TCAS: Disregarding a Resolution Advisory

11 Nov 16 4 Comments

On the 25th of February 2016, a Boeing 737-800 and a Falcon 2000 were involved in an airprox incident over Germany.

The Boeing 737 was passenger flight ENT-511 from Gdansk, Poland to Jerez, Spain. The flight was operated by Enter Air, a low-cost Polish charter airline operating flights to popular holiday destinations all over Europe. Their first commercial flight was in 2010 and the company grew over 300% by 2012, despite rising fuel prices.

Enter Air SP-ENX by Valentin Hintikka
Enter Air SP-ENX by Valentin Hintikka

The Dassault Falcon 2000, a French twin-engine jet, was a business jet departing Memmingem, Germany.

The Falcon had been cleared to climb to FL430 (43,000 feet) and climbed rapidly directly towards the Boeing 737 which was cruising at FL370 (37,000 feet).

Both aircraft received resolution advisories from their Traffic Collision Avoidance Systems.

A Traffic Collision Avoidance System (TCAS) is installed into all modern commercial transport aircraft . It interrogates the transponders of all nearby aircraft, receiving their altitude and distance. The TCAS offers traffic advisories to alert the flight crew of nearby aircraft. When the flight crew receives a traffic advisory (TA), they are not expected to perform avoidance manoeuvres, simply to be aware that there is the possibility of a conflict. It gives the flight crew a chance to locate the other aircraft visually and prepare for the next instruction if the risk of conflict continues.

TCAS predictions are based on the assumption that aircraft are flying straight. In this instance, as one aircraft was climbing rapidly, the TCAS system almost certainly did not offer a traffic advisory but instead went straight to the resolution advisory (RA).

If the TCAS determines that there is a real risk of collision, and if the other aircraft is also TCAS-equipped, the TCAS will send a coordination signal to the other aircraft in order to resolve the encounter.

In this way, the evasive manoeuvres are coordinated: one TCAS system will select an “upward sense RA” (instructing the pilot to climb) and the coordination signal to the second TCAS means that the second system will select a “downward sense RA” (instructing the pilot to descend), thus resolving the conflict immediately.

The Boeing 737 was given the RA to climb while the Falcon was given the RA to descend.

In the Boeing 737, the TCAS sounded with the resolution advisory: CLIMB, CLIMB, CLIMB.

In the Falcon, the resolution advisory was to descend.

The captain of the Boeing 737 made visual contact with the Falcon and decided that there was no need to follow the TCAS resolution advisory. He did not climb although he did report the issue to ATC.

The Falcon followed the advisory and went into a descent.

The Boeing 737 remained at 37,000 feet as the Falcon passed under it at 35,900 feet.

I have written about two instances where the TCAS resolution advisories were ignored. In Why Planes Crash Case Files: 2001, a Boeing 747 and a DC-10 were in a near miss over Tokyo, after Air Traffic Control gave instructions that directly contradicted the TCAS resolution advisory. The Boeing 747 descended, following the controller’s instruction and disregarding the TCAS instruction to climb. The DC-10 also descended, following the TCAS resolution advisory.

The DC-10 passing just above the Boeing 747. Re-creation by “anynobody”.
The DC-10 passing just above the Boeing 747. Re-creation by “anynobody”.

DC-10 First Officer (Pilot Flying): “It felt as if the other aircraft was rapidly rushing toward us, and I wondered why since our aircraft was following the TCAS descent command. Subsequently, I saw the other aircraft become larger and lower its nose when it was just off the tip of our left wing or a little bit inward of that. At that point in time, judging that the attitude of the other aircraft was around 10—15° nose down, at the same altitude as us, and descending, I quickly applied power and pulled the control wheel. The other aircraft was so close that I thought its tail would snag our aircraft.”

In Why Planes Crash Case Files: 2002, I wrote about the Überlingen mid-air collision, in which a Boeing 757 and a Tupolev Tu-154 were put on a collision course after a series of issues with ATC. The Tupolev had made visual contact with the Boeing and was waiting for an instruction for an avoidance manoeuvre. The controller, suddenly aware of the conflict, instructed the Tupolev to descend. The Boeing received and responded to the TCAS resolution advisory to descend. The Tupolev received a resolution advisory to climb, but the captain had already begun his descent. The first officer pointed out that the TCAS said climb. The captain pulled the control column back, reducing the descent rate. A flight instructor in the cockpit, who was monitoring both pilots, snapped back. “He (the controller) is guiding us down!” Investigators believe it was the instructor who retracted the thrust levers. The captain held the control column in place for about two more seconds before pushing it forward to increase the rate of descent.

The ATC instruction for the Tupolev to descend was much too late in any event, even without the TCAS advisory, the Tupolev could not have achieved 1,000 feet below the Boeing. As the Boeing was also descending, there was no chance. The two aircraft collided at 35,000 feet.

