Why Pilots Deliberately Fly A Curved Approach Into Certain Mountain Airports

Credit: Shutterstock | Simple Flying Published Sep 1, 2026, 9:00 AM EDT Brandon's passion for aviation started at a young age. He became involved in the flight simulator and VATSIM communities, which sparked his interest in aviation and eventually led him to earn his private pilot license. Outside of work, he continues to build flight hours, explore new airports, and stay involved in the aviation community. Sign in to your Simple Flying account At most commercial airports, the approach to landing is a straight line. The aircraft intercepts a final approach course aligned with the runway centerline, follows a 3-degree glideslope, and descends continuously until touchdown. The instrument approach procedures that guide this process assume flat terrain on both sides of the flight path and unobstructed airspace for miles ahead of the runway. At mountain airports, those assumptions do not hold. Ridgelines cross the approach path. Valleys constrain the lateral clearance. The runway may sit at 7,000-8,000 feet (2,134-2,438 meters), with terrain rising to 12,000 feet (3,658 meters) on multiple sides. The approaches designed for these airports look nothing like a standard ILS. Localizer courses are offset from the runway centerline. Descent angles exceed 6 degrees, more than twice the normal rate. Step-down fixes lower the aircraft in stages between terrain obstacles. DME arcs curve the flight path around ground stations to position the aircraft for a short final. And the newest generation of GPS-guided RNP AR approaches use radius-to-fix legs to thread curved paths through gaps in the terrain at accuracies within 0.1 nautical miles (185 meters), achieving minimums that would be impossible with any conventional technique. Here is how each of these approaches works and why the terrain at airports like Aspen (ASE) and Eagle County (EGE) demands them. Why A Straight-In Approach Is Not Always Possible Credit: Shutterstock A standard Instrument Landing System approach assumes a straight final approach course aligned with the runway centerline, a 3-degree glideslope extending approximately 5-6 nautical miles (9-11 km) from the runway threshold, and terrain clearance zones on both sides of the approach path that are wide enough to protect the aircraft if it deviates laterally from the course. The FAA's Terminal Instrument Procedures, known as TERPS, define the obstacle clearance surfaces that must be free of terrain and obstructions for an approach to be published. At a flatland airport like Dallas/Fort Worth International Airport(DFW) or Chicago O'Hare International Airport (ORD), meeting those clearance requirements is straightforward because the terrain surrounding the airport is level for miles in every direction. At a mountain airport, the terrain does not cooperate. Eagle County Regional Airport (EGE) in Colorado sits at an elevation of 6,547 feet (1,996 meters) in a valley surrounded by mountains rising to 11,000-12,000 feet (3,353-3,658 meters). Aspen-Pitkin County Airport (ASE) sits at 7,820 feet (2,384 meters) with ridgelines on multiple sides. A straight final approach course extending 6 nautical miles (11 km) from either runway would pass through terrain that is higher than the aircraft's altitude at multiple points along the path. The standard TERPS obstacle clearance surfaces cannot be maintained because the mountains violate them. A conventional ILS aligned with the runway centerline is not possible if the terrain along the approach path is higher than the glideslope the aircraft would fly. The result is that approach designers must find alternative paths to the runway that avoid the terrain while still providing a stable descent to landing. Those alternatives include offset localizer courses that are not aligned with the runway, steeper-than-normal descent angles that clear ridgelines by descending more rapidly, step-down fixes that lower the aircraft in stages between terrain obstacles, DME arcs that curve the aircraft around a ground station to position it for a final, and RNP AR approaches that use GPS-guided curved flight paths to thread through gaps in the terrain. Each technique solves a specific geometric problem arising from the relationship between the locations of the mountains and the runway. Aspen's LOC/DME-E: Offset, Steep, And Step-Down Aspen-Pitkin County Airport (ASE) has one runway, designated 15/33, at 7,820 feet (2,384 meters) elevation. Aircraft land on Runway 15 and depart on Runway 33. There is no option to reverse the flow. The terrain surrounding the airport prevents a standard approach from any direction, and the localizer antenna serving Runway 15 is not aligned with the runway centerline. Because