CAA NZ ATPL Instruments & Navaids common mistakes
CAA NZ ATPL Instruments & Navaids · Understand the traps and how to avoid them.
Treating indicated altitude as true altitude when temperature is non-standard, e.g. assuming the altimeter reads terrain height on a cold day or accepting the published procedure altitude without cold-temperature correction.
Candidates memorise 'high to low, look out below' as a slogan but never internalise that the altimeter is calibrated to ISA and over-reads in cold air below the aircraft, so obstacle clearance on a cold day is less than the chart suggests. The pressure-setting drill dominates study time, leaving temperature error as an afterthought.
What to do instead
For every altimetry question, ask two questions in order: is the setting correct (QNH/QFE/standard), and is the temperature column colder or warmer than ISA? Cold air means the altimeter over-reads and true height is lower than indicated, so add a cold-temperature correction at and below the MSA/approach segment; warm air means the reverse. Practise the rule on a worked example: at an aerodrome 1000 ft above the altimeter subscale with an ISA deviation of -30 degrees C, the aircraft is roughly 120 ft lower than indicated, which is why operators apply a temperature-correction table to procedure minima.
Mixing up which gyro property gives which indication: attributing heading indication to rigidity in space and attitude indication to precession, or claiming a rate gyro measures attitude rather than rate of turn.
Rigidity and precession are learned as vocabulary pairs without being tied to a physical instrument. All three instruments use the same gyroscope, so under exam pressure candidates match properties to instruments by guesswork rather than by reasoning about what each instrument needs to display.
What to do instead
Anchor each instrument to the property it exploits. The attitude indicator needs a stable reference in space, so it relies on rigidity (with erection to gravity via pendulous vanes). The directional gyro also relies on rigidity, but because it is earth-referenced it must be reset against the magnetic compass and suffers apparent drift from earth rate and transport wander. The turn indicator needs to sense rotation about the vertical axis, so it uses precession and is a rate instrument, not an attitude instrument. Turn coordinator vs turn-and-slip: the coordinator's gyro is canted to sense both yaw and roll, which is why it can be used for standard-rate turns in the air but the ball alone is used for balance.
Applying the real-navigation formula with the wrong sign or the wrong component, e.g. adding westerly variation to a magnetic heading to get true, or applying variation to a track when the question asks for heading without accounting for drift.
Candidates learn 'variation east, magnetic least' as a chant but cannot reconstruct it, and they conflate the two separate conversions: true to magnetic (variation) and track to heading (drift/drift angle). Under time pressure the chant is applied to the wrong quantity.
What to do instead
Draw the three-line sketch every time: true north, magnetic north, and the aircraft's path, with the drift angle between heading and track. Work in one direction only, from the known to the unknown, and state the rule you are using in words: variation east means magnetic north lies east of true north, so magnetic bearings are numerically smaller than true bearings. Keep track-to-heading separate: heading equals track plus or minus drift, and drift is into the wind. Check the answer for plausibility: if the wind is from the left, the heading must be to the left of track.
Confusing the static source with the pitot source when diagnosing a blocked system: predicting the altimeter rises when the pitot head ices over, or expecting the airspeed indicator to behave like an altimeter when the static port blocks.
Both systems feed the same cluster and candidates memorise 'pitot affects ASI, static affects the others' without working through what each instrument actually compares. Blocked-pitot and blocked-static scenarios are then answered from memory of a diagram rather than from first principles.
What to do instead
Reason from what each instrument measures. The ASI compares total pressure (pitot) with static pressure, so a blocked pitot with a clear static makes the ASI behave like an altimeter: climb and it under-reads, descend and it over-reads, and it freezes at the value when the blockage occurred if the static is also blocked. The altimeter and VSI use static pressure only, so a blocked static port freezes the VSI at zero and makes the altimeter read a constant altitude, while the ASI drifts toward the equivalent of the trapped pressure. A blocked static also causes the ASI to under-read in a climb and over-read in a descent. Practise sketching the pressure paths for each failure and predicting all three indications before checking the answer.
Mixing up VOR and DME indications and geometry: quoting DME as slant range when the question asks for ground range, assuming a VOR radial is the same as the bearing to the station, or forgetting that a VOR receiver measures the phase difference between a reference and a variable signal to derive the radial.
Candidates learn 'VOR gives bearing, DME gives distance' and stop there. The subtleties, that the CDI shows the selected radial and the TO/FROM flag depends on which side of the station you are, that a radial is a line from the station outward, and that DME is slant range that must be corrected for altitude, are skipped because they seem like detail.
What to do instead
Fix the geometry in your mind: a radial is a magnetic bearing from the station, the aircraft's position is the intersection of a radial and a DME arc or another radial, and the CDI deflection shows angular displacement from the selected course regardless of heading. For DME, remember slant range equals ground range only at the station overhead; at 30,000 ft and 100 nm ground range the error is small, but at close range it is large, so a DME reading of 6 nm overhead at 36,000 ft is mostly altitude. Practise converting between radial, bearing, heading and track on a sketch before selecting an answer.
Answering RNAV/GNSS questions with outdated criteria: assuming GPS standalone gives precision approach capability, treating RNP AR as equivalent to RNAV 1, or ignoring that the navigation specification defines the required performance rather than the sensor.
