EMBRAER Perspective on the Challenges for the Introduction of

Scheduled SHM (S-SHM) applications into Commercial Aviation

Maintenance Programs

Luís Gustavo dos Santos1, a

1Rua José Alves dos Santos 391 Apto.162, Floradas de São José, 12230-081, São José dos

Campos, São Paulo - Brazil

aluis.santos@embraer.com.br

Keywords: List the keywords covered in your paper. These keywords will also be used by the publisher to produce a keyword index.

Abstract. This paper presents an overview of the challenges an original equipment manufacturer

(OEM) such EMBRAER may face to introduce scheduled structural health monitoring (S-SHM)

applications in the maintenance programs of its commercial aviation aircraft models. S-SHM

solutions have the potential to reduce aircraft operators direct maintenance costs and fleet downtime

while keeping aircraft airworthiness at a minimum maintenance downtime and costs. As part of new

approach in terms of scheduled maintenance practices, the replacement or complementation of

traditional structural inspections tasks by new maintenance procedures taking credit of SHM

technologies must be done in ways that meet the expectations and requirements of Regulatory

Authorities, OEMs and airlines maintenance and engineering departments related to topics such as:

safety, continued airworthiness, cost/benefits ratio, S-SHM systems’ built-in redundancies and

reliability to support higher fleet availability, as well as necessary mechanics qualification. Besides

the efforts for validation, verification, qualification and certification of such systems to deliver the

expected effectiveness levels to verify structural integrity and withstanding the operational

conditions to which it will be exposed, an OEM intended to offer their customers with the benefits

of S-SHM solutions will be required initially to revise its policy and procedures handbooks (PPH)

to adopt the new S-SHM Air Transport Association’s Maintenance Steering Group 3 (MSG-3)

Methodology guidelines. This will alter in different ways the current Maintenance Review Board

processes conducted by each OEM to develop and revise the minimum scheduled maintenance

program for a given commercial aircraft type certificate. The contents of the Maintenance Review

Board Reports (MRB) will need to be revised in order to clearly indicate the scope and frequencies

of each approved S-SHM task, and how they will replace, complement and/or be an alternative

means of compliance of the more traditional maintenance tasks types such as general and detailed

visual inspections (GVI and DET, respectively). Additionally, the Airplane Maintenance Manuals

(AMM) will need to be revised to include specific S-SHM procedures on how to perform the

intended inspection, how to proceed when degradation is detected in the monitored structures and

how to repair such systems in case of failures.

Introduction

Maintenance activities are responsible for at least 12% of airline direct operational cost (DOC)

[1]. Although other cost drivers such as aircraft ownership and fuel represent a much larger portion

of DOC (refer to figure 1), reduction of maintenance costs are always a fundamental part of any

airline plan to increase competitiveness and profitability.

An original equipment manufacturer (OEM) such EMBRAER is continuously looking for

opportunities to respond to this need of maintenance cost reduction by its customers. One possible

way to accomplish this is to reduce the complexity of the scheduled inspection tasks as well as the

necessary time to execute them.

Key Engineering Materials Vol. 558 (2013) pp 323-330Online available since 2013/Jun/27 at www.scientific.net© (2013) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/KEM.558.323

All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 193.140.216.7, Hacettepe University, Ankara, Turkey-20/12/14,04:17:33)

Figure 1: Direct Operating Cost Breakdown

When considering that at least half of all maintenance activities of any commercial aviation

aircraft model is related to the inspection of metallic and composite structures, technologies such as

the Structural Health Monitoring (SHM), if correctly integrated as part of the scheduled structural

maintenance program , may have a significantly contribution to the reduction of operators DOC and

their fleet downtime. Nevertheless, there are several challenges for this to happens, starting with the

fact that it is not necessarily clear to airlines what would be the real benefits of introducing SHM in

their operations and how such applications can be implemented successfully in their aircraft and

maintenance programs.

Regarding the benefits of introducing SHM in commercial aviation, Renee et al. [2] provided

cost benefits analysis for 3 different structures (trailing edge, vertical stabilizer and engine mount),

as well as indicated the benefits from a non-economic perspective, showing that a significant

reduction in the life cycle cost could result in an realistic return on investment. According to this

study, for a 30 to 40% replacement of traditional maintenance requirements by SHM solutions, the

time to recover the cost of the initial investment for both the engine mount and the trailing edge

structure would be 2 to 3 years.

