procedure for that aerodrome.
Track Maintenance Track Maintenance
The procedures are defined by tracks to be made good and pilots are expected to allow for the wind. During Precision Approach Radar (PAR) and Surveillance Radar Approach (SRA) procedures, the controller ‘talking the aircraft down’ will adjust the required heading to counter the effects of drift. For ILS approaches pilots are required to maintain aircraft position to within half scale disposition of the deviation displays.
Obstacle clearance is a primary safety consideration in the development of instrument approach procedures. The criteria used and the detailed method of calculation are covered in PANS-OPS, Volume II. The obstacle clearance applied in the development of each instrument approach procedure is considered to be the minimum required for an acceptable level of safety in operations. The protected areas and obstacle clearance applicable to individual types of approaches are specified later.
Obstacle Clearance
Obstacle Clearance Altitude/HAltitude/Height (OCA/H)eight (OCA/H)
For each individual approach procedure an obstacle clearance altitude/height (OCA/H) is calculated for a procedure and published on the instrument approach chart. In the case of precision approach and circling approach procedures an OCA/H is specified for each category of aircraft.
OCA/H for Precision Procedures OCA/H for Precision Procedures
A precision procedure is a runway approach procedure therefore the reference for the calculation of OCA/H must be the elevation of the threshold of the landing runway.
OBSTACLE CLEARANCE
OBSTACLE CLEARANCE
Chapter 10 Instrument Procedures
OCA/H for Non-Precision Procedures OCA/H for Non-Precision Procedures
A non-precision procedure is an aerodrome approach procedure in which the final segment is up to 30° displaced from the landing runway direction. In this case, the reference for OCA/H is the aerodrome elevation as determined by survey. If however, the threshold of the landing runway is 2m (7 ft) or more below the aerodrome elevation, the threshold elevation is used.
OCA/H for Visual Manoeuvre (Circling) (VM(C)) OCA/H for Visual Manoeuvre (Circling) (VM(C))
At the end of a non-precision approach a pilot may manoeuvre the aircraft to land in a different direction from which the approach is made. In this case, the aerodrome elevation is always used as the reference for OCA/H.
Factors Affecting Operational Minima Factors Affecting Operational Minima
In general minima are developed by adding the effect of a number of operational factors to OCA/H to produce, in the case of precision approaches a DA or DH, or in the case of a non- precision approach, MDA or MDH. The general operational factors to be considered are aircraft mass, elevation or the pressure altitude appropriate to the elevation of the aerodrome, temperature, wind, runway gradient and condition. The relationship of OCA/H to operating minima (landing) is shown in the following 3 diagrams.
Margin or Lower Limit Margin or Lower Limit
Operators are required to specify the margin or lower limit to be added to the OCA/H to determine the DH/A or MDH/A. This is determined in accordance with the criteria stated and may be zero. If you have your own aircraft you are not required to add a margin or lower limit.
System Minima System Minima
Regardless of the result of calculation of DH or MDH, for each approach system (procedure or equipment), a minimum height above the datum is established as a ‘never below’ figure. The following table shows the system minima which override a lower DH or MDH. For instance, if the published OCH for a VOR/DME approach (non precision) was 130 ft and the margin or lower limit was 20 ft, theoretically the MDH would be 150 ft. However, the system minimum for VOR/DME is 250 ft, therefore the MDH is 250 ft.
System
System Minimum Minimum (ft) (ft) System System Minimum Minimum (ft)(ft) Full ILS CATII 100 SRA (RTR* 0.5 nm) 250 Full ILS CATI 200 SRA (RTR* 1 nm) 300 ILS (no glide path) 250 VOR 300 ILS back beam (not approved) 250 NDB or Locator 300 PAR (no glide path) 250 VDF 300 VOR/DME 250 SRA (RTR* 2 nm) 350
Table: System Minima
Instrument Procedures Chapter 10
Air Law 10-17
NON-PRECISION APPROACH PROCEDURE NON-PRECISION APPROACH PROCEDURE
The lowest altitude (OCA) or alternatively the lowest height above the aerodrome elevation or the elevation of the relevant runway threshold, if the threshold elevation is more than 2 m (7 ft) below the aerodrome elevation (OCH), below which the aircraft cannot descend without infringing the appropriate obstacle clearance criteria.
Mean Sea Level Mean Sea Level Aerodrome Elevation or Threshold Aerodrome Elevation or Threshold Elevation if more than 2 m (7 ft) Elevation if more than 2 m (7 ft) below Aerodrome Elevation below Aerodrome Elevation
Height of the highest obstacle in the final Height of the highest obstacle in the final
approach approach M M D D A A M M D D H H O O C C A A O O C C H H Altitude Altitude
Obstacle Clearance Altitude (OCA) Obstacle Clearance Altitude (OCA)
Or Or
Obstacle Clearance Height (OCH) Obstacle Clearance Height (OCH) Minimum Descent Altitude (MDA) Minimum Descent Altitude (MDA)
Or Or
Minimum Descent Height (MDH) Minimum Descent Height (MDH)
Minimum Obstacle Clearance (MOC) Minimum Obstacle Clearance (MOC)
for the Final Segment for the Final Segment Fixed margin for all aircraft 90 m (295 ft) without FAF
75 m (246 ft) with FAF Margin or Lower Limit
Margin or Lower Limit Based on operational consideration of:
ground/airborne equipment characteristics; crew qualifications; aircraft performance; met conditions; aerodrome characteristics; location of guidance aid relative to runway
Chapter 10 Instrument Procedures
PRECISION APPROACH PROCEDURE PRECISION APPROACH PROCEDURE
The lowest altitude (OCA) or alternatively the lowest height above the elevation of the relevant runway threshold (OCH), at which a missed approach must be initiated to ensure compliance with the appropriate obstacle clearance criteria.
