The Airplane - Jay Spenser [127]
In the 1930s, airliners such as the DC-3 used a radio-navigation system known as the four-course radio range. Depending on whether the airplane was to the left or right of the intended course, its flight crew heard the Morse code for either A (._) or N(_.) in their headphones. When on course, these two signals merged together to become a constant tone (later on, a needle on the instrument panel displayed these indications, sparing one of the pilots from having to listen for hours at a time).
This system allowed airliners to follow invisible electronic airways in the sky or perform instrument approaches and landings at properly equipped airports. In the latter case, the crew followed published approach procedures involving timed turns and required headings and altitudes. Vertical fan markers, audible when the airplane passed directly over them, marked thresholds bracketing a constant-rate descent to the runway.
If the crew sighted the runway by the time a designated minimum altitude was reached, they went ahead and landed. If not, they declared a missed approach and started all over again or headed to an alternate airport to try their luck there. It was all highly imprecise and extremely stressful and fatiguing for pilots.
With the discovery of radar in World War II came the ground=controlled approach. This military system had trained controllers “talk crews down” (verbally guide them to the runway) through bad weather by reference to two screens. One showed the runway’s extended centerline, while the other showed the desired descent angle. Although effective, ground-controlled approaches soon gave way to better systems.
The key evolution in the postwar era was the instrument landing system (ILS), in which flight crews follow vertical and horizontal guidance bars to track the localizer (extended runway centerline) and descent angle (glide slope). The precise ILS serves the global industry well but is fundamentally complex and expensive. As such, it is not available at the vast majority of the world’s airports. Even at those airports large and busy enough to have ILS, it is generally not available on every runway.
Where ILS is not available, pilots have found themselves back to using nonprecision approaches involving a variety of radio-navigational aids. Right up to the present day, air transport pilots are required to remain proficient in these old-fashioned “step-down approaches” as a fallback in the event that ILS is unexpectedly unavailable.
Satellite navigation is mercifully spelling an end to those “sweaty-palm” nonprecision approaches that look back to aviation’s earlier days. It is also providing an alternative to costly radar and ILS systems. Instead, with very little added costs, virtually any airport in the world can now have precision approach and landing capabilities collaboratively developed by all the participants in the global aviation system. These stakeholders include regulatory authorities and other government agencies, airlines worldwide, aerospace manufacturers (the makers of airplanes, engines, and equipment), and interested nongovernmental organizations (pilots’ unions, passenger organizations, and safety and other advocacy groups).
GNSS allows safe, precise, and flexible approaches that avoid the surrounding terrain of even the most challenging airports. These systems preempt the need for conventional air traffic control based on positive radar coverage. This is great news for developing nations, which are spared the heavy investment burdens of developing conventional aviation infrastructures.
Under the coming air traffic management paradigm, jetliners and other aircraft will automatically report their identity, GNSS-derived position, heading, altitude, vertical speed, and other pertinent information