the localizer course is offset, the approach does not terminate at the runway threshold and requires the pilot to maneuver visually for landing, which is why it is published as a circling approach rather than a straight-in. The approach itself descends at approximately 6.5 degrees, more than twice the standard 3-degree glideslope used at most airports. The descent is not continuous. It uses multiple step-down fixes where the pilot descends to a minimum altitude, levels off briefly, and then descends again at the next fix. The profile resembles a staircase rather than a smooth slope. The missed approach point is 2.6 nautical miles (4.8 km) from the runway threshold, which means that even after completing the entire approach, the pilot still has nearly three miles of visual maneuvering to reach the runway. Minimum descent altitude is 10,980 feet MSL, approximately 3,120 feet (950 meters) above the runway, with 3 miles of flight visibility required. If the pilot does not see the runway environment at that point, the missed approach procedure directs the aircraft to climb away from terrain that rises steeply on both sides of the valley. There is a second localizer approach in the FMS database: the Special LOC-DME Runway 15, which provides lower minimums but requires a specific FAA Letter of Authorization and OpSpec approval before a crew can fly it. In 2024, the FAA issued a Letter to Airmen warning that some FMS systems display both approaches without clearly distinguishing which is which, creating a risk that a crew could inadvertently load and fly the special approach without holding the required authorization. The standard LOC/DME-E is offered by default. Airlines operating into Aspen, including United, American, and Delta during ski season, require their crews to complete Aspen-specific familiarization training covering the approach procedures, opposite-direction traffic protocols, and the high-altitude go-around performance requirements before they are cleared to fly into the airport. Eagle County's RNP AR Approaches: GPS-Guided Curves Through The Mountains Credit: Shutterstock Eagle County Regional Airport (EGE) publishes five instrument approaches to Runway 25: an LDA with a glideslope option, an RNAV (GPS) Y, an RNAV (GPS)-D circling approach, and two RNP AR procedures designated RNAV (RNP) X and RNAV (RNP) Z. The standard approaches have minimum descent altitudes of 1,700 feet (518 meters) or more above the runway and require at least 3 miles of visibility. Those minimums reflect the terrain constraints that prevent a lower, straighter path to the runway. The two RNP AR approaches achieve significantly lower minimums by using curved flight paths that navigate through gaps in the terrain that a straight course cannot safely traverse. The key technology is the radius-to-fix leg. An RF leg is a curved flight path segment with a constant radius, flown using GPS and the aircraft's flight management system to maintain a precise arc through airspace. At Eagle County, the RNP AR approaches use RF legs to curve the aircraft around ridgelines that a straight final approach course would intersect. The aircraft follows the curved path at an RNP value of 0.1 nautical miles (185 meters), meaning the navigation system must keep the aircraft within 0.1 nautical miles of the defined course 95% of the time, with integrity monitoring that alerts the crew if the system cannot maintain that accuracy. The tight containment corridor is what allows the approach designer to route the flight path closer to terrain than a conventional approach could safely permit. Flying an RNP AR approach requires specific aircraft certification, crew training, and an FAA Letter of Authorization. The aircraft must be equipped with an FMS capable of flying RF legs with the required navigation accuracy. The crew must complete an FAA-approved RNP AR training program and review a facility-specific briefing package for Eagle County. Not all aircraft in an airline's fleet are necessarily certified, and not all crews are trained. The RNP AR approaches at Eagle County reduce diversions and cancellations compared to the standard approaches because they keep the airport accessible in weather conditions, particularly low ceilings and reduced visibility during winter snowstorms that would force a missed approach on any of the conventional procedures. DME Arcs And Other Curved Approach Segments Credit: Shutterstock Before GPS-guided RF legs existed, approach designers used DME arcs to create curved flight paths around terrain or to position aircraft for a final approach course that could not be intercepted on a straight course from a convenient direction. A DME arc is a curved segment flown at a constant distance from a ground-based Distance