Building a Family
Fairchild Dornier takes the family approach to developing its various regional jet aircraft.
To remain competitive as well as meet the market demands of regional airlines, Fairchild Dornier has adopted the strategy of developing a family of aircraft. The latest series of aircraft under development by the company begins with the 70-passenger 728JET, which is scheduled to enter service in 2003. This aircraft will be followed by the 50-passenger 528JET in 2004 and the 100-passenger 928JET in 2005.
Strong cost competition has forced major airlines to form global alliances so they could achieve control of the regional air transport systems. Airlines now also play an important role in the decision-making processes of regional aircraft manufacturers. They require comfort equivalent to large aircraft standard, the same level of reliability, all-weather capability, and a turn-around time less than 20 min—but at a lower cost than today's turboprops.
So the challenge is to design a new family of aircraft that better meets the future demands of regional air transport. Several important design aspects have to be considered, such as fuselage layout, the flight control system, cockpit equipment, and aircraft commonality.
Fuselage layout
Shoulder clearance and aisle width are the deciding parameters for fuselage width and passenger comfort. Taking the shoulder width of the 87.5 percentile U.S. male plus 15 mm for summer clothing, a relative comfort factor is defined by cabin width at shoulder height divided by 508 mm per passenger.
 Although length and wingspan may vary, the aircraft family features a similar configuration. Click to enlarge |
The cabin cross section studies of four- and five-abreast configurations indicated the advantage of a five-abreast layout for the aircraft family. The results are: no additional emergency exits over the wing, an obstacle-free cabin between front and rear doors, and sufficient tail angle clearance at takeoff.
For the 100-seat 928JET, the fuselage will be stretched by four frame bays ahead of the wing and three frame bays aft of the wing. The fuselage center section will be redesigned to take the larger wing and higher loads. To fly the 928 on the same slots/routes as the 728, wing size will be increased from 75 m2 to 84.4 m2. The outboard wing will come from the 728, whereas the inner wing will be scaled up to a larger size.
For the 55-seat 528, the aft fuselage section of the 728 will be reduced by three frame bays, and the three-frame-bay-shorter front section will be a redesign of the 728's. It features the same wing as the 728.
Flight control system
Performance and handling qualities of all aircraft in a family should allow for operation by the same crew, with cross-crew qualification possible with minimal effort.
Intensive tradeoff studies were done considering new technologies, pilot workload, reliability, startup cost, operational cost, and technical risk. A fly-by-wire flight control system with pilot-in-the-loop was determined to be the best solution for the jets.
The primary flight control system consists of two elevators with two actuators each in active/active mode for pitch control; two ailerons with two actuators per surface in active/standby mode; three spoilers with one actuator per surface in active/lock-down mode for roll; and one rudder with three actuators all in active mode for yaw. This system is powered by a triplex hydraulic supply.
The secondary flight control system, which is electrically powered, consists of wing flaps in active/active mode, wing slats in active/active mode, and a horizontal stabilizer trim in active/standby mode.
Pilot input is via conventional controls (wheel, stick, and pedal inputs against feel springs) in separate control modules for pitch, roll, and yaw. The modules are dual (left and right) with mechanical disconnect for pitch and roll control and single for yaw control. The mechanical input signal against the feel spring and damper is transformed into an electrical signal by multiplex rotary variable differential transformers (RVDT). Trim actuators pre-load the feel springs according to trim switch inputs, control columns, wheels, and pedals. Autopilot input is mechanical by servomotors.
The primary flight control electronics are composed of two complementary units: the P-ACEs (Primary-Actuator Control Electronic Unit) and the MAU-FCM (Modular Avionics Unit-Flight Control Module).
 Family preliminary specifications. Click to enlarge |
The electrical signals of control modules are input to P-ACE (one per actuator), with two analog actuator control channels, one control path in the direct mode, and a monitor channel. A digital interface connects to the FCM digital section for augmentation. The analog section in the FCM also provides control of spoiler actuators. P-ACE output feeds the power control unit (PCU) of the actuator in active/normal operation mode or damped-in failure mode.
The primary flight control system operates in two modes. The augmented direct mode connects cockpit controls directly to surface actuators. Surface actuator gain scaling, speed scheduling, stability augmentation, and force fighting computation are provided by the FCM. The degraded system operates in direct mode, in which cockpit controls are connected to surfaces by P-ACE.
The selected system layout provides consistent stability and handling characteristics for all three family members. The modular concept with standardized units features fewer part numbers and allows swapping of units to other functions. Fewer mechanical parts reduce system weight, increase reliability, and lower maintenance costs. Failures in the system are reported to the central maintenance computer for easy correction.
