17.
Internal viewing instruments can be either flexible or rigid, designed either for end or angled viewing and, in some instances adaptable for still or
video photography which may be linked to closed circuit television. These instruments are used for examining and assessing the condition of the compressor and turbine assemblies, nozzle guide vanes (fig. 24-2) and combustion system, and can be inserted through access ports located at strategic points in the engine main casings.
18. The engine condition indicators include magnetic chip detectors, oil filters and certain fuel filters. These indicators are frequently used to sub-stantiate indications of failures shown by flight deck monitoring and in-flight recordings. For instance, inspection of the oil filters and chip detectors can reveal deposits from which experienced personnel can recognize early signs of failure. Some maintenance organizations progressively log oil filter
Maintenance
and magnetic chip detector history and catalogue the yield of particles. Fuel filters may incorporate a silver strip indicator that detects any abnormal concentra-tion of sulphur in the fuel.
MAINTENANCE PRECAUTIONS
19.
During engine maintenance, it is necessary to observe certain precautions. The ignition system is potentially lethal and, therefore, before any work is done on the high energy ignition units, igniter plugs or harness, the low tension supply to the units must be disconnected and at least one minute allowed to elapse before disconnecting the high tension lead. Similarly, before carrying out work on units connected to the electrical system, the system must be made safe, either by switching off power or by tripping and tagging appropriate circuit breakers. With some installations, the isolation of certain associated systems may be required.
20.
When the oil system is being replenished, care must be taken that no oil is spilt. If any oil is acciden-tally spilt, it should be cleaned off immediately as it is injurious to paintwork and to certain rubber compounds such as could be found in the electrical harnesses, Oil can also be toxic through absorption if allowed to come into contact with the human skin for prolonged periods. Care should be taken not to overfill the oil system; this may easily occur if the aircraft is not on level ground or if the engine has been stationary for a long period before the oil level is checked.
21.
Before an inspection of the air intake or exhaust system is made it must be ascertained that there Is no possibility of the starter system being operated or the ignition system being energized.
22.
A final inspection of the engine, air intake and exhaust system, must always be made after any repair, adjustment or component change, to ensure that no loose items, no matter how small, have been left inside. Unless specific local instructions rule otherwise, air intake and exhaust blanks or covers should be fitted when engines are not running.
TROUBLE SHOOTING
23.
The procedure for locating a fault is commonly referred to as trouble shooting, and the requirement under this procedure is for quick and accurate diagnosis with the minimum associated work and the prevention of unnecessary unit or engine removals.
24.
The basic principle of effective trouble shooting is to clearly define and interpret the reported symptom and then proceed to a logical and systematic method of diagnosis (fig. 24-3).
Maintenance
25. The reported symptom will frequently originate from flight deck instrument readings and, unless it is apparent from supporting information that the readings are genuine, instrumentation should be checked before proceeding further. Similarly, quick elimination checks should normally be undertaken before more involved tasks. The manufacturers' maintenance manual contains trouble shooting information, usually in chart form and fig. 24-4 shows a typical example.
Maintenance
26. The progressive introduction of improved and more reliable condition monitoring devices (para. 11) will have considerable influence on accepted trouble shooting practice, since to a large extent these devices are designed to pin-point, at an early stage, the specific system or assembly at fault. The development of suitable test sets could eventually eliminate the need for engine ground testing after trouble shooting
ADJUSTMENTS
27.
There are usually some adjustments that can be made to the engine controlling the fuel trimming devices. Typical functions for which adjustment provision is normally made include idling and maximum r.p.m., acceleration and deceleration times, and compressor air bleed valve operation.
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