TCAS RA on Take Off

[Jofm5]

Sorry SLF here so pls be gentle (Feel free to move threads if you think this is an inappropriate thread - I thought the question a little to serious for jet-blast but not really R/N)

I read about the following incident on Incident: Druk Air A319 at Dhaka on May 7th 2009, TCAS RA on takeoff (http://avherald.com/h?article=41974c61&opt=0)


A Drukair Royal Bhutan Airlines Airbus A319-100, flight KB-126 from Dhaka (Bangladesh) to Bangkok (Thailand) with 120 passengers, had just taken off Dhaka and was at 2000 feet less than 2 minutes into the flight, when a military supersonic jet approached from behind and overtook the Airbus just above the Airbus causing a traffic and collision avoidance system (TCAS) resolution advisory having the Airbus descend. The Airbus crew descended about 500 feet before TCAS announced "clear of conflict". The airplane reached Bangkok without further incident.



I am not sure if the report is embelished on the proximity of the the military aircraft but it made me wonder a couple of questions about TCAS and reactions to it.

I have some heavy reading coming tomorrow from amazon so if you think it best me to research the first few questions thats fine :)

At what altitude does TCAS become operational, if not active all the time.

Does TCAS have a minimum altitude for issuing a descend if so does it then issue an advisory of a directional turn if applicable - if not when would a pilot chose to ignore/deviate (I see from earlier threads TCAS/RA SOP is it is to be followed verbatim but in some situations this may not be wise ?

If TCAS does deem it too low to descend - what happens - is it just proximity warning issued and left to the pilots discretion ?

What would be the advised manouvre if any if a TCAS Warning only is issued at low altitude i.e. < 2000 feet ?

If indeed the military aircraft did pass in close proximity above - how serious is the possible wake air turbulence to an A319 size aircraft - I concede type of military aircraft size/profile/propulsion is unknown.

I imagine after this was reported ATC communications between civilian and military will be investigated - if the military aircraft was on approach to an military field adjacent to the civilian is this airspace dealt with by two different authorities or a single handler for all airspace.

Finally - And I will probably be laughed at on this one - would I be correct to say the civilian craft has right of way considering that a supersonic aircraft is typically smaller and more manouverable (agile) than a commercial airliner.


Hope my questions do not sound too silly,

Cheers,

Jof.

[fingureof8]

i thought I would try and answer some of your questions...

1. If memory serves me correct TCAS RA becomes active, if selected, at 1000ft on your RAD ALT. Before 1000ft you will only recieve a TA.
2. TCAS will never descend you below a 1000ft during an RA, based on RAD ALT.
3. TCAS is not capable of giving avoidance laterally, it can only do vertically.
4. As for the right of way, I would expect it depends on varioue things, aircraft on approach vice departure etc etc. Perhaps the civi jet should have been held on the deck?? or perhaps the mil jet was getting some guns practise!!:D

[Jofm5]

Thanks fingureof8 - was pondering whether to delete the thread as being too silly to be answered.


Just for discussion.

1. A Radar Altitude of 1000ft seems sensible to prevent false unwarrented alarms on departure. Thx for info.

2. It makes sense, howeer is TCAS aware of terrain as in is it in anyway linked with the GPWS ? The incident above got me wondering if the two would ever conflict and what would happen?

3. I did not know that - thanks for the info (I can understand it may add to complexity but that raises other questions which I will leave to another time).


It Would be interesting to find out what you would do if you got a TCAS TA at 1000ft - is there any advised evasive manouvre and what info would dictate actions - or is it seat of pants at time ?

Another question I was pondering later on was whether TCAS is capable of identifying a supersonic jet travelling at supersonic speeds. My initial thought would be it would not pick up the transponder of the approaching supersonic in time for it to make any alert to be of use in time to apply collision evasive manouvers - but would be interested to know if my thoughts are along the right lines.

Again apologies for asking so many questions - if you think there is pertinent reading to go research pls feel free to recommend any links/purchases.

Thanks

Jof

[Graybeard]

Radio altitude is the primary ground sensor for the TCAS, and it's also a primary sensor for the GPWS. TCAS is not connected directly to GPWS, other than that, and prioritizing of audio alerts and warnings.

