The risks posed by simultaneous air-traffic-control transmissions will increase with traffic density.
Inadvertent simultaneous transmissions on air-traffic-control frequencies "...can result in messages being misunderstood or lost and have been a factor in some aircraft safety-related incidents". So says a UK Civil Aviation Authority Aeronautical Information Circular (AIC), published in May 1993.
The problem of inadvertent simultaneous transmissions is as old as radio communication itself, but the CAA's AIC was prompted by the increasing number of communication-related incidents, and the rising potential for them, because of increasing congestion. The fact that systems to prevent simultaneous transmissions were, by then, under development and evaluation, and were proving to be "extremely effective", was also mentioned in the same document.
At around the same time, the European Organisation for Civil Aviation Equipment and the US Department of Transportation (DoT) published specifications for anti-blocking devices (ABDs) systems, designed to prevent inadvertent simultaneous transmission.
By May 1995, a system called Contran had been proven to meet these specifications. It was bought by two UK airports, and is highly praised by Britannia Airways - yet it has not been approved for operation.
The Contran system, patented by Lansec of the UK, is installed into existing radio-communications wiring in an aircraft or an air-traffic-control (ATC) station. Neither of the two versions is identical, nor are they interdependent.
Both work on the assumption, that no two transmit signals start at precisely the same time. Contran automatically inhibits the second transmission. The transmit start time difference necessary is calculated in milliseconds.
The pilot or controller whose transmission is inhibited receives the other station's message, even while his own press-to-transmit (PTT) switch is pressed, and an indicator light shows. For a pilot to transmit when the frequency is clear, he has to release his PTT switch and actuate it again as normal: the latter prevents pilots keeping the PTT switch pressed to force their way into the communications queue. Controllers with Contran, however, can push into the queue by keeping the PTT pressed: they hear an audio chime instantly the frequency is clear.
Controllers can override the ATC Contran. Two rapid applications of the PTT switch allow controllers to cut in at any time. A "bypass" switch can override Contran altogether.
Bournemouth Airport is one, which has purchased Contran, but its applications to operate it were initially stalled. Bournemouth senior ATC officer Jeff Berryman says that, although he provided a full trial report which demonstrated that the equipment met all regulatory specifications, the CAA's Safety Regulation Group (SRG) did not approve it, giving no reason.
UK National Air Traffic Services (NATS) tests of Contran, were carried out at Stansted Airport and it was later acknowledged by the SRG, that the only problems had been in NATS' installation of the equipment, not in the equipment itself.
Bournemouth has now received its clearance, and the UK's East Midlands Airport now expects its system to be approved.
Peter Stastny of the SRG's Air Traffic Services Standards Group says that it has no objection to airborne units being fitted, but does not, at present, intend to require it, saying that the case for Contran as reflected in compulsory occurrence reports about ATC incidents is "marginal".
The CAA's Research and Analysis Department, however, says that one factor in the increasingly frequent London terminal-area "altitude-busts" is "...clipped transmissions or replies with incomplete and/or total lack of identifying call signs".
The US Federal Aviation Administration has Contran under test now. In the meantime, the US Airline Pilots Association has barred its members from carrying out simultaneous approaches to close parallel runways using the Precision Runway Monitor system, unless an ABD is in operation.