TCAS, emergent properties and risk trade-offs
There’s been some comment from various regulator’s regarding the use of Traffic Collision Avoidance System (TCAS) on the ground, experience shows that TCAS is sometimes turned on and off at the same time as the Mode S transponder. Eurocontrol doesn’t like it and is quite explicit about their dislike, ‘do not use it while taxiing’ they say, likewise the FAA also states that you should ‘minimise use on ground’. There are legitimate reasons for this dislike, having too many TCAS transponders operating within a specific area can degrade system performance as well as potentially interfering with airport ground radars. And as the FAA point out operating with the AD-B transponder on will also ensure that the aircraft is visible to ATC and other ADS-B (in) equipped aircraft (1). Which leaves us with the question, why are aircrew using TCAS on the ground? Is it because it’s just easy enough to turn on at the push back? Or is there another reason?
At the back of every big jet pilot’s mind is a list of the worst accidents in aviation and a profound desire not to emulate them, and the worst amongst them is the Tenerife air disaster. On the 27 March 1977 at Tenerife in the Canary islands two Boeing 747’s collided on the runway after one began it’s takeoff roll while the other was still taxiing down the runway resulting in the greatest loss of life in any aviation accident (2). While there were many contributing factors that led to the accident, from a situational awareness perspective the runway was fog bound at the time and degraded communications (due to both human and technical factors) prevented effective coordination of runway use by the tower (Weick 1990). What’s important about such air disasters as Tenerife is that they serve as referent examples to inform aircrew’s recognition of patterns of emerging risk, knowing directly where everyone is without recourse to the tower is, in the context of Tenerife, always seen as a ‘good thing’ and inherently a safer position to be in.
The DC9 landed on Rwy 7R. A second aircraft (AC2) was in position and hold on Rwy 19R. LC asked DC9 its position, and pilot responded by intersection of 19R. LC instructed DC9 to turn left on Rwy 19L and stay with him. DC9 responded roger. When LC asked DC9 if he was clear of the intersection, he responded yes. LC cleared AC2 for takeoff Rwy 19R. AC2 declined clearance, saying that he could see lights ahead on 19R. LC asked DC9 if he was on Rwy 19R, and he responded yes. Note that LC could see neither aircraft nor the runways in the fog, but since the aircraft could see each other, a more serious occurrence was prevented.
FAA Report PGLTMKE95003, 1995.
TCAS as it turns out can still provide useful situational awareness data to aircrew on the ground, even though it’s primary collision avoidance functions are inhibited. So where the airfield is visually obscured, ground radar is not available and the operational tempo is hectic TCAS is perceived by aircrew as a valuable ‘in-cockpit’ tool that allows them to directly see what’s out there without needing to rely on the the guy in the tower or whether the other aircraft is ADS-B capable. In turn this makes the system of ground movement control less vulnerable to failures of coordination, thereby reducing the potential for another Tenerife.
Both major regulators clearly recognise the value of TCAS to enhance SA when crossing active runways or taking the active runway, and just as clearly are aware of the drawbacks at other times. All of which leaves the regulators in the interesting if slightly invidious position of having to trade off the safety benefits of ground use against the potential for degraded safety margins for airborne use, at least until ADS-B is fully rolled out.
P.S. And yes if both planes had been fitted with TCAS and were using it on the ground given the poor visibility it is very unlikely there would have been an accident at Tenerife.
On the 16 June 2015, two airliners ended up nearly taking off at the same time on crossing runways at Chicago’s Midway airport when both aircraft responded to the same ATC takeoff clearance. The incident appears to have occurred due to a combination of stepped on transmissions and the closeness of the two callsigns.
1. Given that ADS-B has not been fully rolled out, and your airport may not have an ADSE-X or equivalent system ‘your mileage may vary’.
2. Excluding September 11.