TCAS interrogates all transponders and military IFF within about 60 miles, and calculates distance, reported altitude difference and rate of closure in seconds. 35 seconds to CPA, closest point of approach triggers TA. 20 seconds triggers RA. Individual speeds of the planes doesn't matter, just their rate of closure. Angle of approach is irrelevant, also.

Sounds like the TCAS performed as it should have.

GB

[Jofm5]

Graybeard,

Agreed I think it all performed well and the outcome was as desired....

I find it interesting that GPWS is not correlating with TCAS - surely at such low levels (i.e. +1000ft it is possible that TCAS could issue an advisary that if adhered to GPWS would then issue warnings - how do you deal with that ?).

Call me a devils advocat for stating that scenario, I am just trying to understand what is going on and thinking what possibile issues could arise and how they can be addressed.

I find it interesting how you guys deal with situations when the horns are blaring giving warnings, to me that seems like a huge distraction, but sometimes I read on the herald where instances occur when more than one alarm could trigger at once and the reaction to one could trigger to another. I would like to hope my lack of knowledge at least provokes some thought.

I am coming from a software development perspective and are wondering why the TCAS is not linked to the GPWS as for the prime solution you want to make sure both sets of parameters are included.

I think my question about supersonic planes is as the time for TCAS reaction is greatly reduced by rate of closure thus a supersonic aircraft would have to be identified much further out than one close in travelling subsonic.

You say angle of attack is not important ? does TCAS not identify if an aircraft will pass under/over and by how much to identify what action you should take - if not how does it establish a reaction for yourself that will not conflict with the TCAS resolution on the approaching aircraft.

[HEATHROW DIRECTOR]

<<would I be correct to say the civilian craft has right of way considering that a supersonic aircraft is typically smaller and more manouverable (agile) than a commercial airliner. >>

I don't know about the country where the incident happened but the UK Rules of the Air make no distinction between different types of powered flying machine. There are various rules which should have prevented the incident but ATC should have sorted the problem too. Here are a couple of excerpts from the UK Rules (note "flying machines" does not mention type):

(a) flying machines shall give way to airships, gliders and balloons;
(b) airships shall give way to gliders and balloons;
(c) gliders shall give way to balloons.
(2) Mechanically driven aircraft shall give way to aircraft which are towing other aircraft or objects.

and

11.—(1) Subject to paragraph (3), an aircraft which is being overtaken in the air shall have the right-of-way and the overtaking aircraft, whether climbing, descending or in horizontal flight, shall keep out of the way of the other aircraft by altering course to the right.

[Jofm5]

Cheers heathrow for the response, I was sure there was some sort of "etiquette" to the skys's.

As I suspected it typically is following the line of manouverability which is what I was trying to imply.

Ah well books just arrived - got six to read - it may be a while but I hope your all friendly to questions lol

[GlueBall]

You also have to mitigate the journo's exaggerated, non avation, view as to the mention of "supersonic" at 2000 feet. If that were true then there wouldn't be any windows intact at Dhaka. :ooh:

[john_tullamarine]

was pondering whether to delete the thread as being too silly to be answered.

Generally, we don't fuss too much about what the nature of the question is, so long as it has some relevance to, or interest for, the flying community.

If a thread proves to be of no real interest, it will die a quick and natural death.

Very rare for us to remove a thread from this forum - indeed, sometimes folk will remove their own question and we will reinstate it unless there be some obvious reason why it should stay deleted.

As always, the catch cry is "there are no silly questions .. "

[Graybeard]

Regarding your question of TCAS - GPWS coordination, Jofm5, it would be redundant. Radio altitude is a primary sensor for both systems.

There are 3 basic advisories from TCAS: Descend, Climb, and Maintain Altitude. If one of those commands is unwise, such as ground proximity or max altitude, then it selects the best command from what's left.

Air-air coordination between planes in conflict is based on ICAO sequence number assigned to every plane equipped with Mode S transponders. The one with the lower number is in command of the coordination, and the other plane responds as needed.

TCAS, GPWS, and Windshear advisories are prioritized so that only the highest priority is annunciated by lights and audio.

GB