of 10 Kms around aerodromes and air to ground firing ranges, all transmission lines and structures of height 45 meters or more shall be provided with day and night visual aids.
  • In all other areas, outside a radius of 10 Km from aerodromes, only those portions of transmission lines and structures of any height identified to pose a hazard to aircraft by the Directorate of Flight Safety shall be provided with day visual aids.

Description of Visual Aids :

• Day Marking:
  • Line Markers:
    Coloured globules of 40-50 cm diameter made of reinforced fiber glass or any other suitable material, weighing not more than 4.5 Kg each with suitable clamping arrangement and drainage holes shall be installed on the earth wire's) in such a manner that the top of the marker is not below the level of the earth wire. Up to 400 meter span, one globule shall be provided in the middle of the span on the highest earth wire. In case of double earth wires, the globule may be provided on any one of them. For span greater than 400 meters, one additional globule may be provided for every additional 200 meter span or part thereof. Half orange and half white coloured globule should be used. A typical sketch of marker is given in Fig.I.
  • Structure Marking:
    The structure portions excluding cross-arms above 35 m height shall be painted in alternate bands of international orange and white colours. The bands shall be perpendicular to the vertical axis and the top and bottom bands shall be orange. There shall be an odd number of bands. The maximum height of each band shall be 5 m. A typical sketch of marking a structure is given in Fig.2.
• Night Marking:
  Medium and low intensity obstacle lights as per fig 3 on a complex obstacle such as towers supporting overhead wires should have a night time intensity as per ICAO, requirements in international standards recommended practices. The light on top of the structure should flash at the rate of 20 sequences per minute.