ABSTRACT
The instrument landing system (ILS) is used for precision approaches and landings. This chapter looks at ILS principles and hardware, and explores how the ILS combines with the automatic flight control system (AFCS) to provide fully automatic approach and landing. ILS receivers are often combined with other radio navigation functions, e.g. VHF omni directional range (VOR); these are located in the avionic equipment bay. ILS receivers are based on the super-heterodyne principle with remote tuning from the control panel. The method of selecting and displaying the selected runway heading depends on the type of avionic fit. Electromechanical instruments include the omni-bearing selector (OBS), course deviation indicator (CDI), horizontal situation indicator (HSI) and electronic horizontal situation indicator (EHSI). An alternative ILS approach is the localizer back-beam, or back-course (BC). All localizer antennas actually transmit in two directions; with primary (forward-course) and secondary (back- course) lobes.
