AWB 20-1 Issue 1, 24 September 2001
Spark Plug Care
All piston engine aircraft.
The objective of this bulletin is to highlight to personnel involved in engine maintenance the importance of careful inspection of spark plugs to ensure trouble free operation.
Correct maintenance of spark plugs is imperative for reliable operation of any engine. This AWB is to remind maintainers the important aspects of spark plug maintenance, from selection of the correct spark plug through to cleaning and installation techniques. At all times personnel should consult the Instructions for Continuing Airworthiness (ICA) pertaining to the aircraft, engine or spark plug.
There have been several engine failures that have been attributed to internal breakdown of the ceramic nose core insulator and electrodes of spark plugs. Evidence suggests that many of these failures occurred because of poor cleaning, testing, inspection and handling techniques of spark plugs.
Spark plugs come in many different thread sizes and lengths, heat ranges, electrode types and shielding styles. It is important when selecting a spark plug to choose the correct spark plug for the operating conditions of the aircraft. The Type Certificate Data for the engine lists all the spark plugs that are approved for the engine. It is important to select spark plugs of all the same part number for installation for efficient engine operation.
Currently most spark plug manufacturers do not approve reconditioning of their spark plugs. Most spark plugs are manufactured in such a manner that none of the critical parts can be replaced without destroying the spark plugs integrity.
After removal spark plugs should be degreased using methylethylketone (MEK) or similar. After drying with clean, dry shop air, the spark plugs should be given a preliminary inspection to eliminate those that are obviously unfit for further servicing.
If the spark plug has heavy lead deposits the use of a vibrating cleaner is advised. A common workshop practice is to use either a pick or ground hacksaw blade to loosen lead deposits. This practice should be used with caution since it increases the risk of damaging the ceramic nose core insulator rendering the spark plug unserviceable.
After removal of lead deposits the spark plug should be lightly cleaned with an abrasive compound. Severe abrasive cleaning will erode the electrodes causing premature wear. Always use glass beads, aluminium oxide or similar as an abrasive compound. Silica sand should never be used as an abrasive agent. During the blasting process personnel should wear suitable dust mask to prevent inhalation of abrasive dust.
Either a wire hand brush or a power-driven brush (maximum of 0.005" wire size) can be used to clean the threads on both the shell and the shielding barrel.
NOTE: never wire brush the insulator or the electrodes.
The tools and methods used to set spark plug gaps vary greatly. The main precaution is to avoid putting any pressure on the centre electrode, which could in turn crack the ceramic insulator. The gap should be measured by a round wire gauges and gap adjustment is on the ground electrode only. When adjusting spark plug gaps never place the wire gauge between the electrodes during adjustment, as this will place a side load on the centre electrode and crack the ceramic insulator. If the gap is inadvertently set too close no attempt should be made to widen the gap, since this also could damage the ceramic insulator or electrodes.
Throughout the gapping process for massive-electrode spark plugs every effort should be made to ensure that the side of the electrodes are parallel to each other. This will prevent accelerated wear of the electrodes leading to increased gap dimensions.
After gapping, the electrode should be inspected for wear limits. Many spark plug manufacturers have erosion gauges that act as GO/NO GO gauges to measure wear. However the general rule for spark plugs is to replace them when the electrodes are half it's original dimension.
If during any stage of servicing a spark plug is dropped, it should be scrapped immediately. Many flaws may not be apparent visually or during any stage of the inspection or testing process but could cause failure of the spark plug during operational service.
Inspection and Testing
Inspection should be accomplished with the aid of a magnifying glass (10X or better) in an area with good lighting. The inspection areas are the threads (both the shell and shielding barrel), electrodes, ceramic insulators, shell hex and connector seat. Any spark plug showing signs of deformity, insulator cracking or chipping, excessive electrode erosion, thread damage or any sign of misuse should be discarded. Cavities within the spark plug should be checked for cleanliness and contain no residual abrasive compound or cleaning solvent.
The spark plug should be tested in accordance with instructions of the test unit used. Test units usually require clean, dry shop air supplied at 80-125 psi and are connected to mains power. With both regulated air pressure (appropriate to the gap dimension) and high voltage applied to the spark plug, a consistent spark should be seen through the viewing port. Failure to observe a bright consistent spark means the spark plug is unserviceable and requires replacement.
