Manufacturer suggests earlier accidents may have been caused by phenomenon

Instrument landing system (ILS) indications that wrongly show an aircraft to be on glideslope when it is not are infrequent, but not one-off events, according to Boeing, which has been studying a July 2000 Air New Zealand (ANZ) Boeing 767 incident.

Speaking last week at the Flight Safety Foundation's European Aviation Safety Seminar in Geneva, Switzerland, Capt Dave Carbaugh, Boeing Commercial Airplanes' chief pilot flight operations safety, said he estimated that an average of about two such incidents occur each year.

Events similar to the ANZ incident - at Apia, Samoa - have since occurred to crews on Boeing 777s and Airbus A320s, demonstrating, he points out, that "it can happen to anyone". He speculates the phenomenon may have been responsible for some past accidents but not realised at the time.

Despite the fact that the approach took place on a moonless night over the sea, the ANZ 767 landed safely, because crew members were particularly well briefed and alert, but confused by what they were seeing, says Carbaugh.

After studying the event, ANZ made a video for its pilots explaining the operational implications. This shows how the crew members were surprised by the sudden autopilot glideslope capture, but their inherent trust in the ILS as the most reliable and accurate of the available information providers persuaded them to continue the descent. It was only when they checked the distance-measuring equipment read-out against the aircraft's height that they decided on a go-around 10km (5.4nm) from the runway at 400ft (120m).

So called "erroneous" glideslope signals are not the same as the "false" signals that cause warning flags to show on the glideslope display. No fault flags appear and the ILS installation's morse identification, broadcast on the localiser carrier signal, is received correctly.

Source: Flight International