Airworthiness Notice - B009, Edition 1 - 21 June 1993
Chip Detectors in Aircraft Engines, APU's, Transmissions and Reduction Gearboxes
(This Airworthiness Notice supersedes N-AME-AO 10/89 )
(dated 25 April 1989)
In various aircraft the engine(s), auxiliary power unit (APU), transmission and/or reduction gearbox (RGB) systems may include one or more chip detectors. These devices, whether required or optional, are usually relied upon by operators and maintainers to monitor the health or airworthiness of the engine, APU, transmission or RGB. This is accomplished by detecting iron-based metal in the oil system using one of a variety of techniques.
Transport Canada has conducted an informal survey of engine manufacturers and has found that in spite of high reliability rates, chip detectors do fail periodically. The initial survey was prompted by the failure of a chip detector on a commuter aircraft. As outlined in Airworthiness Directive CF-88-22R2, the dormant failure of the chip detector allowed a progressive failure of the RGB to go unnoticed. This resulted in an uncontained engine failure and penetration of the aircraft cabin. The subsequent survey of over 400 of the same chip detector on the same aircraft and engine models revealed another 38 open circuit chip detectors. Another survey of a different, more sophisticated, type of chip detector revealed 30 removals, most due to failures of the features additional to the basic detector probe. For chip detectors which may have a dormant failure mode, the attrition rate is currently unknown.
Types of Chip Detector
There are a number of types of chip detector available with different indicating systems and applications. Some of these types are:
Magnetic Plug: This type of chip detector has one or more magnetic poles set in a plug which is easily removed from the gearbox casing by a bayonet type mount. This type usually incorporates a self-sealing feature to avoid draining the oil from the gearbox. The magnetic poles attract and retain the chips for visual observation when the plug is removed.
Two Pole, Two Wire: This type of chip detector is similar to the magnetic plug except that it need not be removed to detect chips. The poles are wired to connector pins which, in turn, may connect the chip detector to some form of remote indicator (e.g. flag, warning light, etc.). An alternative indication would be a continuity check across the connector pins, with a continuous circuit indicating chips. This type of chip detector may be self-sealing or not. Where the chip detector is supplied with the engine, APU or transmission an associated remote indicating system is airframe supplied. Two variants of this chip detector type are illustrated below, one using its casing as part of the circuit. In either case, chips at the poles complete the circuit, causing the flag or warning light to indicate the presence of chips. The alternate indication by continuity check is similar to the magnetic plug type, requiring an inspection by maintenance personnel.
WARNING: This type of chip detector has a DORMANT FAILURE mode. This means that a failed chip detector gives the same indication as for a chip-free condition, even if chips are present.
Two Pole, Four Wire: This type of detector differs from the previous type only by incorporating an additional wire per pole, as shown below. The additional feature allows for installing a "press-to-test" function on the aircraft so that the chip detector need not be removed to test it for functionality of the circuitry. Future use of microprocessors may produce a continually self-checking system.
Full Flow: A more sophisticated type of chip detector is the full flow unit. This unit incorporates a chip detector probe, a casing and filter, and a bypass valve to avoid blockage or to bypass cold viscous oil. In this type, the detector probe may take any of the three forms detailed above.
- Fuzz Burners: This type of chip detector is similar to type 3 listed above, but utilizes the alternate circuit to carry current. This circuit is then used to burn off the "fuzz", or light metal dust particles, which may be generated by normal wear, therefore not generating a nuisance warning.
As with all safety devices, chip detectors need to be functionally checked at appropriate intervals. The types of chip detectors discussed in this notice may fail in a variety of modes including loss of magnetism, corrosion and open circuits. To avoid continued "use" of a failed chip detector, Transport Canada strongly recommends that: ALL CHIP DETECTORS MUST BE FUNCTIONALLY CHECKED AT EVERY SCHEDULED OR UNSCHEDULED ENGINE/APU/ TRANSMISSION/RGB REMOVAL, UNLESS A SHORTER INTERVAL IS WARRANTED. Different manufacturers may already have such checks in place (Maintenance or Overhaul Manuals) and may also detail an interval based on operating hours or calendar time.
Operators and owners are urged to review their maintenance programs or schedules to verify that they regularly check their chip detectors for magnetism and functional integrity. If there is any doubt, please contact the manufacturer to establish this necessary interval.
It should be clearly understood that a check of functional integrity involves removal of the chip detector, bridging of the magnetic poles with a conductive material, and a continuity check for completion of the circuit. A second check with the canon-plug connected to an airframe indicator system will check the integrity of the airframe system also.