Mean Sea Level Mean Sea Level Threshold Elevation
Threshold Elevation
Height of the highest obstacle or the Height of the highest obstacle or the highest missed approach obstacle, highest missed approach obstacle,
whichever is the higher whichever is the higher
M M D D A A M M D D H H O O C C A A O O C C H H Altitude Altitude
Obstacle Clearance Altitude (OCA) Obstacle Clearance Altitude (OCA)
Or Or
Obstacle Clearance Height (OCH) Obstacle Clearance Height (OCH)
Decision Altitude (DA) Decision Altitude (DA)
Or Or Decision Height (DH) Decision Height (DH) Margin Margin
The margin is dependent upon aircraft approach speed, height loss and altimetry and is adjustable for steep glide paths and high level
aerodromes Margin or Lower Limit
Margin or Lower Limit
Based on operational consideration of: category of operation, ground/airborne equipment characteristics; crew qualifications; aircraft performance; met conditions; aerodrome characteristics; terrain profile (Rad Alt); pressure error (pressure altimeter)
Identification of obstacles is dependent upon: Category of operation; ILS geometry (GP angle, distance from Loc Ae to R/W threshold; Loc course width); aircraft dimensions, missed approach turn point, use of autopilot (CatII approaches only)
Instrument Procedures Chapter 10
Air Law 10-19
Visual Manoeuvre (Circling) (VM(C)) Procedure Visual Manoeuvre (Circling) (VM(C)) Procedure
The lowest altitude (OCA) or alternatively the lowest height above the aerodrome elevation (OCH) below which an aircraft cannot descend without infringing the appropriate obstacle clearance criteria
Mean Sea Level Mean Sea Level Aerodrome Elevation
Aerodrome Elevation
Height of the highest obstacle in the Height of the highest obstacle in the
circling area circling area M M D D A A M M D D H H O O C C A A O O C C H H Altitude Altitude
Obstacle Clearance Altitude (OCA) Obstacle Clearance Altitude (OCA)
Or Or
Obstacle Clearance Height (OCH) Obstacle Clearance Height (OCH)
Minimum Descent Altitude (MDA) for Minimum Descent Altitude (MDA) for CirclingCircling
Or Or
Minimum Descent Height (MDH) for Minimum Descent Height (MDH) for CirclingCircling
Minimum Obstacle Clearance (MOC) Minimum Obstacle Clearance (MOC) Category A and B 90 m (295 ft). Category A and B 90 m (295 ft). Category C and D 120 m (394 ft). Category C and D 120 m (394 ft). Category Category E E 150 150 m m (492 (492 ft).ft). Margin or Lower Limit Margin or Lower Limit Based on operational consideration of: aircraft characteristics; crew qualifications; aircraft performance; met conditions; aerodrome characteristics
The OCH shall not be less than: The OCH shall not be less than: Cat A 120 m (394 ft) Cat A 120 m (394 ft) Cat B 150 m (492 ft) Cat B 150 m (492 ft) Cat C 180 m (591 ft) Cat C 180 m (591 ft) Cat D 210 m (689 ft) Cat D 210 m (689 ft) Cat E 240 m (787 ft) Cat E 240 m (787 ft)
Chapter 10 Instrument Procedures
Approach Procedure Design Approach Procedure Design
Where track guidance is provided in the design of an instrument approach procedure, each of the five segments of the approach is comprised of a specified volume of airspace, the vertical cross section of which is an area located symmetrically about the centre line of each segment. The vertical cross section is broken down into primary and secondary areas as shown in the diagram below.
Minimum Obstacle Clearance (MOC) Minimum Obstacle Clearance (MOC)
For non-precision approaches, the MOC is defined as a fixed margin to be added to the height of the dominant obstacle in the final approach segment. For precision approaches, glide path information is provided which, by definition, must be free of obstacles.
The angle of the glide path is set to provide the required clearance from obstacles. The MOC for a non-precision approach is made up of two areas; the primary MOC area and the Secondary MOC area.
At any point the width of the primary area is equal to ½ of the total width. The width of each secondary area is equal to ¼ of the total width. Where no track guidance is provided during a turn specified by the procedure, the total width of the area is considered as a primary area. MOC is provided for the whole width of the primary area.
For the secondary area, MOC is provided at the inner edges gradually reducing to zero at the outer edge. The degree of MOC in the primary area depends upon the availability of a Final Approach Fix (FAF) (FAF - see below). With a defined FAF the MOC is 70 m (246 ft) and 90 m
(295 ft) without. MOC MOC MOC MOC Primary Area Primary Area Secondary Secondary Area Area Secondary Secondary Area Area Total width Total width Assumed lowest path
Assumed lowest path
Obstacle Obstacle
Obstacle Obstacle
MOC for a Non-precision Approach MOC for a Non-precision Approach
Instrument Procedures Chapter 10
Air Law 10-21
Fixes and points used in designing approach procedures are normally based on standard navigation systems. These include, but are not limited to:
The initial approach fix (IAF); The intermediate approach fix (IF);
The final approach fix (FAF) or final approach point (FAP); The holding fix, and
When necessary the MAPt.
Fixes Formed by
Fixes Formed by IntersectionIntersection
Because all navigational facilities have accuracy limitations, the geographic point that is identified is not precise, but may be anywhere within an area called the fix tolerance area which surrounds its plotted point of intersection. The diagram below illustrates the intersection of two radials or tracks from different navigation facilities.
Intersection Fix Tolerance Factors
The dimensions of the intersection fix are determined by the accuracy of the navigational system that supplies the information to define the fix. The factors from which the accuracy of a system is determined are:
Ground station tolerance
Airborne receiving system tolerance Flight technical tolerance
Distance from the facility