Maintenance of 747 and 767: Pneumatic Bleed Systems

The pneumatic bleed system on 747 and 767 airplanes has been one of the most frequent contributors to airplane dispatch delays. In response, improvements have been made to the design and overhaul of system components, and pneumatic system health checks...

The pneumatic bleed system on 747 and 767 airplanes has been one of the most frequent contributors to airplane dispatch delays. In response, improvements have been made to the design and overhaul of system components, and pneumatic system health checks have been developed to allow operators to identify failing components before they cause schedule interruptions.

Design improvements

High-pressure shutoff valve and pressure-regulating valve position switch

PlaneSome operators experience erroneous indications that the high-pressure shutoff valve (HPSOV) has not closed when commanded. These erroneous flight deck and air supply control test unit or built-in test equipment (BITE) module indications result from a shift in the actuation point of the HPSOV/pressure-regulating valve (PRV) position switch Debris and plunger wear, which are caused by the angle at which the actuating lever presses against the plunger bore, which increases the friction between the inner plunger and the switch housing. This friction causes the switch to travel too far (i.e., over-travel) before actuation.

Hamilton Sundstrand has improved the wear characteristics and reduced the vibration effects of the HPSOV and PRV position switches by incorporating new material, coating, and design for the plunger and new coating for the plunger bore. Units returned to Hamilton Sundstrand for overhaul since April 15, 2001, have received the redesigned switches. On March 1, 2001, Hamilton Sundstrand issued service information release (SIR) 747BAS141A/767BA032A and incorporated information about the new switches into all its HPSOV-PRV component maintenance manuals. Boeing released service letter 747-SL-36-094 on July 12, 2001, announcing the availability of the redesigned switches.

All 767 and 747-400 airplanes with GE or PW engines delivered since July 2001 have these new switches.

HPSOV-PRV actuator spring

Hamilton Sundstrand also has improved the HPSOV-PRV actuator spring. The service life of the HPSOV-PRV actuator springs varies from 3,000 to 21,000 hours. Spring failures are more prevalent on GE CF6-80C2 engines. The typical failure mode on these engines is spring breakage at the center resulting from high-cycle fatigue.

Hamilton Sundstrand has designed a two-piece spring with guide configuration to address this problem. The new spring requires minor machining in the actuator housing. A 0.0025-in. by 0.7-in. machined cut is made on the inside diameter of the housing in the threaded area. The reworked housing can be used with either the single-spring configuration or the new two-piece spring with guide configuration. The new spring will be available from Hamilton Sundstrand in fourth-quarter 2002.

Maintenance improvements

Component overhaul

PlaneOverhauling pneumatic system components when they are removed for repair - as opposed to only repairing the failed subcomponents - can increase component reliability. Data indicate that this practice keeps mean time between unscheduled removals (MTBUR) near that of the first-time removal MTBUR.

Many operators experience reduced time between component removals each time a repaired-only component is reinstalled on an airplane. However, if the failed component is overhauled as recommended by Hamilton Sundstrand, the time to removal is expected to be at or near that of the first-time removal.

For example, Hamilton Sundstrand recommends overhaul of the HPSOV if it has 8,000 or more hours of service and is removed for repair. If an HPSOV is removed because of a position switch failure at 8,300 hours, the operator should completely overhaul the valve rather than only replace the failed switch. If the switch is the only subcomponent replaced or repaired, the HPSOV will likely fail again in a relatively short time after being returned to service because of other subcomponent failures.

Service bulletins and hard-timing

Boeing and Hamilton Sundstrand also recommend that operators incorporate ATA Chapter 36 service bulletins into their fleets to improve component reliability. Incorporating ATA Chapter 36 service bulletins provides for incremental improvements to the pneumatic bleed system and its individual components.

Operators also may want to consider removing pneumatic components at defined hours of service (i.e., hard-timing). Some operators have indicated that incorporating the hard-timing of pneumatic components into their maintenance programs increased schedule reliability. It should be noted that the hard-timing of components might be inefficient if the hours or cycles are not tracked, if components are removed too early, or if removed components are not overhauled properly.

Operators must decide individually whether or not hard-timing of pneumatic components is economically justified with respect to the potential improvement in the schedule interruption rate. A more economically favorable alternative to hard-timing components is the use of a pneumatic system health check.

Pneumatic system health checks

Boeing pneumatic system health check

Boeing developed a pneumatic system health check (PSHC) for GE CF6-80C2 and PW4000 engines on 767 and 747-400 airplanes with the assistance of United Airlines, other operators, Hamilton Sundstrand, and an ATA Chapter 36 task team. The PSHC improves system reliability by identifying the components that have failed or about to fail before they cause dispatch delays. (Boeing is developing a PSHC for 747-400 airplanes with Rolls-Royce engines, older 767 airplanes with GE or PW engines, and 747 Classic airplanes.)

The Boeing PSHC addresses the following components:

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