SAFE SKIES
Aviation’s long and winding road
CAPTAIN A. RANGANATHAN
|
While China has progressed to using state of the art aviation technologies like RNP that enable its flights to handle the most difficult terrains with ease and safety, we in India are still using outdated procedures.
|
Unless we progress to modern navigation technologies, all highways in the air will lead to Delhi but the rest of the country will remain confined to the mud road era.
If an RNP contained approach procedure is used, the aircraft can descend in a predefined path (depicted by the magenta line in the image) on a steady 2.5 per cent gradient.
Strange resonances between the Halibed temple corridor
The opening words of the ballad “The Long and winding Road”, sung by Paul McCartney of the Beatles, best describes the route in the sky designed to take Chinese airlines to the remote airport in Tibet. Lin Zhi airport is located at an elevation of 9,670 ft, carved into the winding river valley flanked by mountains that are 20,000 ft high. Earlier, flights to Lin Zhi airport was not feasible due to the unreliability of ground-based navigational aids because of the surrounding terrain. The Chinese aviation authorities (CAAC) developed a stringent safety standard. The Required Navigation Performance (RNP) approach and departure procedures were designed and executed by Naverus. Extensive validation flights were carried out and the CAAC authorised the inaugural flight. On September 1, 2006, the first commercial flight landed in Lin Zhi. The 175 km long serpentine approach is the world’s most challenging one for a jetliner to land and the Chinese aviation took the great leap forward.
Indian aviation uses navigation procedures that lack precision while China embraces the latest technology of RNP (Required Navigation Performance) approaches to operate out of the most difficult terrain. ISRO landed the probe from Chandrayaan-1 on the moon but, when it comes to the skies, India continues with the outdated technology which has large errors and which requires space. Unless we progress to modern navigation technologies, all highways in the air will lead to Delhi but the rest of the country will remain confined to the mud road era. The mindset and fixation on the CAT 3 ILS capability of Delhi has numbed the need to access airports in the rest of the country without ground-based aids.
Tunnels in the sky
“The tunnels in the sky”, as Steve Fulton, a former pilot and a founder-member of Naverus, who design the precise RNP routes, calls it, are essentially very narrow and perfectly designed routes in the sky through which the aircraft flies down a tightly contained path using the onboard containment systems to maintain separation with other tunnels or obstacles in the sky. To give an example of the containment path are the perfect-pillared corridors of ancient Hoysala temples in Belur and Halibed or the Meenakshi Temple at Madurai. The bats and pigeons which fly around, keep clear of the walls, just like aircrafts flying down a RNP track are programmed to do.
RNP technology allows an aircraft to fly safer and more reliable approaches and landings. Rather than relying exclusively on ground-based navigational aids, an RNP-guided approach uses a combination of onboard navigation technology and the Global Positioning System satellite network. The pioneer in RNP approaches was Alaska Airline. Hazardous terrain and extreme weather affected the airlines schedules frequently. Aircrafts had to hold or divert from airports where installation of precision approach systems was impracticable. The first RNP-guided flight path was used by Alaska Airlines to land in Juneau, Alaska, in 1996.
Photos: Captain A. Ranganathan. RNP images of approach to Leh by Naverus.
Precise manoeuvres: With RNP, ANP deviation will be less than the position of the aircraft from the centre line
RNAV (Area Navigation) is defined as a method of navigation whereby an aircraft can fly any desired flight path within the coverage of referenced navigation aids or the limits of the capability of self-contained systems, or a combination of these capabilities. Overall, safety is maintained through a combined use of aircraft navigation accuracy, route separation (where the path of the aircrafts flying in opposite directions take different paths) and/or the use of radar monitoring techniques which positively identifies the aircraft position in both horizontal and vertical plane in the sky and communications. The containment system is similar to having a series of needles in the sky and threading them through the narrow heads.
RNP is defined as RNAV operations with onboard navigation containment, monitoring and alerting. A critical component of RNP is the ability of the aircraft navigation system to monitor its achieved navigation performance and alert the pilot if the operational requirement is not being met during an operation. This onboard monitoring and alerting capability can reduce reliance on ground-based navigation aids and to maintain overall safety. When RNP procedure is used with a defined containment value, the procedure becomes extremely accurate. To use RNP as an approach tool, aircraft will need to be certified for RNP 0.3 or even RNP 0.1, reflecting a capability to maintain track with a maximum deviation of 0.3 or 0.1 nautical mile ( 180 or 60 feet ). Aircrafts perform at a much lower value than the required figure and the ANP (Actual Navigation Performance) factor may be as low as 0.02 or 0.03 nm. On board navigation computer systems in modern aircraft can keep the aircrafts within an error of just 12 to 18 feet.