Removal and Installation
Carefully remove the ignition harness connector, avoiding damage to the wire, terminal sleeve or the barrel insulator. It is generally suggested to use a 6-point socket for spark plug removal and installation, as it provides a greater bearing surface than a 12-point socket.
Prior to installing spark plugs an approved anti-seize compound should be applied to the firing end threads. It should not, however, be applied to the first two threads to prevent the risk of misfiring of the electrodes. Never apply anti-seize to the terminal threads of the shielding barrel.
Most modern spark plugs have a solid copper gasket that requires annealing prior to spark plug installation to ensure a tight, gas sealed fit. The maintainer should check that the spark plug has only one washer, is of correct dimensions and is annealed. If the engine is equipped with a thermocouple probe in the form of a spark plug gasket, a normal gasket is not required.
To equalise electrode wear of spark plugs installed in engines with an even number of cylinders due to constant polarity timing, it is recommended that spark plugs be rotated and not be installed in the same cylinder. Generally spark plugs from even numbered cylinders should be installed in odd numbered cylinders and conversely for spark plugs from removed from odd numbered cylinders should be installed into even numbered cylinders.
The spark plug should then be screwed into the cylinder by hand up to the gasket. If the spark plug fouls while screwing into the cylinder there is probably carbon or lead deposits on the threads. Use an appropriate thread cleaning tool to ensure removal of deposits. Cleaning thread inserts (helicoils) with a tap is not recommended as permanent damage to the insert may result.
After torquing the spark plug to the appropriate setting, the ignition harness connector can be installed. Care must be observed not to over torque ignition connector, as severe damage to the harness attaching hardware will occur. Many manufacturers recommend the connector be clean and dry, without the use of a lubricant. However, referral to maintenance data may list a lubricant approved for use in that application.
Testing and Cleaning Equipment
Spark plug cleaner/testers are generally the most overlooked piece of equipment in most workshops. Depending on the type of cleaner/tester, the voltage control and pressure gauges should be recalibrated every 12 months or as directed by the manufacturer. As well, to ensure that the abrasive compound is kept clean the compound should be replaced after cleaning about 75 to 100 spark plugs.
Many cleaner/testers have water traps to assure the air supply remains dry, and these traps should be drained daily. Also, when testing spark plugs, air leakage at the adapter is normal, since this supports the control of air pressure and facilitates the exhausting of ionised air generated in the testing process.
AWB 33-1 Issue 1, 9 May 2001 EMB 120 Cabin Lighting with Bruce Lights
AWB 33-1 Issue 1, 9 May 2001
EMB 120 Cabin Lighting with Bruce Lights
To All Embraer EMB120 aircraft with Bruce Industries cabin lighting installed IAW FAA STC
Recent SDR report and in-service experience has highlighted the possibility of damage by chaffing to the wiring harness installed as part of the Bruce Industries kit. Specifically the high voltage output of the ballast.
Discussion with Bruce Industries has indicated that they have received no reports of damage by chaffing, the major problem reported to them with the installation has been due to arcing at the flourescent lamp fitting.
Extract from SDR :
"As the result of a report indicating a cabin lighting problem, and during defect rectification, it was observed that the ballast output wiring was found to have chafed against the gasper air duct causing the wiring loom and insulation surrounding the duct to burn. Disturbance of the wiring loom to investigate the damage, again caused the wires to short circuit and burn. "
Comments from Bruce industries are as follows :
We have had no reports of arcing attributed to wiring damage associated with this system. However, we do recognize the potential.
There have however been reports of arcing at the lamp/lamp holder interface. Should the lamp not be fully seated against the lamp holder contacts, there is sufficient potential for an arc to bridge the gap. Over time a carbon path may develop which will exacerbate the situation. The same sequence is possible anywhere along the output path from the ballast.
Over the last few years we have actively been developing solutions to eliminate or mitigate arcing attributed to installation conditions. The SDR mentions over current protection as a possible cure. It is unfortunately not as easy as setting a trigger based on current or even voltage. One of the ballast's central design functions is to regulate current to the lamp. During an arcing event the ballast is unaffected an continues to regulate current as per spec. The rated current for a lamp in bright mode is sufficient in combination with the voltage demands of small diameter lamps to generate an arc of destructive potential. Bruce Industries have successfully designed a few circuits that detect an arcing condition and shutdown the ballast without presenting any appreciable nuisance failures in the field. One such circuit employing a microprocessor running a trigger algorithm that monitors operating conditions.
