You are here

Helicopter Transmission Oil Discolouration

ABSTRACT

An increase in the incidence of abnormal discolouration of oil in ADF helicopter tail rotor transmissions has been observed and reported over the last eight years. Aircraft maintainers have initially attributed the discolouration to a range of contamination modes including incorrect fluid addition, hydraulic fluid contamination and degraded fluid. A program of work was conducted at DSTO to identify and characterise the cause of the abnormal discolouration. This report details the identification of a coloured metal quinizarate salt which forms in the aircraft transmissions under normal operating conditions and describes the mechanism of formation and some characteristics of the quinizarate salt to assist in developing maintenance programs to manage the coloured salt when observed in service conditions.

EXECUTIVE SUMMARY

Effective lubrication of modern military aircraft transmissions requires the use of highly specified synthetic polyol ester lubricants. Significant effort can be expended in ensuring serviceability of these oils through a range of oil condition monitoring programs. The current oil used in many ADF helicopter transmissions meets the MIL-PRF-23699 specification and a number of specific brands of oil meeting this specification are in routine service within the ADF.

An increase in the incidence of abnormal discoloration of oil in ADF helicopter tail rotor transmissions has been observed and reported over the last eight years. The most numerous description of the discoloration was of a two phase fluid with a red-purple phase observed at the bottom of the aircraft tail rotor sight glass. Aircraft maintainers have initially attributed the discolouration to a range of contamination modes including incorrect fluid addition, hydraulic fluid contamination and degraded fluid.

A program of work was conducted at DSTO to identify and characterise the cause of the abnormal discoloration. This report details the identification of a coloured metal quinizarate salt which forms in aircraft transmissions under normal operating conditions and describes the mechanism of formation and some characteristics of the quinizarate salt.

Isolation and purification of the ‘red-purple’ contamination was followed by detailed chemical analysis using a wide range of chemical techniques. The red-purple material was found to be formed from quinizarin, which is a metal passivating additive found in some of the brands of oils and copper metal in solution in the oil. The copper was sourced to a bronze component in the aircraft trial rotor transmission.

DSTO has provided advice to the ADF to assist in developing maintenance programs to manage the occurrence of the coloured quinizarate salt.

 

Key information

Author

Andrew Becker and Paul Rawson

Publication number

DSTO-TR-2672

Publish Date

February 2012

Classification

Unclassified - public release

Keywords

Turbine oil, quinizarin, metal passivator additive, oil discolouration