PAUL PHELAN / CAIRNS
ANZ investigators warn of the danger of trusting ILS signals without checking raw height/distance data
A false instrument landing system (ILS) glideslope indication may have been a major contributory factor to the disastrous Korean Air Boeing 747-300 accident at Guam in August 1997. According to recent studies by Air New Zealand (ANZ), the problem may have led the crew to believe that they were on the correct descent when the aircraft crashed into Nimitz Hill 5km (3nm) from touchdown.
ANZ believes that 40 other controlled flight into terrain (CFIT) investigations now warrant review in the light of the new findings.
The airline was led to investigate the false ILS signal phenomenon when a 767 crew approaching Apia, Western Samoa, became suspicious because range and height readouts did not indicate that the aircraft was on glideslope but their instruments did. They abandoned the approach and carried out a second one using ILS localiser and distance measuring equipment (DME) readings only (Flight International, 7-13 November, 2000).
Since then, ANZ group vice-president operations Capt Trevor Jensen has led studies of other similar incidents, and has called for all operators to be made aware of the dangers.
Jensen studied the US National Transportation Safety Board report on the Guam case and believes that it should be reviewed because the NTSB did not investigate the supposedly inoperative ILS glideslope.
The Korean crew was aware that the ILS glideslope was published as inoperative and had mentioned this in their top of descent briefing. Discussions on the cockpit voice recorder, however, indicate that the crew were confused because their flight instruments were showing the aircraft on the glideslope, yet there were no warning flags on the flight director. This, Jensen points out, can happen when the ILS glideslope or localiser is turned off.
At Apia, the maintenance staff had erroneously left the system in a test mode, which sent out a carrier signal but no displacement information. The result was that the crew were given flight director indications which showed the aircraft on glideslope and centreline no matter where the aircraft really was, provided it was within a broad arc 40í either side of the localiser. If the localiser is working properly the crew receive the correct beacon identification, and there are no warning flags as there would be if the ILS was transmitting erroneous or weak displacement information.
ANZ has now adopted a standard operating procedure for all ILS approaches. This requires the crew to check gradual capture of the ILS localiser and glideslope as they intercept them, and to reject the system if it gives sudden indications that the aircraft is on centreline and glideslope.
Source: Flight International
