12/09/15
Overview
Note:
The lightning detection system is an option offered by SB350-34-002.
The lightning detection system finds and calculates the locations of lightning strikes, 360 degrees around the aircraft. The system operates out to a maximum range of 100 nmi. The lightning detection system sends the lightning-detection strike data to the pilot and copilot displays, to become part of the weather radar display. The pilot uses lightning detection data to make decisions about the selection of a course, and to go through or around dangerous thunderstorms.
The lightning detection system includes the components that follow:
- Lightning Detection Processor
- Lightning Detection Processor Tray
- Lightning Detection Antenna
Lightning Detection Processor
The lightning detection processor is a unit installed in a mounting tray in the LH equipment rack. On the front side, the unit has one hook used to attach the unit to the mounting tray and a horizontal handle used to help remove the unit from the mounting tray. Also, on the front panel, the lightning detection processor has three light-emitting diodes (LEDs). The LEDs are used to show the operation status for the processor.
At the rear, the lightning detection processor has two electrical connectors that supply the interface with the antenna and the other external systems. The electrical connectors engage in the two receptacles of the mounting tray.
The lightning detection processor includes internal components that supply detection, amplification, conversion, control, and interface functions. The processor receives the thunderstorm signals from the antenna and processes them to calculate the location and intensity of lightning strikes. The processor uses the relation between the received electromagnetic fields and also the heading input from the attitude heading computer (AHC) to find the relative bearing, range, and intensity of the lightning strikes.
The lightning detection processor includes internal components that supply detection, amplification, conversion, control, and interface functions. The processor receives the thunderstorm signals from the antenna and processes them to calculate the location and intensity of lightning strikes.
The processor uses the relation between the received electromagnetic fields and also the heading input from the inertial reference unit (IRU) to find the relative bearing, range, and intensity of the lightning strikes.
The lightning strikes data is then transmitted to the electronic flight-instrument system (EFIS) display through low-speed aeronautical radio incorporated (ARINC) 429 data bus. The intensity level of the cells can be displayed in one of three different intensity levels. The three intensity levels (light, moderate, or heavy) are defined in relation to the average number of strikes per minute.
The lightning detection processor gets 28 VDC power from the L MAIN BUS. The processor uses this 28 VDC power to supply +5 V, +15 V and -15 VDC sources to all the circuits in the lightning detection processor.
Lightning Detection Processor Tray
The lightning detection processor tray is installed in the LH equipment rack. It holds the lightning detection processor in the equipment rack. The tray is attached to the equipment rack with four screws. The tray has two receptacles at the rear that connect to the interface electrical connectors of the lightning detection processor. The tray has one hold-down clamp on the front used to attach the hook of the lightning detection processor.
Lightning Detection Antenna
The lightning detection antenna is installed on the top of the center fuselage section, at FS551.00. The antenna has three elements: the X-, Y-, and E-elements. The X- and Y-elements sense the magnetic field from a lightning strike. The E-element senses the electrical field from the vertical electrical discharges of a lightning strike.
The electrical and magnetic fields from lightning strikes are in phase with each other. It is this phase relation that the lightning detection processor uses to find the lightning strikes.
The lightning detection processor uses the analog voltages that the antenna senses, to calculate the bearing, range, and intensity of lightning strikes.
12/09/15
Operation
The lightning detection processor is the primary component of the lightning detection system. The different functions of the processor are supplied by the internal component units that follow:
Analog Circuit
Detection and Amplification
The analog circuit receives three differential analog signals from the antenna. Differential amplifier circuits amplify the signals. A synchronous detection circuit makes sure that X- and Y-field signals are in phase with the E-field signal. This is a known property of signals that the antenna senses from the magnetic and electrical fields of lightning strikes. If the E-field signal is out-of-phase (with the X- and Y-field signal), the X- and Y-output signals from the detection circuit are at minimum levels. The synchronous detection circuit keeps the X- and Y-output signals, until the two signals are digitized.
Amplifier circuits change the voltage levels of the X- and Y-signals from the synchronous detection circuit. The full-scale range of the X- and Y-signals is -12 to +12 V. One pair of amplifiers changes the full-scale range of the X- and Y-signals to 0 to 5 V. The other pair of amplifiers expands the first 25% of the full-scale range X- and Y-signals (-3 to +3 V). Then, this pair of amplifiers changes the range to 0 to 5 V.
Analog/Digital Conversion
A 12-bit analog-to-digital (A/D) converter circuit digitizes the X- and Y-signals, after their voltage levels are changed. A multiplexer circuit applies the full-range (0 to 5 V) analog signals to the A/D converter circuit, first.
