You must perform blade tracking when rerigging the helicopter. Tracking is necessary when the blades, the main gearbox, or the main rotor head assembly have been replaced. Unless the blades arc in proper track, vibrations will occur in the helicopter with every revolution of the main rotor. At high rpm settings, these vibrations could cause serious struc-tural damage.

Figure 10-10.Blade trackingStrobex.

Tracking the blades is necessary to be sure that the blades rotate in the same horizontal plane (track). This is accomplished by pretrack rigging of the rotary-wing head and by the use of pretracked blades. Pretrack rigging involves adjusting the pitch control rods until an exact sleeve angle (within 1 minute) is found on all sleeve spindles. A micrometer type of decal is affixed to the adjustable pitch control rods as a permanent reference at the overhaul activity. A pretrack number is stenciled on each blade at the time of manufacture or overhaul. This number is based on the effective angle of the blade. Install prctracked blades on the helicopter by setting the adjustable pitch control rod to the pretrack number stenciled on the blade.

If the pretrack number is MINUS and the pitch control rod decal shows the setting is zero, loosen the locknut. Shorten the rod by rotating the tang clockwise. Keep rotating until it aligns (closest notch) with the appropriate blade pretrack number on the lower scale of the lower decal. Engage the tang by tightening the locknut. If the pretrack number of the blade is PLUS and the pitch control rod decal shows the setting is zero, loosen the locknut. Lengthen the rod by rotating the tang counterclockwise. Keep rotating the tang until it aligns (closest notch) with the appropriate blade pretrack number on the upper scale of the lower decal. After adjusting the remainder of the pitch control rods, tighten the locknuts to the torque specified in the MIMs. Safety wire the locknuts to the tang.

You should perform a ground operational check. With the rotary-wing head engaged, operate the engines at 100 percent. Check for vibrations in the rotary-wing head. If vibrations occur and the adjustable pitch control rods were properly adjusted, use an alternate method of blade tracking. In this case, use a strobe blade tracker to check the blades under actual operating conditions. You must be sure that all blades are rotating in the same horizontal plane. See figure 10-10. Pitch adjustment of each blade may be made to compensate for blade differences.

The Strobex blade tracker permits tracking from inside the helicopter in flight or on the ground. The system uses a highly concentrated stroboscopic light beam flashing in sequence with rotation of the rotary-wing blades, so that a fixed target at the blade tips will appear to be stopped. A soft iron sweep attached to the rotating swashplate passes close to a magnetic pickup attached to the stationary swashplate, causing a once-per-revolution pulse, which synchronizes the lamp flash rate with the rotation of the blades. Each blade has a retroreflective target number attached to the underside of the blade in a uniform location. Tracking of each blade is then determined by the relative vertical position of the fixed target numbers. See figure 10-10. Consult the applicable aircraft MIMs for the proper operating procedures for the Strobex blade tracker. For maintenance infor-mation on the Strobex tracker, refer to NAVAIR 17-15BBA-4.

NOTE: Do not adjust blades by the Strobex method of blade tracking unless problems result from normal tracking procedures.