Sir - As more aircraft are in competition for slots in increasingly crowded routes, air-traffic control (ATC) has resorted to assigning aircraft non-standard levels to facilitate traffic flow.
I witnessed recently a competent controller in a non-radar environment having to berate the crew of a European flag carrier for following the wrong route, which reduced linear separation with other traffic to less than the required minimum. The frequency was almost saturated and the mistake was not discovered for some minutes far too long, had the error had been vertical and not one of tracking.
A high proportion of civil airliners has inertial navigation/reference systems (INS/IRS). The commendable accuracy of these systems, however, has increased the chances of a head-on collision, even when non-standard levels are not employed. Over Turkey and Syria, for instance, it is not unusual to have to change levels - in some cases more than once in a single sector - to avoid conflicting traffic, with ATC imposing tight time constraints on the climbing/descending aircraft, to clear opposing flight levels.
I am sure that most airline pilots flying IRS-equipped aircraft are accustomed to seeing traffic pass directly over or under their aircraft repeatedly. I know of some, who fly (unofficially) offset tracks, as a matter of course.
Is it not time for the International Civil Aviation Organisation to institute a worldwide agreement that all airlines allow for (for instance) a one-mile bias to the right in all computer-generated flight plans when the aircraft is above 15,000ft (4,600m)? The same feature should take only a simple software rewrite for any "direct-to" tracking command. Most air routes are designed around VOR tracking tolerances. An IRS-equipped aircraft flying a one-mile offset would remain well within those tolerances.
This simple measure could increase the safety margin in one area of flight and might avert a tragedy before somebody dies to prove that the problem exists.
Capt. A DIGGER
West Drayton, Middlesex, UK
Source: Flight International
