TCAS RA

[kevenv]

The following happened in class E airspace;

A SE bound B190 at 070 and a SW bound BE20 at 065VFR both on frequency. They are converging on a 90 degree cross. When the a/c are about 20 miles apart I pass traffic to both a/c. Traffic is acknowledged by both. I go on to other things and update the traffic when it's about 10 miles apart. At 3.5 miles I again pass traffic to the BE20. He reports traffic in sight. I pass it to the B190 who replies "we are pretty much IMC". At just under 1 mile, just as the BE20 is starting down for destination the B190 calls and says "we are responding to an RA and climbing". I acknowledged. When he reported able to resume his clx at 070 I issued it and life continued as normal (with me reporting the incident as required by our rules).

My question is, are you never allowed to override an RA? In this case both a/c were on frequency, both altitudes were confirmed, both a/c were in level flight, one a/c was VFR and the VFR had the IFR in sight.

When discussing this at work I found out another incident occurred recently which did not involve me;

At an airport that routinely sees opposite direction arrivals and departures climbing/descending, the arrival was given descent to 160 and the departure was given climb to 150. They were on reciprocal track and traffic was passed both ways, with both a/c being informed of the cleared and intended altitude of the other. In this case when the a/c were quite close in distance the climbing aircraft reported responding to a TCAS RA and descended. The arriving a/c leveled at his cleared altitude, never dipping below it, and did not report a TCAS RA. Both a/c were from the same company. Do the actions of the climbing a/c seem reasonable?

Thanks

[Sidebar]

Do the actions of the climbing a/c seem reasonable?

IMHO, yes. Their hardware alerted them to a threat and provided a resolution, and they followed it.

Remember the midair in Switzerland/Germany aout 10 years ago? The DHL flight followed their RA, the Russian crew did not.

[kevenv]

IMHO, yes. Their hardware alerted them to a threat and provided a resolution, and they followed it.

Remember the midair in Switzerland/Germany aout 10 years ago? The DHL flight followed their RA, the Russian crew did not.

I can see where you are coming from but I have to disagree that the two scenarios are even remotely similar. The second one that I laid out involved two a/c that were cleared to altitudes that provided IFR separation and both a/c were informed of the location of the other as well as the cleared altitude. This was not the case in the incident you mention.

The reason I brought this particular incident to light here as an add on to my original question is we see this scenario virtually daily and that is the first time in the 10 years I have been here that I have heard of this happening.

[Big Pistons Forever]

CAR 602.31

(3) The pilot-in-command of an aircraft may deviate from an air traffic control clearance or an air traffic control instruction to the extent necessary to carry out a collision avoidance manoeuvre, where the manoeuvre is carried out

(a) in accordance with a resolution advisory generated by an Airborne Collision Avoidance System (ACAS) or a Traffic Alert and Collision Avoidance System (TCAS)

The regulations do not required to follow a RA, however some companies require their pilots to always follow an RA.

[Gannet167]

Do the actions of the climbing a/c seem reasonable?

Yes, you have no idea if the other aircraft started to climb/descend unexpectedly for any reason (pilot error, mechanical malfunction, etc. etc. etc) always play it safe and follow an RA.

If ATC tells me that converging traffic will be leveling off 1000' above me, I immediately shallow my climb out so I don't trigger an RA. If the two aircraft are climbing/descending at a few thousand feet a minute - TCAS has no idea where or if they will level off. It only looks at immediate vertical rate and what will happen in the future if nothing changes. If you shallow out your vertical rate, it should prevent an RA.

[kevenv]

Do the actions of the climbing a/c seem reasonable?

Yes, you have no idea if the other aircraft started to climb/descend unexpectedly for any reason (pilot error, mechanical malfunction, etc. etc. etc) always play it safe and follow an RA.

If ATC tells me that converging traffic will be leveling off 1000' above me, I immediately shallow my climb out so I don't trigger an RA. If the two aircraft are climbing/descending at a few thousand feet a minute - TCAS has no idea where or if they will level off. It only looks at immediate vertical rate and what will happen in the future if nothing changes. If you shallow out your vertical rate, it should prevent an RA.

I guess it just hasn't got close enough in the past to trigger an RA. All the planets lined up on this one.

Thanks for the replies. Anyone care to tackle the first one?

[Gannet167]

The problem with ignoring an RA is the traffic you are vigilantly watching go by to de-conflict with may not be the same traffic that is causing the RA. The threat traffic may not be immediately visible to you and you could make matters worse by doing nothing.

From an armchair quarterback position, it would seem to me that the aircraft that got the RA in your first example gave it to themselves. They were advised of traffic 3 miles and 500' away and they decided to descend. Not only were they advised, they also had to have the traffic as a "Non-Threat Traffic" then as "Proximity Intruder Traffic" then a TA, finally an RA. Even at the point of getting an RA, TCAS is suppose to give you enough warning that all you need is a very gentle couple hundred feet per minute climb/descent. So a very slight change in speed or vertical rate well before the RA could prevent it from occurring.

The crew did the right thing by responding to the RA, but I think they could have done a better job of avoiding the traffic in the first place.

[Pratt X 3]

From an armchair quarterback position, it would seem to me that the aircraft that got the RA in your first example gave it to themselves. They were advised of traffic 3 miles and 500' away and they decided to descend...The crew did the right thing by responding to the RA, but I think they could have done a better job of avoiding the traffic in the first place.

According to the original post, the 1900 was the aircraft that received the RA and they were level at 7000 the entire time until the RA. The King Air 200 was VFR at 6500 and had started a descent just before the RA.

