The EGPWS may fail to alert a terrain threat under certain ambient conditions. Which of the following is true?

The EGPWS may fail to alert a terrain threat under certain ambient conditions. Which of the following is true?

• A. Below ISA
• B. During Non-Standard Temperature Lapse Rates
• C. Incorrect Barometric Settings
• D. All of the above

Answer: (D)

EGPWS relies on accurate altitude data and standard atmosphere assumptions to determine how close the airplane is to surrounding terrain and when to issue a warning. When ambient conditions deviate from standard, the relationship between the aircraft’s indicated altitude and the actual height above terrain becomes unreliable, which can blunt or delay the terrain alert. Cold temperatures (below ISA) cause the true altitude to differ from what the altimeter indicates, so the system’s assessment of altitude above terrain can be off. Non-standard temperature lapse rates similarly disrupt the expected vertical relationship used by the warning logic, reducing the reliability of the alert thresholds. If the barometric setting is incorrect, the altitude readout feeding the terrain awareness calculations is simply wrong, further degrading the system’s ability to detect a threat in time. Because any of these conditions can affect the accuracy of the altitude data or the atmosphere assumptions behind the terrain calculations, all of the above are valid factors.

EGPWS relies on accurate altitude data and standard atmosphere assumptions to determine how close the airplane is to surrounding terrain and when to issue a warning. When ambient conditions deviate from standard, the relationship between the aircraft’s indicated altitude and the actual height above terrain becomes unreliable, which can blunt or delay the terrain alert.

Cold temperatures (below ISA) cause the true altitude to differ from what the altimeter indicates, so the system’s assessment of altitude above terrain can be off. Non-standard temperature lapse rates similarly disrupt the expected vertical relationship used by the warning logic, reducing the reliability of the alert thresholds. If the barometric setting is incorrect, the altitude readout feeding the terrain awareness calculations is simply wrong, further degrading the system’s ability to detect a threat in time.

Because any of these conditions can affect the accuracy of the altitude data or the atmosphere assumptions behind the terrain calculations, all of the above are valid factors.