View Full Version : TCAS -Turbulence avoidance applicability

22nd May 2008, 00:09
Is TCAS practical for wake avoidance purposes?

Climbing to alt and falling into line behind other traffic, would the other traffic be displayed on the ND? Being 5 miles ahead on the same track, it's not a threat, but shouldn't it display as proximate traffic, even if you've got the range on the ND greater than 40 NM?

From what I've seen the answer is apparently not.

Any insight into the vagaries of proximate traffic display would be appreciated.

22nd May 2008, 14:59
It is not certified for it so be careful.

Wake vortices decay as a function of time, not distance. Approach controllers convert time into distance as that is what they work with. I would suggest that most TCAS equipped aircraft have a greater TAS in the en-route phase than established on an ILS approach under radar vectoring.

Being 5 miles ahead on the same route can be a threat in en-route.

If you are climbing and falling in behind another aircraft in this situation, you have a high probability of an encounter if the othe aircraft is level and you then climb up through its descending vortex.

Yes, it is a real threat and I have worked on a couple of accidents related to it. However, as there were no regulations that the controllers had broken in these countries and the national investigators were part of the government (as were ATC) then it was never pressed home in the reports. Oh, in both cases the pilots were blamed but they all died and nobody stood up for them with the technical knowledge to prove otherwise.

22nd May 2008, 15:54
"The A340 reduced its speed on its own, miles too early on approach, to increase the distance from the preceding aircraft (based on the TCAS traffic display). It messed up the sequence and an A320 was then only 4 NM behind it! I was obliged to make the A320 perform an "S" for delay".

Be very careful that you do not use TCAS displays for functions that were not intended.

The incident above is from excellent article from Eurocontrol on Incorrect Use of the TCAS Traffic Display (http://www.eurocontrol.int/msa/gallery/content/public/documents/ACAS_Bulletin_6_Mar-05.pdf). The article describes some of the limitations of the display.

One big issue is that the TCAS display shows relative positions. Your aircraft is a moving object, not a fixed point, and your interpretation of the relative motion of a target might be quite misleading.

Even if you are 'in trail' behind an aircraft 5 miles ahead, be alert to the fact that the TCAS traffic might not be the aircraft that you are 'number 2' to. Not all the targets in the display range are displayed.

Finally, it is not unusual for TCAS target bearings to be in error more than 5 degrees, possibly greater than 30 degrees :eek:

I have seen examples of this when my Wx radar paints nearby aircraft -- the Wx return and the TCAS target frequently do not match! In fact, I am more confident in the bearing accuracy of a Wx radar paint from a crossing/preceding/passing, than the TCAS bearing.

Having said that, I do peek at the TCAS to see if I am closing in on a preceding. I just try to remain aware that "it" might not be "him."


Climbing to alt and falling into line behind other traffic, would the other traffic be displayed on the ND?
Not all targets are displayed, including targets that are proximate. There is a lot of fancy mathematics going on in that TCAS computer, and lots of IF and AND and OR stuff to boot!

Excuse me for asking, but what levels do you fly at? Would you be placed only 5 or 6nm behind a preceding at FL330, for example?

22nd May 2008, 19:26
I don't fly, I'm an engineer who evaluates technical safety issues in airline ops ( sort of a Flt Safety Specialist) so my knowledge of operating specifics isn't up to ATPL stds.

I asked the question with a mind to a recent A319 turbulence encounter I've been watching. From what I have heard/understand, the aircraft was climbing up to a position 8 miles behind a 747 on the same airway. It's only hearsay at this point but they had not been properly advised of the traffic and it wouldn't have shown on their TCAS (ND was in PLAN mode @ 300 mi). I was trying to stumble to some kind of understanding of how TCAS, if possible, could have been used so as to have avoided the situation.

Then there's the question - should it be used that way?

23rd May 2008, 09:12
NNC0, to make things simple, the answer to your question is a big NO.
The TCAS is not designed for that and the purpose of the display is not to show the accurate position of targets (other a/c).
It is forbiden to maneuver the a/c to avoid other traffic (as shown on the TCAS) display, in the absence of a TCAS RA (resolution advisory).
It helps indeed for situational awareness, but under no circumstances it should be a tool for the pilot to take the role of the atc controller.