After each conversion is done, the microprocessor on the digital circuit reads the digital data and puts it into a data buffer. If the X- and Y-data values are too small, control signals cause the A/D converter to digitize the quarter-scale X- and Y-signals. This data is then put into the data buffer. The microprocessor and the programmable array logic (PAL), on their related circuit units, control the transmission of data between the two circuit units. The PAL also controls the operation of the multiplexer circuit and A/D converter circuit.
Antenna Fault Detection
The analog circuit has an antenna fault-detection circuit. This circuit senses a small bias current that goes through each of the three antenna elements. If an antenna element or the antenna cable becomes defective, the output signal from this circuit is low (less than 0.7 V). When the antenna and antenna cable are serviceable, the output signal is high (more than 3.0 V). The PAL on the analog circuit gets the antenna status data from this output signal and sends the data to the microprocessor.
Digital Circuit
Control Functions
The digital circuit controls the primary functions of the lightning detection processor. The digital circuit:
- Supplies control logic, address data, and clock signals to the analog and expansion circuits
- Does continuous self-tests of all primary system functions
- Gets the X- and Y-field lightning strike data from the analog circuit
- Gets the digital heading data from the IRU through ARINC 429 receivers on the expansion circuit
- Calculates the relative bearing, range, and intensity of lightning strikes
- Sends lightning strike data (relative bearing, range, and intensity) to the primary flight display (PFD) and multi-function display (MFD) units through ARINC 429 transmitters on the expansion circuit.
Intensity, Range, and Bearing
During the interval between lightning strikes, the microprocessor calculates the position of the lightning strikes that are in the data buffer. The microprocessor uses software to calculate the bearing and range of the lightning strike from the X- and Y-digital data. The microprocessor removes the aircraft heading to change the lightning strike bearing into relative bearing. The lightning-strike position data is put into strike data buffers.
Memory space is allocated in random access memory (RAM) for eight strike data-buffers. Each strike data buffer keeps the data for the newest 256 lightning strikes that are applicable to that display format and range.
The display format can be a 360 degrees view and a 120 degrees view (60 degrees left and right from the aircraft heading). Each display format has strike data buffers for the range zones that follow:
- 0 to 25 nmi
- 26 to 50 nmi
- 51 to 100 nmi
The data for each lightning strike identifies a cell location of the display. The range zone and display format (360 degrees view or 120 degrees view) set the depth and width dimensions of a cell.
Many lightning strikes can occur in a storm cell. Each cell keeps a strike-count for that cell. The lightning detection processor measures the storm cell intensity (with the number of lightning strikes that occur in a cell) during a given period of time.
The microprocessor erases data from the strike data-buffer memory at 2 min intervals. If a strike data buffer receives data for more than 256 storm cells during a 2 min interval, the data received first is erased.
When the aircraft heading changes, the microprocessor calculates the relative bearing again, for all cells in the strike data buffers. This step corrects the relative position of each cell, so that it shows correctly on the display.
The digital circuit also contains video memory and control circuits. These circuits are not applicable to operation of the lightning detection system when the lightning detection processor does not have the expansion circuit.
Transmission Sequence
The microprocessor controls the sequence of storm-cell data transmissions to supply the most important data to the PFD and MFD, first. The storm cells nearest to the aircraft, and in the 120 degrees sector in front of the aircraft, are the most danger to the aircraft. Thus, the microprocessor sends this data, first. The transmission of data for other storm cells in the 120 degrees front sector follows, in the sequence of range. The transmission of storm cell data for the 240 degrees sector behind the aircraft follows (after the storm cell data for the 120 degrees front sector). The sequence of storm cell data transmission for the 240 degrees rear sector is from nearest to farthest from the aircraft.
Display Data
The PFD or MFD calculates the position of each storm cell (lightning strike data) on the radar display with range and relative bearing supplied. The storm cell intensity shows in different colors as follows:
- Storm cells of light intensity (up to eight lightning strikes/min) show as yellow lightning bolts
- Storm cells of moderate intensity (9 to 25 lightning strikes/min) show as red lightning bolts
- Storm cells of heavy intensity (26 lightning strikes/min and more) show as magenta lightning bolts
Expansion Circuit
With the ARINC 429 EFIS expansion circuit, the lightning detection processor does not calculate the video data or applicable control signals. As an alternative, the microprocessor supplies lightning strike data, from the strike data buffers, to an ARINC 429 transmitter circuit on the expansion circuit.
