This incident is another reminder of the problem of complacency in our industry. We easily forget that systems are rarely perfect and thus require some human intervention to mitigate design weaknesses. ILS’s have always required a component to check that the GS is correctly positioned or that a false beam has not been intercepted. Historically this component used markers or a locator beacon; more recently these have been replaced with a distance (DME) check; both require the crew to check the altitude a predetermined point. With the advent of FMS/EFIS maps, RNAV, and very reliable ILS, there are many opportunities to overlook this important check.
IIRC there was a similar incident involving an incorrect ILS test mode at Rio. In this incident the EGPWS alerted the close proximity of terrain. Interestingly the following aircraft, unaware of the problem, crosschecked the marker (DME?) altitude and then checked the QNH with ATC to resolve the apparent altitude error; the QNH was correct, thus a GA flown.
Other examples of error provoking situations (human factors) encountered in low approach incidents can bee seen in
‘TAWS Saves’.
All of these incidents provide examples of the often unavoidable human error (particularly where both crew experience the same error at the same time), which with the failure to use simple crosschecks or safety tools such as altitude-range tables result in hazardous situations.
Note the importance of always checking the altitude element before range, and in referencing distance to the threshold – beware offset DMEs. There is also merit in always displaying EGPWS terrain on at least one EFIS display to avoid a surprise alert or warning.
Situation awareness is a major factor, but not necessarily that it was lost, more likely that it was never adequately gained or provisioned. SA is our understanding of the combination of the real world and our mental model, we have to perceive the world correctly and maintain a good mental model (memory, experience, planning ahead). The latter commences with the approach briefing – the procedure, nav aids, check altitudes, etc. Our understanding is updated as the approach progresses; the mental model must be compared with the real world by the use of crosschecks and monitoring. If a crosscheck fails or the real world does not agree with the mental model (the briefed plan), then most probably an error exists; a normal (SOP) response would be a GA and then the source of the error/discrepancy can be determined at a safe altitude. Avoid the error of making the data (real or memory) fit the picture – confirmation bias.
“The approach briefing is the flight plan for the mind” FSF presentation Phoenix May 06.
"We (pilots) are children of the magenta line" Don Bateman, Chief Designer GPWS/EGPWS, Honeywell.
The NZ video is available on CD from
Flight Safety Foundation.