Minimum equipment lists

Maintenance Unavailability shift supervisor s logs and minimum equipment lists so unavailability of equipment needed to mitigate a shut-down accident. 

A sound Minimum Equipment List (MEL) can be maintained and amended according to experience. 

Master Minimum Equipment List A list of equipment and functions which need not be operative for safe flight and landing based on stated compensating precautions created during aeroplane type certification [ARPS 150]. 

Minimum Equipment List An approved list of items which may be inoperative for flight under specified conditions cooperatively created by operators and regulatory authorities [ARPSISO]. 

Two types of maintenance may be implemented when TLD is apphed. These are minimum equipment list (MEL) maintenance and periodic inspection and repair (PIR) maintenance (FAA Memo 2001). MEL maintenance is most commonly applied to STD faults and PIR maintenance is most commonly applied to LTD faults. However, there are no restrictions in place to this effect. 

Error is an inherent feature of the "real world" (Sil-2) for investigators. It can be managed, with its causes addressed and its occurrence driven down. But there is presumed to be a lower limit to how far errors can practically be reduced error can never be eliminated entirely. Technical equipment is viewed the same way. Faults and failures are understood as a natural part of operating large, complex technologies. Equipment can never be entirely failure free. One clear illustration is the Minimum Equipment List (MEL) for an aircraft. These lists specify the technical faults that it is considered safe for an aircraft to fly with, in different circumstances and with various caveats - an organisational manifestation of this deep presumption of normal failure ... 

Recommendations from this assessment will therefore need to be incorporated in the aircraft flight manual, emergency reference cards, as well as the minimum equipment list. These recommendations will tend to ensure that the safety objectives of the system are not compromised for the remainder of the flight. 

However, at certification, the aircraft was designed to achieve a certain level of safety. When any one system, instrument or equipment becomes inoperative, the designed level of safety is reduced. The question is, how can we be sure that under such conditions the aircraft will continue to be operated safely The solution lies in using the system safety assessment to define minimum equipment lists which will guide the operator as to allowable deficiencies, exposure times and appropriate limitations of use. 

The concept behind a minimum equipment list (MEL) is not new - aviation regulatory authorities have used it for many years (e.g. under the terminology Allowable Deficiencies, Go/No-go list. Dispatch Deviation Manual , etc.). Initially the concept was applied to allow operators to operate their aircraft with certain items of equipment or components inactive, provided that the Authority concerned was satisfied that an equivalent level of safety could be maintained either by (Christy, 1994) ... 

Depending on in-service experience, operational conditions and maintenance procedures the aircraft operator may wish to amend the MMEL by producing a minimum equipment list (MEL). This is allowed by the authorities on the condition that the MEL remains within the limitations of the MMEL (i.e. the MEL must not be less restrictive than the MMEL for the particular aircraft type." ... 

C-05-005-001/AG-001, Technical Airworthiness Manual, Canada National Defence, 2003-02-13. CAP 549, Master Minimum Equipment Lists (MMEL) and Minimum Equipment Lists (MEL), (at www.caa.co.uk. Publications, Design and Production). 

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