The concept and principles of helicopter maintenance

FOR TRAINING PURPOSE ONLY Malaysian Institute of Aviation Technology

WEEK 4:THE CONCEPT AND PRINCIPLES OF HELICOPTER

MAINTENANCE

SABRI OMAR


LEARNING OBJECTIVE & OUTCOMEOBJECTIVE:

1. GAIN KNOWLEDGE ON CONCEPT AND PRINCIPLE OF HELICOPTER MAINTENANCE ACTIVITIES.

OUTCOME:

1. DESCRIBE WHAT IT MEANS BY HARD TIME, CONDITION AND CONDITION MONITORING.

Allen Peter Diman


DEFINITIONS OF HELICOPTER MAINTENANCEThe aircraft is delivered to the airlines along with the Approved Maintenance Schedule. Deterioration of the aircraft due to use and time factor cannot be avoided. If it is allowed to remain unchecked, will lead to non-conformity of its Type Certificate conditions. Or it may even lead to hazardous as well as a catastrophic event. Maintenance activities rectify these conditions and restore the aircraft airworthiness.

Maintenance is the actions necessary to sustain and restore the airworthiness of aircraft, its engines and equipment. Primary maintenance activities are

1. Inspection 2. Overhaul 3. Repair 4. Replacement 5. Modification

of aircraft, its engines and equipment to ensure conformity with the aircraft’s type certificate.

Allen Peter Diman


Maintenance Program Evolution process Allen Peter Diman


The objective of maintenance program evolution is to maintain safety, reliability, at minimum cost. The main means to achieve this is to adjust the initial/current selected intervals. Such exercise is launched when the A/C manufacturer and the ISC consider that sufficient in-service experience is collected, supported by the MRB as advisor.

Airlines report in-service experience in the form of “Nil findings/details of findings” reports for each task. Then all tasks are reviewed individually in MWG meetings.

Allen Peter Diman


MAINTANENCE PROCESSES

Maintenance programs and processes control the maintenance activities of the operator and its fleet of helicopters

There are three primary maintenance processes:

1. Hard time

2. On condition

3. Condition Monitoring• Hard Time and On-Condition

both involve actions directly concerned with preventing failure, whereas Condition Monitoring does not.

However the Condition Monitoring process is such that any need for subsequent preventative actions would be generated from the process.

Allen Peter Diman


Hard Time

This is a life-based concept.

Oldest maintenance process.

Rooted in the assumption that reliability decreases with increased operating age.

Its intended purpose is to prevent failure.

This is a preventative process in which known deterioration of an Item is limited to an acceptable level by the maintenance actions, which are carried out at periods related to time in service.

Hard time applies a fixed time and/or cycles (e.g. calendar time, number of cycles, number of landings) that an item is permitted to operate on an airplane. Upon reaching the limit it must be overhauled or replaced (discarded). This includes:

1. Simple items subject to only one failure mode

2. Components or assemblies which have definite life limits (for example, metal fatigue) or whose failure could have a direct adverse effect upon safety if they malfunctioned in flight

Allen Peter Diman


Hard Time Components

The prescribed actions normally include Servicing and such other actions as Overhaul, Partial Overhaul, replacement in accordance with instructions in the relevant manuals, so that the Item concerned (e.g. system, component, portion of structure) is either replaced or restored to such a condition that it can be released for service for a further specified period

Allen Peter Diman


The general application of the hard time process became outmoded as the industry matured and aircraft became more complex. A methodology evolved that is oriented toward mechanical performance, Reliability-Centered Maintenance (RCM).This is an analytically based concept designed to realize the inherent reliability of a design.

It accepts that the operation of a component or system may fail between required inspections, and that the airplane may be safely operated until the next inspection reveals the failure. Its application is, therefore, limited to items whose failure during airplane operation will not have catastrophic consequences. RCM uses two dominant processes.

