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Difference between revisions of "Circling Approach - difference between ICAO PANS-OPS and US TERPS"
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Revision as of 10:24, 5 April 2011
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 procedure in question. These procedures are based on obstacle 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 most 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:
US TERPS (Km)
- Table 1: Minimum Visibility at OCA
Minimum Obstacle Clearance (MOC)
- ICAO PANS-OPS uses a varying MOC which increases with aircraft category as shown in Table 2:
|Obstacle Clearance m (ft)||90 (295)||90 (295)||120 (394)||120 (394)||150 (492)|
- Table 2: Minimum Obstacle Clearance (MOC) - PANS-OPS
US TERPS uses 300 ft as MOC for all aircraft categories.
Thus the pilot of a Category D aircraft carrying out a circling procedure using TERPS minima would have 94ft less obstacle clearance than using PANS-OPS procedures.
Radius of Circling Domain:
- Both systems use a radius that increases with aircraft category and is based on TAS and bank angle. Both systems assume a 25 kt wind factor.
Aircraft Category is based on threshold IAS (1.3 x Stall IAS) and is shown on Table 3:
|Threshold IAS (kt)||less than 91||91 or more but less than 121||121 or more but less than 141||141 or more but less than 166||165 or more but less than 211|
- Table 3: Aircraft Category - PANS-OPS and US TERPS
For PANS-OPS, the TAS is based on aircraft altitude and the visual manoeuvring IAS (Circling IAS). The latter is shown on Table 4:
|Circling IAS (kt)||100||135||180||205||240|
- Table 4: Visual Manoeuvring IAS used by PANS-OPS
For US TERPS, TAS calculation is based on aircraft altitude and threshold IAS appropriate to category (shown on Table 3).
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 5:
|Bank Angle (degrees)||25||25||20||20||22|
- Table 5: Bank Angle used by US TERPS
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 considerably 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:
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. 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.
Note: some of these references may refer to pre-Change 21 versions of US TERPS.
- ICAO Doc 8168: Procedures for Air Navigation Services - Aircraft Operations (PANS-OPS), Volume II: Construction of Visual and Instrument Flight Procedures