and magnetic north. Lighting To conduct night-time operations, a heliport must have lighting installed that meets specific aeronautical standards. Heliport perimeter lights are generally installed around the TLOF area, and may be flush mounted on the TLOF itself, or mounted just off the TLOF perimeter on short metal or concrete extensions. One alternative to lighting the TLOF if certain criteria are met, is to light the area of the FATO instead. Some locations, due to environmental conditions, illuminate the TLOF and FATO. Lighting should never constitute an obstruction that a helicopter may impact, and for this reason, in the U.S., heliport lighting is not allowed to extend above the TLOF or FATO more than . Current standards recommend that all perimeter lighting be green. Prior standards recommended amber lighting for perimeter lights; however this wavelength has been shown to interfere with night vision goggle (NVG) operations, when used with older incandescent lighting. In the past, lighting has been traditionally incandescent, but increasingly, light-emitting diodes (LEDs) are being incorporated, due to lower power requirements and increased life. While flood lights may be used to enhance surface operations, they should not interfere with flight crew night vision, and should be kept off during flight operations, and only used when conducting ground movement operations. To conduct night operations, a lighted wind cone is also required. At ground-based heliports, lead-in lights may be incorporated to identify the preferred approach / departure direction. Visual slope guidance systems (such as HAPI, PAPI, etc.) are recommended options in both ICAO and FAA document. While airports commonly use 6.6A direct current power, heliport lighting is normally AC powered. Radio control of the lighting by the pilot via an automated ground-based controller is also common. Approach / departure airspace To provide for a safe environment to perform normal helicopter landing and take-off operations, each heliport must have unobstructed approach / departure paths. The minimum recommended separation between flight paths is 135 degrees. A heliport approach path is broken down into two distinct airspace surfaces; the Approach Surface, and the Transitional Surface. The approach surface begins at each end of the heliport primary surface with the same width as the primary surface, and extends outward and upward for a horizontal distance of , where its width is . The slope of the approach surface is 8-to-1 for civil heliports. The Transitional Surfaces extend outward and upward from the lateral boundaries