Allen Peter Diman

Reliability-centered maintenance (RCM) processes

On condition Condition monitoring


On conditionThis also is a preventative process but one in which the Item is inspected or tested, at specified periods, to an appropriate standard in order to determine whether it can continue in service (such an inspection or test may reveal a need for servicing actions). The fundamental purpose of On-Condition is to remove an Item before its failure in service. It is not a philosophy of 'fit until failure' or 'fit and forget it.

On condition avoids predicting hard time failure wear-out points. Rather, repetitive inspections or tests that detect potential failures are adopted. These tests call for the removal or repair of individual components "on the condition" that they do not meet a defined standard of performance.

Allen Peter Diman


A determination of continued airworthiness may be made by visual inspection, measurements, tests, or other means without a teardown inspection or overhaul. The checks are performed within the time limitations prescribed for the inspection or check. Performance tolerances and wear or deterioration limits are defined in the Approved Maintenance Schedule.

On-condition maintenance can involve bench tests and is thus not restricted to on-wing inspections. However, on-wing inspections/tests are preferred. On condition considers specific failure modes. It is based on the likelihood of defining some physical evidence of reduced resistance to the failure mode in question. Until that evidence is present, units remain in service.

Allen Peter Diman

On Condition


Examples ofItems under On Condition

Allen Peter Diman


Condition Monitoring

This is not a preventative process, having neither Hard Time nor On-Condition elements, but one in which information on Items gained from operational experience is collected, analyzed and interpreted on a continuing basis as a means of implementing corrective procedures.

The process applies to items that show deterioration over time.

It consists of observing deterioration of a component or system as it trends toward failure.

Explicit operating parameters of the device, which are indicative of deterioration or wear, are selected. Collecting and interpreting these data “monitors the condition" of the device.

Allen Peter Diman


This form of condition monitoring is best exemplified by engine condition monitoring. Parameters such as altitude, Mach number, inlet pressure and temperature, N1 and N2, burner pressure, and EGT are collected.

These gas path data are normalized and plotted against time. They are compared against known specific deterioration pattern failure modes evidenced by these parameters. These are coupled with oil sample and vibration analyses.

Allen Peter Diman

Accurate identification of incipient failures is thus possible, thereby allowing economical repair before the occurrence of extensive costly damage; it is most beneficial with high-cost items such as engine components. Removal, disassembly, or inspection is not required.


Allen Peter Diman

Honeywell Health and Usage Monitoring System


Maintenance Program

The maintenance program is developed concurrently during type certification. The parent document that results is the Maintenance Planning Document (MPD). There are two primary methods in which the maintenance program is developed.

They are 1. Hard Time methodology for

pre1980 certified aircraft 2. MSG-3 Condition

Monitored Maintenance for post 1980 certified aircraft

This philosophy was however initially only applied to airliners such as Boeing 747. Helicopter comes much later

Allen Peter Diman


Hard Time MethodologyThe manufacturer will analytically determine the maintenance processes required by each aircraft parts, components and engine. The basis of determination will be summed up as follows: Hard time items are:

1. Where the failure of the Item has a direct adverse effect on airworthiness and where evidence indicates that the Item is subject to wear or deterioration.

2. Where there is a 'hidden function' i.e. components that does not give any cockpit indications of its operational status; which cannot be checked with the Item in-situ.

3. Where wear or deterioration exists to such an extent as to make a time limit economically desirable.

4. Where component condition or 'life' progression sampling is practiced.

5. Where limitations are prescribed in a Manufacturer's Warranty.

Allen Peter Diman


On-Condition items are:

1. Where an inspection, or test of an Item to a prescribed standard (frequently in-situ) will determine the extent of deterioration, and hence the 'condition', i.e. any reduction in failure resistance.

Condition Monitoring items are:

1. Where a failure of an Item does not have a direct adverse effect on operating safety, and where Hard Time and On-Condition processes are not prescribed and no adverse age reliability relationship has been identified as the result of analysis of the data arising from a formalized monitoring procedure or program.