Although Renee et al. [2] recognize that their study was limited in some aspects, we can

consider it a valid indication that there are ways to OEMs to develop consistent business cases

around the SHM in order to deliver cost effective solutions for their customers. Therefore, it is

important to focus now on how SHM applications can be successfully implemented by airlines in

their fleets as well as in their correspondent maintenance programs. This will be done in the next

sections of this paper.

Challenge 1: Replacement of traditional structural inspections by new SHM maintenance

procedures

The first part of a successful implementation of SHM solution in a commercial aviation program

is to design structural monitoring systems that can be effectively certifiable by OEM as part of the

type certificate of their aircraft models, or at least, as part of a supplemental type certificate.

Additionally, the certification of such solutions must happens in a way that make it possible to

OEMs to effectively replace or complement current structural inspections procedures (such as

General Visual, Detailed and Special Detailed Inspections – respectively GVI, DET and SDI) by S-

SHM procedures.

The concept of S-SHM is already included in A4A MSG-3 since release 2009.1 [3], but by itself

it was not considered sufficient to convince OEMs to bear the development and certification costs to

make real S-SHM solutions available to their customer.

As the use of S-SHM technologies are part of a new approach for scheduled maintenance

practices, its acceptance by Regulatory Authorities may generate additional difficulties to OEMs

during the naturally demanding, but well known, certification process. Therefore, it is fundamental

that the OEM can refer to clear requirements or guidelines on how to develop and certify such

systems.

This is being addressed currently by a collaborative effort of several members of the SHM

community under the coordination of the Society of Automotive Engineers (SAE) that will

culminate in the issue of the new Aerospace Recommended Practice (ARP) 6461 - Guidance on

Structural Health Monitoring for Aerospace Applications.

Once issued, SAE ARP 6461 will hopefully serve as a common reference for OEMs, Regulatory

Authorities and other parties involved in the development and certification of SHM solutions for a

given commercial aviation program.

324 Structural Health Monitoring: Research and Applications

In EMBRAER perspective, it is fundamental that SHM community increase its support to the

continuous improvement of SAE ARP 6461 contents in order to convince regulatory authorities that

all relevant aspects related to S-SHM systems safety, reliability and continued airworthiness to

replace or complement traditional structural inspections tasks were properly covered. This will

ultimately support the activities of systems and structural working groups involved in the MRB

process of a given aircraft program to revise the applicable analyses to select S-SHM tasks and

finally start taking advantage of the introduction of SHM concepts in the MSG-3 methodology since

2009.

Challenge 2: How to adjust the process to develop a structural maintenance program in order

to introduce scheduled SHM tasks?

A scheduled SHM (S-SHM) task is defined as the act to use, run or read-out a SHM device at an

interval set at a fixed schedule [3]. As explained by Wenk [4], the introduction SHM concepts on

the MSG-3 methodology, which is used to develop maintenance programs for commercial aviation

aircraft models, was the result of a collaborative effort of OEMs and other representatives of the

SHM community.

A general approach to gradually introduce SHM solutions into scheduled maintenance programs

was proposed by Santos [5], where it was mentioned that to successfully introduce a SHM system

into a commercial aviation maintenance program, SHM system developers must remember that as

per 14 CFR part 25 § 25.1529 and Appendix H [6], instructions for continued airworthiness must be

developed for the aircraft and therefore, SHM systems must be properly evaluated in this aspect as

part of the MRB process.

As the MRB process is governed by a Policy and Procedure Handbook (PPH), it must be adapted

in order to allow selection of S-SHM inspections instead of traditional tasks.

An EMBRAER PPH complies with FAA Advisory Circular 121-22A [7] and is a guide to be

used by the EMBRAER, customer airlines and regulatory authorities (RA) representatives during

the MSG-3 process. The MRB process activities are conducted by different Working Groups (WG),

supervised by an Industry Steering Committee (ISC). The PPH states the policies and procedures to

be followed by these groups for development of the initial minimum scheduled maintenance

requirements and their subsequent revisions. Figure 2 summarizes the MRB Process.

Figure 2: MRBR development process

On EMBRAER perspective, the PPH contents that will be affected by the introduction of the S-

SHM concept are primarily related to technical and maintenance development training to

participants of the MRB process, system and structural analysis procedures that must be followed by

WG members and verified by the ISC, industry and regulatory authorities participation roles in this

process.

