Operational consequences of technical malfunctions in commercial aircraft
Ryan Rienhart
FAA Certified Flight Dispatcher/ Aeronautical Engineer/Private Pilot at KLM Royal Dutch Airlines
In the next series of articles, I hope to give a general, non-manufacturer, type, or operator specific understanding of the different ways technical malfunctions or missing items are categorized and structured in the airline industry and what their actual impact can be on the operation of a commercial flight.
To clarify the context, I might use an aircraft type specific case, but the purpose of this article is not to delve in specifics but to give a general understanding of how these deficiencies can affect regular operations. I would first like to explain the philosophy behind the categorization of technical deficiencies, and then give general and a few type-specific examples per aircraft system on what the operational impact can be. As the latter is a very vast subject, considering the specificities of each aircraft or component manufacturer, and local circumstances and limitations I will only limit myself to examples which give a general understanding of what these implications can be, of the more basic systems, and I have opted to publish them separately, per ATA subsystem, in due time.
MEL/CDL
We can categorize these deficiencies in MEL (Minimum Equipment List) items or CDL (Configuration Deviation List) items. MEL items refer to the situation where an aircraft has certain components on the airplane that are inoperative (e.g., the MEL item may be a defective fuel pump) or missing (the CDL item may be a missing landing gear door).
MEL
The MEL is “owned” by the operator and approved by the competent authority overseeing the operator, either in conformity with, or more restrictive than, the MMEL (Master Minimum Equipment List, established which has been established for the aircraft type. (ICAO Annex 6: Operation of Aircraft)
The (MMEL) is a list established, owned and managed for a particular aircraft type by the organization responsible for the type design with the approval of the State of Design, normally the manufacturer.
Two concepts are very important for an aircraft manufacturer to obtain a type certificate for a specific aircraft type:? High reliability of the aircraft components integrated in the various systems and system redundancies (having multiple backups of essential functionalities) incorporated in the design.
The purpose of a MMEL is to give operators a framework to balance an acceptable level of safety with aircraft profitability by authorizing its operation with inoperative equipment in certain prescribed conditions.
Safety
To ensure a safe departure of an aircraft, the MMEL (Master Minimum Equipment List) development process involves various experts, including MMEL specialists, system designers, and safety specialists. These specialists assess several factors for each MMEL item:
Extensive analysis of system interactions ensures that even if multiple failures occur, the safety level remains satisfactory. This analysis not only evaluates the effects of an item's failure but also considers the consequences of subsequent critical failures and their respective probabilities. The MMEL cannot not account for all combinations of these, the final authority on the decision to operate the flight with multiple MMEL items remains with the designated Aircraft Commander subject to guidance promulgated on a proactive or ad hoc basis by responsible parties within the organization of the aircraft operator.
In other words, a captain might still decide not to accept an aircraft where no ovens or lavatories are working even though these are not essential for the safety of the flight.
In any case, if multiple malfunctions exist, the MEL should be consulted for each individual item to check if there are any incompatibilities for each of the associated dispatch conditions.
When an aircraft is dispatched using the MMEL/MEL, an acceptable safety level is upheld through:
The MMEL is a legally binding document necessary for aircraft dispatch. Regulatory authorities review this document before granting approval or acceptance. Consequently, the MMEL/MEL ensures the safe operation of an aircraft, even with one or more inoperative equipment pieces. This is how the actual document looks like.
Profitability:
The MMEL not only ensures the safe dispatch of an aircraft but also enables Operators to make optimal use of their fleet in everyday operations, contributing to their profitability.
One economic benefit is aircraft operation. The MMEL allows the operation of an aircraft even when one or more parts are not functioning properly, whether it's noticed during flight or during ground servicing. This means the aircraft can still be sent out, preventing unplanned maintenance, flight delays, or cancellations.