The expansion circuit sends blocks of serial strike data to the PFD and MFD on the low-speed data bus. Each block of data can contain as many as 63 storm cells.
Power-Supply Circuit
The power-supply circuit receives the +28 VDC aircraft power and makes the different power-supply levels for the internal components. The power-supply circuit also contains the three light-emitting diodes (LEDs) that show through the front panel of the lightning detection processor.
Backplane
The backplane supplies all the electrical connections between the internal circuits of the lightning detection processor and the external system components.
Controls and Displays
Controls
The selection of a weather radar display format is necessary to show lightning strike data on the PFD and/or MFD.
The pilot/copilot uses the TR/WX button on the display control panel (DCP) for selection of the overlay display format on the PFD.
The pilot/copilot uses the TR/WX button on the pilot CCP/copilot CCP for selection of the overlay display format on the pilot MFD or copilot MFD.
The TR/WX OVERLAY menu shows on the display when the pilot/copilot pushes the TR/WX button. The WX/LX selection shows on the TR/WX OVERLAY menu only if the aircraft has the optional lightning-detection system.
The pilot/copilot pushes the TR/WX button to make overlay display format selections. Each push of the button moves the menu selection to the next menu item. The TR/WX OVERLAY menu goes off after 5 sec, if the menu selection does not change again.
Displays
When lightning strike and weather radar reflectivity data must show at the same display location at the same time, only the lightning strike data shows. If weather radar, lightning strike, and traffic alert and collision-avoidance system (TCAS) data must show at the same display location at the same time, only the TCAS data shows.
Lightning bolt icons show the location of storms cells on the displays. The icon colors of yellow, red, and magenta show the storm cell intensity levels of light, moderate, and heavy. The PFD and MFD calculate the location of each storm cell relative to the map center of the radar display.
Annunciators
The TR/WX/LX annunciator, in the lower right corner of the display, shows which overlay display formats are on. The overlay display formats that show in large cyan letters are on. Those that show in small white letters are off.
The PFD and MFD show fault annunciation for the lightning detection system only if the WX/LX overlay display format is on.
When a fail condition of the lightning detection system occurs, the /LX part of the TR/WX/LX annunciator changes to yellow and flashes for 5 sec. After 5 sec, the yellow /LX annunciator stays on continuously.
The PFD removes the lightning detection fault annunciator for the conditions that follow:
- The aircraft roll attitude increases to more than ±65 degrees
- The aircraft pitch attitude increases to more than +30 degrees (up) or -20 degrees (down).
The displays do not show storm cell data (lightning bolts) when a lightning detection system fault occurs. If the display receives storm cell data with an error in the transmission block format, the storm cells from only that block are not shown.
System Interface
The lightning detection system has a direct interface with the input/output concentrator (IOC) unit No. 1 for the heading data, and with the PFDs and MFDs for the display function. The system has also an interface with the radio systems to prevent radio interference.
Heading Input
The lightning detection processor receives digital heading data from the IOC No. 1 on a high-speed ARINC 429 data bus (L-GPBUS-5). The IOC receives this data from the IRU on a high-speed ARINC 429 data bus (L-IRS-1). The lightning detection processor uses the aircraft heading data to correct the relative position of storm cells on the display when the aircraft turns.
Display Output
The lightning detection processor sends lightning strike data (relative bearing, range, and intensity) to the PFD and MFD displays through a low-speed ARINC 429 data bus (LDS-1).
Radio Interference
The lightning detection antenna senses the electrical fields from very high frequency (VHF) and high frequency (HF) radio transmissions. During VHF and HF transmissions, the KEYLINE OUT signal, from the RIUs and the VHF radios, sets the lightning detection processor INHIBIT IN signal to low (less than 0.7 V). This prevents the operation of the lightning detection circuits, and thus prevents incorrect detection of lightning strikes, while the VHF or HF radios transmit.
Power Supply
The L MAIN BUS supplies 28 VDC through circuit breaker E3, on the left secondary-power center (LSPC), to energize the lightning detection system.
System Monitoring
The three light-emitting diodes (LEDs), on the lightning-processor front panel, give the status indications that follow:
- The green LED (in the aft side) shows that the lightning detection processor has +28 VDC aircraft power
- The yellow LED (in the center) shows that the power supply operation has stopped because of an over-voltage or high-current condition
- The red LED (in the forward side) shows that there is a system fault. The cause of the system fault can be a self-test error or a software problem. The software problem causes the watchdog circuit to do a system reset.