Use of transponder on the ground
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TCAS is the Interrogator, not the Reporter
This is misleading: "if u have TCAS, just select the mode XNDR or ALTOFF NOT TA/RA.
It is especially important during low vis weather,so controller can track your progress.TARA only when entering runway in use.It is nerve recking to see alt reporting of an aircraft taxiing on // taxiway when you are taking off in low vis conditions."
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The transponder is the only device replying to TCAS, or replying to ATC SSR. If the Mode S transponder (required with TCAS) is reporting ON Ground, its replies will be ignored by TCAS interrogators in the area.
If the transponder is reporting altitude within 180 feet of runway elevation, as in a Mode C reply, it will likewise be ignored by TCAS in other aircraft. Do not turn off ALT replies (ALT OFF) on your ATC/TCAS control panel unless ordered by ATC, or your transponder will reply with no altitude, maybe causing false TA alerts to other aircraft. (The false alert part depends on the ON Ground priority in the calculations, which may not be consistent between TCAS and transponder manufacturers.)
TCAS coordination between planes is via the transponders. Your TCAS interrogates the other plane's transponder and acts on those replies. Your TCAS reports to the other aircraft via your own transponder. There is no TCAS to TCAS direct link, as there is no ATC SSR uplink to your plane, save for the reply lamp of the Mode AC world. The TCAS processor is just an airborne ATC Secondary Surveillance Radar.
A good way to operationally test your TCAS on the ground is to block the radio altimeter RF signal so it thinks you are above 2500 feet. Your TCAS then thinks it is at altitude, and will give you TAs and RAs from real traffic in the area. It has no effect on airborne traffic, as your transponder is still reporting ON Ground, and within 180 feet of field elevation.
GB
It is especially important during low vis weather,so controller can track your progress.TARA only when entering runway in use.It is nerve recking to see alt reporting of an aircraft taxiing on // taxiway when you are taking off in low vis conditions."
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The transponder is the only device replying to TCAS, or replying to ATC SSR. If the Mode S transponder (required with TCAS) is reporting ON Ground, its replies will be ignored by TCAS interrogators in the area.
If the transponder is reporting altitude within 180 feet of runway elevation, as in a Mode C reply, it will likewise be ignored by TCAS in other aircraft. Do not turn off ALT replies (ALT OFF) on your ATC/TCAS control panel unless ordered by ATC, or your transponder will reply with no altitude, maybe causing false TA alerts to other aircraft. (The false alert part depends on the ON Ground priority in the calculations, which may not be consistent between TCAS and transponder manufacturers.)
TCAS coordination between planes is via the transponders. Your TCAS interrogates the other plane's transponder and acts on those replies. Your TCAS reports to the other aircraft via your own transponder. There is no TCAS to TCAS direct link, as there is no ATC SSR uplink to your plane, save for the reply lamp of the Mode AC world. The TCAS processor is just an airborne ATC Secondary Surveillance Radar.
A good way to operationally test your TCAS on the ground is to block the radio altimeter RF signal so it thinks you are above 2500 feet. Your TCAS then thinks it is at altitude, and will give you TAs and RAs from real traffic in the area. It has no effect on airborne traffic, as your transponder is still reporting ON Ground, and within 180 feet of field elevation.
GB
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Greybeard,
Please explain what in my previous message was misleading...
I NEVER SUGGESTED TO SWITCH TO ALT OFF IN THE AIR
,however TARA is deselected during the AFTER LANDING flow.
ARE YOU suggesting to keep the TARA mode until shut down?so we could all see eachother?
Map is not showing aircrafts with ALTOFF on the GROUND but those with TARA selected.
I wrote seeing an aircraft on your map mode with altitude reporting ZERO on the taxiway parallel to runway interesting to say the least during a low vis take off.
XNDER/ALTOFF is required by many airports before asking for pushback.
Capt STD
Please explain what in my previous message was misleading...
I NEVER SUGGESTED TO SWITCH TO ALT OFF IN THE AIR
,however TARA is deselected during the AFTER LANDING flow.ARE YOU suggesting to keep the TARA mode until shut down?so we could all see eachother?
Map is not showing aircrafts with ALTOFF on the GROUND but those with TARA selected.I wrote seeing an aircraft on your map mode with altitude reporting ZERO on the taxiway parallel to runway interesting to say the least during a low vis take off.
XNDER/ALTOFF is required by many airports before asking for pushback.
Capt STD
Last edited by Boingboingdriver; 2nd Dec 2008 at 12:50.
I can only speak about Asia.
We NEVER turn off the XPDR or the TCAS in our CX 777's ( unless the NNC requires it to TA only )
-The modes S knows it's on the ground and wont trigger a TA/RA in another nearby A/C.
-now days we can NEVER see other A/C on the ground on our ND, even if they have their XPDR switched on. Since they modified the system I have NEVER seen another A/C TCAS target on the ground. ( We used to get the occasional TA from a parked A/C.)