TCAS can issue an unnecessary RA but it is in the best interest of those involved to follow it first and then analyze it afterwards. Sometime the traffic isn't the obvious one and sometimes it is so far off. For example:

The Bombardier BD100 1A10 aircraft (C-****) was on an IFR flight from Fort Lauderdale (US) (KFXE) to Toronto (CYYZ). While descending to 5,000 feet ASL on the downwind for runway 23, the flight crew received a TCAS RA and climbed back to 5,400 feet ASL. The cause was a VFR aircraft at 1,600 feet ASL.

source: CADORS

[Canoehead]

Regarding the first scenario, the policy at my company would require the same thing. An RA in IMC is mandatory.

[Gannet167]

According to the original post, the 1900 was the aircraft that received the RA and they were level at 7000 the entire time until the RA. The King Air 200 was VFR at 6500 and had started a descent just before the RA.

I misread the OP, you're right. Good thing I wasn't reading my TCAS screen.

[xsbank]

Remember also your call is ONLY "Centre, Oilburner One TCAS RA" not "TCAS descent or TCAS climb." Not only might you have to respond to a second alert where you might be blocking the channel, but the new TCAS standard that's being introduced (TCAS II 7.1) will include "Level off now" in its directions.

[Gannet167]

"Level Off" replaces "Check Vertical Rate" and is suppose to help with RA reversals. It should prevent what happened with DHL and Bashkirian over Uberlingen.

[cyeg66]

From this atc's perspective, TCAS is mostly a pain in the a$s but I realize it can also be the last line of defense if everyone is asleep at the switch. It's like that really annoying coworker you have to "endure" until that one day that they save your a$s when you're about to be shit-canned for having forgotten something for the boss. Insofar, I've probably filled in paperwork for about a dozen TCAS RA occurrences and in not one instance was the traffic not aware of the situation/traffic beforehand. I've even received RA "phantom" notifications a couple times. Try filling that one out! As far as I know, crews have to comply with RA's, but I've been wrong many a time before.

[Gannet167]

Eurocontrol estimates that at current traffic density and growth, with the current TCAS II system they will have a mid-air ever 3 years. Thus, TCAS II 7.1 mandate, I believe February 2015 or you're not getting clearance above FL280. TCAS saves. I love TCAS.

[Jastapilot]

I've never heard of anyone ever dying from following an RA... however there are enough examples of people dying because they did NOT follow the RA. I know what I would have done.

[Gino Under]

xsbank

If I may?
You say the correct response to an RA is "Centre, Oilburner One TCAS RA" not "TCAS descent or TCAS climb."
I beg to differ.
If you refer to the Canadian AIM the proper phraseology is "TCAS Climb" or "TCAS Descent". You'll find the FAA is exactly the same. However, EuroControl switched to "TCAS RA" three years ago. It hasn't happened here (Canada) yet.

Gino

[ahramin]

While not a CAR, professional standards dictate that TCAS RAs must be followed. As for pilot controller interactions:

12.16.8 Pilot/Controller Actions

In order to use TCAS/ACAS In the most effective and safest manner, the following pilot and controller actions are necessary:

(a) Pilots should not manoeuvre their aircraft in response to TAs only;
(b) Pilots should notify the appropriate ATC unit, as soon as possible, of the deviation, including its direction, and when the deviation has ended;
(c) In the event of an RA to alter the flight path, the alteration of the flight path should be limited to the minimum extent necessary to comply with the RA;
(d) When a pilot reports a manoeuvre induced by an RA, the controller should not attempt to modify the aircraft flight path until the pilot reports returning to the terms of the existing ATC instruction or clearance, but should provide traffic information as appropriate;
(e) Pilots who deviate from an ATC instruction or clearance in response to an RA shall promptly return to the terms of that instruction or clearance when the conflict is resolved.

In addition, as Gino stated Canada and the US have yet to update their TCAS phraseology, so "TCAS climb" and "TCAS descent" are still correct in North America. ICAO and Eurocontrol phraseology has been "TCAS RA" since 2007. I keep checking every new issue of the AIM to see if they've updated it but so far nothing.

[kevenv]

My thanks to everyone for their replies and for keeping them on a professional and civil tone. We, as in ATC, have a similar system built in to our computers called conflict alert. It can't tell that I have cleared two a/c to appropriate altitudes either. It looks at rates of climb and is a pain in the butt. Again many thanks.

[bcflyer]

At my airline if you get a RA you respond immediately. Even if ATC has told you about the other traffic you never know what the other pilots are thinking. I can't imagine why you would ever ignore an RA. To do so is akin to playing chicken on the highway.

[Eric Janson]

A TCAS RA overrides any ATC clearance at my Airline. It is mandatory to follow the TCAS RA commands as per our SOP.

[CID]

A couple of things to ponder. Depending on the operating rule each aircraft is under, they may not be required to be fitted with TCAS or even a mode S transponder and if the BE20 had one, it would likely be TCAS I which does not issue RAs. So the BE20 likely did not get the RA and had no Mode S transponder to coordinate the actions. The BE20 pilot could have just as easily leveled off or climbed and he may have gotten a TA and again, only if he was equipped with TCAS I. And...pilots are not supposed to maneuver based on a TA.

Second, if you have an active GPWS/TAWS warning, it has priority over a TCAS RA. "PULL UP" has priority over "DESCEND".

Long story short, always act on an RA command immediately unless it's overridden by a GPWS/TAWS warning.

[Sidebar]

I think the way it works is : As long as one aircraft is TCAS equipped, a functioning mode C transponder on the second aircraft will allow the first aircraft's TCAS to provide an RA even though the second aircraft could be unaware of the conflict.

A contributing factor in the midair in Brazil was the bizjet transponder not operating.