Technical data

Product Specification
HIF-2121/R5 CAGE CODE: 97896 SCALE: NONE SIZE: A DWG NO: 965-1176-601 REV: D SHEET 54
6.2.2.1 Mode 2A
Mode 2A is operational when the flaps are not in the landing position. The maximum upper envelope is at 1650 feet radio
altitude for speeds below 220 knots (for fast tuboprop, 190 knots for slow turboprop). As the aircraft speed increases up to
310 knots (for fast turboprop, 280 knots for slow turboprop), the upper altitude increases linearly to a maximum value of
2450 feet radio altitude. For speeds above these values, the upper altitude limit remains at 2450 feet. When the terrain
awareness data is of a high integrity, the upper altitude limit is reduced to 1250 feet.
Figure 6.2.2.1-1 shows the static alert envelope for Mode 2A. Closure rate is the computed change in radio altitude between
the aircraft and the ground, and is considered positive when the radio altitude is decreasing. Actual alerts will occur for
conditions inside of this static envelope.
The lower sloped line of the static envelope for Mode 2A has an equation of:
MIN TERRAIN CLEARANCE (FT) = -1579 + 0.7895 [CLOSURE RATE (FPM)]
The upper sloped line has an equation of:
MIN TERRAIN CLEARANCE (FT) = 522 + 0.1968 [CLOSURE RATE (FPM)]
The lower boundary of this envelope is set at 50 feet radio altitude. The normal upper limit of the boundary is horizontal at
1650 feet radio altitude. As computed airspeed increases from 220 knots up to 310 knots (190 knots to 280 knots for slow
turboprop) the upper boundary also linearly increases up to 2450 feet. The upper boundary is limited at certain airports (via
envelope modulation) to reduce the warning sensitivity and minimize nuisance warnings.
Upon penetrating the envelope, either on the slope or from the top, the caution lights (warning lights for Lamp Format 1)
come on and the voice message is “Terrain-Terrain”. If the envelope penetration lasts beyond these two messages by
approximately 1 second, then the message switches to “Pull Up”, which is repeated continuously until the envelope is
departed.
If the radio altitude monitor logic detects an invalid condition, or excessive closure rate due to a radio altimeter out of track
condition, then all messages are cleared.
Due to previous terrain clearances, aircraft speed, and gear/flap configuration, the actual Mode 2A alert/warning envelope
will be different than the static envelope illustrated in Figure 6.2.2.1-1.
Figure 6.2.2.1-2 shows the actual Mode 2A alert envelopes for two particular sets of terrain closure conditions. Here it is
assumed for both conditions that the aircraft is flying at constant barometric altitude over flat terrain, at 2450 feet radio
altitude. For the maximum expansion condition, airspeed is greater than 310 knots (280 knots for slow turboprop), landing
gear and flaps are both retracted, and no glideslope signal is present. The closure rate begins from the 2450 foot radio
altitude, and continues at a constant closure rate through the alert envelope. This is a typical scenario for inadvertent flight
into a mountain during a holding pattern or initial approach. It is important to note that the actual upper limit is effectively
around 2000 feet AGL, which is compatible with minimum instrument, terrain clearance procedures for mountainous terrain.
For the minimum expansion condition, airspeed is between 150 and 220 knots, landing gear and flaps are both retracted, and
no glideslope signal is present. This set of conditions may not be realistic with the landing gear up, but is given as a
comparative illustration.
The gear position will be equated to flap position on fixed gear aircraft to ensure that this logic work appropriately.