Another economic benefit is in optimizing "initial provisioning" to lower storage costs. Initial provisioning lists all Line Replaceable Units (LRU) along with the number of spare parts needed and their costs. This planning, based on a detailed and complex mathematical model, considers factors like the number of aircraft in the fleet, total flight hours per year, spare part prices, and quantity of items per aircraft. Operators can order spare parts months before aircraft delivery to prepare for any urgent situations that may arise.
In essence, the MMEL significantly boosts operational reliability and offers a pathway for Operators to cut down on operating costs substantially.
MMEL AMENDMENTS
The MMEL needs to be updated, throughout an aircraft’s lifetime, in order to maintain an acceptable level of safety, to alleviate/modify existing dispatch conditions, or to reflect changes in the aircraft’s configuration. To maintain an acceptable level of safety, when the dispatch conditions are changed, the MMEL is amended taking the following into consideration:
The MMEL might be further amended, due to:
The operator shall include in the operations manual a minimum equipment list (MEL), approved by the State of the Operator which will enable the pilot-in-command to determine whether a flight may be commenced or continued from any intermediate stop should any instrument, equipment or system become inoperative.
In other words, the MEL comes from the Master Minimum Equipment List (MMEL), which was created by the aircraft manufacturer and approved during the airplane's certification. Each operator uses the MMEL to make their own MEL, considering their specific equipment, instruments, and how they operate. The operator's MEL might include extra items not in the MMEL, but the competent aviation authority must approve these additional items for administrative control. The operator's MEL might look different from the MMEL but can't be less restrictive.
European Regulations and supplementary information concerning the MEL for fixed wing air carrier aircraft are contained in?IR-OPS?and?EU-OPS. The FAA system is similar except that FAA considers an approved MEL to be a?Supplementary Type Certificate (STC)?issued to a particular aircraft by serial number and registration number as a way of providing authority to fly it in a condition other than that at which it was originally type-certificated.
Limits of MEL Applicability
A MEL is applicable up to the commencement of the flight (i.e. the point when the aircraft begins to move under its own power, in preparation for takeoff). So, in practice this means after pushback, with the engines started and while taxying to the runway and beyond. In most cases, the MEL may need to be consulted even thereafter, in order to decide if it is necessary to perform a return-to gate or an in-flight turn back to the departure airport. However, any decision to continue the flight is subject to the judgement of the pilot in command (PIC).
Factors to be considered in the process of make his/her decision are, among others:
领英推荐
If there is an in-flight failure, the FCOM provides the procedures and limitations that the flight crew should apply.
CDL
Operating the aircraft without some secondary airframe and engine parts is allowed according to the Configuration Deviation List (CDL). Any part not listed here should be seen as essential and therefore a no-go item if missing. Repairing the aircraft should happen at the earliest possible airport where repairs or replacements are possible, as the combination of more malfunctions might mean taking the plane out of service.
MEL CATEGORIES
The MEL sets restrictions on how long an aircraft can operate with malfunctioning equipment. These restrictions serve to:
An aircraft is not airworthy when any item that is not included in the MEL is inoperative, or if the MEL/CDL item specifies this. Some manufacturers also opt to publish a list of EICAS , or ECAM messages in the MEL document specifying this condition. Other than those cases: MEL items can be categorized as follows:
Category A. No maximum repair interval window is specified, however, items in this category shall be rectified in accordance with the dispatch conditions stated in the MEL.
Where a time period is specified in calendar days, it shall start at 00:01 on the calendar day following the day of discovery. Where a time period is specified in the number of flights or flight hours, it shall start at the beginning of the first flight following the discovery of the failure.
Category B. Items in this category shall be rectified within three (3) consecutive calendar days, excluding the day of discovery. For example, if it were recorded at 10:00 on July 15th, the 3-day interval begins at 00:01 on July 16th and ends at 23:59 on July 18th.
Category C. Items in this category shall be rectified within ten (10) consecutive calendar days, excluding the day of discovery. ?For example, if it were recorded at 10:00 on July 15th, the 10-day interval begins at 00:01 on July 16th and ends at 23:59 on July 25th.
