TM 1-1520-238-T-4 5–62 Change 9 5–24. ISOLATING/MONITORING OTHER VIBRATIONS USING AVA EQUIPMENT 5–24 Tools: Nomenclature Part Number Tool Kit, Aircraft SC518099CLA01 Mechanic’s Aviation Vibration 6625-01-282-3746 Analysis Test Set Helicopter Rotor Head 4920-01-180-7610 Balance and Blade Track Run Test Equipment Personnel Required: 152FG Maintenance Test Pilot 152FG Pilot 67R Attack Helicopter Repairer 67R3T Inspector References: TM 1-1520-238-23 TM 1-1520-238-MTF Equipment Conditions: Ref Condition TM 1-1520-238-23 Helicopter safed 1. GENERAL INFORMATION There are two main sources of vibration in helicopters. The rotors produce low frequency vibrations, say 200–5000 RPM or 3 to 80 cycles per second (Hz), at multiples of rotor revolution, and drive train–transmission produces mid, say 5000 to 100,000 RPM or 80 to 1700 Hz, to high, above 100,000 RPM or 1700 Hz, frequency vibrations. Unusual low frequency vibrations are usually a human factors issue and do not indicate a safety of flight problem, but not always. Unusual Hi–Freq vibrations usually indicate an impending failure of some drive train component and so are a safety concern. Figure 5–28 is an example of a spectrum and some of the frequencies that you might see. No limits exist for any of these levels except main rotor once per rev (1P) and tail rotor once per rev (1T). 0.05 FREQUENCY (H z) M58-0294-1 0.3 0.25 0.2 0.15 0.1 0 0 10 20 30 40 50 60 70 80 90 100 4P 1P 8P 4P 1P 1P MAIN ROTOR 1P (288 rpm) MAIN ROTOR 4P (1,158 rpm) TAIL ROTOR 1P (1,404 rpm) MAIN ROTOR 8P (2,304 rpm) TAIL ROTOR DRIVE SHAFT 1P (4,818 rpm) TAIL ROTOR 4P (5,616 rpm) Figure 5–28. Spectrum Example