Key Engineering Materials Vol. 558 325

Maintenance Development and Technical Training. An OEM must provide MSG-3

maintenance task development training to all ISC, MRB Advisors and WG members (including

representatives of vendors or OEM partners) that will participate in a given Maintenance Review

Board. Different methodology and documentation to conduct a MSG-3 analysis (Structures, System

and Powerplant, Zonal etc) must be explained to the training participants. Therefore, the concept of

S-SHM and additional related information must be covered by the training material so this new task

type will be considered by MSG-3 analysts while performing their activities and will be properly

evaluated by the other WG and ISC members.

Additionally, OEM and its partners/vendors must also provide general familiarization training

about the aircraft model being evaluated, including specific lectures with the purpose of analyzing

the design and function of the systems and structures, and discussing failures, their annunciations

and consequences. In these training it is fundamental that the details of each S-SHM solution being

implemented are made available for all MRB Advisors and WG members.

Structural Analysis Procedures. MSG-3 procedures for structural analysis are aimed to

developing an effective aircraft structural maintenance program to detect and prevent the

degradation caused by fatigue, environmental deterioration, or accidental damage. Once the OEM

identify the structural significant items (SSI)1, it is necessary to define what will be the inspections

the cover the remaining structures (designated “other structures”, in opposition to the definition of

SSI). This is the first opportunity to use of S-SHM tasks instead of traditional inspection as they will

not cover, by definition, critical structural elements, and therefore, the selection criteria to accept S-

SHM tasks will be naturally less restrictive than to cover SSI. Therefore, the OEM should clearly

indicate in the part of the PPH how to identify and document the inspection requirements for the so

called “other structures” and what criteria the Structural WG and ISC members must use to verify if

a given S-SHM system designed to be installed in some or all of those “other structures” is able to

deliver equivalent probability of detection (POD) and confidence level that the traditional inspection

that was select to cover those structures. Fatigue effect must be then checked for the damage tolerant SSI. Fatigue Damage (FD) is the

initiation and subsequent propagation of a crack or cracks, due to cyclic loading, with a cumulative effect with respect to aircraft usage (flight cycles). The Damage tolerance and fatigue analysis for fatigue damage is quantitative and assumes a precise knowledge of crack growth, residual strength, and detectability. In addition to the damage tolerance analysis, an FD rating is developed which considers the following indexes: SSI Visibility (related to the visibility of the structure for inspection), sensitivity to damage propagation (established basically by the stress level and the material of the SSI), estimated residual strength after fatigue damage and probability of crack initiation.

Environmental deterioration and accidental damage must be evaluated for all SSI. The environmental deterioration (ED) analysis procedure considers that the aircraft structure may be subject to damage caused by its exposure to adverse environments such as cabin condensation, galley spillage, toilet spillage, cleaning fluids, among others, as well as by the material susceptibility to corrosion and stress corrosion, and the protection applied to the structural components. The environmental deterioration may be time dependent or may be a random discrete event. For the damage susceptibility assessment and its timely detection, the following indexes are considered: SSI visibility, sensitivity to corrosion/deterioration (metal and non-metals), protection (of the structure against the environment) and environmental effects (related to the exposure to adverse environment).

The Accidental damage (AD) analysis procedure considers that the aircraft structure may be subject to damage caused by the contact or impact with foreign objects, or caused by inadequate operation or maintenance practices. The accidental damage analysis takes into account the

1 A SSI is any detail, element, or assembly which contributes significantly to carrying flight, ground, pressure, or control

loads, and whose failure, if it remains undetected, could affect the structural integrity necessary for the safety of the

aircraft.

326 Structural Health Monitoring: Research and Applications

susceptibility of each SSI to accidental damage based on damage exposure frequency and location of one or more sources of damage such as ground/cargo equipment, foreign objects, erosion from rain, hail, lightning, runway debris, spillage, water entrapment and human error during aircraft manufacture, operation, or maintenance. To develop an efficient analysis, the following aspects must be considered: SSI visibility, sensitivity to damage propagation, estimated residual strength after damage and likelihood of damage.

After completion of FD, ED and AD analysis the associated inspection tasks and its subsequent

intervals and thresholds shall be established for each item of analysis. Then, the results are

consolidated in order to select, for each item of analysis, the final requirement that shall be included

in the MRB report proposal. The requirement is composed of threshold (T) and repetitive interval

(I).