Category D. Items in this category shall be rectified within one hundred and twenty (120) consecutive calendar days, excluding the day of discovery. The aircraft may still be able to fly with these malfunctions, however, a penalty will need to be applied to the performance of the aircraft. The regular flight plan may not be possible, and the flight plan may need to be modified. The MEL/CDL functionality in NFP automatically applies the MEL/CDL restrictions to the flight plan so that you do not need to apply them manually.
The MEL is an operational document available for flight crew to review in the event of any inoperative equipment being offered for the flight. Any equipment failures which potentially affect the airworthiness of the aircraft must be assessed in accordance with the MEL prior to acceptance for flight. A minimum equipment list (MEL) is typically based on the master minimum equipment list and provides for the operation of aircraft, subject to specified conditions, with particular equipment inoperative.
The MEL is entirely separate from the?Configuration Deviation List (CDL), which is a list of secondary airframe and engine components which may be recorded as missing for without prejudicing the acceptance of an aircraft for flight. A one-time extension of the repair interval may be granted with appropriate approval. In such cases, the airline can revert to the manufacturer's prescribed interval, (as the airline interval is at least equal or more restrictive than the manufacturer) provided an authorized manager deems it necessary. Captains can request dispensation to operate in violation of the MEL, usually for flights from locations without maintenance support or parts. Such dispensations often come with conditions, like not carrying passengers.
In conclusion, while the MEL provides clear guidelines, a pilot’s judgment and problem-solving skills are crucial in managing inoperative items and ensuring flight safety.
Further Reading:
A complete list of FAA current MMELs for all aircraft types (commercial and GA)?from the FAA Flight Standards Information Management System.
ICAO Annex 6 Part I Attachment G
A Recall on the Correct Use of the MEL, an article in the Safety First magazine, #25, January 2018
EASA
and associated Acceptable Means of Compliance and Guidance Material: EU-OPS 1.030/JAR-OPS 3.030
MEL/CDL Actions
Each MEL/CDL item is broken down into specific action types to be performed by different parties: Generally, they are structured as follows, by a certain letter in the section of the MEL description itself indicating in which department/party scope the specific action to be taken is.
A. The "(M)" letter signifies a particular maintenance task required before using the listed item if it's not functioning properly. The Engineering Department typically oversees these tasks, often carried out by maintenance personnel, though crew members or others may also be permitted. Regardless of who performs the task, it is the operator's responsibility to ensure all maintenance tasks are completed correctly. Unless specified otherwise, maintenance must be performed once before the first flight under the relevant MEL item.
B. The "(O)" symbol indicates that a particular operational procedure is necessary when planning for or using the listed item if it's not functioning. These procedures are usually carried out by dispatch, flight, or cabin crew, though others may be authorized. Regardless of who performs the procedure, it is the operator's responsibility to ensure that all procedures are executed correctly.
MEL Structure
MEL/CDL items are structured in ATA (Air Transport Association) chapters for standardization and ease of reference within the aviation industry.
ATA chapters provide a standardized numbering system that categorizes aircraft systems and components into logical groupings. This system helps aircraft manufacturers, maintenance personnel, regulatory agencies, and operators easily locate information related to specific systems or components. Each ATA chapter represents a particular system or component, such as air conditioning, fuel, hydraulics, etc. When creating MEL and CDL documents, organizing items according to ATA chapters allows for consistency across different aircraft types and operators. It simplifies the process of referencing and cross-referencing items, making it easier for maintenance personnel to locate relevant information and adhere to regulatory requirements.
I hope that this clarifies the philosophy behind the categorization of the technical malfunctions, and in the next articles I will give you a high level, general description of the ATA chapter about the aircraft system in question and add some examples of what the operational consequences of a malfunction can be.
Note: As always, I would like to thank my co-writer Terry Mitchell for his contribution to the design of the article and for ensuring readability also for people outside the aviation industry.