If a circle is drawn around the flight management computer-calculated position of the aircraft, with the RNP figure as the radius, the containment path will be designed keeping this circle clear of the obstacles. The ANP circle will be inside the RNP circle and the aircraft navigation performance accuracy places the aircraft within the ANP circle 95 per cent of the time. The VOR transmission has an acceptable error of two degrees. This translates to an error of 18,000 feet when an aircraft is about 180 km away. Even when the aircraft is within two km, the error is big enough to require a higher obstacle clearance which, in turn, requires a higher visibility criteria. The RNP track error is negligible when it switches to the Approach mode. For the design of the Lin Zhi approach, which commences almost 180 km away, the error is less than 60 feet!
Improving efficiency and safety
RNP containment
Using the increasing capability and accuracy of onboard aircraft systems combined with GPS, RNP approaches and departures at some of the most difficult airports can be performed, with an improvement in safety and efficiency benefits. Access to airports which were difficult with conventional navigation is possible with the RNP approaches. While most of the aircraft flying in the world are RNAV capable, only a third are capable of performing these very accurate RNP approach and departure procedures. These are tailor made for each airfield. A simple comparison is like wearing custom made designer clothes as against wearing free size garments! The cost of commissioning an RNP procedure for a particular airfield will be offset by reduced distances flown and time saved. The benefits are also due to fuel saved and fewer weather-related diversions.
There are three basic hurdles to RNP operations. The aircraft and its avionics must be certified by the regulatory authority. The aircraft must be capable of meeting the requirements of PBN (Performance Based Navigation) with RNP 0.1 or 0.3 criteria. The airline must gain operational approval from the regulator to conduct RNP operations. In addition to having an eligible fleet, the airline must incorporate changes to all procedures similar to the requirements necessary to conduct ILS CAT II/III operations. The airline must also meet stricter requirements for monitoring the integrity of navigation databases and flight-specific availability of levels of RNP performance based on satellite conditions. The airline must have access to RNP approach procedures that are designed for the specific runway end they intend to use. These procedures may be tailored to their specific aircraft and operations, or be public procedures available to all approved operators at that airport. Approach procedures based on RNP often provide improved minima compared to the existing non-precision approaches, in many cases comparable to (but not better than) existing ILS approaches. This allows aircraft to complete landings in a broader range of weather conditions.
In India, we have another hurdle. Insufficient qualified manpower in the DGCA to oversee the implementation of these procedures. The will to update knowledge is also limited. RNP procedures have been in use for more than 10 years, worldwide, but we languish in the inaccurate VOR (Very High Frequency Omni-directional Rangefinder) or NDB (Non-directional beacon) approaches. Because of the acceptable and inbuilt errors in these systems, aircraft need a much higher visibility criteria as well as obstacle clearance limits. These approaches are similar to wearing an “Extra-large garment” on a small-made body and convincing yourself that the fit is good!
Self-contained
The most important aspect of the contained RNP approaches is the freedom from ground-based aids. During the deluge in Mumbai in July 2005, the airport shut down and all the navigation aid installations were damaged due to flooding. When the airport was opened after three days, without ensuring a safe environment, the ILS (Instrument Landing system) for the main runway 27 was unusable. The weather conditions during monsoon season meant that the winds were blowing from the west. The only suitable runway was the secondary runway 32. The VOR approach to this runway required an aircraft to descend only up to 1,400 ft above ground and it required a visibility of more than four km. This was because of the obstruction from Trombay hills on the inbound path of the VOR approach. If we had RNP approach procedures for Mumbai, an aircraft can land on the main Runway 27 as well as on the secondary runway 32 with a much lower visibility or obstacle clearance requirement. All that happened was to make aircrafts land on an unsafe runway in unsafe tailwind conditions in rain. An Air India 747 overshot the runway on July 31, 2005.
Airfields in the Himalayan region like Leh and Kulu are literally shut down for civilian air operation unless the weather conditions are absolutely clear. The procedure for Leh requires clear skies below 26,000 feet for the last 180 km. The aircraft approaches overhead Leh at almost 24,000 feet and descends in a steep box pattern around the runway, keeping clear of the hills in the area. Because of the altitude, the speed of the aircraft will be very high due to the low density at that altitude. The radius of turns become high and the touchdown speeds are also high.
Better alternative
If an RNP contained approach procedure is used, the aircraft can descend in a predefined path (depicted by the magenta line in the image ) on a steady 2.5 per cent gradient. This provides for a positive speed control and a stabilised approach for landing. The RNP contained approach to Leh will have the accuracy required to commence the approach phase from 100 miles, as against the current clear weather operation. Leh will be accessible all through the year. Similar procedures can be designed for all runways in all the airports. The capital expenditure required to install equipment in all the airfields will be avoided. All modern aircrafts are RNP capable. The only expenditure is to design the procedures and make them available for the airlines. This will be a much cheaper option for the authorities and this ensures freedom from damage due to weather conditions. Hundreds of flights get delayed or cancelled out of Delhi and other airports in the northern region every year. Thousands of passengers get stranded or inconvenienced. The choice before us is: “Do we remain in Chandni Chowk era or do we evolve like China”?
Printer friendly
page
Send this article to Friends by
E-Mail
Magazine
