Checklists...

[PanEuropean]

One of the main reasons to turn it to standby is so that aircraft on short final don't have to put up with TCAS traffic alerts and resolutions when they are concentrating on landing... I realize you said that TCAS I and II systems don't do this but I disagree with you. I spend 90% of my career below 400 feet working fires, and the TCAS audio has to be inhibited or you will go nuts with the the advisories. I have also many times had the aural warnings while on the ground or on short final, in many different aircraft.

Northern Flyer:

I'm a bit perplexed by your comments. The TSO standard for TCAS (C119c) mandates compliance with RTCA/DO-185B, the document that defines the performance standards for TCAS systems. Version 7 of the TCAS performance standards - which has been in effect since at least the year 2000 - provides that the sensitivity level of the TCAS must be attenuated to SL 2 when the aircraft is within 1,000 feet of the ground. At SL 2, no RAs are ever issued. Further, the ground estimation logic within the TCAS system looks at the height of other aircraft above ground (by correlating own ship pressure and radar altitude to the altitude differential of the threat aircraft) whenever your own aircraft is below 1,750 feet, and automatically suppress all alerts (both TA and RA) for other aircraft that are determined to be on the ground.

A full explanation of how this works can be found in a FAA publication (a PDF) that can be downloaded here. See page 21 of that PDF for the sensitivity specification, and pages 29 (last paragraph) and 30 for the ground estimation logic specification.

You made reference to flying around at 400 feet doing forest fire work, and the need to inhibit nuisance alerts at that altitude - is it possible that you might have inadvertently confused the TAWS ("don't hit the ground") system with the TCAS ("don't hit the other plane") system? I do agree that if you are doing any kind of low-level work in VMC conditions, the TAWS (not the TCAS) can be a real PITA and it is sometimes necessary to press the TERRAIN INHIBIT button.

I fly aircraft fitted with both TCAS I and TCAS II systems on a regular basis - in fact, part of my job duties includes production test flight of new aircraft to ensure that these TCAS systems work properly. I have never, ever experienced (while in flight) a TA generated in response to an aircraft that is on the ground. I suppose it is possible that certain manufacturers (e.g. Honeywell) might go above and beyond the TSO specifications by flat-out suppressing all alerts when the own ship is below 400 feet AGL (as stated in the Honeywell pilot guide), but for sure, all manufacturers must comply with the specifications set out in the TSO, which are explained in the FAA publication I referenced earlier.

Michael

[square]

Similarly, TCAS I and II systems in other aircraft do not issue traffic advisories or resolution advisories for other aircraft when the TCAS equipped aircraft is less than 400 feet above the ground.

Oh I didn't know TCAS systems were equipped to do that, and that's the only reason I keep my transponder in standby on the ground. I have picked up a few TCAS returns the past few weeks that were indicating at around ground elevation so I thought my understanding of that was correct, and yeah the only reason I keep it off on the ground is to prevent NG-type airplanes like Westjet's from getting annunciations in the cockpit when they're trying to land the plane.

[chipmunk]

Four or five days ago, flying a heavy turboprop, I got a TA ("TRAFFIC") at 500' AGL due to an aircraft with their transponder on ALT taxiing towards the opposite end of my landing runway. It does happen.

[KK7]

Agreed, the system may be designed to suppress TAs and RAs for targets on the ground, but in reality this doesn't always work. RAs are suppressed below 1000' AGL, however you may get a TA if target's or your own transponder is reading incorrectly, such as +/- 100' off. I can also tell you that it is distracting even without a TA, to constantly have a target on your TCAS in very close proximity while on short final.

[iflyforpie]

More distracting than 50...40...30...20......10.......?

[chipmunk]

More distracting than 50...40...30...20......10.......?

If you're trained on an airplane that does that, you're expecting it - it's a normal operation.

Hearing "TRAFFIC" at 500' (especially if in IMC) is not a "normal operation," so to speak.

[Dagwood]

More distracting than 50...40...30...20......10.......?

Yes, but not as bad as 50...40...30...20......10....[thump]...10.......20.....

[Northern Flyer]

One of the main reasons to turn it to standby is so that aircraft on short final don't have to put up with TCAS traffic alerts and resolutions when they are concentrating on landing... I realize you said that TCAS I and II systems don't do this but I disagree with you. I spend 90% of my career below 400 feet working fires, and the TCAS audio has to be inhibited or you will go nuts with the the advisories. I have also many times had the aural warnings while on the ground or on short final, in many different aircraft.

Northern Flyer:

I'm a bit perplexed by your comments. The TSO standard for TCAS (C119c) mandates compliance with RTCA/DO-185B, the document that defines the performance standards for TCAS systems. Version 7 of the TCAS performance standards - which has been in effect since at least the year 2000 - provides that the sensitivity level of the TCAS must be attenuated to SL 2 when the aircraft is within 1,000 feet of the ground. At SL 2, no RAs are ever issued. Further, the ground estimation logic within the TCAS system looks at the height of other aircraft above ground (by correlating own ship pressure and radar altitude to the altitude differential of the threat aircraft) whenever your own aircraft is below 1,750 feet, and automatically suppress all alerts (both TA and RA) for other aircraft that are determined to be on the ground.

