727 to 787: Evolution of Aircraft Maintenance Systems

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727 to 787:

Evolution of Aircraft Maintenance Systems

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航空

727 to 787:

Evolution of Aircraft Maintenance Systems

SPECIAL REPORT

Aircraft maintenance has come

a long way, from push-button,

light-up tests for determining the

health of individual, federated

avionics boxes to centralized

airplane management systems

t h a t i n t e r a c t i ve l y c o l l e c t

data from all systems. It has

evolved to become a valuable

t r o u b l e s h o o t i n g t o o l f o r

maintenance technicians.

Testing mechanical and analog systems in vintage aircraft—such as the Boeing 727

and 737 Classic, and the McDonnell Douglas DC-9 and MD-80—consisted of little more

than pushing a button to supply current to the internal circuitry. A green light would

illuminate to say everything was OK. Such “push-to-test” and “go/no go” systems were

the beginnings of built-in test equipment (BITE) for avionics technicians.

The central maintenance computer on Honeywell’s Primus Epic® system

Star ting in the early 1980s,

digital systems that use electronic

hardware and software

to carry out functions previously

performed by mechanical and analog

systems were introduced on Boeing’s 737

Next Generation, 757 and 767 aircraft,

as well as the McDonnell Douglas MD-

90 and Airbus A320. These new digital

systems brought their own challenges

to aircraft maintainers, as the ability to

troubleshoot these black boxes by detecting

and isolating faults was limited to the

indications provided by the system.

From this challenge came the first

standard for health management: ARINC

604, “Guidance for Design and Use of

Built-In Test Equipment.” Developed by

ARINC and industry partners, this standard

represents the birth of vehicle health

management, where one or more line

replaceable units (LRUs) equipped with

dedicated front panels gave maintenance

technicians the ability to test and query

the system. Such panels included pushbuttons

and rudimentary display capabilities

that showed alphanumeric readouts.

By the mid-1980s, with the introduction

of the first glass cockpits on airplanes

such as the McDonnell Douglas

MD-11, mechanics were able to test and

query several systems through centralized

display panels shared by a number

of LRUs. However, these common display

panels report the results of each

LRU independently and do not have the

capability to consolidate related fault

indications from multiple LRUs. While

this is a significant improvement, maintenance

technicians must still manually

consolidate these results; otherwise they

would remove and replace LRUs that

merely report symptoms rather than

replacing the actual faulty LRU.

In the late 1980s, the 747-400 was

introduced. It contains two Central Maintenance

Computers (CMCs) that receive

fault status indications from most airplane

systems, consolidate these results

to determine the source fault, and correlate

the source fault indication to the

flight deck effects (e.g., flight crew alerts)

caused by the fault. The CMC can display

these results on the Multifunction

Control Display Unit (MCDU) or downlink

these results while in flight to ground stations

to support maintenance planning.

The CMC also provides an integrated

user interface to perform ground tests

on all connected member systems. However,

the 747-400 CMC accomplishes

the fault consolidation via a complex

set of logic equation-based diagnostics.

Development and maintenance of this

“brute force” approach was a great technical

challenge, due to the many interrelationships

between the equations.

This approach requires detailed understanding

of all the equations in order to

ensure that they are consistent and correctly

represent system behavior.

These issues are compounded when

the aircraft is upgraded with new systems.

It took a long time to work out

all the health management system’s

idiosyncrasies, causing an early lack

of confidence in the system by airline

mechanics—“Give me wrong answers

once, and my confidence goes down 50

percent! Give it to me twice, and I stop

using it!” In time, the 747-400 logic has

matured and become a valuable tool for

maintenance technicians.

Following the development of, and

using lessons learned from the 747-400,

Boeing, Honeywell and others in the

airline, aircraft and avionics industries

developed updated standards for maintenance

systems, including ARINC 624,

“Design Guidance for Onboard Maintenance

System.”

Central Maintenance & Airplane

Condition Monitoring

Health management processes include

the following:

➤ Fault detection and isolation philosophy,

➤Optimal sensor quantity and placement

guidelines,

➤Standard built-in-test designs and

practices,

➤Metrics, e.g., fault coverage percentage

or fault isolation accuracy percentage,

➤Verification and validation plans and

procedures,

➤ Fault modeling guidelines, and

➤Interface standards between subsystems

and central maintenance systems.

Coordination and integration of those

processes are key to effective vehicle

health management. The ultimate goal

was to improve testability, isolate failures,

improve system safety and reliability,

and reduce life-cycle costs.

Many of today’s aircraft employ some

type of central maintenance system

that fulfills the role of collecting faults

from all subsystems, performing some

root cause determination and recommending

repair actions. In the Boeing

777, those functions are performed by

the Honeywell-developed central maintenance

system software. It consists of

two portions: the Central Maintenance

Computing Function (CMCF), which

detects faults after they happen, and the

Airplane Condition Monitoring Function

(ACMF), which collects data to enable

prediction of problems in advance.