Bashkirian Airlines Flight 2937 DHL Flight 611 moments before the collision (Re-creation by Anynobody)
Bashkirian Airlines Flight 2937 DHL Flight 611 moments before the collision (Re-creation by Anynobody)

Now I had always been of the impression that if you receive a TCAS advisory, you must follow it. There is not a lot of room here for Captain’s discretion except in the most extraordinary of circumstances, for example that the TCAS is faulty and broadcasting unsafe instructions.

In Europe, this is pretty much the case. The Eurocontrol overview is clear:

Pilots are required to immediately comply with all RAs, even if the RAs are contrary to ATC clearances or instructions, unless doing so would endanger the aircraft. Complying with the RA, however, will in many instances cause an aircraft to deviate from its ATC clearance. In this case, the controller is no longer responsible for separation of the aircraft involved in the RA.

On the other hand, ATC can potentially interfere with the pilot’s response to RAs. If a conflicting ATC instruction coincides with an RA, the pilot may assume that ATC is fully aware of the situation and is providing the better resolution. But in reality, ATC is not aware of the RA until the RA is reported by the pilot. Once the RA is reported by the pilot, ATC is required not to attempt to modify the flight path of the aircraft involved in the encounter. Hence, the pilot is expected to “follow the RA” but in practice this does not yet always happen.

The Boeing procedure regarding TCAS instructions is always to follow the Resolution Advisory with two clear exceptions: if the aircraft is under 1,000 feet or if visual contact requires other action (as was the case at the last minute in the near miss over Toyko).

But what I didn’t know is that isn’t across the board. The U.S. Federal Aviation Administration say the following:

When an RA occurs, the PF should respond immediately by directing attention to RA displays and maneuver as indicated, unless doing so would jeopardize the safe operation of the flight or the flightcrew can ensure separation with the help of definitive visual acquisition of the aircraft causing the RA. By not responding to an RA, the flightcrew effectively takes responsibility for achieving safe separation. In so choosing, consider the following cautions:
(a) The traffic may also be equipped with TCAS and it may maneuver in response to an RA coordinated with your own TCAS.
(b) The traffic acquired visually may not be the same traffic causing the RA.
(c) Visual perception of the encounter may be misleading. Unless it is unequivocally clear that the target acquired visually is the one generating the RA and there are no complicating circumstances, the pilot’s instinctive reaction should always be to respond to RAs in the direction and to the degree displayed.

This means that the flight crew clearly have the leeway to disregard a resolution advisory if they believe that they are visual with the conflicting traffic. But the three cautions are very relevant: the Falcon was responding to its resolution advisory and the captain was taking the risk that his target and his perception of its trajectory was correct.

Now in this case, the flight and the airline were European; I don’t know where the pilot was from but it seems clear that he would be expected to follow the RA as there was no danger to the aircraft by doing so.

The Polish civil air accident investigation agency, the State Commission on Aircraft Accidents Investigation (Państwowa Komisja Badania Wypadków Lotniczych) appears* to agree with me on this. According to the Aviation Herald, the report released on the 7th of November recommended that Enter Air instruct the captain of the Boeing about proper use and principles of the airborne collision avoidance system. The report states that the Captain’s assessment was insufficient to verify that the flight safety was not endangered.

The probable cause was determined as the too high a climb rate by the Falcon 2000.

All’s well that ends well, I suppose, but I can’t imagine ignoring a TCAS warning any more than I would ignore a ground proximity warning.

The key point, I think, is that a TCAS resolution advisory only happens if something has already gone wrong. Commercial flights under IFR should never get close enough to each other for a traffic advisory, let alone a resolution advisory. This means all of the normal safeguards have already failed. It is not a great time to take unnecessary risks. It simply seems sensible to engage in what should be a rather gentle manoeuvre even if not strictly necessary, in order to be confident that separation is kept.


*Note: The Aviation Herald is an excellent website which posts aviation occurrences along with reports and updates as the details become available. Their Monday post is based on access to the Polish State Commission on Aircraft Accidents Investigation report on the incident but they do not link to it.

The Polish transport website shows the event as type zdarzenie which translates as a “happening”, seemingly distinct from an incident and serious incident.

No report is attached to the event listing, which may simply be that the website has not been correctly updated. In any event, I was unable to find a copy of the report either in English or Polish. For the details of the flight and references to the report, I have used the Aviation Herald post as my source.

