BendixKing KMH 920 System KMH 980 - Manual (Page 21)

KTA 970/KMH 980 Pilot’s Guide
Rev 2 006-18273-0000
13
TCAS I Theory of Operation and Symbology
Intruders may be seen in surrounding airspace, but not on the
TCAS I display. The situations in which this may happen are:
•
Most small aircraft have one transponder antenna located on the bot-
tom of the aircraft. When own aircraft is above one of these aircraft,
the transponder antenna can be shaded from the TCAS I interroga-
tions. When this occurs, the TCAS I interrogation may not reach the
other aircraft’s transponder, or the other aircraft transponder’s reply
may not reach TCAS I’s antenna. A lack of replies prevents TCAS I
from tracking intruders. Transponder shading also occurs when the
other aircraft is maneuvering such that line of sight to its transponder
antenna is blocked.
•
The TCAS I directional antennas have a bearing “cone of confusion”.
TCAS I is able to determine bearings for intruders that are located
within –10 degrees to +70 degrees elevation angle with respect to
own aircraft’s horizontal plane for the top directional antenna (+10
degrees to –70 degrees for a bottom directional antenna). Intruders
that are located outside of those elevation angles will be tracked with
no bearing.
•
TCAS I is unable to determine bearings for intruder tracked on the
bottom antenna when the own aircraft has a bottom monopole anten-
na or a bottom directional antenna but the landing gear is extended.
In this case the intruder will be tracked, but not displayed, unless a
Traffic Advisory is issued against it.
•
The other aircraft may have a poor transponder. Ground stations
have more gain and “hear” aircraft at farther distances than TCAS I.
•
TCAS I is required to reduce transmitter power when in areas of high
density so that it does not adversely affect (overwork) other aircraft
transponders and prevent the ground ATC from tracking them. This is
known as Interference Limiting (IL). IL can reduce the nominal TCAS
I surveillance range to around 6 nmi. This means that TCAS I may
not detect a poorly performing transponder until it is much closer.
•
TCAS I has a one-second update rate. When in areas of high density,
TCAS I may reduce its maximum surveillance range to either 10 nmi
or to the range of the 30th intruder in track plus 1 nmi. This allows
TCAS I to maintain its one-second update rate.
•
TCAS I has an altitude surveillance volume of –10,000 feet to +10,000
feet relative to own aircraft’s altitude. Any intruders outside this vol-
ume are not a threat to own aircraft and therefore are not tracked by
TCAS I.