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Failures and the Primary Flight Display

by Flight Learnings

in Electronic Flight Instruments

Instrument System Failure

The competent pilot is familiar with the behavior of each instrument system when failures occur, and is able to recognize failure indications when they appear on the primary flight display. Manufacturers typically use a bold red “X” over, or in place of, the inoperative instruments and provide annunciator messages about failed systems. It is the pilot’s job to interpret how this information impacts the flight.

The inoperative airspeed, altitude, and vertical speed indicators on the PFD in Figure 2-7 indicate the failure of the air data computer. As do all electronic flight displays, navigation units (area navigation (RNAV)/flight management systems (FMS)) and instrumentation sensors rely on steady, uninterrupted power sources of 24 VDC or 12 VDC power. Any interruptions in the power supplies, such as alternator/regulator failure, drive belt failure, lightning strikes, wiring harness problems, or other electrical failures, can completely disrupt the systems, leading to erratic indications or completely inoperative units. Especially in standard category aircraft not designed or built with the redundancy inherent in transport category aircraft, a proficient and prudent pilot plans for failures and has alternate plans and procedures readily available.

Figure 2-7. A PFD indicating a failed air data computer.

Figure 2-7. A PFD indicating a failed air data computer.

The inoperative attitude indicator on the PFD in Figure 2-8 indicates the failure of the AHRS. By understanding which flight instruments are supported by which underlying systems (e.g., ADC, attitude heading reference system (AHRS)), you can quickly understand the source of a failure. It is important to be thoroughly familiar with the operation of the systems and the abnormal/emergency procedures in the pilot’s operating handbook (POH), aircraft flight manual (AFM), or avionics guides.

Figure 2-8. A PFD indicating a failed AHRS.

Figure 2-8. A PFD indicating a failed AHRS.

PFD Failure

The PFD itself can also fail. As a first line of defense, some systems offer the reversion capability to display the PFD data on the multi-function display (MFD) in the event of a PFD failure. Every aircraft equipped with electronic flight instruments must also contain a minimal set of backup/standby instruments. Usually conventional “round dial instruments,” they typically include an attitude indicator, an airspeed indicator, and an altimeter. Pilots with previous experience in conventional cockpits must maintain proficiency with these instruments; those who have experience only in advanced cockpits must be sure to acquire and maintain proficiency with conventional instruments.

Awareness: Using Standby Instruments

Because any aircraft system can fail, your regular proficiency flying should include practice in using the backup/standby instrumentation in your aircraft. The backup/standby instrument packages in technically advanced aircraft provide considerably more information than the “needle, ball, and airspeed” indications for partial panel work in aircraft with conventional instrumentation. Even so, the loss of primary instrumentation creates a distraction that can increase the risk of the flight. As in the case of a vacuum failure, the wise pilot treats the loss of PFD data as a reason to land as soon as practicable.

Essential Skills

  1. Correctly interpret flight and navigation instrument information displayed on the PFD.
  2. Determine what “fail down” modes are installed and available. Recognize and compensate appropriately for failures of the PFD and supporting instrument systems.
  3. Accurately determine system options installed and actions necessary for functions, data entry and retrieval.
  4. Know how to select essential presentation modes, flight modes, communication and navigation modes, and methods mode selection, as well as cancellation.
  5. Be able to determine extent of failures and reliable information remaining available, to include procedures for restoring function(s) or moving displays to the MFD or other display.

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