Potential Safety Benefits of RNP Approach Procedures Sandro Salgueiro Advisor: John Hansman Department of Aeronautics and Astronautics Massachusetts Institute of Technology

Image: Honeywell Aerospace Required Navigation Performance (RNP) Required Navigation Performance (RNP) is a type of Performance Based Navigation (PBN), and is a component of NextGen. RNP is a navigation method that imposes well-defined requirements on the aircraft navigation system (Required Navigation Performance). Because of these performance requirements, the aircraft lateral and vertical trajectories are contained within a narrow corridor. Image: Honeywell Aerospace

Potential Benefits of RNP Approaches Lower approach minimums in areas with challenging terrain. Shorter path length. Lower fuel consumption. Less noise over populated areas. Increased safety. Lower probability of unstable approaches. KSEA RNP RWY 16R KPSP RNP RWY 13R

What’s different about RNP approaches? During an RNP approach, a go-around must be initiated if either the lateral or the vertical deviation limits are exceeded at any point in the approach. 2*RNP (RNP ≤ 0.3 nm) 150 ft 3o

RNP/ILS Comparison Compared to a conventional ILS approach, an RNP approach offers more precise vertical guidance (higher resolution) at distances greater than 1 nm from the runway touchdown zone. RNP approaches are usually captured earlier than ILS approaches. As a result, we hypothesize that RNP approaches are less likely to develop into unstable approaches. Edge of ILS glideslope beam 150 ft RNP vertical track limits 320 ft 1 nm

Approach Stability and Safety Unstable approach: Too fast/too slow. Too high/too low. Not properly aligned with the runway. Aircraft not in landing configuration. Unstable approaches were a factor in 66% of 76 landing accidents and incidents worldwide between 1984 and 1997 (Flight Safety Foundation). Statistically, unstable approaches increase the likelihood of events such as controlled flight into terrain (CFIT) and loss-of-control (LOC).

Elements of a Stabilized Approach Flight conditions Must be stabilized below* Allowed speed deviation Maximum allowed altitude deviation Maximum allowed descent rate VMC 500 ft AGL VREF ≤ IAS ≤ VREF + 20 kt ± 60 ft from glideslope 1000 ft/min IMC 1000 ft AGL ± 120 ft from glideslope VMC: Visual Meteorological Conditions IMC: Instrument Meteorological Conditions AGL: Above ground level VREF: Approach reference speed IAS: Indicated Airspeed *An approach that becomes unstabilized requires an immediate go-around. Source: Flight Safety Foundation

Methodology ASDE-X data was chosen over PDARS due to its higher update rate (1 Hz for ASDE-X versus 4 to 5 Hz for PDARS). Airports were chosen based on the availability of RNP procedures as well as ASDE-X surveillance data. Analyzed ASDE-X data from KSEA (Seattle), KMDW (Chicago Midway), KJFK (New York JFK), and KDCA (Washington Ronald Reagan). Airports with RNP procedures Airports with ASDE-X

Preliminary Data Analysis Eight (8) days worth of surveillance data were analyzed for each airport (KMDW, KSEA, KDCA, KJFK). Selected two days for manual inspection of approach stability (MDW only). Despite high RNP equipage levels among Part 121 carriers (59%), only 64 RNP arrivals were observed. Total number of arrivals 7,640 RNP arrivals 64 RNP percentage 0.84% Airport # of RNP arrivals KMDW – Midway 59 KSEA - Seattle 5 KJFK – New York JFK KDCA – Washington DC

KMDW RNAV (RNP) X RWY 22L

Example Data Sample Aircraft type Altitude (ft) Lateral trajectory Groundspeed (kt) Altitude (ft) Lateral trajectory Vertical profile (ft vs. nm) Glideslope deviation (ft)

Visual Approach vs. RNP Approach KMDW (Chicago Midway) RWY 22L

Visual Approach vs. RNP Approach Visual RNP Visual Sample 1 RNP Sample 1 Visual Sample 2 RNP Sample 2

Example of Unstable Visual Approach Visual Sample 3 60 ft (maximum VMC deviation) VMC stable approach threshold

Example of Unstable Visual Approach (2) Visual Sample 4 60 ft (maximum VMC deviation) VMC stable approach threshold

Example of Unstable RNP Approach RNP Sample 3 60 ft (maximum VMC deviation) VMC stable approach threshold

ILS Approach vs. RNP Approach KMDW (Chicago Midway) RWY 31C

KMDW RNAV (RNP) Y RWY 31C

ILS Approach vs. RNP Approach ILS RNP ILS Sample 1 RNP Sample 4 ILS Sample 2 RNP Sample 5

*Unstable if IMC, stable if VMC. Example of Potential Unstable ILS Approach* ILS Sample 3 VMC threshold IMC threshold *Unstable if IMC, stable if VMC.

Example of Unstable ILS Approach ILS Sample 4 60 ft (maximum VMC deviation) VMC stable approach threshold

*Unstable if IMC, stable if VMC. Example of Potential Unstable RNP Approach* RNP Sample 6 VMC threshold IMC threshold *Unstable if IMC, stable if VMC.

Results

Result 1: Approach Stability Approach stability was evaluated based solely on deviation from glideslope (using ILS stability standards). In the two days that were manually inspected for unstable approaches (MDW), a total of 15 unstable approaches were found, where two of these were RNP approaches. No significant difference in the number of unstable approaches was observed among the different approach types. These statistics are currently not considered significant given the small data set. Approach type Unstable approaches Total approaches Percentage unstable RNP 2 59 3.4% Non-RNP (ILS and Visual) 13 387

Result 2: Vertical Track Variability 59 approaches of each type (RNP, ILS, Visual) were picked from the MDW data set and statistics computed for the RMS value of glideslope deviations. Analysis was done with a single type of aircraft (B737) for consistency. Glideslope deviation RMS (ft) RNP (22L + 31C) ILS (31C) Visual (22L) Mean RMS 36.2 ft 36.8 ft 42.6 ft Std. dev. RMS 12.9 ft 27.8 ft 29.6 ft Lowest RMS 18.7 ft 13.6 ft 19.7 ft Highest RMS 104.2 ft 182.2 ft 163.7 ft Table: RMS values of glideslope deviation

Conclusions No conclusive evidence of improved stability on RNP approaches has been found so far. Because of the small data set used, our results are currently not considered statistically significant. Additional data must be obtained so that a more significant statistical analysis can be performed. Future work will include the development of algorithms for automated evaluation of approach stability based on requirements discussed in this presentation.

Thank you. Questions?

Charts

KMDW RNAV (RNP) Y RWY 31C

KMDW RNAV (RNP) X RWY 22L

KMDW ILS OR LOC DME RWY 31C

KDCA RNAV (RNP) RWY 19

KDCA RIVER VISUAL RWY 19