ILS Localizer 1

28.10.2004

Submission:

Background.

ICAO states that the service volume of a localiser shall be +/- 10 degrees front course signal and +/- 10 degrees to +/- 35 degrees clearance signal. Operationally, an aircraft establishes on the ILS localiser at a range between 25NM and 10Nm and then intercepts the glidepath. The aircraft then follows a continuous decent on the extended centreline down to runway touchdown. An aircraft outside of the +/- 10 degrees course sector will receive a "flag" indication this identifies that the aircraft is outside of the guidance region and should either fly to the right or left.

The ICAO coverage requirements stated in ICAO Annex 10 are mainly historic and are now out dated as other forms of guidance (FMS, GNSS etc) are used for the en-route phase of the flight. Once established in the final approach phase then the guidance azimuth required by an aircraft is very narrow and is less than the requirements stated in ICAO Annex 10.

ICAO and Flight Inspection Requirements.

The International requirements for flight inspection are stated in ICAO document 8071. These requirements specify the localiser coverage and are identical to those requirements in ICAO Annex 10. However, there are many localiser installations throughout the world that do not meet the stated ICAO recommendations due to terrain limitations. In these circumstances the operator simply identifies the limitation on the ILS approach charts and associated documentation. This non - compliance to the ICAO recommendations has no or little impact on the overall Airport Operation although generally there is a reluctance by operators to not to have a non compliance. If wide aperture antenna's were adopted and I see no operational reason not to proceed, then the national regulator, usually the Civil Aviation Authority would require Airport Operator to identify how he has mitigated the risk of an aircraft being outside the new service volume of the wide aperture antenna. This issue should be addressed by both the procedure designer and the engineer responsible for the design of the navigational infrastructure.

Antenna Design Considerations.

Under ILS Category III operations it should be noted that the ILS localiser signal is used for roll out guidance. Additionally, some airline operators use the localiser for departure guidance along the runway. Therefore, the effect of localiser antenna de-phase and the beam forming area should be considered as the aircraft approaches the localiser antenna.

P. Willis