Table of Contents

19. The Maintenance

 Quality assurance

Trying an aero engine specific definition of the term maintenance we find in the english language different notions and approaches. On this account in the following will be tried a differentiation with a definition of the terms which apply for this book. It's absolutely possible that we find in the specialist literature also explanations that aberrate.

Technical terms, definitions and decision processes before particular maintenance tasks are discussed in Ill. 19-01 and Ill. 19-02.

Under maintenance we understand the general care respectively servicing and support. In the aviation, especially aero engines it is to be about care and material preservation. To this, we can count also overhaul and repair. In the literature we find often the shortcut MRO for maintenance, repair and overhaul. These themes own chapters are addicted.

Characteristic for the maintenance in a wider sense an oil charge as care or in an extreme case the exchange if an engine as a whole without its disassembly. In contrast maintenance in form of overhaul and repair requires istead the demounting with the disassembly of the engine.

Thus specific for the maintenance is that the work takes place on site at the installed or the demounted engine and the engine is not brought into the (repair) shop. The maintenance takes normally place in periodic intervals as a sort of continuing process in the considerably longer overhaul intervals. In a special action, for example in akute problems, targeted, not usual maintenance steps can also happen short-term out of the normal intervals.

In a maintenance, necessarily it can come to the exchange of aggregates (e.g., gears, pumps and generators) and acessories (e.g., pipe lines). So a limited disassembly is inevitable. This can even concern the core engine if, for example compressor blades have to be exchanged after a FOD.
The maintenance is described and defined in a multitude of instructions as manuals, specifications and certificates (Ill 19-1). To those belong also documents/notes/instructions in a defined form (Fig. "Maintenance and repair FAA forms").

Typical maintenance tasks and work at the engines are:

  • Inspections.
  • Periodic routine work likt the fill-up of oil, the change of filters and their control.Inspection of the magnetplugs.
  • Cleaning like the washing of the compressor or of the hot parts (e.g., injection nozzle).
  • Checking of the fire detectors and smoke detectors with the related cables and connectors.
  • Function control of equipment and components.
  • Identification and elimination of faults and smaller failures like correction of paint/finish deficitsor equalising of small FODs in the compressor according to the requirements of the appropriate manuals respectively the applicable instructions.
  • When there are „bigger“ problems, initiation of suitable measures ,e.g., exchange of parts or aninvestigation (e.g., chips in a filter or at a magnet plug).
  • At acute problems, implementations and tests of controls and examinations like: Boreskope inspektions,non destructive testing like inspections with penetrant, eddy current or ultrasonic.
  • Work within special actions and backfittings.
  • Testing and actualisation of software of the electronic components, especially control units.
  • Exchange of sccessories (e.g., starter/generators) and periphery (e.g., tube lines).
  • Change of engine modules or whole engines.

The task of the elimination deficits and failures requires an important precondition for directed and successfull action.
Sufficient certain identification of the failure cause as well as the causative concerned parts (Fig. "Problem identification during maintenance"). Necessary is the identification and the evaluation of symptoms and changes at the engine (Fig. "Problem identification during maintenance" to Fig. "External noticable critical operation conditions 6"). For it adequate educated, excperienced and alert personnel is needed. So it's not amazing, that in the specification of the profession for aircraft engine repairers and mechanics (Fig. "Job descriptions of repairers") there is tLithe assumption of the knowledge of important technical terms in connection with failures and problems. Naturally this is also true for the inspection and instruction authorised hierarchy (`certifying staff'). With this requests, we are in the important field for the safety of the engines and with it of the aircraft important area „human factors” (chapter 19.1). The analysis of incidents and accidents has shown a crucial influence of the work environment at the human, employed with maintenance.

The qualification of the maintenance employee (job profile) is regulated in instructions which are published and/or approved by the agency, responsible for flight safety and operators (e.g. military). To this belong also instructions about the responsibility especially in the hierarchy.

Further important influences at the human factores are (Ref. 19.1.2-1):

  • Organisation of the work, e.g., EDP-supported systems, field maps, supply of parts, documentation.
  • Sequence of the work: execution, control, change of shift etc.
  • Work conditions like arrangement of the work environment.
  • Presentation of the work documents like manuals and instructions.
  • Maintenance adequate arrangement of the engines ond the environment: accessibility, checkability,exchangeability.

