Part-66 Module 5: numbers, logic, data buses and glass cockpits

For category B1, Module 5 is 40 multiple-choice questions in 50 minutes. It covers the digital side of the aircraft at a practical level: what the systems do and how to handle them safely.

Numbers and conversion

Binary place values are 1, 2, 4, 8, 16…; 11010₂ = 26. Each hexadecimal digit is four bits (F0 = 1111 0000) and A–F stand for 10–15, so A3 = 163 and FF = 255. Octal digits run 0–7 and are used for ARINC 429 labels. In BCD each decimal digit has its own four bits (0100 0111 = 47). Two's complement represents negative numbers; Gray code changes only one bit at a time.

An ADC turns a voltage into a number and a DAC does the reverse. An 8-bit converter has 256 levels, so a 10 V range gives steps of about 39 mV. Sample at more than twice the highest signal frequency to avoid aliasing.

Logic

AND needs all inputs true, OR any input, NOT inverts, NAND and NOR are the inverted versions, and XOR is true when inputs differ. NAND and NOR are universal gates. De Morgan: NOT (A AND B) = (NOT A) OR (NOT B). Flip-flops store single bits.

Computers and data buses

A CPU has an ALU, a control unit and registers. RAM is volatile; ROM, EEPROM and flash keep their data; an EPROM is erased with UV light. BITE stores fault data for maintenance.

ARINC 429 has one transmitter and up to 20 receivers on a twisted shielded pair, 32-bit words (label in octal, SDI, data, SSM, odd parity in bit 32), at 100 kbit/s or about 12.5 kbit/s. ARINC 629 allows several transmitters; MIL-STD-1553 runs at 1 Mbit/s under a bus controller; AFDX (ARINC 664) is a redundant Ethernet-based network.

Fibre optics and displays

Fibres are immune to EMI, light and fast. Keep ends clean, respect bend radii, and never look into an active fibre. LCDs need a backlight; active matrix (TFT) displays control each pixel with a transistor. CRTs hold dangerous high voltages. Red means warning, amber caution, green normal on most displays.

ESD, software and EMC

ESDS items carry the crossed-hand label. Use a grounded wrist strap (with its 1 MΩ safety resistor), hold boards by the edges and store them in shielding bags. Static too small to feel can still cause latent damage, especially in dry air.

Only approved software may be loaded, usually with a data loader; check the part number afterwards and record it. Airborne software follows ED-12C/DO-178C levels A (catastrophic) to E. Navigation databases change every 28 days. Twisted pairs, shields, bonding and ferrites protect against interference, HIRF and lightning; equipment is tested to DO-160.