Originally Posted by
BOAC
... the problem of when the navigational accuracy of the FMS/Map is suspect ...
This is quite a deep issue; it is not an EGPWS problem per se.
The ‘captaincy’ solution degrades the effectiveness of EGPWS by amending its intended use to alleviate the deficiencies of a navigation system.
If a navigation error exists or is likely to exist then the enhanced part of EGPWS can be switched off (previous FAA solution). The aircraft is then exposed to the hazards of the ‘unacceptable’ risks of CFIT. [Unacceptable because EGPWS is mandated equipment.]
Alternatively if it is not known that the navigation accuracy is degraded, then in day/VMC you could exercise ‘captaincy’, but in night/IMC the only option is to pull up.
Many of these aspects were considered in the initial operational certification - and before the ‘what if’ replies appear, turning flight was (is) not an option after a warning, and any additional risk from a pull up is nil, it is safe flight.
Operators could improve the navigation system; map shifts have contributed to CFIT incidents.
Alternatively adding a GPS module to EGPWS has many advantages including providing CFIT protection with a map shift, and although the cost of GPS may not balance, it would offset the commercial damage of a CFIT accident. Thus the debate might be one of economics and safety culture, but of course the bean counters suffer the same ‘risk aversion’ and error proneness that the crew do.
A cheaper solution which maintains the current ‘EGPWS level of safety’ is to remove the captain’s option. I do not have data on the number of unnecessary pull ups flown, but from about 15 reports of EGPWS ‘saves / ‘near misses’, in a significant proportion (5/15) the crew either did not pull up or they reacted incorrectly. Retaining the ‘captaincy’ option encourages crews to violate SOPs (human nature), and additionally increases workload as discussed previously.
The industry has been very fortunate not to have suffered a CFIT accident with an aircraft fitted with EGPWS; the dominant contribution in the incidents so far has been human error – perception, which is a component of ‘captaincy’. I believe that operations are safer without the EGPWS captaincy opt-out.