Part-66 Module 5 common mistakes
Part-66 Module 5 · Understand the traps and how to avoid them.
Candidates describe EICAS and ECAM as interchangeable names for the same display system, and describe 'the EFIS screen' without separating EADI from EHSI or naming the sensor source (ADC, AHRS, FMC) feeding each symbol.
Instrument systems are usually revised from colour screenshots rather than from signal flow, so the picture is remembered but the source of each symbol is not, and Boeing versus Airbus terminology gets merged into one generic 'engine and warning display'.
What to do instead
For each display, write three columns: what it shows, which computer supplies each element, and what the crew sees if that supplier fails (e.g. red ATT flag on the EADI when the AHRS drops out). Then write one line fixing the naming: EICAS is Boeing, ECAM is Airbus.
When converting a signed 8-bit binary pattern such as 1001 0110 the candidate reads the leading 1 as a sign bit and answers -22, treating negative numbers as sign-and-magnitude instead of two's complement.
The rule 'the MSB is the sign bit' is memorised as a standalone fact, and the invert-and-add-one procedure was learned as a formula for 'making a number negative' rather than as the representation actually used by the processor.
What to do instead
Practise ten 8-bit and five 16-bit two's complement conversions per session, always verifying against the allowed range (-128 to +127 for 8 bits). Work the example backwards: hex 96 -> invert 0110 1001 -> add 1 -> 0110 1010 = 106, so the value is -106. Also state BCD explicitly whenever digits exceed 9.
Questions on data buses are answered by describing ARINC 429 as a bidirectional bus, giving its speed as 1 Mbps or as 100 Mbps, and describing AFDX/ARINC 664 as a slow legacy point-to-point link.
The three standards are learned as a list of names rather than as a table of properties, so characteristics from Ethernet and ARINC 629 bleed into ARINC 429, and the 12.5/100 kbps figures get confused with 629's 2 Mbps.
What to do instead
Build a one-page property table with fixed rows: direction, topology, data rate, word length and error checking. Populate it as 429 = one-way, single transmitter to up to 20 receivers, 12.5 or 100 kbps, 32-bit word with 8-bit octal label and odd parity; 629 = 2 Mbps, multidirectional; AFDX = 100 Mbps switched Ethernet with virtual links. Re-derive the table from memory before each mock.
A truth table for a multi-gate circuit is filled in by tracking only the final gate, intermediate nodes are left blank, a NAND is evaluated as an AND with inverted output only sometimes, and De Morgan's theorem is applied by changing the inversion bar but leaving AND as AND.
Gate symbols are recognised visually but the circuit is never evaluated node by node, and De Morgan is remembered as 'move the bars' without the AND-to-OR swap that goes with it.
What to do instead
Always label every intermediate node (A, B, C) on the diagram and write four separate columns in the truth table plus the output column. For De Morgan, state the rule in full each time: NOT(A AND B) = NOT A OR NOT B, and swap the operator whenever a bar is removed or added. Convert any mixed-gate circuit into all-NAND form to check the identity.
Address bus width and data bus width are used interchangeably, so a processor with a 16-bit address bus is said to address 64 kbit of 16-bit words, or a 32-bit data bus is assumed to mean 2^32 directly addressable locations.
The two buses are drawn identically on block diagrams, so the formulas '2 to the power of n locations' and 'locations times word size' get applied to whichever number appears first in the question.
What to do instead
Fix the relationship in words before calculating: the address bus sets how many locations (2^n), the data bus sets the word size at each location, and total capacity = 2^n x word size. State the answer in both bytes and bits, and sanity-check against known figures such as 8-bit address bus = 256 locations, 16-bit = 65 536 locations.
Multiplexer and demultiplexer are swapped in descriptions, and time division multiplexing is explained as 'splitting the signal into different frequencies' or as sending all channels at once down separate wires.
The names sound like mirror images and are learned as a matched pair of words rather than as opposite data paths, while TDM and FDM are memorised as labels without a physical picture of the shared medium.
What to do instead
Anchor each one in a single sentence with a direction arrow: many inputs to one output over time = multiplexer at the transmitter; one input to many outputs = demultiplexer at the receiver; TDM shares the medium by time slots, FDM by frequency bands. Sketch the transmitter-receiver pair with the single link in the middle and label both ends before answering.
Step-index and graded-index fibre are described as having the opposite core profiles, and optical attenuation in decibels is calculated with 20 log instead of 10 log of the power ratio.
The two index profiles are remembered as two unlabelled curves, and the 10 log / 20 log split is carried over from voltage-gain work in electrical theory without checking whether the quantity is power or amplitude.
What to do instead
Draw the two refractive-index cross-sections with the ray paths overlaid and label them on the drawing itself: step-index has an abrupt core/cladding boundary and modal dispersion; graded-index has a gradual profile and reduced dispersion. For decibels, always write 'dB = 10 log (P1/P2)' for power and reserve 20 log for voltage or current ratios; check the sign and magnitude of each result.
LCDs are described as emissive devices, OLEDs as needing a backlight, and active-matrix panels as being addressed one row at a time with no storage element, so the terms active matrix, passive matrix, backlight and pixel count are used interchangeably.
All flat panels are grouped together as 'modern displays' in revision notes, and the light-source question (does the pixel generate light or modulate it?) is never asked explicitly, so emissive and non-emissive technologies merge.
What to do instead
Sort every display technology into two lists: emissive (CRT, LED, OLED, plasma) versus non-emissive and therefore needing a light source (TN, IPS and other LCDs, which use a backlight). Then answer the addressing question separately: active matrix has a transistor per pixel giving storage and higher contrast; passive matrix relies on row/column scanning and suffers crosstalk. Confirm each answer against the light-source test before moving on.
ESD damage is treated as only ever catastrophic, so a board that still passes test after handling is assumed undamaged, ordinary polythene bags are accepted for storing LRUs, and conductive, dissipative and antistatic materials are used without distinguishing them.
The visible failure is the memorable one, so latent degradation that later causes intermittent faults is not considered, and packaging terms are learned as a list of adjectives with no ranking of surface resistance.
What to do instead
Learn the three outcomes explicitly: catastrophic, latent (degrades but still passes, later intermittent), and no damage. Learn the three material classes in order of resistance, with conductive lowest, dissipative in the middle and antistatic highest, and state that only dissipative and conductive packaging is used for ESDS items, never plain plastic. Tie the wrist strap and grounding procedure to the reason: equalise potential before contact, verify the strap, never touch connector pins.
The three BITE categories are confused in scenario questions, power-up BITE is claimed to detect faults in flight, and candidates state that a no-fault BITE report proves the system is serviceable with no mention of the limits of built-in test coverage.
BITE is learned as a definition rather than as three different test timings, so any question describing a fault during flight gets answered with whichever BITE type was memorised first.
What to do instead
Memorise the three timings as a sequence with what each one can see: power-up BITE runs a self-test on energisation and cannot detect in-service faults; interruptive BITE runs on demand and takes the system offline; non-interruptive BITE runs continuously during operation without disturbing the system. Add one line on coverage limits, and where it fits, distinguish fly-by-wire from fly-by-light in the same note.