ACAS (TCAS)
 

Flying at your MSA in IMC and ACAS gives a "descend" warning, is this possible? And if so what would you do?

this has happened with TCAS, Known as the Ueberlingen diaster where as a result a mid-air collision occurred with a transport plane and a russian charter passenger jet in July 2002.


The best thing to do is follow your TCAS. (I have no professional expierence or any flight expierence).

I agree Mike that descending is better than a possible mid air. I believe that if flying IFR the MSA is 2,000ft above highest point, could be wrong though. However you may have been VFR and have been forced into IMC 1,000ft or less above the highest point.

I realise there are many factors here and obviously you would like to fly around higher than your MSA in IMC. Just thought it was an interesting point and wondered what people opinions were.

MSA always give 1000' obstruction clearance on top of the highest obstacle within the 25 nm radius around the approach reference navaid.
So, if you know the terrain configuration around your destination airport, you might feel safer to descend 500' rather than risk a mid-air.
But, if you are with approach, they may have given you some traffic information that helps understand the picture...

Beware that the 2000' clearance is not for MSA, but for MOCA in designated mountainous area.
In non-mountainous area MOCA clearance is also 1000' over highest obstacle in the 8nm corridor centered on the airway
(+ the angular mid-way protection area + progressive clearance in the 2nm buffer zone).

The MORA (or OROCA) also gives a grid clearance of 1000' for normal region, or 2000' in DMR (designated mountainous region).

Luc

Yes, your senario is possible. But don't forget that that RA's are no longer given when the EGPWS starts to yell at you (TCAS announciation drops to "TA Only"). This also happens within 1,000' RA as well. So the advice would remain, follow the RA. Your technical manuals should have this in them as well.

The mantra is always follow your RA, never ever ever never disobey it. So, you have no choice- either great danger of collision if you do disobey, or obey and increase hazard from terrain. If the RA requires you to significantly use up your MSA safety margin, unless you are over a plateau with high, flat terrain, you may have no problem as the highest peak involved that sets your MSA may well be 30 miles or more away. Therefore, I think obey the RA, cross your fingers behind your back and whistle a happy tune?

Never used TCAS, coz don't fly for a living, so sorry in advance if posting here innapropriate.
A very similar situation occurred a while ago in Christchurch, NZ.I'm slightly fuzzy on the details, having read it in a company safety briefing a few months back, but SF34 immediately after takeoff, entering IMC on a SID climb, descend RA, with no prior TA or warning of any kind. Readout showed the conflicting traffic 500' above, closing from 12 o'clock. Captain elected to level off, rather than descend. The terrain nearby is flat, with no high obstructions. (Unusual for an aerodrome, eh?). Traffic observed on TCAS passing 500' above, then (I think) dissappeared. ("Clear of conflict").
Turned out it was a transponder on test in a nearby hangar. The xpdr was shielded from ATC radar, but in a narrow window on departure, other aircraft could set it off. It had been set for a much higher level but there was something majorly wrong with the altitude encoder.
It was assessed that the crew had probably done the right thing in levelling off, rather than descending or ignoring it completely.