ACMF capabilities existed previously in

earlier federated systems, but the B777

The development of health management systems

reflects the evolution of avionics architectures.

From the B727 to the B777, health

systems have supported mechanical/analog,

digital, federated and modular avionics.

Vehicle Health Management

Evolution for Commercial Aircraft

was the first aircraft to implement them

in the same cabinet with an integrated

advanced user interface, according to

Gautham Ramohalli, the engineering

manager of Honeywell Aerospace’s

Aircraft Diagnostics and Monitoring

Systems. This integration provided the

ACMF with unprecedented access to

aircraft signals and enabled a common,

point-and-click interface on the aircraft

for both CMCF and ACMF.

As opposed to the logic equationbased

diagnostics on the 747-400, the

777 CMCF employs a Honeywell-patented,

model-based diagnostics technology

to drive fault processing and ground

tests, and display textual information to

the maintenance technician. The fault

model encodes the observable symptoms

for each fault condition, based on

the modeled effects of each fault condition

within the member system and the

connectivity of LRUs within the aircraft.

Each system on the air plane is

responsible for fault detection and reporting

in accordance with a set of Boeingdefined

member system requirements.

The reporting system communicates

with the CMCF, using an aircraft-wide

standard protocol that provides for fault

reporting, configuration reporting and

commanded ground tests.

Model information is contained in a

separately loadable database referred to

as loadable diagnostic information (LDI).

The LDI is created by Boeing, using the

Honeywell-developed ground-based Diagnostic

Model Development Tool (DMDT),

the purpose of which is to collect and validate

the fault model for the aircraft.

DMDT is seeded by imports from

various sources. These include the airplane

interface control database, crew

alerting message database, and failure

modes and effects analyses. Aircraft

system designers enter subsystemspecific

diagnostic information and links

to complete the basic model. Boeing,

as the aircraft system integrator for the

777 and 787, completes the aircraft-level

model by performing tasks, including

analysis and correction of DMDT proposed

propagation relationships and

definition of fault isolation relationships

for faults reported by multiple systems.

This aircraft-level modeling determines

fault consolidation and fault cascade

effect removal, and ensures correct flight

deck effect correlation.

The aircraft condition monitoring

function provides a programmable method

for triggering custom data reports.

Report triggers can be defined, using

interface control document signals combined

with logic units, to collect sample

data at a predefined rate and time before

and after the trigger event.

Resulting reports can be stored on

onboard mass storage devices (e.g., the

Maintenance Access Terminal hard drive

on the 777, and the crew information system

[CIS] servers on the 787), downlinked

via the airborne communications addressing

and reporting system (ACARS) or, on

the 787, one of the available broadband

communication paths, such as Gatelink (a

wireless local area network [LAN] technology),

Connexion by Boeing, or Inmarsat’s

Swift64 satellite service.

Such a capability is key to improving

on-time departures because it gives line

maintenance staff time to prepare even

while the airplane is still in the air. “What

that does is extend the maintenance

technician’s ramp time from 30 minutes

to four hours by giving him 3.5 hours

advance notice,” says Don Morrow, Honeywell’s

director, New Boeing Platforms.

The user interface employed by the

CMCF enforces a common look and feel

for all member systems. This reduces the

amount of training required for maintenance

personnel, regardless of the system

they are working on, e.g., landing gear,

environmental control system or avionics.

Differences from Previous Systems

Most previous maintenance systems

depended on the individual member

systems to store the fault data in their

LRU/LRM (line replaceable module). To

display a system’s stored data, a bidirectional

command-request protocol is

performed to retrieve the data each time

a user request is made.

The CMCF uses local fault storage to

store the data, which simplifies the faultreporting

interface for the participating

systems. The CMCF retrieval of fault data

is all contained within the CMCF itself. This

speeds up data retrieval while requiring no

handshaking or protocol with member

systems during the display process.

The CMCF was built upon previous

Honeywell maintenance systems that

added maintenance message text to maintenance

message codes. The objective is

to present maintenance message information

in clear English text that is usable by

the maintenance technician, rather than

having a code that requires translation.

Embraer’s OMS

The Honeywell Primus Epic® onboard

maintenance system, which consists of

a CMC and the airplane condition monitoring

system, is finding its first 70- to

110-passenger aircraft application in the

EMBRAER 170/190 family of commercial

jetliners.

Many of the system health management

capabilities of the Embraer aircraft are

similar to those of the 777 onboard maintenance

system. These include the following:

➤Real-time fault monitoring,

➤Integration with the data loading

system,

➤oint-and-click intuitive navigation,

➤Comprehensive system coverage—

weather radar to auxiliary power unit,

➤Open architecture,

➤Loadable diagnostic infor mation

database,

➤Capability to display bus parameters

and member status, and

➤Remote terminal connectivity.

