CAA NZ AME Instruments common mistakes
CAA NZ AME Instruments · Understand the traps and how to avoid them.
Treating pitot-static instrument errors as one generic 'position error' and failing to separate instrument error, position (pressure) error, and installation/static-port error when asked which is correctable in flight.
Candidates memorise a short error list without building the distinction between errors built into the instrument, errors caused by airflow around the aircraft, and errors from where the static source sits; the terms overlap in casual reading, so they get merged.
What to do instead
Build a three-column table (source of error / physical cause / correctable in flight or on the ground) covering instrument, position, installation, manoeuvre, lag, and density errors, and for each write one sentence on where the pressure actually differs from truth. Test yourself by naming the error from the cause alone, not from the definition.
Reading a resistance thermometer or thermocouple indication as if it were a simple voltmeter reading, and mis-stating which junction or element the meter actually measures.
The two sensor types are taught together and both produce a small voltage or resistance change, so candidates conflate cold-junction compensation, hot-junction measurement, and the location of the sensing element.
What to do instead
Draw each circuit from sensor to indicator and label the measuring junction, reference junction, and compensation element. Then state, in one line each, what happens to the indication if the reference junction is not compensated and if the leads are copper instead of the specified material.
Applying the 'increase in temperature gives increase in resistance' rule to every resistance sensor without checking whether the element has a positive or negative temperature coefficient, or whether the indicator is calibrated for that specific element.
Most classroom examples use platinum or nickel positive-coefficient elements, so the rule becomes automatic; negative-coefficient materials and mismatched indicator calibration then produce wrong answers.
What to do instead
Learn two or three named sensor materials with their coefficient sign and typical resistance, and practise interpreting a resistance-versus-temperature graph for both signs. Add a check step: before predicting the indication, confirm the element type and the indicator calibration.
Confusing the purpose of a synchroscope with that of a phase-sequence indicator or a frequency meter when describing AC generator paralleling instruments.
All three instruments are panel-mounted and all relate to bringing a generator on line, so candidates remember the panel layout but not the specific quantity each displays.
What to do instead
Write a one-line function for each: synchroscope shows phase difference between incoming and busbar voltages; phase-sequence indicator confirms rotation order; frequency meter shows Hz. Then describe the correct switching action when the synchroscope pointer is at the 12 o'clock position and when it is rotating too fast.
Stating that a thermocouple measures absolute temperature directly, rather than measuring the temperature difference between its measuring and reference junctions.
Indicators are marked in degrees Celsius, so it feels natural to say the thermocouple 'reads' temperature; the reference-junction dependence is abstract and easily dropped under exam pressure.
What to do instead
Explain the Seebeck effect in your own words, then answer: what does the meter actually see, and what must the instrument do about the reference junction? Practise questions where the reference junction is at a non-zero temperature and calculate the correction.
Mixing up the deflection direction or scale sense of a ratiometer-type fuel gauge or a moving-coil instrument when the supply voltage changes.
Candidates learn 'moving coil responds to current' and then assume every indicator behaves the same way under voltage variation, ignoring the bridge or ratiometer arrangement that makes the reading supply-independent.
What to do instead
For each indicator type on the syllabus, write what the moving element responds to and how a change in supply voltage affects the reading. Use the ratiometer as the worked example: two coils, deflection set by the ratio of currents, so supply voltage variation cancels.
Describing a magnetic compass turning error and acceleration error as the same phenomenon, or giving the wrong hemisphere for each.
Both errors arise from the vertical component of the Earth's field and the dip angle, so the underlying physics looks identical; only the aircraft attitude and the direction of the resulting swing differ.
What to do instead
Write a two-row comparison: turning error (caused by dip during a turn, largest on north/south headings, leads or lags depending on hemisphere) versus acceleration error (caused by the pendulous mounting during acceleration/deceleration on east/west headings). Then state the correction for each hemisphere explicitly.
Assuming a gyroscope's rigidity in space is perfect and ignoring real drift, or reversing which property (rigidity or precession) explains a given instrument behaviour.
Textbook descriptions present rigidity and precession as separate ideal properties, and candidates memorise the definitions without linking each to a specific instrument behaviour such as apparent drift or turn indication.
What to do instead
For each gyro instrument (attitude indicator, directional gyro, turn coordinator), state which property produces the indication and which produces the error. Then list the causes of real drift and apparent drift, and say which is corrected by a slaving or erection system.
Treating a radio altimeter as a barometric instrument and quoting barometric settings or QNH when asked how it derives height.
Both instruments display height, and revision notes often list them together under 'altitude measurement', so the measurement principle gets blurred.
What to do instead
Write the measurement chain for each: radio altimeter transmits a signal, measures time or frequency shift to the ground return, and displays height above terrain; barometric altimeter compares static pressure with a set reference. Then answer which one is affected by terrain and which by pressure setting.
Answering instrument-system questions from memory of a block diagram without stating the signal path from sensor to indicator, so partial-credit explanation questions score poorly.
Multiple-choice practice rewards recognising the right component name, but written and multi-mark questions require the sequence and the function of each element, which candidates have never rehearsed aloud.
What to do instead
For each system on the syllabus, practise a 30-second spoken walkthrough: sensor, signal conditioning or transmitter, wiring or bus, indicator, and power supply. Record yourself and check that every element is named in order and that you can state what the indication does if one element fails.