An aircraft minimum equipment list (MEL) provides a line of defense for safe flight operations, but the development by aircraft operators with oversight by civil aviation authorities is not always standardized around the world. In the case of MEL authoring, some specialists argue that more efforts are needed to standardize the existing MEL format and to apply current standards of simplified English.
For an MEL to be properly interpreted for safe flight operations, the flight crews that use it need adequate troubleshooting skills as well as knowledge and experience in airworthiness management. On their part, some authorities need to publish more specific guidance materials on MEL development and push for master MELs (MMELs) to be issued by aircraft manufacturers with deeper consideration of the interconnectedness of aircraft systems, several specialists say.
An air carrier fuel-exhaustion incident 30 years ago underscores the importance of MEL-related risk factors. On July 23, 1983, Air Canada Flight 143, a Boeing 767-233, ran out of fuel at Flight Level 410 (about 41,000 ft). The aircraft was at about the halfway point on a trip from Montreal, Quebec, to Ottawa, Ontario, and then to Edmonton, Alberta. The crew glided to an emergency landing at Gimli Industrial Park Airport, a former Royal Canadian Air Force base in Gimli, Manitoba.
One causal factor found in the investigation was inconsistency in the MEL. After being informed of accuracy issues related to the computerized fuel monitoring system and determining that both main fuel tank gauges were inoperative, the captain consulted the 767 MEL, which said that, under these conditions, the flight could not be dispatched. However, this 767 was new, and the MEL contained some blank pages while relevant procedures were still being developed.
Investigators found that 55 changes to the MEL had been implemented in the first three months that Air Canada operated this airplane. The MEL thus had become such a fluid document that the perception grew that maintenance personnel, rather than the MEL, should be the final authority on the aircraft’s airworthiness. In that context, airline maintenance control cleared Flight 143 to depart with the fuel system defects and anomalies, according to one report.1
An MEL essentially is designed to enable safe operation of aircraft, subject to specified conditions, when certain equipment is inoperative. As such, the MEL has a vital function as one barrier in the layers of defenses against an unacceptable risk level. It is the ultimate decision-making tool for flight crews and maintenance personnel to judge whether a planned flight is a “go” or “no-go.”
“MEL authoring to some extent can be considered a science in itself,” says Carl Norgren, an MEL specialist. “An MEL is the synthesis of a number of complex requirements. It is based on the MMEL issued by the aircraft manufacturer and approved by the regulatory authority. It reflects the technical systems installed in the aircraft, the applicable regulation in the state of registry and the operational environment. Condensing this variety of requirements into a manual which is compliant, consistent and practical to use, supporting safe aircraft operations, requires purposely developed skills from the technical writers producing MELs.”
MELs may be infrequently consulted, typically less than once a month per aircraft in business aviation operations. Because of this infrequent use, misunderstanding MEL content may occur.
Technical training to increase proficiency in MEL interpretation, for example, is a sound practice. “For continuing airworthiness management personnel, a brief familiarization with regard to how to handle ‘maintenance’ (M) and ‘operating’ (O) procedures and related categories should be sufficient,” says Martin Eibicht, director of technical services at Jet Aviation Business Jets in Zurich. “Pilots need first to be trained on the airworthiness requirements of a specific aircraft in order to be able to understand the meaning of an MEL and the respective ways of deferring a defect accordingly.”
MEL interpretation training for flight crews is normally covered during one-hour familiarization training, which is part of line-oriented flight training. This standard may not be sufficient, however, some specialists have found. As an example, Jet Aviation Business Jets has a policy that no technical log entry can be made without first consulting the continuing airworthiness management organization.
Eibicht says that the main reason for this policy is the limited knowledge of most flight crewmembers when it comes to airworthiness management. He also believes that the appropriate level of MEL interpretation training should depend on the technical and airworthiness knowledge of flight crews undergoing the training.
