Will the sun ever set on ILS?

One of the paradoxes of modern aviation is that while highly automated passenger aircraft navigate around the world by satellite, with their pilots receiving much of their flight information via computerised datalinks, when it comes to the most exacting phase of flight – the 'hands-off' automatic landing in almost zero visibility – we depend for precise guidance to the runway on a system first used more than 70 years ago.

This is the Instrument Landing System (ILS), installed at virtually every commercial airport in the world.

Although today's ILS equipment embodies the latest technology, its basic guidance principle has remained the same since those early years. A radio transmitter adjacent to the runway touchdown point provides a glide slope beam, angled up at around 3 degrees above the horizon, that guides an aircraft down a smooth descent path to the runway. A second transmitter, called the localiser and located beyond the far end of the runway, transmits a separate beam that provides left/right guidance to align the aircraft precisely with the runway centre line. The concept is relatively simple – so simple, effective and reliable, in fact, that changing to newer landing guidance techniques that offer additional benefits is expected to take many years to accomplish.

Landing guidance

Landing guidance system categories (Cats), describe the combination of the aircraft's height above the ground – Decision Height (DH) – and the visibility – Runway Visual Range (RVR) – at which the pilot must have the runway in sight before attempting to land. If it is not visible at either the DH or the RVR, the pilot must carry out a 'missed approach' procedure and climb away.