Can TCAS be a nuisance on the ground???

[Mister Geezer]

Can anyone help me out with this one???

Right here is the story.....

I was flying a Cessna 152 from a certain Scottish Regional airport earlier on today and with pre-take off checks done and clearance received I was waiting at the holding point awaiting line up clearance from ATC. I was waiting at a holding point which was about 300 meters along the runway from the threshold since it was going to be a departure from an intersection.

Anyway.. while waiting for a Midland Commuter EMB-145 to line up at the threshold and depart one of the guys on the EMB-145 asked ATC if there were any aircraft overhead the field. He asked just while they had lined up and were waiting for T/O clearance. The question appeared to me as quite an odd one since it was a good VMC day and you would be able to see if there was any traffic.

However while I was waiting at the holding point I had my Transponder on. Since that particular Transponder does not transmit ALT info, then it twigged!! Could the Midland guys in their EMB-145 have seen an aircraft with no alt info on their TCAS, which 'appeared to be over head the field'???? Question is if they had could it be me at the holding point, that they were looking at on their TCAS display?????

I am just assuming that it is TCAS because it is an odd question to ask when lining up along with the WX being good.

Finaly I am assuming that BMC's EMB-145s have TCAS!!!!! :rolleyes:

Thanks for any replies!!!

MG


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When I'm not in my right mind, my left mind gets pretty crowded.

[BIK_116.80]

YES YES AND YES!!!!

The situation you describe is a pain in the proverbial every working day for crews of TCAS equipped aircraft. Transponders without altitude information should be banned.

All airline aircraft operating in UK airspace are now required to have TCAS fitted. TCAS is a wonderful system and works extremely well, but its whole principle is based on the idea that other traffic will have either (1) A transponder which provides altitude information; or, (2) No transponder at all.

TCAS does not work at all well when there are aircraft with transponders which do not provide altitude information. This can happen for two reasons; (1) The aircraft does not have an altitude encoder fitted; or, (2) The pilot has not switched the transponder altitude information on.

The real rub comes when a TCAS equipped aircraft is operating in controlled airspace which has non-controlled airspace below it.

For example, the TCAS equipped aircraft might be in cloud at FL90 and the lower limit of controlled airspace might be FL50. Flying below the lower limit of the airway is an aircraft with a transponder but which is not providing any altitude information.

To the TCAS equipped aircraft, the other aircraft might appear at 12 o'clock, opposite direction, one mile, with no altitude info. Although the two aircraft are in reality well separated vertically, the TCAS system will assume that the other traffic might be at the same level - but of course it has no way of knowing for certain.

Unable to generate a resolution advisory, the TCAS system can only generate a traffic advisory and will announce, "TRAFFIC, TRAFFIC" over the audio system and will present the crew with a big yellow blob on the TCAS display, representing the target.

The hapless crew then has to decide how to deal with this information. There is no easy answer to this. Whilst dealing with this dilemma is no big deal in the cruise, in busy airspace during an approach to land with multiple radio frequency / heading / altitude / speed changes and checklists to deal with these TAs can be an unnecessary distraction. On one approach into London I had seven unnecessary TAs on traffic with no altitude information within 10 minutes.

All the while the non-TCAS aircraft has no idea of the consternation he is causing those around him.

Please please please, owners and operators of aircraft with transponders :

If your aircraft has an altitude encoder please turn it on! If you don’t know how to turn it on, please ask someone. Most GA transponders have a position marked "ALT" which is the setting required. The position marked "ON" does not provide altitude information from most GA transponders.

If your aircraft does not have an altitude encoder and you don’t want to get one, please consider turning off your transponder unless it is absolutely needed. Alternatively, I have seen altitude encoders advertised for about £100. You might consider fitting one of these to your aircraft as it would be of mutual benefit.

[Mister Geezer]

Thanks very much BIK!

So by your reply I assume that we did show up on TCAS!

That has cleared up that 'grey area'. I was just about to say to ATC while I was at the holding point, that we could of maybe triggered a display on his TCAS but I did not want to be a pain!!! I have seen TCAS in use many times and on the jumpseat on a 737 once I saw a 'Non-Altitude' aircraft which the crew 'assumed' was below trigger the TCAS on board. A pain in the backside I agree and I do not like flying aircraft with 'half transponders'.

I will have a word with the company that I rent the aircraft from. I think Alt Encoders are a great thing but the company seem to spend as little money as possible on their aircraft. :mad:

Would the crew on the ground have received a aural 'TRAFFIC TRAFFIC' while they were at the threshold.

Thanks for your reply.

MG

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When I'm not in my right mind, my left mind gets pretty crowded.

[This message has been edited by Mister Geezer (edited 08 January 2001).]

[Capt Pit Bull]

They wouldn't have recieved an aural, as all the aurals are inhibited based on radio altitude (500' in v7 IIRC).

They may still have been experiencing a TA, with a yellow blip on their traffic display, or they may have simply been concerned about having a non altitude reporting blip in their vicinity.

CPB

[BIK_116.80]

Capt Pit Bull is correct - no TCAS aurals on the ground.

However if there was a TCAS equipped aircraft flying overhead the runway then they WOULD get an aural "TRAFFIC TRAFFIC" as well as a visual TA - even though they may be at 5000 feet and you are close to sea level.

The CAA has mandated TCAS for certain aircraft. But they have not mandated that all transponders should provide altitude information. Unfortunately, this is a case of half a regulatory job being done.

