View Full Version : TCAS question

6th May 2015, 00:44
Anybody have an explanation for what happened in this case...

The Arctic Rides LLC. Dassault-Breuget Falcon 50, N900KE, was enroute from PANC, Anchorage
to KFAR, Fargo at FL390. Winnipeg Centre passed an advisory of crossing traffic 1000 feet above
at FL400. The Fast Air Ltd. Gulfstream G150, C-FREE, operating as PBR150 was enroute from
CYWG, Winnipeg to CYYC, Calgary at FL400. Both aircraft were in visual contact and were
receiving erroneous TCAS Resolution Advisories (RA). N900KE was being advised to climb and CFREE
was being advised to descend. The TCAS RAs were disregarded by both crews. The aircraft
subsequently passed each other with approximately 1 mile lateral and 1000 feet vertical
separation. The event was discussed on a company frequency by the crews of each aircraft. The
crew of N900KE had noted a similar event approximately one hour previously while in Edmonton
Centre airspace. As a result of the two events, the crew of N900KE selected Traffic Advisory (TA)
only on their TCAS. Subsequent testing of the system did not reveal any faults however further
analysis of the TCAS processor will be carried out by the manufacturer. The crew of C-FREE
cancelled RVSM for the remainder of the itinerary. Subsequent testing did not reveal any faults and
the system was returned to service.

6th May 2015, 01:11
just for a question

what are the maximum authorized altitudes for each type of plane?

6th May 2015, 02:03
Do you mean service ceiling? It's 45,000 ft for the G150 and 49,000 ft for the Falcon 50.
I stand to be corrected.

7th May 2015, 06:01
relative to topic: have seen garbling of the TCAS signal received by ATC. The airplane would appear simultaneously at 4 positions at the same time on the radar. One of the TCAS blocks was found faulty. Could be similar case in vertical plane.

7th May 2015, 16:15
There goes my confidence.

8th May 2015, 05:49
Was just at a safety conference. It was stated that a crew is not authorized to disregard a RA....not sure when one can...

Set to TA only appears to violate the TCAS rules as well...

Again, just what was stated on TCAS RA at a safety conference.

8th May 2015, 06:25
>Was just at a safety conference. It was stated that a crew is not authorized to disregard a RA....not sure when one can...

Set to TA only appears to violate the TCAS rules as well...

Again, just what was stated on TCAS RA at a safety conference.<

You are living a bit too much by a book. Common sense, not to fly straight into a plane one can see, overrides everything in MY BOOK.

Amadis of Gaul
8th May 2015, 17:59
Was just at a safety conference. It was stated that a crew is not authorized to disregard a RA....not sure when one can...

What safety conference was that? Here's a scenario for you, this one is from my CRJ days. We were approaching 33L at BOS with one of Cape Air's 402s approaching 33R. Beautiful day, clear and a million, we were getting an RA off of the 402. We had him in sight the whole time, landed no problem, he landed no problem. We did let the tower know we got the RA, so they can maybe work on spacing a little better (fat chance), but there was no problem.

9th May 2015, 00:01
GCAA Risk Management Seminar, there was a panel discussion specifically on TCAS RA.
I did admit that the information was from a conference, and I really dont know the parameters, but that is what was stated....

I know the common sense part, but it appeared that an RA requires some action by the crew.....again, it was just fresh from last week.

9th May 2015, 08:49
Crews may indeed ignore an RA if the situation calls for it, such as the scenarios described in this thread.

From FAA AC 120-55C (http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC_120-55C_CHG_1.pdf):

(2) When an RA occurs, the PF should respond immediately by directing attention to RA displays and maneuver as indicated, unless doing so would jeopardize the safe operation of the flight or the flightcrew can ensure separation with the help of definitive visual acquisition of the aircraft causing the RA.

By not responding to an RA, the flightcrew effectively takes responsibility for achieving safe separation.

Some TCAS horror stories from the past, compiled from NASA reports:

TCASII--Genie Out of the Bottle? (http://asrs.arc.nasa.gov/publications/directline/dl4_tcas.htm)

10th May 2015, 19:40
To the Gallic poster above, if I remember correctly (retired) we were supposed to set the TCAS to TA only if on approach to parallel runways when both were in use for landing.
PS not relevant to original post, which sounds like a system or calibration error.

12th May 2015, 09:25

From what is available in your description I would hazard a "pop-up" RA.

I used to have an Eurocontrol RVSM+TCAS presentation created in about 2002 that explained this with some simple graphics. The gist of it (please excuse some loosely used technical terms):
- the sampling of altitude reported is in 25 or 100 ft increments, depending on the installed equipment; the values being processed are discrete, not continuous
- if the source air data would change from 38849 feet to 38851 feet (difference of mere 2', a sure occurence), the numerical value further down the processing chain goes from 38900 to 39000. Over the sampling cycle this (percieved instanteous) difference will yield quite high pseudo vertical rate seen by the other aircraft
- provided other altitude/range/CPA tests are satisfied, TCAS computer will issue an advisory. (Trash-in, trash-out)

I suppose there are elaborate filtering algorithms implemented, but certain geometric configuration will eventually create the result you describe. Some usual suspects:
- what were the actual levels of the two A/C as reported by their respective XPDRsd
- any turbulence experienced at the time
- any previous "misbehavings" of altitude reporting / keeping systems of the two A/C
- what are the sampling rates of TCAS installations (25 or 100 ft)
- were A/Ps engaged during the occurence
- were the A/Ps correctly slaved to the same air data source as the active TCAS XPDR
- are the TCAS boxes involved OEM or TSOed retrofits

12th May 2015, 17:12

We used to get a question on our TCAS exam about a Pop-up RA where two aircraft very close to each other could have a situation where there was no TA but it went straight to the RA. Turbulence could be a reason for this happening. Not enough detail is given here but maybe that is the case for this incident.

Has anybody out there ever had a pop-up RA. The details would be interesting.

12th May 2015, 17:42
Pleasure is mine to review some old knowledge. The paper is here: http://www.skybrary.aero/solutions/levelbust/ResPool/E_RVSM.pdf, page 26.