Fundamental terms MRO

Figure "Fundamental terms MRO" (Ref. 19-10): For example the DIN 31051(norm) and the JAR 66, 145, 146 (Ill 19.1.2-1) contain generally accepted definitions of important terms which can also be applied to the region of maintenance, overhaul and repair:

Maintenance: procedures to conserve and restore of the nominal condition as well as its establishment and evaluate of the actual condition. Purpose is the reliability respectively the availability at cost effectiveness.

Maintenance incorporates the following portions:
Inspection: measures to assess the actual condition.
Repair: measures to restore the nominal condition.
Personnel: maitenance is carried out by technicans (repairers, mechanics, engineers).

Definitions of fundamental terms:
Nominal condition: enterity of the characteristic values.
Actual condition: entirety of the characteristic values at a given point of time.
Failure: falling below the design limit (threshold level of damage) of an `attrition reserve' with an impermissible interference of the function.
Attrition: degradation of the' attrition reserve'.
The diagram above shows the changing of the attrition reserve over the operation time. To a new part an attrition reserve of 100% can be assigned. Inspekctions take place in fixed time intervals („1“, „2”, „3“…). Thereby the consumpted attrition reserve (Zi - Z1+1 ) will be determined. The attrition reserve declines over the operation time as respective actual condition. An overhaul must be in time, i.e., before the threshold level of damage is reached. So, with a sufficient safety margin, the breakdown can be avoided.
Disturbance: accidental annoyance/disruption of the function.
Reliability can be characterised with the availability. This is the likelihood of the deviation from the target specification in a predetermined space of time.

In the aviation we find the lower displayed maintenance concept. It's divided in

  • scheduled work (scheduled maintenance). This is carried out after requirements from the OEM for maintenenace, inspection and overhaul.
  • unscheduled work (unscheduled maintenance) in the overhaul/repair. This can be triggered by complaints during the scheduled work, information by the pilot, or by technical departments of the operator. The complaints are documented in forms and so a defined process with the documentation of the work and the checks will be established. To this belongs the determination of priorities and deadlines according to security relevance and the logistic of the spare parts and the exchange parts.

 Maintenance program development

Figure "Maintenance program development" (Ref. 19-10): From the Air Transport Association (ATA) the so called MSG-3 of the „Maintenance Steering Group” (MSG) is issued. In it reside representatives from the airplane-OEM, the operator and the aviation agencies (JAA and FAA). The MSG-3 contains guidelines for the minimum requirements of an overhaul program to meet the airworthiness.
The MSG-3 contains systematic procedures of logic decision processes. This counts especially for the type of inspection.

The shown processes apply for so called „Maintenance Significant Items“ (= MSI) to which also the engines count. MSIs are characterised by

  • Safety relevance,
  • Limitation of the operation,
  • Impact on the economics,
  • Possiblility of hidden failures (during flightnot cognizable).

The left column contains a rating system that uses function, tasks and consequences that leads to the safety classification in so called CAT. Altogether there are 20 assessment steps (Hazard Categories, Volume 1 Ill. 2-3.2). Up to CAT 4 it's to be about classifications where the safety, respectively the default risk during operation stands in the foreground.

Above that CAT 5 and CAT 8 (grey areas) have priority because their safety relevance.
From this the required actions are determined equivalent to the right column. Thereby the individual consequences of a functional failure will be evaluated by the possibilities of the overhaul.

Concluding the deadlines/time limits for the necessary work must be determined. Thereby the experiences of the operator and the OEM are used.

 Maintenance instructions and specifications

Figure "Maintenance instructions and specifications":

Aircraft Safety Alerts (Ref. 19-1) are informations and instructions about operation experiences in the aviation. They serve the safety and the procuct reliability. The alerts are issued, generated and published by aviation authorities, for example the FAA (Federal Aviation Administration). Basis are released informations of the operators and the `shops'. Those documentes usually can be received by internet from the responsible aviation authority. Forms are available by internet from the FAA (Fig. "Maintenance and repair FAA forms"). To Aircraft Safety Alerts count the following aero engine relevant documents:

