Technical data

ManualsBrandsBoss Audio Systems ManualsAutomotiveREV-965

Product Specification

HIF-2121/R5 CAGE CODE: 97896 SCALE: NONE SIZE: A DWG NO: 965-1176-601 REV: D SHEET 53

6.2.2 Mode 2 -- Excessive Terrain Closure Rate

Mode 2 provides two types of alerts based on aircraft gear/flap configuration, radio altitude (terrain clearance), and how

rapidly that radio altitude is decreasing (closure rate). Barometric altitude of the airplane is not important in initiating this

warning.

These two alerts are commonly referred to as Mode 2A, described in sections 6.2.2.1 and 6.2.2.2, and as Mode 2B, described

in section 6.2.2.3.

Differentiating and scaling radio altitude generates closure rate. As the closure rate term is inherently noisy, especially over

irregular terrain, extensive rate limiting and filtering must be used to obtain an accurate closure rate value for computation.

The computer uses a number of different sets of sophisticated rate limits and filter methods to allow maximum sensitivity

during cruise, while providing progressively less sensitivity during the landing phases of flight. These rate limits vary as a

function of gear and flap position, aircraft speed, and whether or not the aircraft is on an ILS approach. It is this rate limiting

and filtering that determines the effectivity of Mode 2 in providing advance alerts, while avoiding unwanted or nuisance

alerts.

Altitude rate is combined with closure rate in the filtering method to provide “lead” information. Increasing the altitude

descent rate will tend to speed up the alert occurrence. Reducing the altitude descent rate, or initiating a climb, will tend to

delay the alert occurrence, or reduce the time that the alert is on.

Figure 6.2.2-1 shows the block diagram for Mode 2 alerts.

CLOSURE RATE DETECTION

RADIO RATE

MODE 2

OUTPUT

5733 FPM

30 FT

CLOSURE RATE (THOUSAND FPM)

MODE 2 ENVELOPE

(FOR GA FAST, GA SLOW NOT SHOWN)

MODE 2B INHIBIT

ILS MODE 2B

FLAPS DOWN

9800 FPM

3545 FPM

1220 FT

1650 FT

@ LE 220 KTS

2450 FT

@ LE 310 KTS

2038 FPM

AIRSPEED

FLAPS DOWN

GEAR DOWN

TIME CONSTANT

RATE LIMITS

FILTER

FLAPS DOWN

GEAR DOWN

ALTITUDE (ASL)

MODE 2

AUDIO

MODE 2

VISUAL

MODE 2

ALTITUDE

GAIN

(RADIO ALT RATE)

RADIO ALTITUDE

MODE 2 TAKEOFF

FIGURE 6.2.2-1: MODE 2 BLOCK DIAGRAM

Radio altitude, indicating the vertical distance between the aircraft and the underlying terrain, is differentiated to determine

the rate of change in this vertical distance. Any decrease in this vertical distance indicates potential ground contact for the

aircraft. Decreasing radio altitude may be the result of reducing the aircraft’s altitude, an increase in height of the terrain, or a

combination of both effects.

The computed closure rate is applied to the appropriate alert envelope, where the closure rate value is compared against the

actual radio altitude value, to determine the alert conditions as described below.