9  Alarm Testing
9.1  The alarm points of all monitors shall be checked and recorded at commissioning (2364).
9.2  The alarm points of all monitors shall be checked and recorded at intervals not exceeding 12 months (2365).
9.3  Where it is not possible to establish the amount of change of the transmitted parameter when checking the alarm points, then independent test equipment shall be used (2366).
10  Glidepath Commissioning Alarm Conditions
10.1  At commissioning the alarm condition given in Table 3 shall be set and checked by flight inspection.
Table 3 Phase Advance and retard
CAP 670  Air Traffic Services Safety Requirements

GP type  Method  Pass criteria 
Sideband reference  Adjustment  of relative phase between upper and lower aerials to give sufficient alarms to cause shutdown (2367).  Alignment and Displacement Sensitivity within the limits in Table 1. Below Path Clearance and Coverage within the limits in ILS 02 Table 2 (2370). 
Null reference  Adjustment  of relative phase between upper and lower aerials to give sufficient alarms to cause shutdown (2368). 
M-Array  Adjustment of the relative phase of the middle aerial, compared to the upper and lower aerials to give sufficient alarms to cause shutdown (2369). 
Alignment and Displacement Sensitivity 
Condition  Pass criteria 
Width wide and angle low (1872)  Alignment and Displacement Sensitivity within the limits in Table 1 (2380). 
Width normal and angle high (1873) 
Width normal and angle low (1874) 
Width narrow and angle normal (1875) 
Width wide and angle normal (1876) 

11  Routine Monitor Recording
11.1  All equipment monitor readings for all transmitters capable of operating into the aerial shall be taken at monthly intervals or as prescribed by the equipment manufacturer or engineering authority (1882).
12  Alarm testing following Engineering Work
12.1  Following any engineering work involving the aerial distribution unit, feeder cables, aerials or monitor-combining unit, the following glidepath alarm condition shall be set and checked by flight inspection:
a) Angle low and width wide simultaneously (1867);
b) Phase advance alarm (1868); and
c) Phase retard alarm (1869).
NOTE 1:If the monitor phase tests are not successful and transmitter adjustments are required, parts of the normal flight inspection will have to be repeated. Unless staff are highly confident that all phasing is correct, it is advisable to consult the guidelines given in Annex 1 before starting the main flight inspection.
NOTE 2:On certain systems it is difficult to adjust the aerial phasing so that the system is just at the alarm point. For this reason, it is permissible to make the tests with the monitor near to or just beyond the alarm point. Provided that both the monitors and the flight inspection figures show reasonable symmetry, the behaviour at the alarm points can be calculated.
Calculation of Displacement Sensitivity
In many cases it is more usual to measure the signal width rather than the displacement sensitivity as defined by ICAO. For example, on a glidepath system the flight inspection will normally measure the angle between 75 μA fly-up and 75 μA fly-down. To convert changes in displacement sensitivity to changes in the angle, the following formulae should be applied:
An X% REDUCTION in displacement sensitivity (decrease in sideband power) will produce a GREATER width angle of (1190):
(nominal angle) ÷ [(100 -x)/100]
An X% INCREASE in displacement sensitivity (increase in sideband power) will produce a NARROWER width angle of (1191):
　
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