This unit is available as a replacement ballast, identical in form, fit and function, to the unit supplied for the EMB120 retrofit kits. P/N BR9000-21 a non-dimmable ballast incorporating arc detection. Bruce Industries are modifying their STC to reflect the change.
Part Number BR9000-22 is a currently in use ballast, the only difference is that it has the capability of dimming. The BR9000-22 is in service on the CRJ200, CRJ700, ERJ135 and ERJ145. With a jumper installed at the connector, the BR9000-22 may be used on the EMB120. Bruce Industries recommendation is that the dimming circuitry be removed or disabled internally and the part number changed to BR9000-11. Please Contact Bruce Industries for further details : Email Steve Jaffe email@example.com
CASA strongly recommends that operators carry out, as soon as practical, an initial inspection of the wiring looms and harnesses affected by the retro fit of the Bruce Light Kits. Determine that they are free from damage, do not chaff or abrade any structure or fittings and are adequately secured to prevent or reduce damage in the future. Defects found as part of the investigation are required to be submitted to CASA as an SDR pursuant to regulation 51A of CAR 1988.
CASA also recommends that operators review their maintenance program to ensure that an adequate inspection of the wiring associated with the cabin lighting system is in place. As this installation was carried out as a result of the application of an STC, neither the manufacturers Maintenance Planning Document nor the Maintenance Review Board Document will address the requirements regarding maintenance of this installation. It would be prudent to review the STC and ensure that any continuing Airworthiness Requirements are also incorporated.
Maintenance personnel should be reminded of the requirement to be vigilant during the installation of these fluorescent lamps to ensure that they are correctly installed, it would be prudent to inspect the base for signs of deterioration arcing or tracking each time the tube is replaced.
CASA recommends that consideration be given to replacement of the older style ballasts on an attrition basis with the newer style that provides arc detection.
AWB 00-5 Issue 1 - Depleted uranium in aircraft
Depleted uranium in aircraft
AWB 00-5 Issue 1, 5 April 2001
The purpose of this bulletin is to remind all individuals working on aircraft manufactured utilising Depleted Uranium (DU), of the need to follow the aircraft manufacturers and the Occupational Health and Safety recommendations.
This subject was previously covered in Airworthiness Advisory Circular 150-1.
Aircraft manufactures have for some time utilised Depleted Uranium in the manufacture of aircraft i.e. flying control system balance weights. The aircraft manufacturers maintenance manual provides the safety procedures to be followed when working on such systems.
Lift a piece of DU and the most obvious feature is immediately apparent, it is surprisingly heavy. This is why it is often utilised as a counter or balance weight. Balance weights for aircraft control surfaces must fit into limited spaces and although several materials can provide a heavy concentrated mass for installation into a small space, DU is uniquely suited to this purpose.
DU is however, slightly radioactive. To minimise radiation hazards, the DU balance weights are cadmium plated during the manufacturing process. The DU would therefore normally pose no danger, that said, it should still be handled with caution. The primary hazard associated with DU is the harmful effect the material could have should it enter the body. If the particles of the parent metal or its oxides are inhaled or ingested, they can be chemically toxic and cause a significant and long lasting irradiation of internal tissue.
The cadmium plating coating on the DU balance weights not only attenuate radiation emissions but also provide corrosion prevention as unprotected DU corrodes fairly rapidly, producing a black, dusty oxide.
During installation, no penetration of the DU balance weights is permitted. The balance weights must not be sanded, filed, drilled, reamed or reworked in any way. The weights should fit so that they can be simply bolted into place.
Whenever the protective cadmium plating is breached, the weight should be removed from the aircraft or the part may be temporarily repaired by a cleaning/repainting procedure in accordance with the manufacturers maintenance manual.
CASA would remind all individuals to follow the aircraft maintenance manual and all Occupational Health and Safety procedures when working on any aircraft system where DU or any other radioactive material, has been utilised.Disposal of the depleted Uranium balance weights, and any other components containing DU or other radioactive material, may only be undertaken by a state licensed toxic waste recipient.