MEL Provisions - Historical Background to TCAS/ACAS II Development
As there seems to be quite a lot of interest regarding MEL provisions whereby, as it is mentioned, flights might depart without the TCAS (essentially ACAS II) being fully serviceable, I would just like to provide some background information as to why this can be permitted. In the late 1980s when it was starting to become apparent that TCAS would be adopted as a potential aid to alert pilots of impending undue proximity to another aircraft I, as a Flight Operations Inspector in the UK Civil Aviation Authority, was tasked with representing the Authority's interests from a transport aircraft pilot's perspective. Consequently I established contact with the FAA, 'flew' an early version in a flight simulator, and observed how the equipment worked from within the flight deck of a Piedmont Airline aircraft transiting between two aerodromes. (The FAA also very kindly let me fly a breadboard version of what was being developed as equipment to satisfy an ACAS III standard, that is to say equipment where both vertical and horizontal advisories might be posted: it worked very convincingly, but as there seemed to be no identifiable need for this level of alerts it didn't go much further at that stage.)
In the UK, an ACAS (for those who don't know this acronym, this stands for Aircraft Collision Avoidance System) Operational Safety Working Group was set up, involving air traffic controllers, scientists, military interest and of course UK airline and CAA pilots, to understand what TCAS II - as a solution for ACAS II (ie the vertical-only advisory generating equipment) might do, what operational guidance might be published, how aircrews might be trained when their aircraft had this equipment installed and certificated, and what air traffic controllers might expect to observe.
Amongst many topics that were discussed was how, if there should ever be a requirement that a certain class of aircraft, such as aeroplanes over a certain mass, helicopters, etc. must have TCAS II installed, flights might or might not be permitted to take place if the system became inoperable. Whilst one point of view was that the aircraft should not be allowed to get airborne, or if it were to fly should remain clear of controlled airspace and remain in VMC (Visual Meteorological Conditions), others pointed out that this could be quite impractical in airline operations and that concessions should be made via the MEL (Minimum Equipment List). After discussion it was agreed that if, for example, an aeroplane normally based in the UK were to find its TCAS equipment unserviceable when away from base, somewhere in Australia, possibly, it would be sensible, taking into account all the other means by which separation between aircraft was normally applied, to permit this aeroplane to fly for a limited period sufficient for it to land at a place where a repair could be made. But this period of grace would be subject to certain conditions such as a maximum number of days, not to proceed beyond an airfield at which a repair could be made, and (later on when separation standards were reduced) not to fly within certain airspace in which serviceable TCAS was prescribed. It is worth remembering that 30+ years ago aeroplanes transiting across the world would make more stops en route that they would today, and that reserves of spares that might be needed to rectify an inoperable TCAS - with the emphasis on the 'S' for System - might not be readily available.
This decision was agreed within the Safety Working Group, was later taken up by the Joint Aviation Authorities (when I was the UK representative) and by ICAO Flight Operations Panel (ditto). Safety is the product of managing risks, and I'm not aware that flight operations have suffered unduly where flights have taken place with inoperable TCAS. It is worth remembering that if two aircraft normally required to have TCAS are converging and one has inoperable TCAS, it is highly unlikely that the other will also have its TCAS equipment inoperable.
I hope that this little essay helps clarify how it came about at the outset that flights by aircraft required to have TCAS installed might be permitted to get airborne with the equipment unserviceable, and that this concession has been agreed on a universal basis subject, as always, to conditions.