Another issue associated with the infrequent use of the MEL is speed in identifying the applicable MEL item once a flight crew observes a failure indication, such as a light flashing on an instrument panel. Technology is now helping. “Most aircraft of the newer generation are capable of generating a fault history, which can be downloaded and sent to the manufacturer for further advice,” says Eibicht. “Only so-called ‘hard faults’ (clear failures of a component or system) do not require the involvement of the manufacturer with regard to troubleshooting and related possible ways of dispatching the aircraft.”
Because of their infrequent use, MELs must be produced in a format that is not only user-friendly but also standardized for quick consultation — whether published on paper or on digital displays such as electronic flight bags (EFBs).
As noted, another problem some specialists have identified has been the absence of a standard, universally implemented MEL format. “While there are some guidance materials available, many give conflicting or confusing information,” says David Burk, owner of Aerodox, a consultancy.
“As of today, there is no strict requirement mandating a particular layout of the MEL,” says Giovanni Cima, operations rulemaking officer at the European Aviation Safety Agency (EASA). “The traditional ‘five-column’ format and the ATA [U.S. Air Transport Association, now called Airlines for America (A4A)] numbering system are used in most cases, but they are not the only possibility — and whether they should be followed depends very much on the format of the MMEL upon which the MEL is based. Several manufacturers currently produce their MMELs in the ‘message-oriented’ format, which is more consistent with the presentation of failure messages on aircraft displays and with electronic documentation intended to be used on EFBs.”
Aerodox’s Burk adds, “While TGL 26 [the Joint Aviation Authorities Guidance Document for MEL Policy] states that a five-column format is preferred, it does not state that this must be followed and that it is at the discretion of the operator. TGL 26 recommends that the MEL follow the MMEL page format of five columns, but if various examples of MMELs are observed carefully, it is clear that even these documents do not follow the standard. Some number their columns to show five columns, but actually have six columns. Some others, while having five columns, show only four numbered. With all this in mind, how can the operator be held to a ‘standard’ MEL format?”
Considering the average flight crew’s infrequent use of the MEL and limited training to interpret its content, regulatory authorities may have to require authoring of MELs so that layout and paragraph numbering are standardized, some specialists say.
“One solution for authorities to enforce how the MEL looks is to publish their own manual stating how they want the MEL to look,” Burk says. “Working with various authorities around the world, some have documents giving an idea of how they want the manual to look, but most only offer suggestions, or no guidance at all for the operator to follow. Most authorities’ main guideline is that the format should be consistent and clear to the user of the manual.”
Nevertheless, EASA’s Cima says, “While standardization is always advisable, we believe that imposing one particular format or layout for every MEL would be an unnecessary constraint for operators, also implying — in those cases where the MMEL is different — additional reformatting work with the potential of introducing mistakes. What is instead required is that the content of the MEL is based on the content of the MMEL, is not less restrictive, and the format and presentation of items and dispatch conditions are clear and unambiguous.”
Among voluntary solutions gradually winning acceptance by airlines worldwide is aircraft manufacturer–developed EFB content that incorporates the MMEL into a highly legible, easily accessible package of operational documentation for crewmembers. This enables the airline to author MELs using MMEL-customization tools, and the manufacturer to provide related technical and training support. International working groups of manufacturers and airlines developed the data standards and presentation formats for this solution, which participating manufacturers have designed partly to resolve legacy problems such as the MMEL/MEL format inconsistencies noted.
MELs should be written in Simplified Technical English — defined in the ASD-STE100 international standard — to be easily interpreted by the typical line pilot or maintenance technician, who is often working in physically and mentally demanding conditions and for whom English may not be the first language.
Simplified Technical English is intended to eliminate the need to translate content into other languages. The military and aerospace industries have developed Simplified Technical English for international maintenance. It has been adopted as a standard for a simple language that can be written, read and translated with minimal strain or chance of error.
Simplified Technical English is specific for each company. First, the terminology must be defined: not only terms to use but forbidden terms. Then, simple grammar must be defined to generate machine-readable sentences, with no complicated constructions such as those in literary works, says Tomislav Matievic, key account manager at Tanner, a documentation management company.
