In normal conditions when the rotor is coupled to the engine,  both needles move together in the same direction. However,  with a sudden loss in engine power the needles “split”  showing that the engine and rotor are no longer coupled as  the clutch has disconnected.  [Figure 4-19] Many newer aircraft have what is referred to as a glass  cockpit, meaning the instrumentation is digital and displayed  ensure free rotation. Helicopter transmissions are normally  lubricated and cooled with their own oil supply. A sight gauge  is provided to check the oil level. Some transmissions have  chip detectors located in the sump, to detect loose pieces of  metal. These detectors are wired to warning lights located  on the pilot’s instrument panel that illuminate in the event  of an internal problem. Some chip detectors on modern  helicopters have a “burn off” capability and attempt to correct  the situation without pilot action. If the problem cannot be  corrected on its own, the pilot must refer to the emergency  procedures for that particular helicopter.  The primary purpose of the main rotor transmission is  to reduce engine output rpm to optimum rotor rpm. This  reduction is different for the various helicopters. As an  example, suppose the engine rpm of a specific helicopter  is 2,700. A rotor speed of 450 rpm would require a 6:1  reduction. A 9:1 reduction would mean the rotor would turn  at 300 rpm.  Most helicopters use a dual-needle tachometer or a vertical  scale instrument to show both engine and rotor rpm or a  percentage of engine and rotor rpm. The rotor rpm indicator is  used during clutch engagement to monitor rotor acceleration,  and in autorotation to maintain rpm within prescribed limits.  It is vital to understand that rotor rpm is paramount, and that  engine rpm is secondary. If the rotor tachometer fails, rotor  rpm can still be determined indirectly by the engine rpm  during powered flight, because the engine drives the rotor at  a fixed, one-to-one ratio (by virtue of the sprag clutch). There  have been many accidents where the pilot responded to the  rotor rpm tachometer failure and entered into autorotation  while the engine was still operating. Look closer at the markings on the gauges in  Figure 4-18.  All  gauges shown are dual tachometer gauges. The two on the  left have two needles each, one marked with the letter ‘T’  (turbine) the other marked with the letter ‘R’ (rotor). The  lower left gauge shows two arced areas within the same  needle location. In this case, both needles should be nearly  together or superimposed during normal operation. Note the  top left gauge shows two numerical arcs. The outer arc, with  larger numbers, applies one set of values to engine rpm. The  inner arc, or smaller numbers, represents a separate set of  values for rotor rpm. Normal operating limits are shown when  the needles are married or appear superimposed. The top  right gauge shows independent needles, focused toward the  middle of the gauge, with colored limitation areas respective  to the needle head. The left side represents engine operational  parameters; the right, rotor operational parameters. Figure 4-19.  A “split” or divided needle condition is a result of a  sudden loss of engine power. Figure 4-18.  Various types of dual-needle tachometers.  Dual Tachometers Main Rotor Transmission 110 100 90 80 70 60 50 110 100 90 80 70 60 50 E R % RPM % RPM NR NP 120 110 105 100 95 90 80 70 60 40 0 RPM X100 ROTOR TURBINE 2 3 4 5 1 0 25 30 5 10 40 20 15 35 R T ROTOR PEEVER TURBINEA PERCENT RPM 0 70 30 10 40 110 60 50 20 80 90 100 120 R T 110 100 90 80 70 60 50 110 100 90 80 70 60 50 E R % RPM 4-11