MAKE SURE THAT THE IGNITION SWITCH IS OFF BEFORE REMOVING THE SPARK IGNITER OR SPARK IGNITER LEAD ASSEMBLY AS DANGEROUSLY HIGH VOLTAGES MAY BE PRESENT. ALLOW FIVE MINUTES AFTER OPERATION FOR ELECTRICAL DISSIPATION BEFORE DISASSEMBLY.
CAUTION:
THE EXCITER AND THE IGNITER CONNECTOR WELLS SHOULD BE KEPT DRY AND FREE FROM FOREIGN MATERIAL. CLEAN BY WIPING WITH A CLEAN DRY CLOTH, IF REQUIRED. DO NOT WASH WITH SOLVENT.
A.
Removal (1) Disconnect the ignition lead at the igniter (See Figure 201, 74–00–00). Hold the lead to prevent its twisting while removing the nut. Separate the lead from the igniter by pulling straight out with no rotational motion. (2) Unscrew the igniter. Remove shim if one has been used. (Refer to para 2.A.(3), this section.)
B.
Installation (1) When replacing or reinstalling a serviceable spark igniter, use shim, if required by para 2.A.(3), this section. Apply a light coat of antiseize compound (NS–165) to the threads. Install the spark igniter, tighten to 150–200 lb in. (17–23 N.m) and lockwire to fuel nozzle. (2) Connect ignition lead; tighten to 70–90 lb in. (7.9–10.2 N.m).
2.
Inspection/Check A.
Inspection (1) Inspect the center electrode; replace igniter if the electrode is loose (See Figure 201). (2) Inspect semi–conductor coated ceramic and body for cracks. If any cracks are visible through the carbon (soot) coating, replace the igniter. (3) Inspect spark igniter for fretting wear. Wear of 0.032 in. (0.81 mm) max. is serviceable without corrective action. Wear of 0.032 in. (0.81 mm) or more is acceptable if it does not affect the function of the spark igniter and if the following corrective action is taken: (a) Fabricate a shim from corrosion resistant steel shim stock to the dimensions given in Figure 202. (b) At installation of the spark igniter, place the fabricated shim between the outer combustion case and the igniter. NOTE:
A shim sized per Figure 202 will reduce the spark igniter installed thread length by 1/2 turn. Localized fretting is thereby moved 180° from the wear area of the previous installation. Be sure to remove the shim when a new spark igniter is installed.
(4) Check the general condition of the spark igniter; replace if excessive erosion is evident. (5) Check the operation of the spark igniter after it has been removed from the engine. Apply 28 volts dc to the exciter and observe the rate of firing. Normal operation is six sparks per second minimum. Replace the spark igniter if it fails to fire or fires intermittently. NOTE:
Do not replace the spark igniter as defective until the ignition exciter is known to be acceptable. (Refer to Inspection Check, para 2., 74–10–01.)
74–20–01 Page 201 Nov 1/89 Rolls-Royce Proprietary Data – Uncontrolled Printed Copy
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250–B17F SERIES OPERATION AND MAINTENANCE
Spark Igniter Figure 201
3.
Cleaning CAUTION:
UNDER NO CIRCUMSTANCES SHOULD THE SPARK IGNITERS BE CLEANED BY WIRE BRUSHING, SAND BLASTING, VAPOR BLASTING OR SCRAPING ON THE IGNITER TIP. ANY OF THESE PRACTICES CAN DAMAGE THE SEMI–CONDUCTOR MATERIAL BETWEEN THE TWO ELECTRODES AND RESULT IN SHORTENED SPARK IGNITER LIFE OR IMMEDIATE SPARK IGNITER FAILURE.
Normal soot or carbon formation on the tip is not detrimental to the operation of the spark igniter and need not be removed. If cleaning is desired, for some special reason, wipe the metal tipe with a soft cloth only. Remove any sizeable lump of carbon with a blunt instrument. Be careful not to damage the semi–conductor material.
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