Cap’n P B, I support your calls for altitude encoding transponders and revision of the EGPWS MMEL; however, I wonder if you are overstating the weakness of ACAS.
My previous simplified position on the safety of ACAS should have balanced the probabilities of encountering such rare situations that you describe. For the scenario that you envisage, consider the following geometry; forgive the generalizations and any inaccuracies in the equipment specs, I no longer have access to these details.
Assuming that RA commands are limited to excursions of plus/minus 1000ft (no adverse manoeuvre by the intruder), then the host aircraft would have to be flying in an area with less than 1000ft obstacle clearance. This normally occurs during take off or approach. During take off the climb vector may be assumed to bias the probability of an alert towards a ‘do not climb’ or ‘climb’, after the ACAS becomes active above 500ft. During the approach the probability of obstacle or ground collision may only be of consequence when below 500 ft. The significant obstacle or terrain defining the MOC may not be on track, and then the RA has to be ‘descend’, which at low altitude requires unique collision geometry, particularly where the host aircraft should be descending on an approach to land. I would suggest ‘do not climb / maintain VS’ are more likely.
Therefore I would conclude that the chance of a CFIT accident is most unlikely, even more so when considered in conjunction with a MEL’ed EGPWS.
The point is that safety systems may not be able to cope with every scenario that can be envisaged, but from the balance of probabilities of encountering such a unique scenario, the safest course of action is to follow the RA. The dangers appear where pilots invent scenarios (areas on concern) that are of little consequence (probability of occurrence) and allow their concerns to bias decisions following an alert in other, and very valid scenarios.
The solution to these dangers involves classic airmanship; the need for discipline, control of hazardous attitudes (I know better, macho, press on). This also requires knowledge of the safety system, the operating logic, and regulatory rationale behind the approved procedures; for ACAS this appears somewhat deficient. The airmanship skills are those required to act quickly to an alert, the reaction should be a ‘trained for’ response in a predetermined, an announced situation – the RA. This is defence in depth, which should also include altitude encoding transponders and revision of the EGPWS MMEL.
I fear that too often some pilots outthink safety systems or unnecessarily challenge regulatory assumptions, and in doing so, create new hazards that the certification process should have accommodated on the basis of probability; but I stand to be corrected on my limited knowledge of the detailed workings of, and the certification of ACAS.
P.S. I am sure that you realise, but for other Ppruners, turning the EGPWS Terrain mode off does not inhibit the basic GPWS functions. Also, are Honeywell addressing the lack of runway data? Their response to data omissions and the on-line update service is normally very good.
Last edited by alf5071h; 27th Apr 2005 at 15:18.