TCAS II: two tread detection logics, which one is true?

[upsangel]

I currently doing some research works on the TCAS, the references online is quite unclear about its working logic, lets assume:

DHL and FEX are flying in parallel, @same height, horizontal separation far enough. DHL flies 10 times faster than FEX and takes over it.

Logic1: TCAS tracks and then projects both planes route on a global coordinate, for sure it will found that the two routes are parallel and has no interception, alarm would not be triggered.

Logic2: TCAS only tracks the range and relevant closing speed of the two, that means the TCAS shows the planes is rapidly coming closure to it. TCAS will alarm.

Which logic is true to TCAS???

[rudderrudderrat]

Hi upsangel,

"The system includes:
A single channel TCAS computer
Two TCAS antennas
Two mode S ATC transponders, one active the other in standby.
These transponders allow:
o
Interface between the ATC/TCAS control panel and the TCAS computer
o
Communication between the aircraft and intruders equipped with a TCAS system.

The TCAS interrogates transponder of intruders. From the transponder replies, the TCAS determines for each intruder:
Its relative bearing
Its range and closure rate
Its relative altitude if available (ATC mode C or S)

Then the TCAS computes the intruder trajectory, the Closest Point of Approach (CPA) and the estimated time (TAU) before reaching the CPA.

Each time the relative position of the intruder presents a collision threat, aural and visual advisories are triggered.

TCAS optimizes vertical orders to ensure a sufficient trajectory separation and a minimal vertical speed variation considering all intruders."

Logic 2 is correct.

[hawk37]

Rudderrudderrat,

Does the tcas unit use the "relative bearing" in any of it's calculations?

Or is this relative bearing only used as a pilot aid, showing up on the display, to help him/her acquire the traffic visually?

[upsangel]

thanks rudderrudderrat for your quick reply!

I also read "Then the TCAS computes the intruder trajectory, the Closest Point of Approach (CPA) and the estimated time (TAU) before reaching the CPA" in some online documents. It is the sentence which caused confusion,

1. the CPA concept is a objective one,

2. "computes the intruder trajectory" implies that the trajectory is in a 3D objective coordinate. If TCAS runs under the relative approach, then the "trajectory" is always pointing towards the TCAS plain itself, rather than pointing to the CPA.

You are very welcome to further discuss on this very curious topic!~:)

[rudderrudderrat]

Hi,

The simplest and most robust way to calculate if anything is going to hit you or miss is to see:
1) How close is it?
2) Is it getting closer?
3) Is the relative bearing constant or changing rapidly?
4) Will it hurt?

If the answer is getting closer & constant relative bearing & relative height < X ft then the answer is yes it may hurt therefore warn.
If the answer is closer but bearing changing rapidly or relative height > X ft - then just advise.
If the relative height is 1,000 ft or more (+/ a bit for error) then don't say anything.

[Capt Pit Bull]

Relative bearing is not used in the collision avoidance calculation.

It is a key learning point for TCAS: if bearing data is not available for any reason (e.g. some installations can have a non directional lower antenna) any RA generated is still valid.

Bearing data is only used for for traffic display plotting.

[Natstrackalpha]

In your question though: Logic1: TCAS tracks and then projects both planes route on a global coordinate, for sure it will found that the two routes are parallel and has no interception, alarm would not be triggered.

Well, yes, but don`t forget proximity of the aircraft to each other, too.

TCAS

Answer - 2

[fellman]

Capt Pit Bull is essentially correct except in one detail - with TCAS Version 7 the relative bearing IS used by an internal TCAS collision avoidance function called the Miss Distance Filter: an estimate of the predicted minimum horizontal separation is made based on a linear projection of the aircraft trajectories. If TCAS diagnoses that this prediction is reliable (i.e. neither aircraft is manoeuvring by turning or changing speed) AND that the minimum separation is large enough (i.e. greater than the TCAS parameter DMOD - 0.2NM at low altitude up to 1.1NM at high altitude) then the generation of an RA can be suppressed.
If the predicition is not reliable or the predicted separation is not large enough then an RA will be generated in the normal way.
So the relative bearing is not used to generate RAs, but it can be used (in favourable circumstances) to suppress unnecessary RAs.

[Bleedvalve]

Sorry for a bit of jumping in here with a different question.

What will cause a TCAS system to intermittendly not display other targets? See them visually but nothing indicating on TCAS.

[Natstrackalpha]

Sorry for a bit of jumping in here with a different question.

What will cause a TCAS system to intermittendly not display other targets? See them visually but nothing indicating on TCAS.

BV Maybe its because . . . the pilots in the other aircraft have switched to Standby momentarily in order to change squawk.:)

[upsangel]

I should re-phrase the question like this:
the DHL is flying absolutely to the EAST
the FEX is flying absolutely to the WEST
they are at the same altitude,
horizontal separation is 1.1nm

any chance the TCAS will alert??

If no, Logic 1 wins.
If yes, Logic 2 wins, if relative bearing is not used in the collision avoidance calculation.
right?

[Natstrackalpha]

(If in the cruise):

Avoid impact

When back on ground, consider, changing TCAS, alternative line of work - burning licence getting seriously drunk.

If on approach, then answer = Yes. and TCAS RA unless both aircraft are going away from each other, even without RA for whatever reason you would have already taken avoiding action - admittedly not as dramtically as above, you should study TCAS its a great subject.