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Study Overview
The study investigated incubation of MicrobMonitor2 tests at four different temperatures (22°C, 25°C, 28°C and 30°C) after testing fuel samples deliberately contaminated with three common fuel degrading microorganisms; a bacterium (Pseudomonas aeruginosa), and two fungi (the yeast, Yarrowia lipolytica and the mould, Hormoconis resinae). For each contaminant, three levels of contamination were investigated, broadly corresponding to low (negligible) contamination, moderate contamination and heavy contamination in accordance with IATA Guidance limits.
The full technical report can be provided if required.
Study Findings
Comparison of colony counts after 3 days and 4 days incubation
A statistical comparison (One-way ANOVA) of the numbers of colonies that developed in MicrobMonitor2 tests established no significant difference between colony counts obtained at 3 days and colony counts obtained at 4 days incubation, for all test microorganisms, when incubated at 25°C, 28°C and 30°C.
Example showing 5 Hormoconis resinae colonies in a MicrobMonitor2 test after 3 and 4 days incubation
Comparison of categorisation of contamination in accordance with IATA Limits after 3 days and 4 days incubation
The study did show that there can be a slight increase in the number of colonies between day 3 and day 4 which can, in a small number of cases, result in a change in contamination category as defined in the IATA guidance; for instance, a change from ‘Negligible’ to ‘Moderate’ level contamination, or from ‘Moderate’ to ‘Heavy’ level contamination. The incidence of these changes was low and was as follows for each microbial contaminant tested;
- For fuel contaminated with the yeast Yarrowia lipolytica, the contamination category changed between day 3 and day 4 of incubation in 2 of 27 tests (7.4%).
- For fuel contaminated with the mould Hormoconis resinae, the most common aviation fuel contaminant, only 1 of 27 tests conducted (3.7%) showed a change in contamination category between day 3 and day 4.
- For the bacterium Pseudomonas aeruginosa there was no change in contamination category between day 3 and day 4 of incubation in any of the 27 tests conducted.
Inferences for routine microbial monitoring of aircraft fuel tanks
Our experience suggests that, for a typical aircraft fleet, most fuel samples are entirely free of contamination and only about 10% show some level of microbial contamination (i.e. in the proportions and range investigated in this study). If so, the study infers that in a typical fleet monitoring program, less than 1% of MicrobMonitor2 tests would show a contamination category change between day 3 and day 4 of incubation. In other words, for a typical fleet, providing MicrobMonitor2 tests are incubated at a temperature between 25°C and 30°C, tests examined at 3 days would show the same contamination category as a test incubated for 4 days for more than 99% of tests conducted.
Precautions in considering changes to incubation time
- We would recommend that users continue to check MicrobMonitor2 tests after 4 days incubation, even if results after 3 days incubation are used to determine aircraft operational actions. This will further help to establish whether the data from our study applies to the user’s particular operations.
- If MicrobMonitor2 tests are incubated at temperatures below 25°C, then the final results must always be obtained after 4 days incubation (even if incubated at 22°C which is within the specified range of incubation temperatures for the test).
Updated June 2022
