Boeing Aero Contents

Published: April 1999

Advanced Maintenance Techniques for the 757-300

The Boeing 757-300 airplane is the first major derivative in the 757 family. With additional equipment and several new features, it provides operators with increased passenger capacity and improved operating economics over the 757-200. The 757-300 incorporates the latest advances in technology - many of which are used in other Boeing models - since the development of the 757-200 in the late 1970s. Operators of the 757-300 should be aware of these advances and the accompanying maintenance techniques to help them achieve the highest possible dispatch reliability with this derivative.

As the successor to the highly reliable Boeing 757-200, the 757-300 incorporates new and proven technology used on the 737-600/-700/ -800/-900, 777, and other airplanes throughout the Boeing fleet. Changes include upgraded built-in test equipment and related revisions to manuals and system architecture to help operators successfully troubleshoot problems, establish the required preventive maintenance, and maintain a high dispatch rate. In addition, new maintenance techniques have been adapted from the latest Boeing models for use on the 757-300. The technology, maintenance, and system improvements on the 757-300 are found in several categories:

  1. Built-in test equipment
  2. Air Data Inertial Reference Unit
  3. Vacuum lavatories
  4. Tail skid
  5. Yaw damper/Stabiliser Trim Module
  6. Flap slat accessory module and flap skew detection system
Built-In Test Equipment

Built-in test equipment (BITE) on the newest line replaceable units (LRU) in the 757-300 follows the methodology set on the 737-600/- 700/-800/-900 and 777 (figure 1). During development of the 777, a methodology for presentation and content of BITE was developed. This specifically defines how a front-panel BITE will operate and how the mechanic will use it. The new front-panel BITE incorporates features that were agreed upon by airline mechanics and Boeing chief mechanics, human factors experts, engineers, and customer services personnel. Their joint goal was to define a standard for BITE to help mechanics troubleshoot and return an airplane to service in less time. All-new LRUs with front-panel BITE comply with this Boeing standard, increasing commonality in the Boeing fleet and easing troubleshooting.

Air Data Inertial Reference Unit

The Air Data Inertial Reference Unit (ADIRU) is the latest technology in navigation that is also standard equipment on 737-600/-700/-800/- 900 airplanes. ADIRU combines the inertial reference units (IRU) and the air data computers (ADC) into one LRU. ADIRUs have higher reliability and weight savings compared to conventional IRUs with separate air data computers.

The three air data inertial reference units (ADIRU) on the 757-300 are located in the main equipment centre. The left ADIRU normally supplies the air data and inertial reference function to the captain's instruments, and the right ADIRU normally supplies the first officer's instruments. The centre inertial function is continuously used in systems that require three sources of inertial reference information. This is common with the current-production 757 and 767 models.

The most notable change with this system compared with the 757-200 is in the ADC function, which resides in the ADIRU. This function of the ADIRU receives inputs from the pitot and static system through conventional probes and ports. The pitot probes and static ports collect air data information and then send this air data to the air data modules (ADM) through standard flexible tubes. The ADMs receive these analogue air pressures and convert them to digital values, which are then sent to the ADIRUs. The 757-300 has a total of seven ADMs: three pitot and four static. The ADMs are located in the main and forward equipment centre. The flight deck interface to the ADIRU BITE is the inertial reference mode panel (IRMP), almost identical to that on the 757-200. The function of the IRMP is still initialisation and troubleshooting of the ADIRUs.

Two maintenance enhancements to the ADIRU are the additional BITE capability and centre ADC switching function. In addition to being self monitoring, the ADIRU monitors the health of the ADMs. ADIRU troubleshooting starts at the IRMP. The BITE structure for the IRMP has significantly changed with the incorporation of ADIRU. The flight- deck-mounted IRMP does not fully comply with the Boeing BITE standard because commonality with the 757-200 IRMP was essential and did not allow for the standard BITE panel to be included on the face of the ADIRUs. However, the IRMP includes additional troubleshooting capability derived from the BITE standard, with a total of 21 maintenance messages that can be displayed on the IRMP front face. These messages relate directly to the maintenance message index in the fault isolation manual (FIM) to help mechanics troubleshoot the ADIRU.

A new capability on the 757-300 allows for quick dispatch if a primary ADC function fails. This is available in the form of the centre ADIRU air data function, which is used as a warm spare for either the captain's or the first officer's instruments. The main equipment centre contains two switches to direct the centre ADIRU air data computer information to either the captain's or the first officer's instrument panel. Selecting the centre mode for either the left or the right side switches the centre ADC function to that side.

Vacuum Lavatories

Through lessons learned with previous Boeing models, the 757-300 includes improvements for a state-of-the-art vacuum lavatory system. Commonality of existing parts was a high priority during design of the water and waste system and led to a common system and spare parts provisioning. The two-tank vacuum waste system was also designed for high operational reliability, made possible through system redundancy and proven reliable components. Each waste tank has separate supporting equipment. For example, if a vacuum blower fails or a waste line clogs for one tank, the other waste tank will continue to operate normally.

Other features include easier blockage removal, straight waste lines, and large diameter bends for clog reduction. A tank rinse has been included to help reduce inadvertent waste-tank shutdown. BITE is performed through the lavatory waste modules.

Boeing has issued an all-model service letter that outlines several options for maintaining a vacuum waste system. Operators who follow these recommendations have reported greater reliability with their vacuum lavatories.

