TCAS RA during ILS approach.

[dirk85]

Hi!

I would like to share with you fellow aviators what happened to me recently during a flight, and hear your thoughts on what I think is quite an interesting situation.

Sunny day, little to no wind, in a medium business jet of the Citation family.
The event took place at an airport in France, after about one hour of flight.
We are in the approach phase, ready to begin the ILS procedure, still in contact with the approach controller. At about 15/20 miles from the airport we are able to see a target on the TCAS around the airport area, that at the moment does not seem to constitute a problem, being out of our approach path.
No information about any other traffic is given by the controller.
The FO, myself, and the captain do not speak french, and perform all the required communication with the ATC in english, as per company SOP.
After being cleared to perform the ILS we are handed over to the tower frequency.
We are unable to establish visual contact with the other traffic, that is a couple of miles off to our right.
We are instructed to continue the ILS and to report 3 miles. In the mean time the controller keeps talking in french with what we suppose being the other traffic: far from standard communication is used, since the frequency is blocked for a long time.
We don't understand a single word of that conversation and we are not aware of the position and the intentions of the other traffic.
We try to communicate with the tower to obtain informations, but we are unable, due to the frequency being congested by the small traffic and the controller still transmitting.

We receive the first TA, "traffic, traffic".
We still don't have the traffic in sight.
We are at about 8 nm, established on the glide, AP on, when we receive the RA, "descent descent".
The FD is commanding an increase of the rate of descent.
The terrain well in sight, the pilot flying disconnects the AP and follow the FD indications. The pilot not flying executed the standard call out: "Tower, XXX TCAS descent" on the frequency, that is still blocked.
After a few seconds we are clear of traffic, and find ourselves about one dot and a half below the glide slope, speed still under control, and in sight with the terrain and the airport environment.
We decide to proceed to the landing.

Only on the ground, after touchdown, the tower seems to understand that something happened.
The controller later acknowledged there was a mistake in the separation of traffics due to a deviation of the small traffic from the clearance given (?).

ASR filed, still waiting for the feedback from the french side, not holding my breathe.

A few considerations.

At about 1000 feet radio the TCAS RA function is automatically inhibited, and only TA is available.

The TCAS is not linked with the GPWS: the descent instruction received during the ILS at about 3000 ft of height left us a little bit surprised.

Luckily we were in VMC, but the very same circumstances in IMC could have lead to a much more "interesting" situation.

Your thoughts?
How does it work on your planes, is the TCAS linked somehow with the GPWS to give proper indications in case of RA in these particular conditions?
How would you behave in case of marginal weather conditions?

[Denti]

"Our" TCAS will inhibit increase descent commands below 1500ft, descent commands below 1100ft and all aural alerts below 1000ft. However, all TCAS alerts will be inhibited by GPWS and windshear warnings. There is a priority order, and TCAS is pretty low as another traffic might move, the earth will not.

Added to that at least one pilot should display EGPW information on his ND which will give additional situational awareness. In the situation above even in IMC i would follow the RA as i still have terrain protection while it is active.

[grounded27]

I believe all TCAS systems to have a floor of 1500' RA AGL as Denti stated above, sounds like the RA was legit. More damn problems like this are a result of non standard communications.... Enter ADS-B.

[TheNoFlyZone]

Interesting.

First, the controller was obviously not doing his job properly, in that he neglected to pass you the traffic which was headed your way, or make sure the traffic stays out of the way.

Also, the fact that you couldn't get in touch with the controller is also weird, since it seems he was having a long conversation with a pilot.

Had he been talking in english or french with that traffic, you still wouldn't have been able to get in contact with the controller. And its no guarantee that you would have spotted the traffic either. So dual languages is really a secondary issue here.

Now i understand that had the controller spoken to that plane in english, you might have tried to figure out where he was and what he was doing, but that's a lot of if's and maybe's. Again, no guarantee that "english only" would have spared the TCAS RA.

Seems to me poor ATC procedures is entirely to blame.

Thenoflyzone

[dirk85]

I agree the language problem is not the main issue but I reckon that being able to understand their communication could have given us a better overall picture of the situation.
We understood later it was a training flight, and that the chatter was due to the fact that they were tryng to coordinate a couple of IFR procedures.
Mistakes can happen, but it's also interesting how a situation that was far from being "complex", just two traffics, sunshine, terrain flat like a pancake etc etc can easily turns to s**t in a matter of seconds.
We went to the very same airport just a few days ago: still lot of talk on the frequency, and still wide use of local language. :=

"Funny" also how just a couple of nm later in the approach and the RA would be inhibited... would have the traffic crossed out path at that point probably I wouldn't be here writing on pprune. :\

[172_driver]

Just a thought, what kind of airspace was it where the situation occurred? I used to instruct in the US and as standard the Class D airspace around many relatively busy airports only stretched to a 4 nm radius, with Class E outside. The clever VFR pilot would stay away from the ILS corridor, but VFR traffic could operate legally without two way radio communication (and in some cases without transponder) just outside the Class D circle at any altitude from GND to 18 000 ft. It sounds in this case the controller was communicating to the aircraft involved, and just failed to give you proper information about it, but just thinking if airspace design could have anything to do with it? Also was the airfield radar equipped? Did the controller understand that the aircraft was straying onto the final approach path until you advised of the TCAS RA? You said the also the chatter was to coordinate IFR procedures.. therefore quite possibly legit? Not trying to play down the seriousness of the situation, just trying to elaborate on how a few unfortunate circumstances could lead to a good day becoming a bad day… quickly, as you experienced.

[PT6A]

172,

That's the problem with US Airspace that GA traffic can legally operate in this way without talking to ATC... They can also fly along in the airway... Without talking to ATC.

The NTSB advised against this sometime ago.... But the likes of AOPA are too strong for the FAA to change it to a more "European" style ATC system.

There is a reason they call the lower US airspace "Indian Country"

[172_driver]

I agree..personally enjoyed the "freedom" of American airspace but always knew it was at the expense of something else... Then again, I wouldn't expect a European airliner do S-turns on final to build up separation to a PA28 ahead ;)

I started my flying in a Scandi country and even there the 8 miles could stretch outside the CTR. Not sure about France. Not many captains I fly with seem to remember VFR operating rules and what airspace they're flying in… because it's not relevant to their type of operation… but is it?

[dirk85]

The airport and the surroundings are in class D airspace, just checked on the french AIP.
On the jepp plates airspaces are not depicted, a part from restricted or prohibited areas: when a procedure or part of it is in uncontrolled airspace there is usually a note, so I assume in any other case it is totally in a controlled area.

[CanadaKid]

Had a similar RA ("descend, descend") on the ILS at an African airport. The PNF remained on the glide slope descending at approx 1000 fpm. Also in VMC conditions but without the continuous ATC chatter you experienced. I have used 121.5 on the No.2 VHF to speak to tower during a critical time when the main freq was blocked.

Like you, very grateful it was day VMC when it happened.

Cheers CK

[dirk85]

I admit I didn't even think about the guard frequency at the moment. In general I think it is a smart idea, but in that particular case there are other two quite busy ifr airports at about 20/25 nm. The result could have been hilarious... :}:}

[Checkboard]

That's the problem with US Airspace that GA traffic can legally operate in this way without talking to ATC... They can also fly along in the airway... Without talking to ATC.

It's the same in the UK, so I wouldn't go pointing fingers at the US too quickly. :uhoh:

In the UK, you can take off, and legally fly IFR in "G" airspace without filing ANY flight plan, and without talking to ATC. The only reason no one appears worried about it is the amount of radar ATC has access to.