Effective Reliability Oversight with insufficient data
An article by our Guest Blogger and Senior Reliability Trainer Rustom D. Sutaria of Avia Intelligence.
With the inclusion of general and corporate aviation types into more formal continuing airworthiness management, a somewhat thorny issue presents itself where low or even ultra-low aircraft utilization is concerned.
The simple question is, is there sufficient levels of data in sufficient volume to be able to conduct a meaningful aircraft maintenance reliability analysis? This is somewhat of a major concern to anybody from the Fixed Base Operational environment to Charter or even season operations.
These guys have told me, repeatedly, that some of these aircraft, although under the requirements of an Air Operator Certificate simply are not being flown enough to generate sufficient hours and cycles for a meaningful reliability analysis. One engineering manager even informed me that they are responsible for an aircraft that is doing at most 40 or 50 hrs per year, and have asked me how to project potential failure rate of systems and components on-board their aircraft.
There are several approaches that can be considered. All of which consider the level of aircraft complexity involved, operational environment and off-course degree of utilization in terms of the expected number of hours and cycles operated in any given 12-month period.
Weibull Analysis
The most effective approach involves, the utilisation of Weibull, which provides engineers with an understanding of life data analysis. Where aircraft maintenance is concerned, the Weibull plot is extremely useful for maintenance planning, particularly where reliability centred aircraft maintenance is concerned. It tells the engineer/analyst whether scheduled inspections and overhauls are needed, and to a certain extent helps us to identify an almost cyclic or rhythmic pattern in terms of predicted failure of aircraft systems and associated components.
Utilising two defining characteristics of the Weibull line (The slope, β (beta), and the characteristic life, eta η) the analysis offers the following advantages:
1. The ability to provide reasonably accurate failure analysis and failure forecasts with extremely small samples.
2. It allows for Small samples or 100% population data where there may only be two or three failures in a period of three-months (as an example).
3. It provides a simple and useful graphical plot of the failure data.
From the low or seasonal utilization perspective, Weibull analysis may still be useful even with inadequacies or inconsistencies in the data. (Even bad Weibull plots are usually informative to engineers trained to read them.)
In all fairness, determining the slope and the characteristic life of an aircraft component or system is the hard part, particularly if you do not have the resources available to the blue-chip airlines. This is because historical data is still required, to make effective predictions. Very often smaller low or seasonal utilization operators dismiss Weibull because of a simple lack of resources.
The key would be for Aircraft Type Certificate holders and manufacturers of aircraft components, to provide at least fundamental beta & eta ranges that correspond to typical utilizations on a bi-annual basis. This information can then be utilized by reliability engineers to project failure rate for their fleet.
Pooling Arrangements
Another approach to solving the problem of data that is too low or too slow, is to “pool” data (i.e. collate data from several operators of the same type of aircraft) for adequate analysis. For the analysis to be valid, the aircraft concerned, mode of operation, sector length, utilisation and maintenance procedures applied must be substantially the same.
The problem with pooling arrangements, is that often, the data is seen to be ‘commercial in confidence’, and it is difficult to find another operator that is even willing to talk to an operator about sharing their data. Regardless, flying clubs, Part M SubPart F or Part 145 businesses might envisage this as an opportunity to provide a service that reports reliability and maintenance programme effectiveness to their clients.
Obviously, there will be a few confidentiality issues to overcome, however, and on a positive note organisations like ‘Airbus India’ are already offering a similar service for maintenance programme effectiveness oversight & development to several carriers in the sub-contintant and APAC regions.
If you would like to arrange an EASA Compliant Reliability Training Course please email: office@sassofia.com or see www.sassofia.com for more details of how SAS may help you.
