Strange TCAS Behaviour

[Transition Layer]

Sitting at FL350 the other night, company aircraft, same type (both B744s), was sitting directly beneath us at FL330. He was so accurately under us in fact the RAD ALT was showing 2060'-2080' continuously. I've seen the radio altimeter flash up when flying over aircraft in the past but this was permanently on, very impressive!

Anyway, the TCAS initially showed him as 2000ft below, which was true, but for periods of 5-10 mins the TCAS return disappeared. Talked to him on 123.45 and sure enough, we had disappeared from his display too. Just wondering if there is some sort of "black hole" which exists for aircraft remaining stationery relative to each other, directly on top or beneath of one another.

Cheers,
TL

[bflyer]

Hi
TCAS works on the basis of 'TALKING' to other TCAS-equipped aircrafts,..if an aircraft is not ACAS-equipped, it will not show on your screen..so depending on your aircraft's particular antenna installation..both your aircrafts were so perfectly positioned on top of each other the line of transmission was blocked..hence..no return
my topence worth...cheers

[EngineOut]

bflyer,

that is not quite correct. TCAS will display an aircraft with a transponder turned on. Even in Mode A (so you don't get any height read out from the target, just the target displayed). The TCAS II systems talk to each other in a RA, so that they both don't climb together or both do descend together. You will even get an RA in response to standard Mode C equipped aircraft. I had one recently sent off by a cherokee. Now as TCAS II is in the region of $300,000USD plus, I doubt the PA28 would have had it fitted!!

TL,
My guess would be, that at a particular relative alignment of the two aircraft, the your lower TCAS antenna was line of sight blocked from the lower aircraft's transponder or TCAS antenna simply due to the close proximity and much greater angles.

[Capt Pit Bull]

Well, the directional qualities of the antennae are (if memory serves) only specified for +/- 15 degrees in azimuth (although many will function well beyond that, 90 degrees might be pushing it). As you don't get 'NO BEARING' proximate traffic (only for TAs and RAs) I guess that might explain it.

TCAS will display an aircraft with a transponder turned on

It won't always. Another cause for target loss is interference limiting mode, but I wouldn't have though this was the case (kind of short range to be filtered).

If the lower antenna was blocked shouldn't matter, TCAS should contact via the upper antenna, in fact the upper antenna is the default, to avoid indirect transmission paths from the ground where possible.

As far as I am aware, every modern TCAS installation includes upper and lower antenna for transponders and the TCAS itself. There was problematic function of TCAS in its early days due to restricted line of sight from single antenna.

Having said that, on some installations the lower TCAS antenna is not directional at all. Since the upper antenna is the one that usually does all the work, it has to be specified to be directional, a few quid can be saved on the lower antenna. Maybe thats the case on your aircraft.

Note that the other aircraft would still be tracked, and a no bearing TA and/or RA issued accordingly.

Finally of course, maybe your lower antenna actually was broke. Again, since the upper is the main, I would imagine that a u/s or weak lower antenna could be carried for quite a while before anyone noticed (supposition here).

pb

[Sallyann1234]

"Finally of course, maybe your lower antenna actually was broke. Again, since the upper is the main, I would imagine that a u/s or weak lower antenna could be carried for quite a while before anyone noticed (supposition here)"

But this wouldn't explain why "we had disappeared from his display too"

[763 jock]

Had exactly the same yesterday with a 330 2000' directly above. Must be a blind spot or similar.

[Graybeard]

There have been lots of options with TCAS installations of antennas. However, the 747-4 would have been delivered from BAC with dual directional TCAS antennas, and yes, transponder antennas top and bottom.

The antennas are monitored electronically, so any failure would be flagged. Due to FOD, etc., most of the directional antenna failures are the bottom ones, btw. That's certainly more convenient for the mechanic than replacing the upper one.

Rate of Closure and Range are the primary determinants of TCAS alerts. You had zero rate of closure, so TCAS wasn't worried about a collision, and hence, no need to display a "No Bearing" alert.

I agree that the directional antennas could not resolve the bearing angle, so dropped the target from display. Note, determining target bearing has nothing to do with collision avoidance: it is there just to help the pilot acquire the traffic visually.

GB

[ManaAdaSystem]

I've seen this on a number of occasions. Other aircraft 2000 ft below and catching up with us, the TCAS target starts to move around when the other aircraft is approaching directly below us. It often moves rapidly inside a radius of about one mile, disappears completely, pops up again, moves around and finally stabilizes when the other aircraft is a mile or so ahead of us.

First time I actually thought we had a playful fighter on our hands, but we soon realized it must be a spot where TCAS has some problems detecting the other aircraft.

[Graybeard]

Back in the early days of TCAS, I was reviewing pilot squawks that had resulted in the TCAS processor being removed. There were a few strange writeups, centered around southern Nevada, such as, "We had four targets, our FL350 altitude, approaching from four directions. ATC said no other a/c in the area."

I called a meeting of the airline's engineering and Flt Ops, some of which were USAF Reserve pilots. I explained this was surely the work of ECM, Electronic CounterMeasures at Nellis AFB, (or even Area 51?). I suggested they tell the USAF to knock that sh*t off. We never saw another such squawk.

GB

[gearpins]

Hello everyone
TCAS has a priority logic that helps to optimise transponder interrogation time and prioratize it in such a fashion that it will display(depending on the model) 7 to10 a/c simultaniously.
so could it not be that, other tcas traffic popped up on screen at the same time as the one below/above dropped off ?