When the OEM receives the approval of its damage tolerance analysis (DTA) report by the

appropriate regulatory authority (certification) and the latest results of the on going full fatigue test,

all fatigue limitations generated by the damage tolerance analysis, for the SSI’s categorized as an

airworthiness limitation item (ALI), shall be evaluated by the WG, in order to determine if the

correspondent inspection level remains practical and effective. Subsequently, the FD tasks which

are associated to any limitation generated by DTA (SSI inspection level, threshold, and interval),

shall be classified as an ALI and be listed in a dedicated section or appendix of the MRBR.

The final goal of the previous procedures is to determine which task type, interval an threshold

are applicable and most effective to limit or eliminate the effects of fatigue, environment and

accidental damages. Therefore, as indicated before for the “other structures”, the PPH must clearly

indicate which criteria must be used to verify if a S-SHM task can provide equivalent probability of

detection (POD) and confidence level that the correspondent traditional inspection that was select to

cover those structures.

System Analysis Procedures. MSG-3 procedures for system and powerplant analysis provides

procedural steps for the development of tasks and determination of their intervals, associated with

the aircraft certificated operating capabilities. Each proposed task must be assessed in accordance

with an applicability and effectiveness criteria. Similarly to the structural analysis procedure, it is

necessary to define maintenance significant items (MSI)2 among all systems of the aircraft model

being evaluated. In EMBRAER perspective, any pure S-SHM systems design will never be

classified as a MSI and therefore, no MSG-3 system analysis will be required. Consequently, no

scheduled task would be selected for the S-SHM systems components that will be installed in the

aircraft. Nevertheless, EMBRAER understands that built-in tests (BIT) must be implemented as part

of any ground support equipment (GSE) that will interrogate the S-SHM sensors installed in the

aircraft, in order to evaluate if the data collection and analysis that will be performed as an

inspection tasks are valid.

Industry and Regulatory Authorities participation. The ISC and WG are formed by

representatives of aircraft operators, OEM, and its partners and major vendors. It is chaired by an

airline representative and co-chaired by an OEM representative, but regulatory authorities’

representatives are included as advisors to the ISC and WGs. Regarding the S-SHM, the ISC

participation must ensure that the Structural WG properly evaluated the use of available S-SHM

systems to replace or complement traditional inspections, in accordance with the PPH guidelines.

The regulatory authority maintenance review board (RA MRB) has the authority to approve the

proposed initial inspection requirements that will be published in the MRB report. This board is

formed by the MRB Board Chairman and MRB Executive Chairman (both are representatives of the

regulatory authority of OEM’s country), foreign authorities’ representatives, MRB members and

advisors, and ISC Chairman and Co-Chairman. There are two responsibilities that have a direct

2 A MSI is one whose failure could affect safety, be hidden to the operating crew (pilots and flight attendants), or have a

significant potential economic or operational impact.

Key Engineering Materials Vol. 558 327

effect on the acceptance of S-SHM tasks as part of MRB Reports: A. To review and confirm

acceptance of the PPH (concurrently approved by the ISC and obviously containing all the

guidelines for the evaluation of S-SHM tasks), and B. Offer guidance and assistance to the ISC and

WGs, including the aspects/requirements that will be observed in order to accept S-SHM tasks as

replacement or complement of traditional ones.

In our perspective the best way to make what was suggested in this subsection is that the SHM

community increases the involvement of representatives of the major regulatory authorities around

the world, to promote the structural health monitoring solutions not as a promise for the future but

as a concrete option for improvement of the maintenance programs of the current commercial

aviation fleet.

Challenge 3: Changes on the Instructions for Continued Airworthiness

The instructions for continued airworthiness (ICA) of a given airplane must contain a series of

manuals containing, but not limited, the following information [6]:

A. Scheduling information (maintenance instructions) for each part of the airplane and its

engines, auxiliary power units, accessories, instruments, and equipment that provides the

recommended periods at which they should be cleaned, inspected, adjusted, tested, and lubricated,

and the degree of inspection, the applicable wear tolerances, and work recommended at these

periods.

B. Airplane maintenance manual, introducing information that includes an explanation of the

airplane's features and data to the extent necessary for maintenance or preventive maintenance.

C. Details for the application of special inspection techniques including radiographic and

ultrasonic testing where such processes are specified.

D. A section titled Airworthiness Limitations that must set forth each mandatory modification

time, replacement time, structural inspection interval, and related structural inspection procedure

approved under 14 CFR part 25 §25.571, and each mandatory replacement time, inspection interval,

related inspection procedure.