Category: Accident Reports,

4 Comments

  • Nowadays TCAS is mandatory equipment for ALL commercial airliners, at least so in West Europe, not just in “new” aircraft but also retrofitted in older ones. The Fokker F27 I flew many years ago now were all TCAS equipped.
    It would seem to me that the problem arises exactly when logic says that there should not be one: 1. if visual contact is made with the other aircraft, 2. when ATC issues instructions conflicting with the TCAS .
    For those who are not all that familiar with what the pilot sees through the windscreen: Especially when flying at high altitude, an aircraft on an opposite track may appear to be approaching at a very different altitude than it’s real level. Perhaps this is due to the distortion in perception caused by the earth’s curvature which is just about to become visible at altitudes let’s say above 35000 ft, but still not really all that obvious. So an aircraft may seem to be at the same level when still distant when in reality it isn’t. The actual aircraft may appear as no more than a tiny speck in the windshield. Often the contrail is the first thing visible. Next, it seems a long time for it to close in but when near the gap suddenly closes very rapidly. And the aircraft that seemed alarmingly at a head-on collision course only a short time previously passes harmlessly 2000 feet below. At a closing rate of nearly 1000 miles per hour, this is not the time to start guessing!
    The TCAS has a few sectors that will start giving signals if and when a potential conflict is perceived by another aircraft penetrating it, first a flashing bleep, then an audio calling “traffic – traffic”. At that stage the pilot should be alert and ready to take action if necessary.
    If action is required, and only then, should the crew follow the instructions from the TCAS: “climb” or “descend” as indicated by a needle generated to give an “up” or “down” indication. The pilot holds the controls until the needle indicates that it is in a green section of an arc (vs. red). This indicates that the rate of climb or descent is bringing the aircraft out of the danger zone and the controls (pitch attitude) and power should be held until the danger has passed.
    Since the system will clearly show whether it is functional or not, it is usually very reliable and should be followed by the crew exactly as they should react when they hear a warning like “whoop whoop sink rate pull up” or “terrain pull up”. These warning leave no room for “let’s see what happens”, they must be followed INSTANTLY.
    The second problem can be when the human factor, in some cases ATC, thinks that they have the situation under control. In 99 out of 100 cases THEY DON’T unless their instructions are the same as coinciding with TCAS.
    ATC instructions rarely contravene GPWS because I cannot imagine any situation where an aircraft in imminent danger of CFIT can be directed by a human controller (other than perhaps a member of ISIS after taking over ATC by force) to continue on a path that leads into the ground. TCAS is designed to operate in an environment where aircraft are at higher altitudes, not near surface or obstructions and under some form of ATC control or at least operating with an ATC clearance and, of course, a transponder “squawk”. It is also designed to operate giving an added level of safety to aircraft operating under IFR.

  • P.S.
    Sorry, I started flying jets before RVSM was introduced, so the flight levels were FL 280-310-350-390 etc. in one direction and 290-330-370-410 in the other: Above FL 290 the minimum separation was 2000 feet between aircraft flying in opposite direction. With the introduction of RVSM, the separation between levels was reduced to 1000 feet but only for aircraft RVSM equipped, the crew had to be licensed and the opertator needs RVSM approval, Above FL 420 RVSM is no longer prescribed and I have flown aircraft that had all equipment but the operator had no approval, If the aircraft was able to climb directly and at a minimum climb rate of 500 fpm all the way without a “step”, ATC could allow the aircraft to climb through RVSM airspace.
    RVSM stands for “Reduced Vertical Separation Minima” (sorry, I did Latin at school and even more than 55 years on, still have problems with calling it “minimums”).

  • PS 2
    Re-reading the article I realise that the authorities very cleverly introduce escape clauses, leaving the crew to decide whom to believe if ATC instructions contradict the TCAS.
    So again, set the crew up for “pilot error”.
    In general, TCAS should (as I tried to explain, perhaps not too convincingly) be regarded as virtually inviolate when operating at higher altitudes between cruising aircraft or aircraft cruising and climbing or descending. Reacting to the TCAS at low altitudes, often in more congested airspace, may lead to a few ATC controllers needing resuscitation and to triggering a GPWS warning, or can it? Conflicting advise from two systems? Is that possible?
    It may be worthwhile setting up a scenario in a simulator and see what happens? One system screaming “Traffic – Traffic dive” and the other: “Terrain terrain whoop whoop pull up”. With ATC screaming “Make an immediate turn left”. Could be interesting but is it a technical possibility? I don’t know.. It could perhaps lead to a cockpit crew not reporting for duty for their next few flights..

  • On some of the Fokker F27s I used to fly the GPWS could give a contradictory warning: If full flaps were used we would hear the GPWS calling “Too low – flaps”.
    These aircraft had a modification to allow them to be operated under Cat 3. They also had a special autopilot installed for the purpose. This was a special modification, only for Air France internal post “Aeropostale” flights. On a cat3 approach, use of full flaps was not allowed hence the GPWS warning. The F27 cannot be fitted with an autothrottle so Air France solved it by putting a 3rd crew member in the jump seat who was trained to operate the power levers during a cat3 ILS approach.
    Our problem was that the autopilots in these aircraft were produced especially for Air France only. Spares were no longer available so if it was broken it stayed broken. We did a lot of hand flying ! And of course needless to say ILS cat 1 only!
    But the TCAS was required equipment. The warning “traffic-traffic” could be ignored as long as we monitored the situation. It was not infrequent and only advisory, usually for a brief time only.

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