Nowlan and Heap's reliability-based maintenance method

This article, aimed at readers with a basic knowledge of industrial maintenance, presents the origins, principles and characteristics of the first Reliability-Centered Maintenance (RCM) method, which was developed in 1978 at the request of the US Department of Defense (DoD) by two United Airlines aircraft maintenance managers: Stanley Nowlan and Howard Heap. Indeed, in the commercial aviation sector, the first aircraft maintenance requirement was laid down in the 1930s, requiring that "instruments and accessories be overhauled at adequate intervals to ensure their correct operation at all times". This concept, known as "Hard Time" maintenance, was the reference method until the 1960s. With the arrival of the first jumbo jet, the Boeing 747, the American authorities developed maintenance standards MSG-1 and MSG-2, which introduced new concepts of maintenance tasks, condition checks and behavior monitoring. In 1978, the DOD commissioned a study by Nowlan and Heap to develop a maintenance method that would focus primarily on the consequences of failures, rather than the maintenance tasks recommended by MSG-1 and MSG-2.

Feedback from the use of the Hard Time Concept on their company's fleet led Nowlan and Heap to highlight the paradox of this practice. They discovered that very few components (11%) of an aircraft had a reliable service life, and that scheduled scrapping was of no interest. So as not to betray Nowlan and Heap's approach, the article is based on the main chapters of their report, and scrupulously respects their concepts and terminology.

The first section is devoted to their thoughts on maintenance and the description of this paradox, which led them to define reliability-based maintenance.

The main definitions useful for understanding Nowlan and Heap's RCM method, hereafter referred to as RCM-N&H, have been extracted and translated from their glossary and are the subject of the second section. The third section provides the main elements contained in their report for defining a reliability-based maintenance program: basic principles, failure classification, the four basic maintenance tasks, the procedure for drawing up the initial maintenance program and the use of feedback to identify weak points in the initial program and make improvements. To illustrate how their RCM method works, the case of two critical functions of an aircraft main landing gear hydraulic braking assembly is discussed. It provides an insight into the process of selecting applicable and effective maintenance tasks. The final section is devoted to an analysis of their method, and underlines the importance of their work, which is at the origin of the reliability-based maintenance methods currently in use.