David Learmount/NEW ORLEANS
Incomplete procedures after maintenance on instrument landing system (ILS) ground installations could lead aircraft to disaster while indicating to the pilots that the signal is normal, according to two major airlines which came close to suffering accidents. This has taken the industry by surprise, because ILS has been in use for more than 40 years without such an occurrence being formally recorded.
In February 1999, an Air France Boeing 777-200ER could have hit the ground 7.4km (4nm) short of the runway at Rio de Janeiro, had the crew followed the ILS signal and not received warning from the enhanced ground proximity warning system. The Brazil civil aviation authority says it has completed an inquiry but has not yet filed it to the International Civil Aviation Organisation.
Last July, an Air New Zealand (ANZ) 767-300 also picked up an ILS signal with the same fault on approach to Apia, Western Samoa, indicating that the aircraft was on centreline and glidepath, but the crew became suspicious fairly early in the approach because the distance measuring equipment (DME) readings did not tally with the aircraft's altitude. The cloudbase was high enough to allow them, especially with their heightened vigilance, to abandon the approach when the ground was in sight.
The phenomenon can occur when the ILS ground installation sends out a carrier signal only because it is left in test mode after maintenance, but it still sends out an identification and pilot instruments show no warning flag indicating that the signal is false.
The result is that the pilots' primary flight display shows the aircraft on the localiser (lateral guidance) and on glidepath anywhere in the 80¹ arc in which the glidepath signal normally propagates, with no signal fault indication appearing on the instrument.
ANZ has, as a consequence, worked out a standard operating procedure for ILS approaches, which includes the crew checking for gradual convergence with the localiser or glidepath while the aircraft is establishing on the ILS beam, instead of receiving sudden indications that it is on centreline and glidepath.
Source: Flight International