-However you CAN see the airborne A/C coming and going.
They modified the system logic about 3 years or more ago, or was it when they introduced mode S? Not sure but it works fine.
We NEVER turn off the XPDR or the TCAS in our CX 777's ( unless the NNC requires it to TA only )
-The modes S knows it's on the ground and wont trigger a TA/RA in another nearby A/C.
-now days we can NEVER see other A/C on the ground on our ND, even if they have their XPDR switched on. Since they modified the system I have NEVER seen another A/C TCAS target on the ground. ( We used to get the occasional TA from a parked A/C.)
-However you CAN see the airborne A/C coming and going.
They modified the system logic about 3 years or more ago, or was it when they introduced mode S? Not sure but it works fine.
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I have seen on my PFD a yellow tcas symbol showing alt 0 during take off many times(paris and Vienna)...I believe it was due to both of us (me taking off) having TA or TARA mode selected.
I am aware there is no Traffic advisory /or resolution advisory triggered on the ground
Capt STD
I am aware there is no Traffic advisory /or resolution advisory triggered on the ground
Capt STD
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On the PFD in a 737?
Besides, it really depends what kind of box you have fitted and what the company advised SOPs for that box are. For ours (yes 737s as well) it is to let the transponder in TA/RA all the time except during degraded performance where we switch to TA. TCAS as such shouldn't have any problem with that as was described above very aptly by Graybeard.
Besides, it really depends what kind of box you have fitted and what the company advised SOPs for that box are. For ours (yes 737s as well) it is to let the transponder in TA/RA all the time except during degraded performance where we switch to TA. TCAS as such shouldn't have any problem with that as was described above very aptly by Graybeard.
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"I have seen on my PFD a yellow tcas symbol showing alt 0 during take off many times(paris and Vienna)...I believe it was due to both of us (me taking off) having TA or TARA mode selected."
Capt STD
If you have seen that, Capt, your TCAS System has failed. Based on your own radio altimeter, any transponder return reporting altitude within 180' of ground is to be suppressed - not displayed. Now, if you have seen a yellow blip with no altitude tag (not 0), that is quite normal, as without an altitude in the reply, your TCAS cannot determine the other aircraft's altitude, only distance and rate of closure.
That is precisely why you are wrong in turning ALTOFF on the ground. You should never, ever, turn off ALT unless specifically instructed by ATC. There are only two conditions in which ATC will ask that:
1. Your altitude reporting is erroneous and you have no backup.
2. ATC SSR is overloaded. That's unlikely.
Putting your transponder in Standby will of course stop the altitude replies, so the ALT switch position is irrelevant.
When Mode S was in final development in 1987, I argued with the design engineers to remove the ALT switch from the control panels entirely, but they couldn't without an FAA mandated change.
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A bit off topic, but relevant:
The FAA stated that for Mode S (and TCAS) retrofits, the altitude source for the transponder be the most accurate in the airplane, i.e. the CADC digital or synchro output (with integrity monitoring), and not the Gilham code that is used by the older ATCRBS transponders. There is no monitoring of Gilham code, so errors in altitude reporting happen.
Sure enough, the KAL 747 fleet was retrofitted with TCAS (and Mode S), but they didn't use the best CADC altitude output. A few years later an error in the Gilham code had one KAL 747 reporting 10,000 when it was actually at 7,000, and it nearly collided with another 747; JAL, I believe.
GB
Capt STD
If you have seen that, Capt, your TCAS System has failed. Based on your own radio altimeter, any transponder return reporting altitude within 180' of ground is to be suppressed - not displayed. Now, if you have seen a yellow blip with no altitude tag (not 0), that is quite normal, as without an altitude in the reply, your TCAS cannot determine the other aircraft's altitude, only distance and rate of closure.
That is precisely why you are wrong in turning ALTOFF on the ground. You should never, ever, turn off ALT unless specifically instructed by ATC. There are only two conditions in which ATC will ask that:
1. Your altitude reporting is erroneous and you have no backup.
2. ATC SSR is overloaded. That's unlikely.
Putting your transponder in Standby will of course stop the altitude replies, so the ALT switch position is irrelevant.
When Mode S was in final development in 1987, I argued with the design engineers to remove the ALT switch from the control panels entirely, but they couldn't without an FAA mandated change.
---------
A bit off topic, but relevant:
The FAA stated that for Mode S (and TCAS) retrofits, the altitude source for the transponder be the most accurate in the airplane, i.e. the CADC digital or synchro output (with integrity monitoring), and not the Gilham code that is used by the older ATCRBS transponders. There is no monitoring of Gilham code, so errors in altitude reporting happen.
Sure enough, the KAL 747 fleet was retrofitted with TCAS (and Mode S), but they didn't use the best CADC altitude output. A few years later an error in the Gilham code had one KAL 747 reporting 10,000 when it was actually at 7,000, and it nearly collided with another 747; JAL, I believe.
GB