A full explanation of how this works can be found in a FAA publication (a PDF) that can be downloaded here. See page 21 of that PDF for the sensitivity specification, and pages 29 (last paragraph) and 30 for the ground estimation logic specification.

You made reference to flying around at 400 feet doing forest fire work, and the need to inhibit nuisance alerts at that altitude - is it possible that you might have inadvertently confused the TAWS ("don't hit the ground") system with the TCAS ("don't hit the other plane") system? I do agree that if you are doing any kind of low-level work in VMC conditions, the TAWS (not the TCAS) can be a real PITA and it is sometimes necessary to press the TERRAIN INHIBIT button.

I fly aircraft fitted with both TCAS I and TCAS II systems on a regular basis - in fact, part of my job duties includes production test flight of new aircraft to ensure that these TCAS systems work properly. I have never, ever experienced (while in flight) a TA generated in response to an aircraft that is on the ground. I suppose it is possible that certain manufacturers (e.g. Honeywell) might go above and beyond the TSO specifications by flat-out suppressing all alerts when the own ship is below 400 feet AGL (as stated in the Honeywell pilot guide), but for sure, all manufacturers must comply with the specifications set out in the TSO, which are explained in the FAA publication I referenced earlier.

Michael

It's clear to me that you are far more knowledgeable than me about this issue. You are correct, the GPWS system we use is inhibited at low level. However our TCAS/ACAS system is very active at low level. Just a thought but 90% of our aircraft are not equipped with radar altimeters, would this affect the attenuation suppression?

An example I was flying our DHC-6 the other day and on the ramp waiting to taxi to the active and had a "TRAFFIC TRAFFIC" alert while another aircraft was landing.

[Beefitarian]

More distracting than 50...40...30...20......10.......?

Yes, but not as bad as 50...40...30...20......10....[thump]...10.......20.....

I got a good giggle out of that one. Reminds me of the story of that lady with the fake license, "Stupid nose wheel."

[PanEuropean]

...our TCAS/ACAS system is very active at low level. Just a thought but 90% of our aircraft are not equipped with radar altimeters, would this affect the attenuation suppression? An example I was flying our DHC-6 the other day and on the ramp waiting to taxi to the active and had a "TRAFFIC TRAFFIC" alert while another aircraft was landing.

Ah, yes, I guess if the 'own ship' does not have a radar altimeter, then the TCAS would have difficulty doing the necessary math to figure out that the other aircraft was on the ground.

I "kinda-sorta" thought that a radar altimeter was a prerequisite component for TCAS I and II installations, but maybe not - I honestly don't know. The aircraft I fly (DHC-6 Series 400) are equipped with Class A TAWS (basic equipment on the aircraft) and either TCAS I (basic equipment on the aircraft) or TCAS II (a popular customer option). I do know that a radar altimeter is required to support the Class A TAWS installation. So, on DHC-6 Series 400 aircraft, the TCAS is 'for sure' also using the radar altitude to work out the ground estimation logic.

The Series 400 Twin Otter knows whether it is on the ground or in flight - this determination is made using airspeed. If the IAS is less than 40 knots, the aircraft is deemed to be on the ground. When it is on the ground, the transponder is put into a special 'ground operations' mode, in which it remains alert and fully ready to function, but will not make any emissions except in response to interrogations that are specifically addressed to the aircraft by name (meaning, by registration mark). This provides support for ADS-B out based Airport Surface Detection Equipment (ASDE) that Narc referred to in his post on page 1, but ensures that unless the ASDE - or another aircraft - specifically asks the own ship by name where it is, the aircraft behaves as if the transponder is in standby. The display on the transponder will show TA (or, TA/RA) and GND while on ground (see photo below), and the GND indication will disappear as soon as the aircraft speed exceeds 40 KIAS.

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Anyway, back to the original thread hijack : I honestly don't know if it still makes sense (in 2011) to switch conventional Mode C or Mode S transponders (that don't have ground/air mode detection capability) to standby while on the ground. When first posed this question, I didn't mean to suggest to anyone that we ought to operate this way, my question was posed as a 'genuine question', because I really don't know if we still need to be moving transponders to standby while on the ground. I think not, but I don't have enough information to support this proposal.

So far as ATC radar systems go, I don't think we need to switch to standby, for reasons explained earlier. But, so far as TCAS systems in other aircraft go, it appears that different experiences are being reported by forum members who have posted replies.

Hopefully other forum members will continue to post their thoughts and interpretations.

Michael

Transponder in 'Ground' mode (for aircraft that can supply the transponder with the ground/air state of the aircraft)