Therefore, to use S-SHM solution as part of any commercial aviation maintenance program it is

necessary to revise the correspondent ICA with information related to S-SHM inspections. The

MRBR, maintenance planning document (MPD) and the aircraft maintenance manual (AMM) are

some of the ICA used by EMBRAER in its programs and in our perspective it would not be

necessary dramatic changes in their current layout and content to cover the S-SHM solutions. In the

following paragraphs it will be presented more details about the possible changes in the above

mentioned documents for the introduction of S-SHM. Other considerations may apply for other

ICA, such as the ones related to the airworthiness limitation items, but will not be discussed in this

paper.

Maintenance Review Board Report (MRBR). The contents of the MRBR will need to be

revised in order to clearly indicate the scope and frequencies of each approved S-SHM task, and

how they will replace, complement and/or be an alternative means of compliance of the more

traditional maintenance tasks types such as general and detailed visual inspections (GVI and DET,

respectively). Figures 3 and 4 show examples of a hypothetical S-SHM system being introduced

respectively in the MRBR and MPD, replacing a special detailed inspection (SDI) that uses a

borescope to inspect the internal structure of both left and right hand elevators.

328 Structural Health Monitoring: Research and Applications

Figure 3: Example of a S-SHM replacing a SDI in a MRBR.

Notice that in the MRBR example, the term “S-SHM” replaced the term “SDI”, but the

inspection threshold and interval remain the same as in the original task (the values were purposely

removed from the example). In the MPD, it will be also necessary to indicate that while the number

of technicians required to perform the S-SHM is still one (as originally indicated for the SDI), only

0,25 man-hour will be used for the complete accomplishment of this inspection (the SDI takes much

more than that only to open the access panels for the borescope).

Figure 4: Example of a S-SHM task replacing a SDI in a MPD.

Airplane Maintenance Manual. Additionally, the Airplane Maintenance Manuals (AMM) will

need to be revised to include specific S-SHM procedures on how to perform the intended

inspection, how to proceed when degradation is detected in the monitored structures and how to

repair such systems in case of failures.

Also, as indicated in figure 5, the correspondent maintenance procedure in the AMM could be

significantly simplified. Depending on the positioning of the S-SHM design and its GSE connection

point, the technician would not need to open several circuit breakers or use a rear fuselage

workstand to perform the inspections, or even set the elevator a neutral position. The mechanic

would connect the GSE to the S-SHM system, then activate it and wait for the results (including the

self-diagnostics results that will be necessary before assessing the monitored structure condition).

Additionally, the current SDI procedures would be then considered as readily available alternative

maintenance procedures in case the S-SHM is defective.

Figure 5: Example of a S-SHM procedure replacing a SDI in an AMM.

Key Engineering Materials Vol. 558 329

Conclusion

There are challenges for the immediate introduction of S-SHM tasks as part of any commercial

aviation maintenance program. Three of them were presented in this paper along with suggestions

on how to overcome them. Different approaches for the introduction may be used by different OEM

and SHM solutions providers, but whatever they are it will be necessary to propose a method to

certify S-SHM inspections by regulatory authorities as valid replacement of more traditional tasks,

make adjustments on the process to develop maintenance programs and revise instructions for

continued airworthiness affected by the introduction of S-SHM tasks.

References

[1] Maintenance Cost Management – A Reference Guide, EMBRAER S.A. Available at

https://www.flyembraer.com

[2] Kent, Renee M., D. A. Murphy, 2000. Health Monitoring System Technology Assessments –

Cost Benefits Analysis. NASA / CR-2000-209848.

[3] MSG-3: Operator / Manufacturer Scheduled Maintenance Development; Revision 2009.1,

Airlines for America (A4A), available from A4A at http://www.airlines.org.

[4] Wenk, L., “Status of MSG-3 (Maintenance Steering Group 3) Guidance on Using SHM for

Scheduled Maintenance” 7th International Workshop on Structural Health Monitoring, Stanford

CA, Sept 9, p103, 2009.

[5] Santos, L. G., “Embraer Perspective On The Introduction Of SHM Into Current And Future

Commercial Aviation Programs” 8th International Workshop on Structural Health Monitoring,

Stanford CA, Sept 9, p103, 2011

[6] Appendix H to Part 25 of Title 14 of the Code of Federal Regulations, Federal Aviation

Administration (FAA). Available at http://rgl.faa.gov/

[7] Advisory Circular 121-22A – Maintenance Review Board Procedures, dated 3/7/97, Federal

Aviation Administration (FAA). Available at http://www.airweb.faa.gov.

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