Airworthiness Directives (ADs) are instructions accordant to the Federal Aviation Regulation Part 39 for operators of distinct aircraft types, aero engines or systems (Fig. "Problem identification during maintenance"). Also aviation authorities of other countries edit AD - equivalent documents. For example the German Luftfahrtbundesamt (LBA) in form of a Lufttüchtigkeitsanweisung (LTA). Usually those are based on a message of a malfunction or a failure report (defect report) in a (not obligatory) form (Fig. "Maintenance and repair FAA forms"). ADs are issued by the responsible aviation authority if an unsafe situation is identified. An AD describes measures, work, inspections, situations/conditions and limits if the concerned products can be further used. It's forbidden to operate a component that does not accords to the requirements of the AD. The attention respectively execution of an AD must be suitable documented (Ref 19-2).

We differ:

  • Voluntary ADs: The operators can decide about the application themself.
  • Mandatory ADs: The operator must follow the directive in the predetermined time frame.
  • Emergency ADs: The operator has to follow the directive immedietely.

Unapproved Parts Notification (UPN) are notes (assumed) not approved parts and components (suspected unapproved parts = SUPs, see chapter 20.2.1). Those notes with instructions to the aviation authorities, expecially the FAA, are possible by the internet with the there available forms (Ref. 19-6). They deal with

  • Identification and reporting,
  • Relevance and assessment,
  • Aircraft parts, aero engines and propellers,
  • Quality and identification of approvedaviation parts.
  • Spare parts,
  • Report program for voluntary messages.

Special Airworthiness Information Bulletin (SAIB) is an information tool that warns, instructs and gives recommendations to the aviation. Those contain nonbinding informations and instructions which don't justify an AD (Ref. 19.1-5).

(Aviation) Maintenance Alert: Those warnings give the possibility to exchange experiences from the operation (of the aero engines). Aspired are icreased durability, reliability and safety. Basis are informations from employees out of the operation and the Maintenance.

Service Difficulty Alert (SDA) or Service Difficulty Report (SDR): This Alert (warning) points at a potencial danger (example Ref. 19-3). An Alert is frequently triggered by the report of a reporting system and issued by the responsible aviation authority. This is no duty action and doesn't necessarily demand an AD-note.

A multitude aero engine specific documents are used during maintenance and overhaul. They partly need a authority permission and/or an approval by the OEM. They shall guarantee the required safety of the work, hardware and the operation. The validity is approval specific, e.g., for the operator and/or the (repair) `shop'. Typical documents are:

Maintenance Manual (MM, chapter 19.1.3): It contains accepted and recommended processes and procedures, specific for the mounted (on wing) engine type.

Engine Manual (EM, overhaul manual): Usually it covers binding the activities on the dismounted aero engine. Its structure is prescribed (e.g., ATA-system). Overhaul manuals are normally published by the OEM and must not be changed without permission. Besides disassembly instructions, repair limits (chapter 21.1) and processes to apply are contained. In an illustrated description of the engine parts (parts catalogue =IPC, parts list) all visible parts of the aero engine are listed respectively described with sketches.

Standard Practices Manual: Contains accepted (by the OEM and/or responsible authorities) processes and practices. Thereto belong the necessary informations, instructions and frequently uses processes.

 Maintenance and repair FAA forms

Figure "Maintenance and repair FAA forms": The safety of airplanes is in a high degree governed from the function safety of the engines. For this the communication with operators, overhaul shops and repair shops about safety relevant occurrences and/or observations is of particularly importance. Appropriate informations are facilitated and intensified with the use of the internet. There can also be found instructions in connection with acceptance persons and/or inspection personnel as well as for the certification of tasks like repair procedures. The responsible authorities offer in their web sites („Home Pages”) forms and guidances for an optimal message. Exemplary are forms of the US aviation administration FAA. We see a plurality of problem specific informations, which enable the user a directed information to the responsible agency.

Figure "Problem identification during maintenance" (Ref. 19-7): The described case can be exemplary for an AD. It adresses an acute problem in connection with an engine type, which is used in many airliners. Because of the urgency, from the European Eviation Safety Agency (EASA) an „Emergency AD“ was published. This happens without pre-publishing for comments of the concerned operators, the maintenance and the shops. Short term the safety is ensured by suitable measures.

The AD-note contains amongst others detailed instructions about actions and assignments (action & compliance) about:
Application with information of all potential concerned engine types and airplane models, an explanation of the grounds for this instruction (see text in the Ill.) and the validity date.

