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Circling Approach - difference between ICAO PANS-OPS and US TERPS
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Comparison between ICAO PANS-OPS and US TERPS
Aerodrome operating minima (AOM) are calculated by operators based on information supplied by national authorities in their AIPs. This information typically consists of approach and departure procedures which assure safe separation between the aircraft and known obstacles located close to the intended flight path of the procedure in question. The procedures themselves are based on obstacle clearance domains constructed using internationally accepted standards.
Two main sets of procedures exist:
- ICAO Procedures, described in ICAO Doc 8168 Procedures for Air Navigation Services (PANS-OPS). PANS-OPS procedures are the international standard and are used throughout Europe and in many other countries world-wide.
- US Terminal Instrument Procedures (TERPS), described in FAA Directive No 8260.3B. US TERPS are used in USA and in certain other countries. These include Canada, Korea, Saudi Arabia and Taiwan. Some NATO military procedures are also based on US TERPS standards.
Pilots should be aware that there are significant differences in obstacle clearance criteria between procedures designed in accordance with ICAO PANS-OPS and US TERPS. This is especially true in respect of Circling Approaches where the assumed radius of turn and minimum obstacle clearance are markedly different (see below). US TERPS Change 21 (May 2009) introduced measures which have reduced the differences between the two methods; nevertheless, significant differences remain.
Both PANS-OPS and US TERPS assume values of minimum visibility available to the pilot at the lowest obstacle clearance altitude (OCA). These values are calculated differently and therefore, may result in different AOM. Table 1 shows the lowest value of visibility assumed by each method:
|1.9 km1.026 nm
|2.8 km1.512 nm
|3.7 km1.998 nm
|4.6 km2.484 nm
|6.5 km3.51 nm |
|1.6 km0.864 nm
|1.6 km0.864 nm
|2.4 km1.296 nm
|3.2 km1.728 nm
|3.2 km1.728 nm |
Minimum Obstacle Clearance (MOC)
ICAO PANS-OPS uses a varying MOC which increases with aircraft category as shown in Table 2:
|Obstacle Clearance||295 ft89.916 m||295 ft89.916 m||394 ft120.091 m||394 ft120.091 m||492 ft149.962 m|
The FAA has modified the criteria for circling approach areas via TERPS 8260.3B Change 21. The circling approach area has been expanded to provide improved obstacle protection. As a result, circling minima at certain airports may increase significantly.
TERPS 8260.3B Change 21 - Circling approach areas for approach procedures developed beginning in 2013 use the radius distances (in NM) as depicted in Table 3. These distances, dependent on aircraft category, are also based on the circling altitude which accounts for the true airspeed increase with altitude.
|Circling MDA in feet MSL||Approach Category and Circling Radius (NM)|
|CAT A||CAT B||CAT C||CAT D||CAT E|
|1000 or less||1.3||1.7||2.7||3.6||4.5|
|1001 – 3000||1.3||1.8||2.8||3.7||4.6|
|3001 – 5000||1.3||1.8||2.9||3.8||4.8|
|5001 – 7000||1.3||1.9||3.0||4.0||5.0|
|7001 – 9000||1.4||2.0||3.2||4.2||5.3|
|9001 and above||1.4||2.1||3.3||4.4||5.5|
From May 2, 2013, the FAA started publishing new instrument approach plates that include an enlarged segment of airspace to protect aircraft during circling approaches. The new airspace also offers pilots additional obstacle clearance while considering their MSL altitude above the MDA, which affects true airspeed. The boundaries of protected airspace for circling approaches are defined by arcs drawn from the threshold of each runway at an airport. The higher the speed of the aircraft, the larger the arc. Previous versions of the FAA’s terminal instrument procedures (Terps) used a radius of 1.7 nm from the end of the runway for a Category C aircraft such as a Hawker 800. Under the new criteria that radius will increase by 65 percent, to 2.7 nm. Chart providers U.S. Terminal Procedures and Jeppesen both plan to use new chart symbology to identify the updated approaches. The government plates will show an inverse “C” in a black box in the approach minimums area of the plate, while Jeppesen will use a “C” inside a black diamond. Charts without the new designation will continue to be guided by the old, smaller-radii criteria.
