Originally Posted by
nnc0
The TCAS document referenced above states RA's are inhited below 1000 plus or minus 100. In practice I believe - RAs are inhibited below 900' on approach and I think below 1100 after climb. RA's become TA's when inhibited.
RA's are also suppressed during Windshear warnings.
Correct. Threats are displayed as TA below the inhibit altitude, and during windshear and Ground Prox warning.
Aircraft reporting within 180' of runway height are considered to be on the ground, and not displayed on TCAS.
I was riding jumpseat in a 737-3 into rwy 30 at KLGB one dark December early evening rush hour a dozen years ago when TCAS was still in its youth. When we were at about 1200' on the ILS, the TCAS said, "Monitor Vertical Speed," and the whole upper arc of the RA/TA/VSI went red. There was a hover bird at about our altitude right over the middle of the airport, giving us a red target.
Then again, "Monitor Vertical Speed," and the whole lower arc of the VSI went red too, leaving just a tiny green arc at 0' VS. There was a light aircraft on short final to 25L, the second red target. The captain (not flying) turned around to me, "What do we do now?"
Almost before I chuckled the words out to go ahead and land, the traffic on 25L sank below 200' and we crossed 900', shutting off the warnings.
As for parallel approaches, they are a nuisance when one or more of the aircraft are on base leg, and a collision appears likely to a TCAS. Once both planes are on final, the rate of closure between them goes to near zero, and the TCAS shuts up. The key to TCAS is projected altitude difference and rate of closure.
GB