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

2. 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

3. 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

4. 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

5. 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

6. 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).

7. 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

8. 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

9. 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

10. KMDW RNAV (RNP) X RWY 22L

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

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

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

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

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

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

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

18. KMDW RNAV (RNP) Y RWY 31C

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

20. *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.

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

22. *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.

23. Results

24. 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

25. 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

26. 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.

27. Thank you. Questions?

28. Charts

29. KMDW RNAV (RNP) Y RWY 31C

30. KMDW RNAV (RNP) X RWY 22L

31. KMDW ILS OR LOC DME RWY 31C

32. KDCA RNAV (RNP) RWY 19

33. KDCA RIVER VISUAL RWY 19