Radius of Circling Domain
Both systems use a radius of the circling domain, Obstacle Evaluation Area for US TERPS and Visual Manoeuvring Area for PANS-OPS, that increases with aircraft category and is based on TAS and bank angle. Both systems assume a 25 kt46.3 km/h
12.85 m/s wind factor, always added as a constant, without an assumption for the direction of the wind.
Aircraft Category is based on threshold IAS (1.3 x Stall IAS) and is shown on Table 4:
|Threshold IAS||less than 91 kts168.532 km/h
|91 kts168.532 km/h
46.774 m/s or more but less than 121 kts224.092 km/h
|121 kts224.092 km/h
62.194 m/s or more but less than 141 kts261.132 km/h
|141 kts261.132 km/h
72.474 m/s or more but less than 165 kts305.58 km/h
|166 kts307.432 km/h |
85.324 m/s or more (but less than 211 kts390.772 km/h
For PANS OPS only*
For PANS-OPS, the TAS is based on aircraft altitude and the visual manoeuvring IAS (Circling IAS). The latter is shown on Table 5:
|Circling IAS||100 kts185.2 km/h
|135 kts250.02 km/h
|180 kts333.36 km/h
|205 kts379.66 km/h
|240 kts444.48 km/h |
For US TERPS, TAS calculation is based on aircraft altitude and threshold IAS appropriate to category (shown on Table 4).
PANS-OPS assumes a bank angle of 20° for aircraft of all categories. US TERPS assumes a bank angle which varies with aircraft category, but is never less than 20° - see Table 6:
|Bank Angle (degrees)||25||25||20||20||22|
Because the IAS used for TAS calculation is greater and the assumed bank angle is lower, the radius of the circling area used in PANS-OPS is larger than that used in US TERPS. This means that an obstacle within the assumed circling area calculated using PANS-OPS criteria might fall outside the obstacle area calculated using US TERPS.
This is believed to have been a major factor in the following fatal accident, which resulted in considerable loss of life: B762, vicinity Busan Korea, 2002
Additionally, pilots not aware of the increased bank angle expected in US TERPS calculations might, in certain circumstances, stray outside the circling area due to the increased circling radius.
In summary, circling procedures based on US TERPS calculations afford considerably lower safety margins than those based on ICAO PANS-OPS. It is therefore essential that pilots understand these differences and are aware of the basis of calculations for all airfields at which they intend to operate, including alternates. The basis for calculation of minima is usually printed on the approach plate (see Further Reading: Jeppesen Chart Basics - a presentation).
National authorities or operators may require adjustments to be applied to operating minima to compensate for these differences.
Some other less significant differences exist between ICAO PANS-OPS and US TERPS. For details refer to the relevant documents listed below.
- Aerodrome Operating Minima (AOM)
- Approach and Landing Risks
- Approach and Landing Accidents (ALA)
- Circling Approach
- Circling Approach Discussion Paper, FSF European Advisory Committee, 24 January 2011
- ICAO Doc 8168: Procedures for Air Navigation Services - Aircraft Operations (PANS-OPS), Volume II: Construction of Visual and Instrument Flight Procedures
- US FAA Directive 8260.3B Standards for Terminal Instrument Procedures (TERPS) Including Change 21
- Jeppessen Briefing Bulletin: Implementation of new circling criteria based on TERPS 8260.3B Change 21
- TERPS Review - Circle to Land Tactics
- TERPS Review - Circling and the Visual Segment
- Jeppesen Chart Basics - a presentation
- Circling Traps - a presentation
- Article: "TERPS, CFIT and me" from June 1998 edition of "IFR Refresher" magazine
Note: Some of these references refer to pre-Change 21 versions of US TERPS.