“These go beyond having a system

that just reports faults to one that quickly

gives maintenance technicians a snapshot

of all the member systems on the

aircraft,” says Eric Heinzer, Honeywell’s

A maintenance technician accesses data

using a wireless terminal

Customer and Product Support leader

for regional OEMs. “A mechanic having

to deal with a series of faults now can

easily determine what systems are not

on-line and start there.”

Like the system now being developed

for the Boeing 787, the EMBRAER

170/190 Primus Epic system has a

separately loadable database, so that

loadable diagnostic information can be

updated without having to alter the CMC

functional code. In addition, the CMC is

navigable, with a cursor control device

comparable to that on the 777. Another

common feature with the Boeing widebody

is the ability to query the CMC on

the EMBRAER 170/190 with a commercial

off-the-shelf laptop.

On the EMBRAER 170/190, the CMC

is part of Honeywell Aerospace’s Primus

Epic integrated avionics system. The

CMC resides on a dedicated module

within the Primus Epic modular avionics

unit (MAU) and utilizes a commercial offthe-

shelf (COTS) operating system. The

CMC software stores maintenance data

locally on the CMC module in a format

that is later downloaded and analyzed

with common PC software applications.

Honeywell evaluated the need to

bridge airborne embedded software with

PC software and made a design decision

to use the PC software on the CMC line

replaceable module in the Primus Epic

MAU cabinet, according to Gary Bird,

Honeywell’s product portfolio leader,

Telematics and Diagnostics. This decision

meant that PCs could be connected

directly to the CMC module, using COTS

hardware and protocols.

In turn, this provided the ability to create

a CMC interface console, using a PC

instead of specialized hardware. The CMC

is specifically designed so that its operational

interface in the cockpit requires no

keyboard. The interface is point-and-click

and works with a variety of cockpit-mounted

cursor control devices.

777 to 787

Boeing’s 787, now under development,

will also utilize the Honeywell-patented,

model-based CMCF and ACMF technology,

and in this application, the health

management function is part of the crew

information system/maintenance system

(CIS-MS). The CIS-MS provides a

networking infrastructure that enables

airborne functions to interact with ground

components and a computing environment

capable of hosting RTCA/DO-178B,

Level D and Level E, software applications.

Hosted by the CIS are various standard

applications, including the maintenance

system, electronic flight bag (EFB), data

loader, flight deck printer and terminal

wireless LAN unit (TWLU).

The preceding systems provide interfaces

to airplane communication systems,

airline applications and information systems

in an open architecture that can be extended

and adapted. Maintenance system control

and display are available through several

user interfaces. Web-based technology is

used for the primary interface, and ARINC

661 is provided as a backup.

The primary interface to the maintenance

system is a commercial off-theshelf

PC that uses a typical Web browser

interface. Unlike the Boeing 777, these

devices are not certified or installed

in the airplane, and an operator may

choose to stow a laptop computer on

the airplane for convenience. If a laptop

computer is not available, the cockpit

multifunction display is equipped to

provide the minimum CMCF display

functionality necessary to prepare the

airplane for dispatch.

Basic to the 787 airplane is the terminal

wireless LAN unit, which provides

the airplane side of the Gatelink connectivity

and lets maintenance technicians

electronically access the airplane when

it is located at the gate. An optional crew

wireless LAN unit (CWLU) provides the

ability to use a wireless laptop computer in

the proximity of the airplane to perform all

of the available maintenance system control

and display functionality. The CWLU

system provides necessary security and

allows for multiple simultaneous users.

Easy Access

With this wireless capability, maintenance

technicians will not have to fight

their way through departing passengers

to get into the cockpit or even have to

hook up their computers to external connections

on the tail or engines to access

troubleshooting data. They can access

data while roaming freely around the

exterior of the airplane. Up to four remote

terminals can be connected to the CMC

at one time, allowing for a faster aircraft

build in the Boeing factory, as well as

expedited return to service in airline

maintenance operations.

In addition, the maintenance system

will provide links to the electronic airplane

maintenance manuals (AMMs), allowing

access to detailed maintenance and troubleshooting

procedures without leaving

the airplane. “By auto-linking to electronic

documents on the 787—something that

was not available on the 777—we’re creating

an essentially paperless and medialess

airplane,” explains Bird.

How soon can 787 operators see

the benefits of Honeywell’s crew information

system/maintenance system?

The new Boeing aircraft’s first flight is

scheduled for August 2007, with certification

in March 2008 and first deliveries

by May 2008. There were, through

January 2006, 291 orders for the twinaisle

Boeing 787.

B777 OMS menus, including higher-level page

and (right) flight deck effects correlation page.

bocome

727 to 787: Evolution of Aircraft Maintenance Systems

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