displacement between the terrain elevation and the current aircraft altitude (left
side of Figure 4-4). If the aircraft is more than 2 000 ft above the terrain, there is no terrain information displayed.
-Peaks mode (refer to 4.1.5.4 – EGPWS: Peaks Mode): it displays terrain regarding to the absolute terrain elevation (i.e. referring to the sea level instead of the aircraft elevation). It improves the terrain awareness of the flight crew (right side of Figure 4-4). Practically, if the aircraft is more than 2 000 ft above the terrain, the terrain is still displayed with a gradient of green colors. In addition, the Peaks mode provides two figures in the bottom right corner of the display, which are the highest and lowest terrain elevations. The lowest terrain elevation refers to the lowest terrain information contained in the terrain database.
4.1.3.1.2. EGPWS Terrain Clearance Floor (TCF)
The TCF function provides an additional terrain clearance envelope around the runway against situations where Mode 4 provides limited or no protection. When the aircraft penetrates the terrain clearance envelope, the EGPWS triggers aural and visual alerts.
TCF alerts take into account the current aircraft location, the reference point of the destination runway, and the radio altitude.
The terrain clearance envelope is defined as in Figure 4-5.
4.1.3.1.3. EGPWS Runway Field Clearance Floor (RFCF)
The RFCF function complements the TCF function. It provides a circular envelope centered on the selected runway, extending up to 5 NM from the runway end. The inner limit of the RFCF envelope is set at K NM (K depends of the position error, the runway data quality and geometric altitude quality).
The RFCF function provides alerts for cases where the runway is at high elevation compared to the terrain below the approach path. In these cases, the radio altitude may be so high that the EGPWS does not trigger TCF alerts, whereas the aircraft could be below the runway elevation.
The field clearance is defined as the current aircraft altitude (MSL) minus the elevation of the selected runway.
4.1.3.2. PREDICTIVE T2CAS FUNCTIONS
4.1.3.2.1. T2CAS Collision Prediction and Alerting (CPA)
The CPA function provides the flight crew with alerts indicating that the current flight path is hazardous due to the presence of terrain ahead. The alerts permit a timely initiation of the suitable escape maneuver to avoid a CFIT. A pull up is considered as the basic escape maneuver (PULL UP warning). When the pull up is not possible, the T2CAS announces a turn around maneuver (AVOID TERRAIN warning).
For the prediction of CFIT, the CPA detects the terrain profile (augmented by an additional margin above the terrain element height, called Minimum Terrain Clearance Distance – MTCD) with a clearance sensor. Refer to Figure 4-8.
The terrain profile is extracted from the terrain database. The MTCD depends on the distance to the nearest airport, the height to the airport elevation, and the aircraft vertical speed. The MTCD is composed of a basic term and an offset term. 中国航空网 www.aero.cn 航空翻译 www.aviation.cn 本文链接地址：getting to grips with surveillance(68)
