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LLZ 421
Operation and Maintenance Maintenance
ILS 420
6−1
Ed. 01.10 SOAC
CHAPTER 6
MAINTENANCE, FAULT LOCATION AND REPAIR
6.1 MAINTENANCE
6.1.1 Introduction to Maintenance Activities
Maintenance of a system can be defined as preventive and corrective maintenance (corrective main-
tenance may also be defined as ’Repair’ activity in case of a fault). Within this chapter two basic types
of preventive maintenance are described, namely
periodic maintenance and normal maintenance.
It is essential to ensure that the radiated signals remain within the fixed tolerances at all times after
the navigation system has been handed over for service to air traffic. With this aim in mind, ICAO (see
document ICAO DOC 8071, Vol. I, chapter 4 for ILS) recommends verifying the most important signal
parameters of the ground station within the framework of periodic maintenance to maintain the opera-
tional integrity and serviceability. In addition, flight testing is required to confirm the correctness of
the setting of essential signal−in−space parameters to determine the operational safety and accept-
ability of the ILS installation.
NOTE: Change of parameters is allowed only by maintenance people with specific access level
(i.e. > level 2) to the PC user program (ADRACS or MCS). Changes to the established
monitor alarm limits should only be done during flight check procedure.
In the Navaids ILS 420, all the main parameters are maintained at the preset values throughout the
entire life of the system by means of a digital controlled transmitter, so that drift as a result of ageing
should not normally occur. In addition, the radiated signals are checked by two high−precision micro-
processor controlled monitors. These prevent faulty signals from being emitted by either switching
over to the standby transmitter or shutting down the system completely. Since, moreover, no parts
which are subject to mechanical wear and tear are used, the periodic maintenance intervals can be
made longer than usual and the number of measurements restricted to a minimum. In addition to the
periodic maintenance work, there are a few activities of normal maintenance which should be per-
formed as and when necessary.
NOTE: The responsible authorities (e.g. ATC/Controls) must be informed before commencing
any maintenance work in accordance with national regulations!
6.1.2 Work on Equipment sensitive to electrostatic Charge
A grounding strip with a large cross−sectional area is connected between the shelter grounding ter-
minal, the equipment racks and the worktable to act as the system ground and to eliminate electro-
static charges. It is firmly connected to the table and a grounding bus on the working surface. This
system ground should not be connected to either the mains protective ground conductor or to hous-
ings and grounds of external consumers, i.e. consumers not associated with the worktable, except
at the potential equalization bus for the overall installation. The protective ground wire offered with
mains cable or plug connections is not suitable for potential equalization of equipment on a worktable.
Depending on local circumstances, it can carry quite high RF interference voltages.
Mains−supplied equipment, power supplies and test equipment should be connected via two−wire
cables and two−pin plugs. For measures to be taken with respect to components sensitive to elec-
trostatic charging (MOS, Low Power Schottky) please refer to the customer service documentation
of the manufacturer. If an employee is required to handle subassemblies for transport purposes, he
should place both hands flat on a grounded surface beforehand (e.g. on the ground bus of the work
table). The module can be picked up by its insulator immediately following this potential equalization.
It is safer to avoid touching the terminals. Only remove short−circuit links where absolutely neces-
sary. Place printed circuit boards only on tables with a conductive, grounded working surface. Leave
individual subassemblies in the antistatic plastic bags for as long as possible.