Allen Peter Diman


Condition Monitored Maintenance (MSG-3)

Condition Monitored Maintenance, as a program, is the formalized application of the maintenance processes Hard Time, On-Condition and Condition Monitoring to specific items as prescribed in the Approved Maintenance Schedule.

The controlling activity of Condition Monitored Maintenance is Condition Monitoring irrespective of whether Condition Monitoring is prescribed as a primary maintenance process in the Approved Maintenance Schedule or not.

Allen Peter Diman


Condition Monitoring is repetitive and continuous, the key factor in its use being the introduction of aircraft embodying failure tolerant designs, which allow for replacement of some traditional failure preventative maintenance techniques by non-preventative techniques.

Condition Monitoring is not a relaxation of maintenance standards or of airworthiness control; it is, in fact, more demanding of both management and engineering capabilities than the traditional preventative maintenance approaches. Each Condition Monitored Maintenance Program is required to be approved by the Airworthiness Authorities.

Allen Peter Diman


SCHEDULED MAINTENANCE PROGRAMME (MSG-3)

The Hard Time methodology is very expensive and did not significantly improve maintenance. Consequently, Condition Monitored Maintenance is currently preferred. Standards adopted are formally known as MSG-3.

MSG-3 currently includes three revisions, the most recent adopted in 1993. Airplanes certificated during the 1980s and 1990s use MSG-3 for the development of scheduled maintenance tasks. MSG-3 is dominantly task oriented rather than process oriented.

Allen Peter Diman


MRB PROCESS

EASA and FAA require the A/C Type Certificate holder to prepare and revise the initial minimum scheduled maintenance requirements that are applicable to a dedicated aircraft (Regulatory Requirement CS/FAR 25.1529). This document is called the Maintenance Review Board Report (MRBR), and provides the scheduled maintenance tasks and their frequencies (intervals) for the aircraft systems (including engine), structure and zones. MRBR development is based on the MSG-3 method.

Allen Peter Diman


Allen Peter Diman

Maintenance Review Board Process


Objectives

The objectives of scheduled maintenance are to

1. Ensure safety and reliability of aircraft

2. Restore safety and reliability that was lost by deterioration

3. obtain sufficient necessary to improve items with inadequate target reliability

4. Accomplish these goals at minimum costs including maintenance and costs of resulting failure

Allen Peter Diman


Bell engineers were involved from the start and were able to steer the direction of the design. From the routing of systems to the placement and size of maintenance panels, this group has helped develop an aircraft that is more efficient and easier to maintain. One result is that all of the 429’s systems are routed outside of the cabin, either above the roof beams or below the floor structure. The interior of the 429 does not need to be removed during an annual inspection and that cuts time and cost.

Allen Peter Diman

The 429 is the first, and, so far, the only helicopter designed using the Maintenance Steering Group 3 (MSG-3) process that has been the foundation of airliner reliability since 1970. Using this process, lead mechanics from the customers, representatives of Transport Canada, the FAA and EASA, and


Benefits of implementing MSG - 31. Maximizes aircraft availability

• Major Tenet of Aircraft Availability Improvement Plan (AAIP)

• Extended inspection intervals frees up assets

2. Safeguards inherent safety and reliability

3. Ensures Operational Safety, Suitability, & Effectiveness

4. Reduces Costs / Cost Avoidance

5. Creates program credibility and instills confidence by involving all stakeholders

6. Integrates all levels of maintenance activity

7. Outcome has logic that is defensible at all levels of scrutiny

8. Assures that all areas of the aircraft are thoroughly covered and have the proper level of inspection

Allen Peter Diman


HOW MAINTENANCE ON HELICOPTER IS BEING CLASSIFIED

Allen Peter Diman


TYPES OF MAINTENANCE ON HELICOPTER

Allen Peter Diman


Allen Peter Diman


Allen Peter Diman


Allen Peter Diman

Engineering

The concept and principles of helicopter maintenance