Operators and aircraft management companies should ensure that technical writers are proficient in Simplified Technical English and capable of standardizing the content authored by their technical writers, both in recurrent sentences and words. “An author must know the specifications of ASD-STE100 in which the structure of the sentences, the rules and the basic terminology are defined,” says Matievic. “As with other languages, what matters the most is continuous training, best if performed on the job. As far as the standardization of authored content is concerned, guidance can be found, for example, in the Functional Design Method, which has become during the past 20 years a ‘quasi-standard’ for technical authors.”
Reliability of Content
The life cycle of an MEL includes multiple steps from MMEL issuance and revision to MEL approval. Multiple potential sources of error exist: Both the authors and the reviewers can make mistakes, in particular, if MEL authoring and review do not follow a standard process.
“The main problem for both the author and the reviewer is that most do not understand what the basic intent of the MEL is,” says Burk. “The MEL should be written for the operator, not to make it easier for the person doing the review. Many MMELs apply to multiple aircraft models, and some reviewers want the MEL to match the MMEL to make their job easier. What is easier for the reviewer does not necessarily make the MEL a good or accurate MEL for the user.”
Opportunities become available to improve technical reliability when authoring in the MEL production process only occurs a few times a year.
“To control and improve the MEL production and review process is a task for the quality system of the operator,” says EASA’s Cima. “Avoiding mistakes that could generate safety hazards should be also part of the safety management system of the operator to identify risks and implement preventive and/or corrective actions. Training of all involved staff (both those authoring and reviewing the MEL and those using the MEL) is a key to improving the quality of the final outcome. The outsourcing of the MEL production and review process does not guarantee the quality of the final outcome, but simply shifts the issues from the operator to the contractor.
“If an operator does not have enough expertise to produce and review the MEL, outsourcing may be a solution. This is possible under the [typical contractual] provision on contracted activities, where the activities are outsourced under a quality control scheme that ensures compliance of the contractor with all applicable requirements. In this respect, it is important to understand that responsibility for the final outcome of the contracted activity cannot be delegated and rests with the operator.”
Aerodox’s Burk says, “The first step in producing an accurate MEL is good guidance from the issuing authority. Then the author needs to understand what the intent of an MEL is — not just a copy and paste of the MMEL. An MEL author has traditionally been someone who is handed the job and told to produce an MEL. No training, no guidance, just a copy of the MMEL and the procedures manual from the manufacturer. Companies need to have someone dedicated to the MEL process, not just a person with multiple jobs of which the MEL is one, and very minor in most cases, compared to their other assigned jobs.”
How can reliability be improved in MEL production? “Having someone assigned to the MEL process who has had training and experience with both MEL and the MMEL processes — and has been involved with, for example, a flight operations evaluation board meeting where the MMEL final documents are processed — would be very helpful,” says Burk. “Understanding not only what the MMEL states, but having some idea of the intent of the MEL item, is very helpful if not essential.”
Some regulatory authorities have started to outsource oversight of operators’ MEL review processes with the stated intent of improving the quality of the MEL.
“The competent authority shall approve, in any case, both the initial issue of an MEL and any subsequent amendment, assessing each item affected to verify compliance with the applicable requirements,” EASA’s Cima says, for example. “The approval process to be followed for each case — whether a thorough review or a lighter process — depends on many factors, including the content and the complexity of the document, the staffing, expertise and organization of the authority, and so forth. Therefore, it cannot be prescribed by rule. Authorities may also allocate tasks related to the certification and oversight of operators to qualified entities, if they have in place a process to initially and continuously assess such entities.
“However, as with operators subcontracting certain activities, responsibilities cannot be delegated when an authority task is allocated to a qualified entity. The issuance of certificates, licenses, ratings and approvals will continue to be done by the authority.”
Burk has found that inspectors or reviewers of MELs sometimes lack relevant training beyond guidance material. “Most have the idea that if a requirement is in the MMEL, it should be in the operator’s MEL,” he said. “But the MEL should be customized. All guidance material states that the MEL must not be less restrictive than the MMEL; nowhere do guidance materials say the MEL must be an exact copy of the MMEL.
