Aeroplane Categories for Departure Procedures
Chapter 8 Approach Procedures
8.27 DA/H for Precision Approach Procedure. This is defined as the lowest altitude or height at which a missed approach must be initiated to ensure compliance with the appropriate
obstacle clearance criteria. The reference datum for a precision approach is always the threshold of the landing runway.
8.28 OCA/H for Non-precision Approach. This is defined as the lowest altitude or height below which the aircraft cannot descend without infringing the appropriate obstacle clearance criteria. For non precision procedures the reference datum is the aerodrome elevation or the elevation of the relevant runway threshold (if the threshold is more than 2 m (7 ft) below the aerodrome elevation).
8.29 OCA/H for Visual Manoeuvre (Circling) (VM(C)). This is defined as the lowest altitude or height above the aerodrome elevation, below which the aircraft cannot descend without infringing the appropriate obstacle clearance criteria. It is based on the highest obstacle in the VM(C) area with respect to the aerodrome elevation.
156
Chapter 8 Approach Procedures
8.31 System Minima. To ensure that the minima are realistic and give sufficient ‘buffer’ to allow for anomalies, for each method of conducting an instrument approach, a minimum height above the datum is specified below which an approach is not permitted to continue without the necessary visual reference. This height is known as system minima and overrides a lower DH or MDH. For instance, the system minimum for a CAT 1 ILS is 200 ft. If OCH plus the upper margin for a CAT 1 ILS approach is calculated at 170ft, then system minimum would prevail and DH would be 200ft. If on the other hand, DH is calculated at 230ft then, as this is higher than system minimum, DH remains 230ft. For Air Law, the student is only required to know the system minima for ILS CATs I/II/III. Note that for CAT III, DH can be zero; therefore system minimum for CAT III must also be zero.
Category System minimum
CAT I 60m (200ft)
CAT II 30m (100ft)
CAT III Not applicable
8.32 Calculation of DA/H. The diagrammatic representation of the method of calculation of DA/H is shown below.
MSL
Threshold elevation
Height of the highest obstacle or the highest missed approach obstacle (whichever is higher) Obstacle clearance altitude/height (OCA/H)
Decision altitude/height (DA/H)
DA DH OC A OC
H Margin
Dependant upon the aircraft approach speed, height loss and altimetry and is adjustable for steep glide paths and high
level aerodromes Margin or lower limit
Based on operational consideration of ground/airborne equipment characteristics, crew qualifications, met conditions, aerodrome characteristics, terrain profile (rad alt), pressure error
Method of determining DA/H for precision approaches
Identification of obstacles is dependant upon: category of operation; ILS geometry; aircraft dimensions; missed approach climb gradient; missed approach turn point;
use of autopilot (CAT II ops only)
Figure 8.10: ICAO system minima for ILS.
Figure 8.11: Method of determining DA/H for precision approaches.
Chapter 8 Approach Procedures
8.33 Specific Data for ILS/MLS. In determining the criteria for DA/H for ILS/MLS other data needs to be taken into account and requirements specified. Wing span and the vertical distance between the wheels and the GP aerial limitations are specified as follows:
Aircraft category
Wing Span
(m) Vertical distance between the wheels and the GP aerial
H 30 3
A, B 60 6
C, D 65 7
DL 80 8
Note: Category DL has been included to cater or the A380. Category H refers to Helicopters.
Other criteria for ILS include:
CAT I is flown with pressure altimeter
CAT II is flown with radio altimeter and flight director
Missed approach climb gradient is 2.5%
Glide path angle is 2.5° minimum and 3.5° maximum. CAT II/III requires 3°
8.34 Calculation of MDA/H. The diagrammatic representation of the method of calculation of MDA/H is shown below.
Figure 8.12: Limitations for ILS.
158
Chapter 8 Approach Procedures
8.35 Calculation of MDA/H for VM(C). The diagrammatic representation of the method of calculation of DA/H is shown below
DESCENT GRADIENTS
8.36 Descent Gradient. The design of procedures must allow adequate space for descent from the published height crossing the facility, to the runway threshold. This is achieved by establishing a maximum allowable descent gradient for each segment of the procedure with the most critical being the final segment where threshold speed (or the ability to decelerate to it) will be a function of gradient. The optimum descent gradient in the final approach should not exceed 5% (50 m/km; approx 300 ft/nm) which is equivalent to a 3° glide path. Where a steeper gradient is necessary the maximum permissible is 6.5% (65 m/km (400 ft/nm) which is equivalent to a 3.8° glide path). In the case of a precision approach the operationally preferred glide path angle is 3° and this is mandatory for CAT II/III. An ILS GP in excess of 3° is used only where an alternative means of satisfying obstacle clearance requirements is impractical.
8.37 High Rate Descents. Gradients over 6.5% may result in descent rates exceeding the recommended maximum rate of descent for some aircraft. Pilots flying such approaches should be aware of this before starting the approach. High rate descents are not permitted as a means of avoiding noise abatement procedures. Where GPs greater than 6.5% are established (i.e.
9.5% /5.5° at London City), the authority of the state in which the aerodrome is situated must give specific approval; the operator must be approved to carry out high rate descents; specially approved aircraft must be used and pilots specially trained.
Figure 8.14: Method of determining DA/H for circling approaches.
Chapter 8 Approach Procedures
TRACK REVERSAL AND RACETRACKS
8.38 Requirement. If a straight in approach is not possible (or feasible), a procedure may be established using a facility on the aerodrome that serves both as the IAF and the MAPt. In this case, some form of track reversal procedure will be required in which the aircraft is flown outbound from the facility on a defined track, and then turned to fly inbound back towards the facility. This may be a procedure turn or a base turn.
8.39 Procedure Turn. A procedure turn is defined as a turn from outbound to inbound in which the tracks flown are reciprocal. There are two types:
8.40 45°/180° Procedure Turn. This requires track guidance to a point (timed or DME) where a 45° turn is made followed by a straight leg of either 1 minute (category A or B aircraft) or 1 minute and 15 seconds (category C, D and E). At the end of the timed leg a rate 1 turn is made through 180° to bring the aircraft into a position intercept the reciprocal of the outbound track at an interception angle of 45°.
CAT A and B = 1 min Figure 8.15: Track reversal - procedure turn.
160
Chapter 8 Approach Procedures
8.41 80°/260° Procedure Turn. This requires track guidance to a point (timed or DME) where an 80° rate 1 turn is made followed immediately by an opposite direction 260° rate 1 turn. In still air, this should bring the aircraft on to the reciprocal of the outbound track. Also in still air, the procedure should take exactly 2 minutes.