pneumatic system health checks have been developed to allow
operators to identify failing components before they cause
According to Boeing data
collected from operators, pneumatics (as defined by Air Transport
Association [ATA] Chapter 36) is the third most frequent cause of
schedule interruptions for the 747-400 and the fourth most frequent
cause for the 767 and the 747 Classic
(i.e., 747-100/-200/-300). During 2000 and 2001, the pneumatic bleed
system accounted for nearly 7 percent of all schedule interruptions
for 747-400 and 767 airplanes.
Some operators experience
much greater reliability, with schedule interruptions attributed
to ATA Chapter 36 as low as one-tenth of the fleet average. Other
operators experience much poorer reliability, with schedule interruptions
as much as 3.9 times greater than the fleet average. Although the
reasons for this wide range of reliability are not easily determined,
maintenance practices by operators can be key to improving reliabilityparticularly
those that check the health of the pneumatic system and allow for
replacement of failed components at a scheduled maintenance check
rather than at an airport gate.
This article discusses
three factors that improve the reliability of pneumatic bleed systems
on 767 and 747 airplanes with General Electric (GE) or Pratt &
Whitney (PW) engines.
system health checks.
shutoff valve and pressure-regulating valve position switch.
Some 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, increase the friction between the inner plunger and the switch
housing. This friction causes the switch to travel too far (i.e.,
overtravel) before actuation.
Bleed component supplier
Hamilton Sundstrand (Windsor Locks, Connecticut) 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
(fig. 1). 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/767BAS032A
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 the new switches.