The PBN framework is recent relative to the rest of the syllabus, and candidates learn sensor names (GPS, VOR/DME, INS) rather than navigation specifications (RNAV 1, RNAV 2, RNP 1, RNP APCH, RNP AR APCH). The distinction between RNAV, which requires on-board performance monitoring and alerting, and RNP, which adds containment, is easily blurred.
What to do instead
Learn the specifications as performance contracts. RNAV 1 and RNAV 2 require total system error within 1 or 2 nm respectively for 95 per cent of the flight time and apply in terminal and en-route airspace; RNP 1 adds on-board performance monitoring and alerting. For approaches, LNAV, LNAV/VNAV, LP and LPV are distinct minima with different vertical guidance and integrity: LPV uses SBAS and gives vertical guidance to a decision altitude, LNAV/VNAV uses baro-VNAV or SBAS, and LNAV is lateral only. RNP AR APCH has radius-to-fix legs, a required navigation performance below 0.3 nm and specific crew and aircraft approval. Whenever a question mentions a specification, ask what performance and alerting it demands, not which sensor is fitted.
Misreading TCAS/ACAS and GPWS/TAWS logic: giving a traffic advisory the same response as a resolution advisory, assuming TCAS issues manoeuvres in the vertical and horizontal planes, or treating a 'too low, gear' GPWS warning as a pull-up command.
Warning-system questions are answered from a general sense of urgency rather than from the defined aural messages and the required crew response. Candidates also mix up the two families of system, so TAWS modes are attributed to TCAS and vice versa.
What to do instead
Memorise the decision tree. A traffic advisory is advisory only: acquire visual contact and prepare, but do not manoeuvre on the TA. A resolution advisory is an instruction: respond immediately, and TCAS RAs are vertical manoeuvres only (climb or descend), never turns. Traffic advisories and RAs are inhibited below defined altitudes and above the aircraft's ceiling. For GPWS/TAWS, learn the modes by number and aural message, and note which are inhibited by configuration: mode 1 excessive descent rate, mode 2 excessive terrain closure, mode 3 altitude loss after take-off, mode 4 unsafe terrain clearance (gear and flap calls), mode 5 excessive deviation below glideslope, mode 6 callouts, and the reactive/predictive windshear functions. A configuration warning such as 'too low, gear' calls for a go-around or configuration change, not a maximum-performance pull-up, which is reserved for the terrain-closure and sink-rate warnings.
Confusing autopilot modes and their capture logic: assuming altitude hold captures on selection rather than on reaching the selected altitude, treating a selected heading as a track, or expecting the autopilot to maintain a glideslope without an armed approach mode.
Mode annunciator logic is learned from screenshots rather than from the sequence of arming, capture and hold. Candidates do not separate lateral from vertical modes, or selected from managed modes in an FMS-equipped aircraft.
What to do instead
Learn the state machine. Lateral modes (heading select, VOR/LOC, LNAV) and vertical modes (altitude hold, vertical speed, level change, VNAV path, glideslope) are independent, and each has an armed state before capture. Altitude hold engages only at the selected altitude; until then the aircraft follows the vertical mode in use. Localiser capture precedes glideslope capture, and the glideslope is only captured when the approach mode is armed and the aircraft is established. Selected modes follow dialled values (heading, altitude, speed); managed modes follow the flight plan and performance database. Read the annunciator to confirm which mode is active versus armed, and remember that a mode reversion (for example from VNAV path to vertical speed) changes the flight path.
Ignoring chart and plotting conventions: reading a bearing from a chart with a protractor against grid north, plotting a position without converting between chart datum and magnetic, or misreading a Jeppesen-style approach plate's plan view as if it were to scale.
Candidates treat chart work as mechanical and skip the datum and projection reasoning. On Lambert conformal conic charts the convergence of meridians means grid north differs from true north away from the standard parallel, and plan views on approach plates are deliberately not to scale.
What to do instead
Before plotting, identify the projection, the chart graticule and whether bearings are true, magnetic or grid. On a conic chart, grid north at the central meridian equals true north; elsewhere apply convergence, and remember the chart's scale varies with latitude. For approach plates, use the plan view for orientation and the profile and minima for vertical and lateral limits, not for measuring distances. Always state the datum and the north reference in your working, and cross-check a plotted position against a second navaid or a DME arc before committing to an answer.
Treating the magnetic compass as a reliable heading reference in turns and acceleration: expecting it to read correctly in a sustained turn, or applying the correct lead for a turn through north but not through south, or vice versa.
Candidates learn the dip and deviation causes separately from the turning and acceleration errors and then cannot apply them. The compass is the only instrument that depends on the earth's magnetic field, so its errors are geometry, not malfunction, and the exam tests whether the candidate can predict the direction of the error.
What to do instead
Learn the four errors as a set with direction. Turning error: in the northern hemisphere the compass under-reads (lags) when turning through north and over-reads (leads) through south, so on a northerly turn you roll out late and on a southerly turn early. Acceleration error: in the northern hemisphere the compass indicates a turn to the north when accelerating and to the south when decelerating in level flight, because the centre of gravity sits ahead of the pivot. Variation and deviation are corrections to be applied, not errors to be tolerated. For any compass question, sketch the hemisphere, the manoeuvre and the resulting card movement before answering.