xpdr TCAS conflict

Center_Mid

Well-Known Member
A question for pilots who fly anything TCAS-equipped:

I've heard a rumor that when a GA airplane leaves its transponder in altitude reporting mode when on the ground, it sets off TCAS alarms in nearby taxiing airliners. Is this just one of those half-informed "facts" swirling around my FBO or is there truth to this?

I fly small planes out of an airport with regular jet traffic, so I was curious...
 

MikeD

Administrator
Staff member
[ QUOTE ]
A question for pilots who fly anything TCAS-equipped:

I've heard a rumor that when a GA airplane leaves its transponder in altitude reporting mode when on the ground, it sets off TCAS alarms in nearby taxiing airliners. Is this just one of those half-informed "facts" swirling around my FBO or is there truth to this?

I fly small planes out of an airport with regular jet traffic, so I was curious...

[/ QUOTE ]

Don't know specifically. But there's been times when after I've landed and am in dearm, ground will make a blanket call of "aircraft in dearm, squawk standby". Now I don't know if this was for TCAS or for the RAPCONs radar; but the last couple of times it's happened, there's been a heavy taxiing or inbound.

So I'm curious to.
 

IrishSheepdog

Sitting in the median
It won't provide a problem for the aircraft on the ground (since most airliners leave their TCAS in standby mode until ready for takeoff, as we do). It will, however, possibly create a problem for an aircraft on approach. As the aircraft is coming in, say for a visual approach, it may recognize the aircraft on the ground taxiing and provide a breakaway maneuver with a climb away from the airport.
 

junkstream

Well-Known Member
"It will, however, possibly create a problem for an aircraft on approach. As the aircraft is coming in, say for a visual approach, it may recognize the aircraft on the ground taxiing and provide a breakaway maneuver with a climb away from the airport."

Not really. Some class B airports have aircraft taxi with the transponder on and this creates NO problem for inbound aircraft.

TCAS interrogates other transponders to determine the position and azimuth of conflicts. TCAS has two "modes" TA or Traffic Advisory and RA or Resolution Advisory. As we fly around we are able to see other transponder equiped aircraft and their relative position on the TCAS display. They appear as blue diamonds with an indication of their altitude relative to us and whether they are climbing or descending.

If the TCAS determines a possible conflict it will give a TA and announce "TRAFFIC, TRAFFIC" the blue diamond changes to yellow. This happens all the time on approaches to parallel runways--no big deal if you have visual contact with the other guy.

If the TCAS plots a possible collision within the next 5-15 seconds (based on closure rate) it will give a RA and announce "CLIMB, CLIMB" or "DESCEND, DESCEND" at the same time the target aircraft diamond turns red and the VSI has a green arc that appears at the vertical speed you need to be at and a red arc at the vertical speed you must avoid. It will announce "CLIMB FASTER" if you are not complying. When clear of the conflict it first announces "MONITOR VERTICAL SPEED" and then "CLEAR OF CONFLICT."

We get TA's all the time, but I've only had a couple RA's. One just this week on a very clear VFR day under ATC control at 9000 feet on descent. We would have hit a cherokee without the RA. About the same time as the RA, ATC announced "traffic 12 o'clock 2 miles" and you could hear the ATC collision alarm in the background.

Anyway my point here, leaving a transponder on on the ground will not cause a TCAS equiped plane to engage in some kind of "breakaway manuever" because the RA mode is disabled when the TCAS equiped plane extends it's gear--usually at the marker--a good 5-8 miles from the threshold. The TCAS is disabled completely with weight-on-wheels.

More than you ever wanted to know . . .
 

Jason

Well-Known Member
MikeD - One of the reasons that they have asked you to squawk standby may have simply been to clear the radar screen. Some larger airports have a ground control radar system that interrogates transponders. Having a a group of aircraft in a small area with their transponders on may cause the controller's radar to have a fit as it would probably see it as a conflict situation and set of the alert. Or maybe the heavy taxiing toward the dearm area would cause an alert if bothe aircraft's transponders are on. I don't really know - that's just a guess on my part BUT I do know that it wouldn't be because of a TCAS issue.

Jason
 

Cheechako

Well-Known Member
Jason, we had that very thing happen in DFW a couple weeks back. There were about 8 of us sitting on one of the taxiways waiting for our gates to open. DFW is asking everyone to keep their transponders turned on when taxiing around. The controller said something to the effect "All the RJs waiting for their gate down there, turn your transponders off- it's creating a jumbled mess on my screen."

Back to the TCAS thing, I think most airliners' TCAS units inhibit RAs below a certain height AGL. That way you don't get escape guidence when on short final because of another airplane on the taxiway or parallel runway.

Some will turn their TCAS on when waiting to depart behind a heavy to see how far it is when cleared for takeoff.
 

SaabFO

New Member
I know in our Saabs the TCAS TAs and RAs are inhibited below 900 feet AGL while we're descending and 1100 feet AGL while we're climbing. I'm not sure about other aircraft though. Some airports like Milwaukee actually require aircraft to leave their transponders on while on the taxiways and runways.
 
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