Change in aviation is met with heavy resistance, and even a ten year old technology is considered relatively new. With the introduction of Performance Based Navigation (PBN) in the Indian Airspace, confusion still exists on RNAV (aRea NAVigation), RNP (Required Navigation Performance), and where this RNAV/RNP are implemented in the Indian ATS.
The basic airway system (in India and the world over) was constructed based on sensors: the VOR and the NDB stations and receivers on board the airplane, which provide the capability to fly to, or from a radio station along one of its “radials”. These radio stations are scattered, purposely, across the country, and the airway system is constructed by simply “connecting the dots”, and an aircraft’s position is always relative to one of these stations. Example: Waypoint LATID is 77NM from Bangalore International Airport’s VOR (BIA), on a radial of 012°of BIA.
When an aircraft’s navigation system has a little more intelligence: the ability to scan and receive signals from multiple such radio ground stations, or from self contained navigation aids, such as the Inertial Reference System (IRS), or from the globally available GPS satellite constellation, and determine the aircraft’s position in terms of the World Geodesic System 1984 (WGS-84) coordinates, it provides the ability to determine the aircraft’s absolute position, rather than referencing it to a sparse set of radio stations. Example: Waypoint LATID is N14° 28.6’ E077° 56.9’.
The advantage with absolute position is freedom in the lateral: an aircraft can determine its absolute position, and fly to another waypoint whose absolute position is known, without having to stick to a “radial” or a VOR station. The ability to fly “Direct-To” another waypoint from the present position offers an easily comprehendible advantage: fuel savings through shorter, more direct routes. This freedom in the lateral, and the ability to navigate freely in an area, gives rise to RNAV, or Area Navigation.
Indian airspace is comprised mostly of “W” routes, which are, as per AAI, exclusively available for domestic operators only. According to ICAO Annex 11, a “W” route is NOT an Area Navigation Route, which means, the airway is constructed with reference to ground radio beacons, and are mostly direct from one beacon to another.
The other airways in India are “A”, “B”, “G”, “L”, “M”, “N”, “P”, “Q”, “R”, “UL”, “UM”. Of these, “L”, “M”, “N”, “P” and “Q” are area navigation routes. This means that these routes are not constrained to fly between ground based radio stations, but are instead optimised, more direct routes that save fuel. The “Q” routes were recently introduced in 2012, in July.
Since flying these routes implies a reliance on the aircraft’s complex navigation system (which authorities have no operational control of) rather than the simpler ground referenced navigation system (which authorities maintain), it is imperative that in the interest of safety, the complex area navigation system be capable of a certain navigation accuracy, also termed the navigation performance.
Certain routes, and certain procedures may require a higher navigation accuracy and its associated certainty, while others may be less demanding. To quantify these “higher and lesser” accuracies, the term “Required Navigation Performance” (RNP) was introduced, which stipulates the minimum navigational accuracy that must be guaranteed, with a certainty of 95% availability.
With RNP, of the many requirements, the aircraft must be capable of displaying the Actual Navigation Performance (ANP). As long as the actual navigation performance is within the limits of the RNP, everyone’s happy. But if the ANP gets worse than the RNP, that’s when Air Traffic Control must be notified so they can keep close eye on you and other airplanes in relation to your aircraft, and direct you based on conventional navigational practices.
The Area Navigation Routes – “L”, “M”, “N”, “P” – are all RNP 10 in India. The newly introduced “Q” routes, are all RNP 5. This means that your aircraft’s navigation accuracy must be better than 5 NM if it is to fly along the newly introduced 7 “Q” routes: Q1 – Q7. If however the ANP of the aircraft is 5.5 NM, then the accuracy is not enough to fly the “Q” routes, but accurate enough to fly thee RNP 10 routes: “L”, “M”, “N”, “P”.
The benefits of the RNP routes are evident. The newly introduced “Q” routes connect Delhi to Mumbai, Ahmedabad, Udaipur, and Vadodra. Picking “Q1”, which is Mumbai to Delhi (BBB- DPN), there are 13 waypoints in between the starting (BBB) VOR and the ending (DPN) VOR. Except for one, none of the other waypoints are ground based radio aids. The total ground distance between Mumbai and Delhi along Q1 is 633NM. The domestic non-RNAV “W13N” route between Mumbai and Delhi, has 5 waypoints in between, three of which are ground based radio aids (VOR). The ground distance along W13N is 653NM. A347, another non-RNAV route between Mumbai and Delhi, has 9 waypoints in between, three of which are ground based radio aids. The ground distance along A347 is 735NM. Compared to W13N and A347, Q1 saves 20NM and 102NM of ground distance, which translates to a saving of between 2 minutes and 14 minutes of flying time. A heavy Airbus A320, flying at FL350 at 76Tonnes, can save between 124 kg and 634 kg of fuel, which translates to a saving of between INR 11,000 and INR 56,227 per Mumbai-Delhi flight. Another advantage is the smooth flight path, as opposed to the zig-zag of non-RNAV routes.
Indigo’s 11 daily direct flights from Mumbai to the capital can save the airline about INR 1,21,000 per day, one way alone! Air India, with 12 direct flights, saves INR 1,32,000 one way, per day.
Aircraft with high navigation performance are allowed to fly the RNP routes. With higher accuracy, more airplanes can be squeezed on an airway. The “Q” routes allow aircraft to aircraft longitudinal separation of 50NM, while W13N allowed for a 10 minute separation, which translates to around 75NM. Theoretically, up to 13 airplanes may now fly on Q1, at any point of time, as compared to 9 on W13N. The capacity of the Indian Air Traffic System (ATS) has increased 44% on this route alone.
RNP and RNAV arrivals and departures are already in use, explained in another article which shall follow soon.