backup system is not isolated, as normal combined
before the aircraft were released for flight. In some
system pressure exists within pressure and return lines.
cases, accidents occurred on such aircraft shortly
The return shutoff valve remains open. Combined
thereafter.
system pumps maintain high pressure at the rudder and
When an aircraft experiences a control
stabilizer actuators. Flow demand on the backup pump
discrepancy during flight, a thorough investigation
is not excessive at high rates. The cockpit indicator light
should be conducted immediately. In cases where
should remain on, indicating backup system operation.
aircraft have safely returned from a flight during which
3. When the flight and combined systems fail, the
a control discrepancy was experienced, a thorough
backup flight control system performs as an isolated
investigation is necessary. This investigation must be
system. Surface rates available at the rudder and
made before further flight. All parts of the affected
stabilizers are reduced by the limited output of the
control system should be inspected for proper rigging,
backup pump. There is no flaperon actuator control. The
clearances, and potential causes for interferences. All
cockpit indicator will flicker out if the pilot applies
sealed units that are suspect must be replaced. Primary
inputs to the controls that exceed the capacity of the
cause factors that should not be overlooked include
pump. The cockpit RUDDER THROW light will also
maneuvers that have exceeded the operational design
be illuminated, indicating that approximately 33
of the control systems. Hydraulic system
percent of normal rudder throw is available.
contamination, corrosion and/or distorted or
Q16-17. When is a backup flight control system
disconnected linkage may have caused the problem.
activated?
Inadequate lubrication and external contamination in
Q16-18. At what pressure does the backup hydraulic
the form of preservative compounds, such as grease
system get activated?
combined with dirt and dust, may have caused the
problem. An increasing number of flight control
Q16-19. When does the backup flight control system
system malfunctions are related to system
perform as an isolated system?
contamination, and this ever-important aspect of
hydraulic system maintenance should be given the
PRIMARY FLIGHT CONTROL
attention it deserves. Checking of system filters and
SYSTEM MAINTENANCE
contamination inspection of suspected systems are
within the capability of organizational activities. If a
LEARNING OBJECTIVE: Recognize the
system is found to be contaminated, the source of
procedures for primary flight control system
contamination must be eliminated and the system
maintenance to include trouble analysis,
cleaned by recycling or flushing in accordance with
rigging/alignment, and operational checks.
instructions provided in the appropriate MIM.
Maintenance of primary flight control systems
Contaminated components must be replaced as
consist of periodic maintenance, trouble analysis,
necessary to restore proper system operation.
removal and replacement of flight control components,
Disposition instructions for removed hydraulic
rigging and adjustment of the flight controls, and
components vary with the production status of the
operational and alignment checks of the primary flight
aircraft model. Diligent care must be taken to retain the
control system. Most primary flight control
component in the as-is condition, with no change in
component maintenance is limited to removal and
a d j u s t m e n t , d i s a s s e m b l y, o r c l e a n i n g . I f t h e
replacement of the component. Check the applicable
component has slides or pistons that are jammed, no
MIM for proper repair procedures.
attempt to free them should be made.
MALFUNCTION OF PRIMARY FLIGHT
The aircraft must not be released for further flight
CONTROLS
until the cause has been determined and corrected. If it
is not readily apparent why the component
There have been many cases reported in which
malfunctioned, you should submit a Hazardous
pilots have found flight controls jammed while the
Material Report/Engineering Investigation request. If
aircraft was on the ground. Because the controls were
the discrepancy cannot be duplicated or cause
freed by excessive pressure before an inspection could
determined, an appropriate entry must be made in the
be made, the causes for the jammed condition could
Miscellaneous History section of the aircraft logbook.
not be found. No positive corrective action was taken
16-21