The execution in the case at hand will be described with informations of the following steps:

  • Identification of the concerned engines.
  • Briefing of non-partisans about the safetycontrol of the oil filling cover.
  • Replacing of concerned oil filling covers.

How are conclusions possible about the cause of a problem from observations and symptoms possible in line with the maintenance

 Observation of problem symptoms during maintenance

The safety and effectivity of maintenance work depends essential from an identification and correct analysis of the problem as soon as possible. To this belong:

  • Assessement of the safety relevance. To this belong possible failure chains and results.
  • Identification of causal and contributory influences and components. Therefore is the knowledgeof the failure mechanism especially important.
  • Definition of suitable measures and its execution with the responsibles on site, consulting the OEMand responsible authority.

The Pilot respectively the crew get important informations from observations during operation. They must, possible without time delay draw conclusions and initiate remedies (Fig. "Problem symptoms registered by the user"). To this belong type specific experience and expertise.
From the observations of the crew the maintenance personnel can also often afterwards correlate causes. That improves the likelihood of an identification of the concerned components and the causes and so it servs the safeguard of measures in the maintenance phase. .
For this the maintenance personnel must have as much as possible expertise. The illustrations 19-5 up to 19-11 schould not only give a first aid to the crew, but also for later analysis of the symptoms.

 Problem symptoms registered by the user

Figure "Problem symptoms registered by the user" (Ref 19-8): This picture shows, that some symptoms can „fit” to severqal causes, that means they are ambiguous. In such cases a conclusive analysis is understandably limited. Thereby the danger of faulty or not sufficient conclusions exists. The problem can even worsen by the introduced measures (disimprovement, chapter 21.3.3 and volume1 Ill. 3-3). An example is the shut down of the wrong, unproblematic engine.

 External noticable critical operation conditions 1

 External noticable critical operation conditions 2

 External noticable critical operation conditions 3

 External noticable critical operation conditions 4

 External noticable critical operation conditions 5

 External noticable critical operation conditions 6

 External noticable critical operation conditions 7

Illustrations 19-5, 19-6, 19-7, 19-8, 19-9, 19-10 and 19-11 (Ref. 19-8): In fact the literature on which this compilation is based is designed for the crew of airliners. So, at problems during operation, spontaneous as much as possible suitable measures with enough safety should be enabled.
As symptoms a crew has as well visual, acustic and smell anomalies as also indications by the cockpit instruments.

The classification in the pictures uses several steps:

  • Identification of possible causes and verification with the observations. To this belong:
  • Typical operation condition,
  • features, symptoms,
  • suitable (remedy) measures,
  • possible results of the measures,
  • risks of misinterpretations.


19-1 Federal Aviation Administration „Aircraft Safety Alerts“, Update August 11, 2005.

19-2 M.J.Kroes, T.W.Wild, „Aircraft Powerplants” Seventh Edition, Page 240.

19-3 Transport Canada, „Service Difficulty Alert“, „Honeywell T54 Series Engines, Engine Fuel Control Unit, Drive Shaft and Spline Wear”. Sep 28, 2004.

19-4 Federal Aviation Administration „SUP-Related Advisory Circulars“, Update October 11, 2005.

19-5 Federal Aviation Administration „Special Airworthiness Information Bulletins (SaAIB)”, Update August 08, 2006.

19-6 Federal Aviation Administration „SUP-Related Advisory Circulars“, Update October 11, 2005.

19-7 Emergency Airworthiness Directive der EASA „AD No: 2005-0025” October 26, 2005. ATA 79 „Engine Oil tank-Oil filler cap assembly - Inspection/Replacement“.

19-8 C.Brady. „Turbofan Engine Malfunction Recognition and Response” The 737 Information Site http://www.b.737.org.uk/, 1999.

19-9 Transports Canada, Zeitschrift „Feedback, Canadian Aviation Service Difficulty Reports“, Issue 1/2003, Page 25-30.

19-10 S.Schwab, „Neugestaltung und Anpassung eines Instandhaltungsprogramms für die McDonnell Douglas MD - 11 auf LTU Standard” ,Diplomarbeit an der FH Aachen, Oktober 1996, Page 14 - 49.

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