The lightning sensor system (LSS) is used to
detect and locate areas of lightning activity within a 100
nautical mile radius around the aircraft The system
provides the operator with a visual display of the
position and rate of occurrence of the lightning activity
on the EHSL. The lightning sensor system detects both
visible and high energy invisible electromagnetic and
electrostatic discharges (lightning) indicating areas of
turbulent activity. After evaluating the LSS display, and
its relation to precipitation as indicated by the weather
radar display, the operator can effectively plan the
proper course to avoid hazardous weather. The
lightning sensor system consists of a receiver/processor,
an antenna, a mode selector switch (located on the
radar control panel, fig. 3-29), and the pilot's and
copilot's EHSL. The system is powered by a 2-ampere
circuit-breaker placarded LSS, located on the overhead
circuit breaker panel (fig. 2-7). Because the system is a
passive device (it does not transmit), it can be operated
safely on the ground. Weather in all directions around
the aircraft may be monitored, even before starting
Lightning Sensor System Mode Selector
Switch (fig 3-28).
SBY. In the standby (SBY) mode,
lightning data is not displayed. However, the receiving
and processing equipment is active and lightning strikes
are being counted and accumulated.
LX. The LX mode is the normal
working mode of the lightning sensor system. Lightning
strikes are collected, processed, and displayed.
CLR/TST. When clear/test is selected,
all memory of past strikes and symbols are erased.
After 3 seconds the system enters the test mode.
In the test mode, simulated lightning signals are
fed into the test loop in the antenna and a lightning
strike is simulated at a bearing of 45 degrees at 25
nautical miles. A lightning alert is also generated along
the outermost range ring at a bearing of 45 degrees.
This simulated strike will progress in severity through
level three within 15 seconds of entry into the test mode.
If the mode selector is left in the CLR/TST mode, the
alert and strike symbols will reduce in severity and
disappear. The lightning rate symbol will disappear after
approximately 2 minutes.
The lightning sensor system antenna is used
in this test, and as a result, any real
lightning activity that occurs while the test is
in operation may also be displayed.
Lightning Sensor System Display. The LSS
shows areas of lightning activity on the EHSI with
symbols. Each lightning symbol represents the center
of a circular area with a radius of nine nautical miles.
Three different lightning symbols are used to represent
three different rates of occurrence of the lightning within
each 18 nautical-mile diameter circle. The lightning rate
symbols represent the lightning rates-of-occurrence
during the last two minutes. The symbol's location on
the display represents the average position of the
lightning that has occurred during the last two minutes
inside each 18 nautical-mile diameter area. Lightning
may not be occurring at the center of the symbol.
Because it is easier for the LSS to detect lightning at
close distances than at far distances, the number of
lightning strokes required for each rate symbol is
adjusted for distance to the storm. Figure 3-32 shows
the number of strokes required for each symbol versus
range to the storm. Figure 3-33 shows the three rate-of-
Lightning Alert System. It is possible to see
a lightning flash visually, and see no change in the
display. This is normal, because, unless one of the rate
thresholds are crossed, there will be no change in the
symbol, other than a slight position update. The
lightning alert function shows the operator that the unit is
operational, and shows real-time lightning activity. The
lightning alert function places a magenta rate-one
symbol near the outer range mark of the display, at the
proper bearing, for each lightning flash (fig. 3-33). The
lightning alert symbol is displayed for five seconds, and
gives the operator confidence that the system is
functional even though there may be no change in the
white lightning rate symbols. It is also an indicator of
Annunciations. The following mode annunciations may
appear on the EHSI display.
LX/F (amber). Indicates that self-test
has detected a fault.
LXIS (green). Indicates that the system
is in the standby (SBY) mode.
LX/CL (green). Indicates that the
system is in the clear (CLR) mode. This annunciator will
appear for approximately 3 seconds after the CLR/TST
mode has been selected. After this time the mode
annunciator will switch to LX/T.
LX/T (green). Indicates that the system
is in the test (CST) mode. This annunciator may be
replaced with LXMN.
LXmn (amber). In the CLR/TST mode
an amber LXn will be displayed, where mn is a two digit
failure code. Table 3-8 explains these codes.