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Improve Your Knowledge (IYK)

Reliability, Maintainability and Availability: The Trinomial

of the Operability

Berquó, Jolan Eduardo – Electronic Eng. (ITA)·.

Aerospace Product Certifier (DCTA/IFI)

Government Representative for Quality Assurance – RGQ (DCTA/IFI)

Post-graduated in Reliability Engineering and System Safety Engineering (ITA)

Specialization in Systems Engineering and Analysis (Italy)

jberquo@dcabr.org.br/jberquo@uol.com.br YIK 48 – OCT 13, 2014

The Availability has something to do with the

binomial Reliability and Maintainability? The

answer is yes. Incidentally, it seems intuitive that

there is a relationship between these three

design features. Let's explore this topic in this

MSC.

We have spoken a lot about Reliability in our

MSC and, occasionally, we have mentioned the

maintainability. Now, let's see how these

characteristics come together to characterize the

availability. There is in English the acronym RAM

that was extracted from the initials of the terms

Reliability, Maintainability and Availability.

Like Reliability & Maintainability, Availability is

a probabilistic feature, as we will see.

We have defined the term reliability1 as the

probability of a system having a satisfactory

performance during a certain time interval

under certain conditions.

We already have said that reliability, in the

allocation of functional requirements of an

aircraft, is the exponential distribution:

R = e-t or R = e-t/MTBF (2)

1 See IYK 05

where is a constant named failure rate, t is

the time, and MTBF is the Mean Time

Between Failures.

As we have also said, the above expression only

applies to repairable systems.

The maintainability is a design and installation

feature, which is expressed as the probability

that an item is restored to a defined condition

within a given time interval, when maintenance

is done according to the specified procedures

and specified technical resources.

Sometimes we use the term Reliability (eg, the

allocation of safety requirements), sometimes

use their parameter or its inverse, MTBF. On

the other hand, when working with

maintainability, in general we use the various

parameters of this characteristic, for example:

CT (Mean Corrective Maintenance Time), pt

(Mean Preventive Maintenance Time), MTBM

(Mean Time Between Maintenance), (Mean

Active Maintenance Time, which includes CT

and pt) and MDT (Maintenance Downtime).

With the information above, we can now speak

about the availability.

Availability - is the probability that a system or

equipment, when used under stated condition,

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will operate satisfactorily at any point in time as

required.

Availability (A) is a function of Reliability and

Maintainability. We consider the availability on

three occasions in the life cycle, namely:

Inherent Availability (AI) - is the probability that

a system or equipment, when used under stated

conditions in an ideal support environment (i.e.,

readily available tools, spares, maintenance

personnel, etc.), will operate satisfactorily at any

point in time as required (it excludes preventive

or scheduled maintenance actions, logistics

delay time, and administrative delay time).

It is expressed as Ai =

(3)

where MTBF is the Mean Time Between

Failures and is the Mean Time Between

Corrective Maintenance.

This situation occurs when you already know the

, but do not yet know the average time for

preventive maintenance ( pt), to be defined in

the activity of Reliability Centered Maintenance

(RCM), normally developed by the method called

MSG-3 .

Ex.: MTBF = 200h e = 5h. So, Ai = 0, 975 or

97,5%.

This means that in every 100 times that we want

use the system, it probably will be available at

about 98 times.

Achieved availability (Aa) – is the probability

that a system or equipment will operate

satisfactorily at any point time as required, in an

ideal logistical support environment.

É dada por Ao =

(5)

It is expressed as Ao =

(5)

Where MDT, as already seen, is the

Maintenance Downtime. It Includes

logistic time delays (time to provide test

equipment (GSE), spare parts and

transportation).

Ao is really the availability at the beginning of

the operational phase of the system. From that

moment on, it will be improved, mainly with the

improvement of Mean Time Active Maintenance

and reduction of logistic times delays. This

occurs with the learning of technicians who take

care of the logistic support (maintenance, spare

parts, transportation, etc).

Well, let's end here. We believe we spent some

useful information, as title of familiarization on

the Trinomial of operability (RAM).

References:

1. M. Modarres, What Every Engineer should

Know about Reliability and Risk Analysis.

Marcel Dekker, Inc., USA, 1993.

2. Blanchard, Benjamin S.; Verma, Dinesh;

Peterson, Elmer; Maintainability: A Key to

Effective Serviceability and Maintenance

Management. Jonh Wiley & Sons, Inc., New

York, 1995.