PBL,
I would have thought it was a straightforward exercise to list the differences between a ground proximity warning and an RA.
1. Ground proximity is based on radio altimetry, not on self-advertisement from another electronic device.
2. That altimetry is measuring something definite and very close
3. What it is measuring will not move and requires no projected CPA that must be monitored during the manoeuvre in case a different advisory is required
4. You are way out of legal airspace when you get that warning, so the likelihood that there is someone else around you is just about zero and you can pretty much do what you like.
I agree with 1. to 3. but would you agree that 4. is likely to happen near an airport (you have to approach the ground when landing and get near it when taking off) and airports are well known as places where aeroplanes like to congregate... Anyway, the points I'm trying to make are:
a) GPWS and TCAS (and to a lesser extent EGPWS) are last-defence systems, conceived to
possibly stop/reduce the chances of an accident occurring. They are
not designed or certified to replace ATC or good piloting - except at the last minute when either or both have been lacking.
so,
b) As the alerts/advisories/warnings given by the above equipment are somewhat time constrained in their period of usefulness (!), then a consistent response over a short period is required to allow these systems to function as intended, i.e. you can't pontificate at great length about what action to take: it must be a memory drill - "recall item". And you have to perform it correctly.
I introduced a specific scenario. Maybe if airmen doesn't want to address it, then you can? Why is manoeuvring against the RA the "worst possible option" in the specific scenario I gave?
I assume that is the Überlingen scenario? I say that it was the worst possible because it led to a collision; almost by definition any other action would have produced a "miss". I know this is slightly simplistic but if you think about it in reverse, to generate a "hit" both aircraft have to occupy the same small space over the same very small time period. Any changes to the trajectories of either will quickly disrupt this meeting. Finally, it's because the manual says: "
NEVER MANOUVER AGAINST AN RA".
Those are not the only two options. Another arises when you have an advisory to manoeuvre to avoid a conflict from same-level or higher, and you have an aircraft below you in sight. What would your decision be and why?
Follow the RA. The danger in aviation comes mostly from the aircraft you can't/haven't seen or aren't aware of. I would posit that you are unlikely to hit an aircraft that you have in plain sight as you can tell if it is on a constant relative bearing or not; also you have the option of a lateral manoeuvre to de-conflict. If the a/c below has TCAS, there might be coordination going on that you are unaware of. If it doesn't, then they'll (like you) probably follow the rules of the air to avoid a collision (if they've seen you!)
I find detailed technical discussions about most things to do with aviation absolutely fascinating... but the job has shown me that for some scenarios, you have to have a fairly rote response prepared for immediate use. I applaud scientific examination of the limits of aircraft systems but when you get to 99.99% confidence (or whatever) in a particular one, especially if it involves time-critical warnings, you just have to say: "It works, do what it says" as there isn't any room left to do a risk analysis on an alternative response.
I have had several RA's in real life, one caused by an aircraft directly above deciding to descend at a great rate through our level. Did TCAS save our lives? Don't know but it was f***ing close when we did eventually see it.