Monday, October 23, 2017
Transport Canada - Aviation Safety Letter Ahead to the Past—A Little History

Ahead to the Past—A Little History

by Tom Fudakowski, Chief, ANS Operations Oversight, National Operations, Civil Aviation, Transport Canada

The old saying “the more things change, the more they are the same” probably has extensive application with everything to do with aviation. Yes, we have made huge leaps in technologies, but the underlying principles remain the same. Our domestic airspace structure has evolved over the past five decades in concert with increasing air traffic volumes. Nevertheless, controlled airspace in southern portions of domestic low-level airspace represents a miniscule volume of the total—not surprising, at least in the Canadian context, when you consider that the global aircraft fleet could probably be contained in a cubic nautical mile of airspace. That comparison is no doubt a little far-fetched given that aircraft are not stationary and move in four dimensions. For that reason, the class of airspace is tailored to traffic densities and diversity of operations.

“see and avoid”

When you operate in visual meteorological conditions (VMC),
the principle of “see and avoid” has primacy

The term controlled airspace in and of itself may mislead some people. Too often, it tends to be associated exclusively with air traffic control (ATC) when in fact controlled airspace has two separate meanings. The more important association should be with regulated airspace, that is, airspace in which specific operating rules and minima apply. The ATC functions may apply, but only to flights operating under IFR rules. The VFR flight, on the other hand, may not be subject to any ATC requirements, but flight visibility and horizontal and vertical distances from cloud apply. By the same token, uncontrolled airspace simply means there is no ATC separation service provided, but the rules of flight are regulated and must be complied with, as minimal as they are. All of this is naturally in the interest of safety and collision avoidance. When you operate in visual meteorological conditions (VMC), the principle of “see and avoid” has primacy, albeit the responsibility shifts to ATC as you move up the airspace classification scale. Nonetheless, self-preservation would suggest that an attentive look out for other traffic is wise under any situation.

For those reasons, particularly when operating in Class E or lower classification airspace, we’ve always had operating rules to mitigate the risks of collision. The universal hemispheric rules regarding proper altitude for direction of flight is a case in point. We started out with the rule of odd or even altitudes when in level flight for both VFR and IFR operations. There was, however, a caveat that required a VFR flight to cross a controlled airway at an angle of 45° or greater. Otherwise, if you were crossing an airway at a lesser angle or actually navigating along the airway, you cruised at the odd or even plus 500 ft altitudes. All this made eminently good sense since IFR operations tended to be concentrated at the lower levels and cruising speeds were not that divergent. Then, as higher performance aircraft were introduced into service and operating altitudes rose into the teens, block airspace was introduced. A VFR flight required the filing of a controlled VFR (CVFR) flight plan and an ATC clearance to operate along airways. This requirement applied above 9 500 ft above sea level (ASL) east of 114° west and 12 500 ft ASL west of this meridian. With the increase in controlled airspace and airways, it became apparent that the odd or even plus 500 ft airway crossing requirements were no longer practical nor met their intended purpose. This resulted in the cruising altitude order requiring all aircraft, irrespective of the mode of flight, to maintain an odd or even altitude when in level flight. Fortunately, by that time, larger and ever-higher speed aircraft had migrated to the high teens and flight levels and the VFR operations were left the lower strata to themselves. Interestingly, as traffic volumes continued to grow, there was a need to revisit some of these rules and adapt them to existing realities. One of those realities was harmonization with our neighbours to the south, which created the VFR rule for flight above 3 000 ft above ground level (AGL) of odd or even plus 500 ft altitudes.

Airways may not have been very crowded, but some wise operating practices were applied. When flying along an airway, particularly after the introduction of VHF omnidirectional ranges (VOR), you always shaded slightly right of centreline. Prior to that, navigation aids did not offer the level of accuracy to generate great concern as their cross-track error provided lots of dispersion along the routes. With the four-course low frequency ranges (LFR), an intersection formed by two courses could cover a couple of counties. In fact, the width of the courses allowed ATC to climb or descend aircraft in opposite direction by instructing the aircraft to maintain well right of the course—an approved separation criterion. Putting this into today’s environment would make some folks’ hair stand on end. In today’s navigation-rich environment, with precision measured in meters, if not centimeters, and point-to-point navigation capability, some of these same operating practices are formally returning. On the North Atlantic, with daily unidirectional tracks published to take best advantage of minimum crossing times, off sets of up to 2 NM are now a standard operating practice. As you can see, in some instances, we have come full circle.

Airport control zones evolved over time. Some may be surprised that a control zone was not automatically associated with ATC service. Control referred to regulating the weather limits required to conduct VFR operations, which was really analogous to what we know as mandatory frequency (MF) areas, except that radio communication or position reporting was not a requirement. Control zones with operating control towers, on the other hand, were there for the purpose of airport control service and simply controlled aircraft and vehicle movements on runways and taxiways. Air traffic controllers did not per se control movements in a control zone; that came later. You could fly no radio (NORDO) into an international airport without pre-arrangement, execute proper circuit joining procedures and wait for the green light. That obviously did not last and radio communication became a mandatory requirement, but only at those locations whose control zone was designated as a Positive CZ.

The proliferation of various categories of airspace and nomenclature, each with differing operating rules, was not conducive to easy comprehension. Canada developed— and was in fact the first state to introduce—the now universally accepted airspace classification scheme. Irrespective of what a particular airspace may be referred to, it is its classification from A to G that governs the rules of conduct. Thus a control zone simply describes a volume of airspace around an aerodrome, or likewise, a terminal control area (TCA), but its classification governs the operating rules.

This is all food for thought when operating in today’s increasingly complex airspace structure. But, that fundamental principle of maintaining vigilance and watching for other traffic applies ever more today than in the past. No matter what class of airspace you are operating in, a clear understanding of rules and alertness may save you anxious moments if not more.

Keep a sharp look out and watch for other aircraft.



This article was published by Transport Canada in TP 185E -. Reprinted with permission

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