[HugMonster]

I'd add one point here - it's probably better practice not to switch the TXPDR on until lining up on the runway...

[ALKIN Hold]

Interesting one. It's always surprised me how many GA pilots and instructors in the UK routinely leave encoding switched off on Mode C equipped transponders. It surprises me even more that this regularly happens under the London TMA where I would have thought it causes the most problems. The instructors at least should know better and be passing on the habit of using Mode C. I believe it comes from the fear of being caught on a temporary incursion of a couple of hundred feet into controlled airspace from below, something which frequently happens in that section of tight airspace.

Maybe it is time to make some mandatory mode C areas, say, under the London TMA?

[This message has been edited by ALKIN Hold (edited 09 January 2001).]

[Mister Geezer]

It is obviously time to even make Transponders with Mode C mandatory in order for these spurious TCAS indications and alerts to stop.

The days of having 'half transponders' must surely come to an end!!!

Regards

MG

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When I'm not in my right mind, my left mind gets pretty crowded.

[MrBig]

The same problem arises when operating around sea ports. Apparently ships have transponders, but, not surprisingly, no altitude read-out. The problem is that in certain curcumstances crews dismiss TCAS contacts as ships. One day it won't be?

[static]

Mr. Big,
I could be wrong ofcourse, but I find it hard to believe that ship`s transponders work on the same frequency. ATC would go crazy in e.g HKG or SIN.
I`ve never seen a TCAS symbol that could be passed as a ship.

Please correct me if I`m wrong.

[BIK_116.80]

I recently spoke to a controller who looks after an area just to the north of London. This area is very popular with light aircraft. She tells me that she quite often handles aircraft which are equipped with a working transponder, but which are not providing any altitude information. She tells me that when she asks them to "Sqwark altitude if available" that 8 out of 10 of them (yes - 80%) come up with a correct altitude read out within a few seconds.

WHY IS THIS SO?

There are several reasons why an aircraft which is fitted with an altitude encoding transponder might not be providing altitude information.

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(1) The altitude encoding equipment is broken altogether. The crew may or may not be aware of this, but can always ask ATC.

(2) The altitude encoding equipment is working but is providing incorrect altitude information, and the crew has been asked to turn off the altitude information by ATC.

(3) The crew has switched the transponder to "ON" rather than "ALT" position, thinking that "ON" is the correct setting, and possibly not understanding what the "ALT" position is for.

(4) The crew has switched the transponder to "ON" in the knowledge that this will not provide altitude information because they don’t want to get caught for being in the wrong place at the wrong altitude - just in case their navigation and altitude keeping is not as good as it might be. ie an inadvertent penetration of controlled airspace.

(5) The crew has switched the transponder to "ON" in the knowledge that this will not provide altitude information because they know that they are or soon will be in the wrong place at the wrong altitude. ie a deliberate penetration of controlled airspace.
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Situations (1) and (2) can and do happen, although rarely.

Situation (3) is a pilot education issue, and I would ask all those who are instructors, or who might be involved with aero clubs, or who might otherwise have contact with pilots in general to spread the word that transponders need to be switched to "ALT".

On transponders which do not have an altitude encoder fitted, the “ALT” position will work exactly the same as the “ON” position. A pilot who is unsure whether the aircraft has an altitude encoder or not should be aware that you can not do any harm by selecting “ALT” even if an encoder is not fitted.

(4) is understandable but unwise, while (5) is inexcusable.

Please, everyone, switch your transponders to “ALT”.

[Mister Geezer]

Thanks for all your replies.

When flying aircraft with Transponders with no Alt Encoder then I have always found a sticker on a placard on the transponder display which says 'No Alt Info', or words to that effect. The Cessna that I flew yesterday did not have an encoder on board and had the associated sticker on the Transponder display. Despite that, as a matter of course the selector was set to ALT as a routine procedure.

MG

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When I'm not in my right mind, my left mind gets pretty crowded.

[Maint99]

I seem to recall one a/c type that I worked on had a forced standby for the xpdr. I think it was a Jetstream 4100, but could be wrong. This disabled the xpdr until weight off wheels. This sounds like the way to go to eliminate spurious TCAS indications.

Would it be possible to do this for TCAS as well, from a flight ops point of view?? i know a few wx radar systems use this.

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"You want it on the gate for WHEN??"

[This message has been edited by Maint99 (edited 10 January 2001).]

[Mister Geezer]

That is a very interesting point about the transponder that is only on when airborne.

However Should the transponder be on when you are on an active runway. i.e. on the roll when taking off?

MG


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When I'm not in my right mind, my left mind gets pretty crowded.

[HugMonster]

Yes - IMO it should be switched to ALT when lining up, and to standby again as part of the after-landing checks.

[foghorn]

BIK

IMHO in the GA world number 4 is by far the most prevalent reason for not switching on mode C.

[Capt Pit Bull]

Maint 99,

DOn't know if this is of interest.

TCAS won't work unless the aircrafts transponder is turned on.

So if your Transponder is WOWed then that automatically means TCAS would be at Standby on the ground.

CPB

[BIK_116.80]

So we need two new rules then?

One rule that all transponder equipped aircraft must also have an altitude encoder fitted, and a second rule that all pilots must turn the transponder to ALT, even if they think it likely that they will be temporarily navigationally embarrassed.