TCAS reliability
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Some things to remember about TCAS II (which is what is fitted in most of the TCAS equipped aeroplanes).
a.k.a. TCAS urban myths.
1 "the TCAS tracks targets within +-2700'"
What the TCAS displays and what your TCAS is tracking are two different things. Selecting 'above' or 'below' is only to reduce clutter on the pilot's TCAS display.
A TCAS II unit will track targets +-9,200 above, level with and below you at several times the slant distance compared to the 6nm or 12nm user selectable display range, regardless of the pilots' display selections.
2. "ATC can see my SSR so his TCAS should see it too."
Not always. My TCAS relies on your TXDR squawking in response when my TCAS interrogates it, not in response to a ground based SSR interrogation. So the fact that a very powerful ground based transmitter elicits a response from your TXDR, does not guarantee that my TCAS will see you too. It SHOULD
but it ain't always so!
Remember, the TCAS equipment is just that -- electronic gadgetry. Very clever and ingenious electronic gadgetry, but an electronic gadget designed by humans.
Using a GPS without being aware of its function and limitations, and without cross checking, is lazy and unprofessional. Using an altimeter without being aware of its basic function and limitations, is also lazy, and unprofessional. And when added to the mix on a bad day, can be fatal.
Same with TCAS. A very clever aid useful in avoiding collisions, yes. Use it in blind faith, then maybe you will hit someone!
There are a couple of excellent topics on TCAS in the current Flight Safety Aus mag --- have a read!
a.k.a. TCAS urban myths.
1 "the TCAS tracks targets within +-2700'"
What the TCAS displays and what your TCAS is tracking are two different things. Selecting 'above' or 'below' is only to reduce clutter on the pilot's TCAS display.
A TCAS II unit will track targets +-9,200 above, level with and below you at several times the slant distance compared to the 6nm or 12nm user selectable display range, regardless of the pilots' display selections.
2. "ATC can see my SSR so his TCAS should see it too."
Not always. My TCAS relies on your TXDR squawking in response when my TCAS interrogates it, not in response to a ground based SSR interrogation. So the fact that a very powerful ground based transmitter elicits a response from your TXDR, does not guarantee that my TCAS will see you too. It SHOULD
but it ain't always so!Remember, the TCAS equipment is just that -- electronic gadgetry. Very clever and ingenious electronic gadgetry, but an electronic gadget designed by humans.
Using a GPS without being aware of its function and limitations, and without cross checking, is lazy and unprofessional. Using an altimeter without being aware of its basic function and limitations, is also lazy, and unprofessional. And when added to the mix on a bad day, can be fatal.
Same with TCAS. A very clever aid useful in avoiding collisions, yes. Use it in blind faith, then maybe you will hit someone!
There are a couple of excellent topics on TCAS in the current Flight Safety Aus mag --- have a read!
Thread Starter
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I think i understand TCAS a little better know. As a follow up question If the TCAS equipped aircraft queries an aircrafts transponder and the transponder responds also the aircraft is responding to the SSR poll what are the limits to how many TCAS/SSR response a transponder can give in say a minute. Is this fixed or say do older transponders have performance less then newer ones.
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Here is some basic information from the Honeywell site site about ACAS/TCAS systems. There is lots of useful stuff inside.
ACAS/TCAS
AIRBORNE/TRAFFIC COLLISION AVOIDANCE SYSTEM
Understanding ACAS/TCAS II
{snip by ITCZ}
TCAS LIMITATIONS
A flight crew may visually identify an intruder that is not displayed in the cockpit or which disappears from the display even though the system is functioning properly.
Certain situations exist in which TCAS cannot provide traffic awareness, target display, or collision
protection, but these limitations are the result of the intruder aircraft not carrying functioning, altitude-reporting transponders.
In other cases, TCAS is limited by system design parameters, equipment installation configuration, and control panel settings, causing an intruder to be absent or disappearing (dropout) from the display. These will be explained in the following paragraphs.
{snip by ITCZ}
ATCRBS and Mode S Transponders will NOT reply if they do NOT hear a TCAS interrogating signal. A transponder may not hear a TCAS interrogation signal for any one of the following reasons: the signal may be blocked, too weak, or too far off center frequency; the intruder aircraft may have only a lower antenna and be shielded from TCAS interrogations from above; or the transponder may have poor receiver sensitivity. If transponders don't reply, TCAS cannot track these intruders.
Hands up all Ppruner's that think the wattage of the airborne TCAS equipment is equal to that emitted by the ATC SSR interrogator...?
Hands up those Ppruner's that can identify TCAS/SSR antennae mounted on upper and lower surfaces of their aircraft?
Hmm!
When TCAS is Limited
TCAS does not display all traffic. Even with the cockpit controls set properly, sometimes intruders will not appear. However, pilots should not be concerned. Even though the following list of items are limitations of the system, TCAS is designed to protect the aircraft. These conditions do not affect the collision avoidance function. You will receive TAs (traffic advisories) and RAs (resolution advisories) if near mid-air geometry is predicted in any of the following conditions.......
{snip by ITCZ}
"the TCAS does not display all traffic"
Remember that phrase - its from the manufacturer!
It was not designed to help a pilot "see and avoid" traffic.
That is why it does not say "Traffic! Three o'clock, below!"
It was designed to protect the aircraft.
Very different.
Use this equipment, like everything else in an aircraft, within its design limitations.
ACAS/TCAS
AIRBORNE/TRAFFIC COLLISION AVOIDANCE SYSTEM
Understanding ACAS/TCAS II
{snip by ITCZ}
TCAS LIMITATIONS
A flight crew may visually identify an intruder that is not displayed in the cockpit or which disappears from the display even though the system is functioning properly.
Certain situations exist in which TCAS cannot provide traffic awareness, target display, or collision
protection, but these limitations are the result of the intruder aircraft not carrying functioning, altitude-reporting transponders.
In other cases, TCAS is limited by system design parameters, equipment installation configuration, and control panel settings, causing an intruder to be absent or disappearing (dropout) from the display. These will be explained in the following paragraphs.
{snip by ITCZ}
ATCRBS and Mode S Transponders will NOT reply if they do NOT hear a TCAS interrogating signal. A transponder may not hear a TCAS interrogation signal for any one of the following reasons: the signal may be blocked, too weak, or too far off center frequency; the intruder aircraft may have only a lower antenna and be shielded from TCAS interrogations from above; or the transponder may have poor receiver sensitivity. If transponders don't reply, TCAS cannot track these intruders.
Hands up all Ppruner's that think the wattage of the airborne TCAS equipment is equal to that emitted by the ATC SSR interrogator...?
Hands up those Ppruner's that can identify TCAS/SSR antennae mounted on upper and lower surfaces of their aircraft?
Hmm!
When TCAS is Limited
TCAS does not display all traffic. Even with the cockpit controls set properly, sometimes intruders will not appear. However, pilots should not be concerned. Even though the following list of items are limitations of the system, TCAS is designed to protect the aircraft. These conditions do not affect the collision avoidance function. You will receive TAs (traffic advisories) and RAs (resolution advisories) if near mid-air geometry is predicted in any of the following conditions.......
{snip by ITCZ}
"the TCAS does not display all traffic"
Remember that phrase - its from the manufacturer!
It was not designed to help a pilot "see and avoid" traffic.
That is why it does not say "Traffic! Three o'clock, below!"
It was designed to protect the aircraft.
Very different.
Use this equipment, like everything else in an aircraft, within its design limitations.
Last edited by ITCZ; 28th Sep 2004 at 16:06.