Tail Skid

The 757-300 includes a tail skid similar to that on the 777-300 to accommodate the reduced rotation clearance that resulted from increasing the length of its fuselage. Controlled by the landing gear lever, it retracts when landing gear is selected UP and extends when landing gear is selected DOWN. The tail skid helps to reduce costly body contact by absorbing energy in the event of a tail strike on landing or takeoff. The energy is absorbed by crushing a cartridge inside the tail skid shock absorber.

Two indications will alert maintenance personnel to check the tail skid crushable cartridge:

If either or both of these indications are found, additional inspections of the tail skid system are required. Maintenance personnel will inspect the cartridge by inserting 1/8-in rods (similar to proximity switch electronics unit (PSEU) open rods) into the side of the cartridge that houses the inspection holes (figure 3). If the rod passes through the housing, the cartridge is crushed. To determine the extent of the cartridge crush, the same pin must be inserted into the tail skid upper indication hole. If the pin does not pass through the upper hole, the cartridge is only partially crushed, and the airplane may be returned to service in accordance with the operator's minimum equipment list. If the cartridge is crushed beyond the upper inspection hole, then additional structural inspections may be required along with replacement of the cartridge before the next revenue flight. The cartridge was designed for easy replacement.

The 757-300 tail skid system also includes a body contact indicator, which is a small blade detector just aft of the tail skid. If the tail skid is fully compressed during a tail strike and the body makes contact with the runway during one of these events, the engine indication and crew alerting system (EICAS) will display a TAIL STRIKE (caution-level) message. The body contact indicator has a dual-loop indication system for redundancy and nuisance message reduction. Both loops must be open for EICAS to annunciate a tail strike. If a single loop opens, EICAS annunciates a status-level message indicating a fault with the body contact indicator. The tail skid system is monitored and tested by the PSEU.

Yaw Damper/Stabiliser Trim Module

Another new LRU on the 757-300 is the yaw damper/stabiliser trim module (YSM). The YSM is a combination of the rudder ratio changer module, stabiliser control module, and the yaw damper. A primary reason for combining the three LRUs into a single box was a lack of available update functionality in the existing LRUs. Combining the three LRUs increased the reliability, lowered the cost by more than 200 percent, decreased the weight of the airplane by 46 lb (20.9 kg), and freed up equipment shelf space.

In addition to its current functions, the YSM on the 757-300 provides the following major new functions:

Elevator feel limit (EFL).
The EFL was added as a result of the increased rotation takeoff speeds of the 757-300 relative to those of the 757-200. The elevator feel computer (EFC), a hydro-mechanical device that determines column forces, was modified to add the EFL. The EFL limits control column forces at rotation and for 7 sec after the airplane is airborne. Without the EFL function, which is controlled by the YSM, the flight crew could experience a heavier-than-expected column force (based on 757-200 experience) at and immediately after rotation for some takeoff configurations. If a fault condition is detected in the feel limit, EICAS will display an ELEV FEEL LIM status message. YSM BITE will detect the EFL fail condition and display the fault condition in a maintenance message format on the YSM front-panel BITE. The mechanic can now use the maintenance message index in the FIM to continue troubleshooting and find the failed component. The components were designed as LRUs for easy maintenance in case of failure.

Elevator feel shift module (EFSM).
The EFSM is a new component on the 757-300 and is identical to the EFSM on the 737-600/-700/-800/-900. It was installed to comply with the latest regulatory revisions to certification requirements for stall identification. (When full airplane stall is encountered, hydraulic pressure increases between the EFC and the feel and centring unit.) This increases the column force gradient or "stiffness" and results in a nose-down airplane response that aids in stall recovery.

Flap Slat Accessory Module and Flap Skew Detection System

The 757-300 has incorporated a flap skew detection system. Flap skew occurs when either the inboard or outboard edge of the flap moves farther than the rest of the flap. The flap skew detection system monitors, detects, and shuts down the flaps if a skew condition occurs. The heart of this system is the flap slat accessory module (FSAM), taking the place of the flap slat electronic unit (FSEU) number 3. The new LRU was introduced because the existing configuration of the FSEU would not accommodate the addition of the flap skew circuits.

The flap skew sensors are mounted on the trailing edge of the flap jack screws (figure 4a). The sensor (figure 4b) is fixed to the flap track, and a target (which contains magnets) is mounted to the jackscrews. As the jackscrew begins to turn, the target also turns. This will pulse the sensor, and a signal (distance travelled) is sent to the FSAM, which compares this signal with the opposite side of the same flap. If the distances travelled by either side of the flap differ by a predetermined amount, the FSAM will shut down the flap system to help eliminate damage to the flap and surrounding structure and the possible loss of a flap in flight. The alternate flap mode will override the normal flap skew detection function, allowing maintenance personnel to move the flaps after maintenance has been performed. Maintenance personnel must verify the flap fault and also ensure that moving the flap will not cause any damage. Standard front-panel BITE is located on the FSAM.

Summary

Wherever possible, the 757-300 incorporates new maintenance philosophies established during design of the 777. This derivative also includes several new features designed to make the 757-300 easy to maintain. Among them are a common front-panel BITE, fault reporting through EICAS, and additional correlation between the front-panel BITE and the FIM. The new systems in the 757-300 were also designed for easy fault detection and troubleshooting. The combination of new systems and maintenance techniques was intended to provide operators with the highest dispatch reliability possible on the latest version of the 757.

Launch customer Condor Flugdienst took delivery of the first 757-300 in March 1999. — ED.

Eric White
Chief Mechanic - Standard-Body Programs
Service Engineering
Boeing Commercial Airplanes Group

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