Measuring Equipment station. The pilot holds a fixed DME reading, typically between 10 and 30 nautical miles (18.5-55.6 km), while flying a curved path around the station until intercepting the final approach course inbound to the runway. The arc is not a GPS-guided path. The pilot maintains it by monitoring the DME distance and adjusting heading to keep the needle centered, a technique that requires continuous attention and crosswind correction. DME arcs remain published at airports across the United States and internationally, including at several mountain airports where they serve as the curved segment that positions the aircraft for a short straight-in final. They are also used at airports where terrain or airspace restrictions prevent a long straight-in approach from a standard direction. The precision of a DME arc is lower than an RF leg. The pilot is maintaining a distance from a ground station using a cockpit instrument, not following a computed curved path with 0.1 nautical mile (185 meters) accuracy. The protected airspace around a DME arc is wider than the containment corridor of an RNP AR approach, which means the approach designer must allow more room for terrain clearance on each side of the arc. That wider buffer limits how close the arc can be routed to terrain, which in turn limits how low the approach minimums can be set. The LDA approach at Eagle County (EGE) uses a different non-standard technique. The LDA, or Localizer Type Directional Aid, provides lateral guidance on a course that is offset from the runway centerline, similar in concept to Aspen's offset localizer but with a glideslope option that provides vertical guidance as well. The pilot flies the LDA course down to a decision altitude and then must visually maneuver to align with the runway for landing. The LDA exists at Eagle County because the terrain prevents a standard ILS localizer from being installed on the runway centerline. Each of these techniques, the DME arc, the LDA, the offset localizer, and the step-down approach, was the best solution available before RNP AR technology made it possible to design curved precision approaches with minimums that rival a standard ILS at a sea-level airport. Which Airlines And Aircraft Are Certified For RNP AR At Mountain Airports Credit: Eagle County Airport Not every aircraft and not every crew can fly an RNP AR approach. The FAA requires three separate authorizations before an operator can use the procedure. The aircraft must carry an FMS certified for RF legs at RNP 0.1 or better with integrity monitoring. The flight crew must complete an FAA-approved RNP AR training program and review a facility-specific briefing package for each airport where they intend to fly the procedure. The operator must hold FAA Letters of Authorization C384 and C081 covering RNP AR operations. If any of the three elements is missing, the crew flies the conventional approach with its higher minimums, regardless of what the aircraft's avionics are capable of. Major US airlines, including United, American, and Delta, operate into Eagle County Regional Airport (EGE) during ski season with more than 28 daily flights between them serving destinations including Atlanta (ATL), Chicago (ORD), Dallas (DFW), Denver (DEN), Los Angeles (LAX), Miami (MIA), and New York (JFK/EWR). These carriers operate RNP AR-certified aircraft including the 737 MAX and A321neo, on these routes, and their crews are trained for the Eagle County RNP AR approaches. The lower minimums mean flights that would divert to Denver or Grand Junction on a day when Eagle County's ceilings drop below the 1,700-foot (518-meter) threshold for the standard approaches can instead continue to the RNP AR minimum of 282 feet (86 meters) and land. The reduction in diversions is the commercial justification for the investment in aircraft certification and crew training. Aspen received its own RNP AR approach in March 2026 when the FAA approved the RNAV (RNP) N Runway 15 procedure developed by Honeywell. The new approach provides minimums of 537 feet (164 meters) above touchdown, 1.25 miles visibility, and a 3.5-degree descent angle, substantially shallower than the 6.5 degrees required by the conventional LOC/DME-E. Initial approval covers Gulfstream G350, G450, G550, G500, and G600 and Dassault Falcon 8X aircraft, meaning the approach is currently available to business aviation operators rather than airlines. Airline certification on additional aircraft types is expected to follow. Honeywell currently offers more than 400 public RNP AR approaches across the United States, and the number of airports and operators using the technology continues to grow as the FAA approves new procedures and more aircraft complete the required avionics certification.