By Steve Schmidt
The evolution of Performance-Based Navigation is entering a new phase called Advanced Required Navigation Performance (ARNP). Airspace designers in the U.S., Europe and New Zealand are already creating new departure, arrival and approach procedures to exploit its benefits.
What will these new procedures look like? Imagine an RNAV arrival that constantly descends by weaving through other traffic flows, with idle throttles. Imagine it flowing into an approach with satellite-based augmentation system vertical guidance down to 200-foot decision altitude. The idle descent will save time, fuel and money. Better data about the aircraft’s position and trajectory will decrease air traffic control’s use of radar vectors to maintain safe separation.
Required Navigation Performance (RNP) is waypoint-to-waypoint navigation using a system that monitors performance and issues alerts when requirements are not met. An RNP-qualified aircraft provides cockpit indication of the navigation system’s performance, often displayed as a value in meters or nautical miles, and labeled EPU or ANP. Aircraft with RNP can accurately fly a prescribed flight path repeatedly, with almost zero chance of exceeding the established bounds. This repeatability allows designers to fill terminal airspace with closely intertwined skyways that provide safe separation.
ARNP adds one required element to the basic concept of RNP – the ability to fly Radius-to-Fix (RF) legs. It can also add any of the following five optional elements:
Gulfstream G450/G550/G650 aircraft already have an RNP navigation system that can fly RF legs, meeting the minimum requirements for ARNP. They also already have the optional vertical navigation, with other options possibly being added later.
An RF leg is a curved flight path that resembles the familiar distance measuring equipment (DME) arc. They are defined by a certain radius, arc length and a fix. To a flight management system, it’s different than an arc because the RF leg has more stringent track-keeping requirements. For example, a GIV can fly a DME arc, but not an RF leg. These additional track-keeping requirements are important from an RNP perspective. To support TOAC, an RF leg can be assigned a certain time constraint.
The TPO element allows an overtaking aircraft to pass well right of the slower aircraft so it can continue to climb or descend. It’s different than a standard offset because the principles of RNP are applied to keep both aircraft within established bounds.
At cruise altitudes, when an aircraft starts a turn from one leg to the next, FRTs keep the aircraft within established bounds. FRTs are like RF legs with either a 15- or a 22.5-nautical mile radius. However, they are unlike RF legs because they are not legs at all, but just a prescribed way to transition from one leg to another as they meet at a waypoint. RNP principles are applied to limit any deviations.
RNP holding is similar to standard holding, but the principles of RNP are applied to significantly reduce the size of the protected airspace. Because RNP aircraft can easily stay inside established bounds, air traffic control can spend less time monitoring safe separation.
TOAC or RTA require the aircraft to pass waypoints at a certain time, in effect, transferring speed management to the flight management system. It allows air traffic control to better control the separation of aircraft flying the same procedure, maximizing the airport’s capacity.
The pace of change in airspace design has been breathtaking since GPS became available for civilian use in 1994. The first phase changed the way we fly routes and approaches. In the second phase, RNP provided the necessary confidence that aircraft are very close to where they ought to be. Then, RNP authorization-required approaches added a new flexibility in approach design called RF legs.
And now we have the transformative ARNP phase, which incorporates the best part of RNP AR without onerous training and equipment requirements.