To: Capn Notarious
Most engines and most helicopter transmissions and intermediate and tail rotor gearboxes are equipped with chip detectors. Some of these detectors are simple devices that magnetically attract a chip and if the chip bridges a gap a circuit will be made and a light will illuminate. Other more complex chip detectors work in the same way but an additional electrical circuit will allow the chip to be burned off. If the chip is too big to be burned off then there are impending problems in the gearbox. Even if the chip is of non-magnetic material it will if it bridges the gap cause the light to go on. Most chip detectors can be removed from the gearboxes with out losing any oil due to a spring-loaded valve built into the chip detector body.
There are some jet engines that do not employ chip lights. (Mainly Rolls Royce). The engine is protected with chip collectors who are nothing more than magnets. These detectors are removed on a periodic basis and the chips are collected on clear adhesive tape and this tape is transferred to an inspection card. This card is then sent to Rolls Royce or an assigned lab and the chips are analyzed. Records are kept and when any of the chips build up at a specified rate the engine is pulled and sent for sectional overhaul. On this type of chip collector it is imperative that the “O” rings be replaced prior to reinstallation. An Eastern Airlines L-1011 was almost lost due to oil starvation when the oil leaked out past the un “O” ringed chip collectors.
I do not believe the Apache intermediate or tail rotor gearboxes are protected by chip detectors because they are lubricated by very viscous grease, which would limit the migration of chips. (At least that is the way they were designed).
One other point is that chip detectors do not protect the unwhetted parts of an engine such as turbines and compressors. That is why they specify periodic bore scope inspections.
Chip detectors or chip collectors indicate that there is a problem in the system being monitored (not including spurious chip lights due to initial break-in of the transmission or engine). That is why during the first (X) hours the oil filters must be changed. A Spectrometric Analysis of the oil on a periodic basis will indicate premature wear long before the chips develop.