focus on maintainability and reliability targets.
To help ensure that established goals would be met, the design team
focused on airplane design, system design, power plant design, and
scheduled maintenance program development.
The first step in the airframe design process was to examine the
causes of nonroutine maintenance on previous Douglas designed twinjet
airplanes. MD-80 dispatch delays were attributed to 1,699 components
(i.e., 1,699 six-digit ATA chapter classifications). Of these, 116
components caused 50 percent of the delays, and their improvement
was given the highest priority by the 717 design team.
The team also focused
on retaining airplane design elements that had proved successful
on previous Douglas-designed twinjet airplanes, including the 100,000-cycle
airframe structural design and the simple, reliable, low-maintenance
primary flight control system. Figure
4 shows some of the significant 717 airplane design improvements
made to lower maintenance costs and improve reliability.
System designs on 717 predecessors were revised to improve component-level
design and ease of maintenance on the 717:
environmental control system uses three-wheel air-cycle machines,
which eliminate the need for air-cycle machine ground cooling
fans and reduce scheduled maintenance. The system has 27 percent
fewer line-replacable units (LRU) than the DC-9 and MD-80 design.
- The integrated electrical
system reduced the number of major components from 60 to 9, which
eliminated 150 wires compared with the DC-9 and MD-80 design.
The 717 system features an integrated drive generator, no-break
power transfer, and interchangeable power conversion distribution
- The airplane interior
features removable window escutcheons that permit inner and outer
windowpane replacement without the removal of seats or sidewall
- Electrically controlled
aileron trim, rudder trim, and spoilers simplified the flight
deck pedestal, or aisle stand, and eliminated many cables compared
with the DC-9 and MD-80 design.
- The in-line (i.e.,
straight-shaft), engine-driven hydraulic pumps have higher reliability
than the bent-axis (i.e., articulated-shaft) pumps on earlier
airplanes. The flareless fittings in the hydraulic lines significantly
improve reliability because they are less prone to cracking and
- The integrated flight
deck has state-of-the-art displays, communication and navigation
equipment, and digital flight guidance system, which cumulatively
reduce the number of flight deck LRUs by 57 percent, compared
with the DC-9 and MD-80 design.
- The landing gear system
incorporates steel brakes that are attached with 10 pins rather
than traditional fasteners, which reduces installation time by
60 percent, compared with the DC-9 and MD-80 design.
- The vacuum waste system,
which features modular lavatories, permits single-point aft servicing
and is designed for corrosion prevention.
- Built-in test equipment
is an integral element of many 717 digital components, reducing
troubleshooting and inspection times. In addition, the integrated
centralized fault display system (CFDS) receives inputs from more
than 30 LRUs and presents information on the flight deck multifunction
control and display units for ease of line maintenance.
- The auxiliary power
unit, based on a proven design, requires no specialized ground-support
equipment for transport.
- The 717 structures
are based on the proven design of its predecessors, with improved
corrosion protection and appropriate material choices.