ACCURACY OF FIXES
Nominal Fix Nominal Fix
Fix Tolerance Area Fix Tolerance Area
Chapter 10 Instrument Procedures
Accuracy of Facility Providing Track Accuracy of Facility Providing Track
Certain radio navigation aids provide track guidance information. When attempting to fly along the defined track from an aid, the accuracy with which the pilot accomplishes this is allowed for in an addition to the basic accuracy of the equipment. The pilot accuracy addition is known as flight technical tolerance.
VORVOR ± 5.2º (this value includes a flight technical tolerance of ± 2.5º) ILS Localiser ILS Localiser ± 2.4º (this value includes a flight technical tolerance of ± 2º) NDBNDB ± 6.9º (this value includes a flight technical tolerance of ± 3º)
Overall Tolerance of the Intersecting Facility Overall Tolerance of the Intersecting Facility
When intersecting fixes are based on radio nav aids, the overall tolerance of the fix is assumed to be:
VORVOR ± 4.5º when used in an approach procedure to establish a step down fix where less than 300 m (984 ft) of obstacle clearance prevails; accuracy is considered to be ± 7.8º
ILS LocalizerILS Localizer ± 1.4º
NDBNDB ± 6.2º when used in an approach procedure to establish a step down fix where less than 300 m (984 ft) of obstacle clearance prevails; accuracy is considered to be ± 10.3º
Other Fix Tolerance Factors Other Fix Tolerance Factors
Where no radio nav aid is available or other factors constrain the use of such aids, positions and turning points may be referenced by other means. For instance, when leaving an airway, an aircraft may be given radar vectors to a point marking the beginning of the arrival route, or the start of the final segment of a CATI ILS may be marked by a 75 Mhz marker beacon. The fix tolerance factors of such aids are:
Surveillance Radar Surveillance Radar
Radar fix accuracy is based on consideration of: radar mapping accuracy, azimuth resolution, flight technical tolerance, controller technical tolerances, and the speed of the aircraft in the terminal area.
Terminal Area Radar (TAR) within 37km (20 nm) Terminal Area Radar (TAR) within 37km (20 nm) Fix tolerance is ± 1.5 km (± 0.8 nm).
En-Route Surveillan
En-Route Surveillance Radar (ESR) within 74 ce Radar (ESR) within 74 km (40 nm)km (40 nm) Fix tolerance is ± 3.1 km (± 1.7 nm).
DME DME
Fix tolerance is ± 0.46 km (± 0.25 nm) + 1.25% of the distance to the antenna.
75 MHz Marker Beacons
75 MHz Marker Beacons Fix tolerances for ILS and “Z” markers for use with instrument approach procedures are calculated using the aerial polar diagram. Typically fix tolerance is +/- 0.8 km (0.45 nm) at 6000 ft and 0.35 km (0.2 nm) at 1000 ft.
Instrument Procedures Chapter 10
Air Law 10-23
Fix Tolerance Overhead a Facility Fix Tolerance Overhead a Facility
Most procedures require the aircraft to be flown over the facility providing track guidance at some point in the procedure. The accuracy of the ‘on top’ is a factor of the width of the cone of confusion over the beacon which is a function of the aerial design of the ground installation.
VOR VOR
VOR provides excellent track guidance but fix tolerance overhead a VOR is based upon a cone of confusion 50º from the vertical. At 3000 ft the accuracy of the ‘on top’ is given by:
2 x tan 50° x 3000 = 2 x 1.19 x 3000 = 7140 ft or 1.17 nm
NDB NDB
Fix tolerance overhead an NDB is based upon an inverted cone of ambiguity extending at an angle of 40º either side of the facility. At 3000 ft the accuracy of the ‘on top’ is given by:
2 x tan 40° x 3000 = 2 x 0.84 x 3000 = 5040 ft or 0.83 nm
Locator Locator
To improve the accuracy of ‘on tops’ some procedures employ an NDB with a 75Mhz marker co-located. This gives reasonable track guidance with an accurate ‘on top’. This system is called a locator and is shown on the chart as NDB(L).
Approach Area Splays Approach Area Splays
The tolerances described above are used to narrow and widen instrument approach areas as the aircraft flies to and from a facility respectively. The areas where the primary and secondary areas are increasing/decreasing in width are known as ‘splays’.
FAF Location FAF Location
For the final approach segment (contained between FAF and MAPt), the optimum and maximum distances for locating the FAF relative to the threshold are 9 km (5 nm) and 19 km (10 nm) respectively. VOR = 2.0 nm VOR = 2.0 nm NDB = 2.5 nm NDB = 2.5 nm Splay width at facility Splay width at facility Secondary area
Secondary area Primary area Primary area
Final Approach Splay Final Approach Splay
Reducing in width closer to the facility Reducing in width closer to the facility
FAF FAF Missed approach Missed approach Point (MAPt) Point (MAPt)
Chapter 10 Instrument Procedures
In designing instrument approach procedures adequate space is allowed for descent from the facility crossing altitude/height to the runway threshold for straight-in approach or to OCA/H for circling approaches.
Establishing a maximum allowable descent gradient for each segment of the procedure provides adequate space for descent.
The optimum descent gradient in the final approach should not exceed 5% (50 m/km, approximately 300 ft/nm) which is equivalent to a 3º glidepath.
Where a steeper descent gradient is necessary, the maximum permissible is 6.5% (65 m/km, approximately 400 ft/nm) which is equivalent to a 3.8º glidepath. In the case of a precision approach the operationally preferred glidepath angle is 3º. An ILS glidepath in excess of 3º is used only where alternate means of satisfying obstacle clearance requirements are impractical.
In certain cases the maximum descent gradient of 6.5% (65 m/km) results in descent rates that exceed the recommended rates of descent for some aircraft. Pilots should consider carefully the descent rate required for non-precision final approach segments before starting the approach.
There are five segments to a standard instrument approach procedure.
Segments of an Instrument Approach Procedure Segments of an Instrument Approach Procedure