Easa learning objective 022.13.03.01.03 states: Explain the need for standby instruments to supplement the EFIS in the event of all the display units failing and the challenge of using these standby instruments, namely their size and position on the flight deck.

Easa learning objective 022.13.03.01.02 states: Describe the typical layout of the EFIS display units and how there may be a facility to transfer the information from one display unit on to another if a display unit fails.

Aircraft with electronic flight instrument systems (EFIS) must have an independent means of providing vital flight information (as shown on the PFD) to the pilots in the event of the failure of all the EFIS displays OR their sources of information.

EFIS installations are fed with data from an air data computer (ADC) and inertial reference unit (IRU) or attitude and heading reference system (AHRS) , this data is processed before being displayed on the multifunction displays.

The electronic standby instrument display is fed from micro-electromechanical (MEMS) air data sensors connected to independent standby pitot and data sources. Attitude and heading information is provided by a MEMS gyro and the system is fed from a separate power source. These standby instruments are small and often placed in less than optimal positions on the flight deck which makes them difficult to read.

Another reason for having standby instruments is in case of failure of the source of information supplied to the EFIS system. Incorrect information fed from the pitot/static system, air data computer or inertial reference unit will result in erroneous information being processed and displayed on the EFIS displays. Because standby instruments use their own independant sources they can be used to verify the infomation displayed on the main instuments, such as when there's an ASI disagreement.

Recent feedback confirms that the correct answer option in the exam is:

'The electronic standby indications may be difficult to read but are essential since the failure of an EFIS display means a significant loss of flight data'

Unfortunately the phrase '....failure of an EFIS display...' is incorrect because the loss of a single display would not result in a significant loss of flight data because the PFD information can be switched to the other screen.

We think that the correct answer should say ' ....failure of the EFIS displays' or '...failure of all EFIS displays' but our feedback suggest that 'an' is still in use.