Aircraft Operations. Volume II - Construction of Visual and Instrument ...

Part III — Section 3, Chapter 6 III-3-6-13
6.4.8.8.3.5 Height loss (HL)/altimeter margins for a specific speed at threshold (Helicopters). For helicopter
operations the concept of Vat is not applicable. Height loss margins are listed in Table III-3-6-3.
6.4.8.9 Effect of obstacle density on OCA/H. To assess the acceptability of obstacle density below the OAS, the
CRM described in 6.4.9 may be used. This can provide assistance by comparing aerodrome environments and assessing
risk levels associated with given OCA/H values. It is emphasized that it is not a substitute for operational judgement.
6.4.9 Obstacle clearance of the precision segment — application of
collision risk model (CRM) for GBAS operations
Note.— A specific GBAS implementation of the CRM is in preparation.
6.4.9.1 General. The ILS CRM is a computer programme that establishes the numerical risk which can be
compared to the target level of safety for aircraft operating to a specified OCA/H height. This ILS CRM can be used for
GBAS Category I operations while the specific GBAS CRM is in preparation. A description of the ILS CRM
programme and instructions on its use, including the precise format of both the data required as input and the output
results, are given in the Manual on the Use of the Collision Risk Model (CRM) for ILS Operations (Doc 9274).
6.4.9.2 Input. The CRM requires the following data as input:
a) Aerodrome details: name, runway threshold position and runway orientation, threshold elevation above MSL,
details of preceding segment;
b) GBAS parameters: category (Cat I only), glide path angle, GARP – LTP distance, GBAS course width and
height of DCP;
c) Missed approach parameters: decision height/altitude (obstacle clearance height) and missed approach turn
point;
d) Aircraft parameters: type, wheel height (antenna to bottom of wheel), and wing semi-span, aircraft category (A,
B, C, D or DL) and missed approach climb gradient; and
Note.— The CRM does not consider Category E aircraft.
e) Obstacle data: obstacle boundaries (either as x and y coordinates relative to the runway threshold or as map
grid coordinates) and obstacle height (either above threshold elevation or above MSL). For density assessment,
all obstacles penetrating the basic ILS surfaces described in 6.4.7.2 must be included.
6.4.9.3 Output and application. The output of the programme is the overall (total) risk of collision with obstacles
to the aircraft of operating to the specified OCA/H and through the missed approach. Other information may also be
produced using various output options.
6.4.9.3.1 For example, the risks associated with individual obstacles may be given, and these risks can be ordered,
either in terms of obstacle range, or more usefully in terms of risk magnitude, so that the user may see at a glance
which obstacles are the major contributors to the total risk.
6.4.9.3.2 The user, by rerunning the CRM with the appropriate parameters, can assess the effect on the safety of
operations of any alteration in the parameters, typically varying the glide path angle, or increasing/reducing the
OCA/H. The computed risk is compared with a prespecified acceptable level of risk (not worse than 1 × 10 -7 per
approach) which meets the overall safety target.
23/11/06