Besides thrust, bleed air can be used for engine
Air pressure, bled from the compressor,
controls oil leakage on nonrubbing labyrinth or
clearance-type bearing seals. These bearings use
Cross-bleed Air Engine Starting System
the differential in sump pressures to keep oil loss
to a minimum. The sump scavenge pump capacity
Most aircraft in the fleet using two or more
is greater than that of the oil system pressure
engines employ a cross-bleed starting system.
pump. Not only does the sump scavenge pump
See figure 6-11. This system provides regulated
scavenge all the oil in the sump area, it also
air pressure from one engine to start the remain-
scavenges air in the sump, creating a lower air
ing engine(s). The first engine must be started by
pressure than that of the area surrounding the
an external source of air pressure. External
bearing sump. This action allows the compressor
sources may be auxiliary power units or ground
bleed air external from the sump to flow from
support equipment. Subsequent engines can then
outside the sump area across the bearing seal,
be started using bleed air from the running engine.
preventing oil leakage in the opposite direction.
Opening the cross-bleed air valves allows regulated
The airflow also helps cool the bearings. See
bleed air from the running engine to supply air
to the other engines' starter.
Compressor bleed-air valves reduce the load
on the compressor, making it easier for the starter
The guide vanes of a turbine-powered engine
to turn the compressor. During starts, air is bled
from the compressor through ports on the com-
are used to direct the flow of inlet air into the
compressor section. The air is coldest at this point,
pressor housing. The bleed valves are held open
and is subject to icing. The biggest problem
by compressor air pressure until the engine starts.
resulting from ice forming at this point is the
After starting, the speed-sensitive valve directs
blockage of inlet air, which causes air starvation,
compressor discharges air to close the bleed
and thus engine failure. Another problem is the
Figure 6-12.-Engine bleed-air distribution.