TCAS: Disregarding a Resolution Advisory
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.
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.
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.
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.