1 Availability

Availability is the probability that a system or component is performing its required function at a given point in time or over a stated period of time when operated and maintained in a prescribed manner ⁽⁷⁾. This top-level definition leads us to the top-level equation for calculating availability:

However, this top-level equation does not portray the whole story. To begin to understand system performance we need to be able to describe the reliability and reparability of the system in availability terms. This is termed Inherent Availability, sometimes referred to as Intrinsic Availability, (Ai). The equation for calculating Ai is as follows:

Where MTBF is the Mean Time Between Failure and MTTRc is the Mean Time To Repair (Corrective).

To further understand system performance we need to add the dimension of preventive maintenance. This produces the performance parameter of Achieved Availability (Aa), which is a function of reliability and maintainability, both corrective and preventive:

Where MTTRp is the Mean Time To Repair (Preventive).

The final dimension to understanding system performance is to understand how well the support system has been designed. This completes the picture of Operational Availability (Ao):

Where MTTS is the Mean Time To Support and includes parameters such as Administrative Delay Time and Logistic Delay Time.

2 Operational Availability

Having seen how operational availability calculations are derived and how important they are to measuring system performance it begs the question why so few programme managers and ILS managers understand them.

During my time as a Consultant, I have been in receipt of many Invitations to Tender and ILS Plans that call into question how well systems availability is understood:

E.g. 1. The stated Operational Availability requirement for a Command and Control Information System was 99.5% when planned downtime was accounted for.

This requirement is asking for only 0.5% loss of availability due to corrective maintenance (repairs) and the support chain activity - an unrealistic aspiration.

E.g. 2. The following is drawn from an R&M Plan to support a Fuel Tanker programme:

AVAILABILITY REQUIREMENTS

The Tanker Specification document does not detail any availability requirements.

RELIABILITY REQUIREMENTS

Table 1 below identifies the reliability requirements for the Tanker taken from the Specification:

Table 1 - Tanker Reliability Requirements

The Reliability Requirements are in a standard MOD format, which have been transposed to provide target MTBFs. The Contractor has, however, been unable to find amongst the ITT documentation any usable definition of Mission and Basic Failure. These key definitions for this Reliability programme have been developed by the Contractor:

Mission Failure

A Mission Failure is any incident that prevents the starting or completion of a refuel mission unless the operator can rectify it (having no specialized training) within 15 minutes from the time the incident occurred, using the tools and spares carried with the vehicle.

Basic Failure

A Basic Failure is defined as any maintenance relevant failure that results in an unsatisfactory equipment condition that requires unscheduled maintenance to restore the equipment to its peacetime serviceability state. It excludes scheduled maintenance actions but includes unscheduled maintenance highlighted by scheduled maintenance. All Mission Failures are also Basic Failures.

Failure Exceptions

The following failure exceptions are offered:

A failure of a component beyond its specified life;

A failure caused through misuse, accident, human error or maintenance not in accordance with defined schedules;

Any use of the equipment in excess of its design limit;

Secondary failures;

Failures rectified by adjustment;

Usage Indicators.

MAINTAINABILITY REQUIREMENTS

The Tanker Specification document does not detail any maintainability requirements. However, there is a maintenance requirement stated as follows:

'Other than daily and weekly replenishment and replacement of consumables, the refueller and trailer shall not need maintenance at less than 10,000km or 6-monthly.'

Furthermore, the Mission Failure definition requires recovery from a failure condition within 15 minutes. However, this value has no relevance in the establishment of Maintainability Requirements except that it is a statement of the maximum permitted recovery time for the Tanker following a mission failure.

The above example is typical of the (operational) availability requirements being issued to Industry to fulfil. No further comment is necessary except to say that a commonly understood approach is necessary and Industry must buy in to the approach.

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Ebeling C (1996) An Introduction to Reliability and Maintainability Engineering, McGraw-Hill. Back to Text

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last update:  December 27, 2004