Had a TCAS RA responded to last week where an aircraft was descending out of 13,000 for 11,000 feet (get underneath side by side traffic at 12,000 feet), a departure level at 10,000 feet and at 11,700 feet descending to 11,000 feet aircraft said they were climbing due to a RA. That's a very large buffer to have to respond to a RA...
TCAS RA question
- Thread starter NovemberEcho
- Start date
ctab5060X
Well-Known Member
It's a time until collision more than an altitude that sets off the TCAS.
BobDDuck
Island Bus Driver
Slight necropost...
The closest I ever came to dying in an airplane was right there. I was in a RJ7 and before we switched to CTAF, Center pointed out a flight of 4 F18s traversing VR-84 at 1500 feet. We had them on TCAS showing 600 feet below us (we were at 2100 on the approach) and figured it would all work out. What we didn't know was that F18s were in non standard formation and the second element was +500 (and not squawking). My FO so them about a mile from us, just as they went for the deck to get out of our way.
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ASpilot2be
Qbicle seat warmer
I had an interesting TCAS RA the other day.
Coming into PDX from the South can be interesting on a nice day, there is a lot of VFR traffic on the Arrivals, including sky diving. Still havent figured that one out. I am sure @JordanD knows the pain.
Anyways, we were level at 10,000. There was a VFR aircraft coming at us at 9500. Our TCAS called out the traffic and said not to descent. Makes sense. That aircraft says he has us in sight, and center said altitudes his discretion. What does he do? He climbs, and sets off the RA. But we were clear of the conflict before we could react because he was just passing us as he climbed.
Coming into PDX from the South can be interesting on a nice day, there is a lot of VFR traffic on the Arrivals, including sky diving. Still havent figured that one out. I am sure @JordanD knows the pain.
Anyways, we were level at 10,000. There was a VFR aircraft coming at us at 9500. Our TCAS called out the traffic and said not to descent. Makes sense. That aircraft says he has us in sight, and center said altitudes his discretion. What does he do? He climbs, and sets off the RA. But we were clear of the conflict before we could react because he was just passing us as he climbed.
DE727UPS
Well-Known Member
I can see how that could happen. Would be nice if ATC would realize that TCAS requires greater separation than just one guy having a visual. Maybe they do and the controller screwed up. I've never had that happen before. Wonder to what extent TCAS is covered in the 7110.65?
NovemberEcho
Dergs favorite member
TCAS isn't covered at all. TCAS says you need 600', FAA says you need 500'. When we bring you 500' over/under a VFR we are perfectly legal whether you see him or not. I've seen TCAS cause more conflicts than avert them.
BobDDuck
Island Bus Driver
A constant 500 foot buffer will not trigger an RA. We'll probably get a TA but that's it. If the guy is CLIMBING up to the 500 foot buffer underneath us (or descending on top of us), TCAS has no way of knowing the target is going to stop 500 feet away and as such may, depending on convergence vectors and a bunch of other logic, throw an RA.
NovemberEcho
Dergs favorite member
In theory yes, but I have had multiple RA's from level flight with a 500' buffer. I've had RA's from 1500' buffers when one is climbing or descending. Half the time those RA's tell you to climb into other traffic!
Isnt the RA supposed to tell you to descend if they are climbing? Why would they have you climb INTO the traffic?
NovemberEcho
Dergs favorite member
I've had the TCAS Give an RA for a VFR that was 500' below them and they climbed into the IFR guy that was 1000' above them.
In that case if the vfr and ifr planes arnt changing altitude shouldnt the RA be MONITOR VERTICAL SPEED. You would think the TCAS would realize there are 2 planes there one above and one below and have you climb shallow so you dont hit the guy above or not climb or descent so you hit neither of them.
Joe blow vfr may be bumping up and down 100fpm when he appears level. Probably has something to do with it, and then an overcorrection exacerbates the issue
NovemberEcho
Dergs favorite member
You would think so.
Autothrust Blue
Welcome aboard the Washington State Ferries
absolutely not!Not sure if theres any rules against memes but I think this one really encapsulates the TCAS issue.
BobDDuck
Island Bus Driver
Probably because it saw a closure rate with the VFR intruder and the only out was to climb. It also knew it had 1000 feet on the other IFR guy which is plenty of time to trigger an RA for him to climb as well. Aircraft equipped with TCAS will "talk" to each other to decide the best course of action before triggering an RA. Obviously "dumb" non TCAS aircraft don't have that capability.
What about TIS or ADSB aircraft? The birds I fly get TAs but NOT RAs. All I get is a TCAS display popup and the aural, Traffic. 9 oclock. same altitude. 2 miles.
BobDDuck
Island Bus Driver
TIS just "sees" transponder pings in the area, triggered by ground ATC radar. It doesn't talk to anybody else and won't provide escape guidance.
ADBS has the capability to provide RA functionality, but I don't believe it does that as of yet.