“Some authorities have hired individuals who specialize in MELs for their reviews. This is good as long as they have guidance on what the authority requires in its MEL, and puts consistency into how the MELs are reviewed and avoids the [issue of an] inspector wanting the MEL to look exactly like the MMEL. Overall, I believe that outsourcing the MEL review helps not only the authority, but the operator by adding consistency in the MEL review process.”
MEL updating generally follows any further installation of aircraft systems or components and includes updates to the MMEL by the aircraft manufacturer. The MEL, therefore, is a dynamic document, constantly subject to change due to technological developments and revised risk assessments. Authorities commonly grant 90 days, starting from the issuance of an MMEL revision, for operators to revise their corresponding MELs.
Achieving compliance within 90 days normally should not be an issue. “The operator is informed in advance by the type certificate holder of an MMEL revision, before the MMEL is approved by regulating authorities,” says Eibicht of Jet Aviation Business Jets. “The only exception is Dassault, whose MMELs are normally approved up to 30 days prior to the official publication of the MMELs themselves by the manufacturer.”
Compliance monitoring of MMEL requirements is normally a straightforward process, he says. “We keep track of all MELs on our fleet by using a simple Excel worksheet where the status of each MEL is clearly visible. Compliance checks with the authorities’ websites are done monthly.”
Compliance with regulatory requirements is the starting point of any MEL update; however, it is advisable for the operator or aircraft management company to perform additional risk assessments during the process to develop safer operating procedures. “Since the MMEL, and to some extent the MEL, may not consider special environments or areas of operation, we always have to consider all factors before deferring a defect,” says Eibicht. “On some occasions we have grounded the aircraft due to its flight schedule and the limitations given by the MEL in order to avoid an aircraft being grounded at a remote destination.”
A New Complexity
The August 2008 crash of Spanair Flight 5022 in Madrid — in which the flight crew neglected to set flaps and slats before takeoff and the takeoff warning system (TOWS) was inoperative — was also in part characterized by MEL inconsistencies (ASW, 12/11–1/12). Prior to the accident, the flight crew had taxied out for departure but had to return due to a fault associated with the ram air temperature (RAT) probe heater. The ground-sensing relay controlling the RAT probe heater is shared with the TOWS.2
No troubleshooting was performed to establish whether the RAT probe was defective or whether the ground-sensing relay controlling the RAT probe heater was defective. The aircraft was released to service according to the MEL entry for “RAT probe heater inoperative.” In many previous instances in the worldwide McDonnell Douglas MD-80 fleet, inoperative TOWS had been linked to faults with the RAT probe heater.3
“As electronic aircraft systems become ever more complex and interlinked, understanding failures within such systems becomes more demanding,” says Norgren, the MEL specialist. “A single point of failure can have multiple consequences. Similarly, a single symptom can be caused by multiple failures.”
The Spanish Civil Aviation Accident and Incident Investigation Commission recommended after the Spanair accident that EASA “issue an interpretation regarding the need to identify the source of a malfunction prior to using the MEL and assure that national authorities accept and apply the same standards with regards to their procedures for overseeing operators in their respective states.”4
Thomas Fakoussa, owner of Awareness Training Fakoussa, a consultancy, argues that additional troubleshooting skills are needed by pilots precisely because of the interconnectedness of many faults today. He says, “From a pilot’s perspective, it is not just important to realize that a system and its adjacent components have failed. Only flight crews know what kind of operation they have ahead of them: weather, terrain, airport, crew (experienced or inexperienced), time of day, type of territory (water, desert, etc.), and which systems or components will be needed. … Without analyzing what that means for the flight, [the MEL] is just a help to remind the crew of the absolute minimum.”
Mario Pierobon works in business development and project support at Great Circle Services in Lucerne, Switzerland.
- Williams, Merran. “The 156-tonne GIMLI Glider,” Flight Safety Australia, July–August 2003.
- CIAIAC (Spain’s Civil Aviation Accident and Incident Investigation Commission), Final Report A-032/2008. 2011.