The Technician's Bench
Troubleshooting Intermittent or Uncommanded Pitch Activity
by Butch Nimmo, Tech Rep
Avionics installers and bench technicians alike shudder when they hear the word "Intermittent". The term is very nebulous and can cover a variety of problems both mechanical, and electrical in nature. Such problems occur with no regularity and are difficult to duplicate during the troubleshooting process.
One key to successfully resolving an intermittent pitch motion problem is getting an accurate description of the complaint in the form of symptoms which the system is displaying. Some examples of questions to ask are:
1. Does this intermittent pitch motion or activity occur randomly, or can it be associated with a particular type of flight condition such as smooth air or turbulence?
2. Does this intermittent pitch motion only occur coincidentally with flap extension or retraction?
3. Does this intermittent pitch motion occur after the aircraft sits on the ramp in a rain storm?
The answers you receive from pilots operating the airplane will dictate the area of the autopilot system requiring further investigation.
There are several possible explanations for these intermittent pitch anomalies.
1. The absolute pressure transducer, even though it is of a very rugged design, can develop a sensitivity to aircraft vibration. This may cause an instantaneous change in output, which in turn causes a pitch deviation from the desired altitude. To determine if this is the case, locate the transducer. Check for a properly engaged mating connector. Pull the control yoke or stick back to a neutral elevator position. Engage the altitude hold mode. While holding the controls in neutral, have an assistant roll the knurled handle of a screwdriver on the transducer case. Any response which can be felt in the controls, as a result of this induced vibration, indicates a defective transducer.
2. Improperly installed flap compensator components are another source of intermittent pitch changes. Flap compensators are used to drive the pitch servo, and in some cases the trim servo, in response to flap movement. This compensates for the tendency of the aircraft to "balloon" or "settle". In general, flap compensators employ either a single or dual potentiometer, an actuator arm, a cable for attaching to the flap actuator, and a retract spring. There are some which attach by means of a gear, rather than cable/actuator arms. If troubleshooting indicates a possible flap compensator problem proceed by inspecting anything which would allow the flap compensator potentiometer shaft to move without flap motion, as follows:
a) Inspect for loose or missing set screws which attach the actuator arm or gear to the potentiometer shaft. Inspect for missing or improperly installed retract springs. Inspect for loose mechanical attachment of the potentiometer itself.
b) If the installation is mechanically sound, inspect the wiring for a loose or missing airframe ground wire at the flap compensator potentiometer. Inspect the wiring at the rear of the Pitch Flight Guidance Computer (PFGC) or, in a System 55, the programmer/computer, for wires not properly insulated from airframe ground. Some aircraft only use two of the four flap compensator wires. Cap and stow any wires not required for the particular aircraft in question. Due to the routing of the flap compensator wiring, and its affinity for upholstery screws, it may become necessary to isolate the flap compensator and its associated wiring at the PFGC or the System 55 programmer/computer. For System 60-2's, PSS's, and System 65's, cut each flap compensator wire (wires are single, white, 22 Gauge wire with pin number and identification stamped on each) and install them into a four pin Molex connector, leaving the two connector halves separated. For a System 55 locate the Flap Comp/CWS option connector behind P2 on the mounting rack. Extract pins 8, 9, and 10. Identify and insulate the pins. If the subsequent test flights indicate normal operation, further investigation of the flap compensator wiring will be required. Once the defective wire or wires are located and repaired or replaced, re-install pins 8, 9, and 10 into the Flap Comp/CWS option connector in the case of a System 55, or simply connect the two halves of the four pin Molex previously installed in the case of a System 60-2, PSS, or System 65.
It is important to note here that even though a particular installation may require all, some, or none of the flap compensator wires, all of the wires are present in each harness shipped. Improperly capped and stowed flap compensator wires can and do cause intermittent problems even in installations not requiring flap compensation.
3. Improperly installed static ports which allow moisture to collect and aircraft static systems which allow for the accumulation of moisture, can cause intermittent pitch activity as well.
a) Inspect for proper routing of the tubing in installations which require separate static ports independent of the aircraft static system.
b) Inspect for low spots which would allow moisture to collect. Our STC installation routing is designed to prevent low spots.
c) In installations which use the existing aircraft static system, disconnect the pressure transducer from the static system and cap off the static line. Test fly the aircraft with the transducer vented to ambient cabin air pressure. (Don't open or close any vents during this test). If the test flight reveals normal autopilot operation, investigate the aircraft static system further.
If you have any questions implementing these procedures give us a call in Technical Support.
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