DGCA CPL Technical General common mistakes
DGCA CPL Technical General · Understand the traps and how to avoid them.
On a hydraulic-system question, selects the 'series' system as the one in which two actuators can operate simultaneously, or says system pressure adds across actuators.
The words series and parallel are borrowed from electrical circuits, so candidates transfer the rule that series circuits share current and parallel circuits share voltage. Aircraft hydraulic schematics use the terms differently: parallel/open-center systems connect actuators across a common pressure line, while series systems route fluid through one unit to the next.
What to do instead
Trace the pressure line: if both actuators tee off the same pressure manifold, it is a parallel system and they can move together. If the return from one actuator feeds the next, it is series and they move in sequence. Do not use electrical voltage/current analogies for hydraulic pressure and flow.
Labels a balance tab as a trim tab and predicts that it moves in the same direction as the control surface; or labels an anti-balance tab as a servo tab.
All three are small surfaces hinged to a primary control, and diagrams often show only the linkage. The candidate remembers 'tab moves opposite' without tying it to the aerodynamic purpose.
What to do instead
Use purpose first: trim tab = pilot-adjustable tab that holds a control position and moves opposite the surface; balance tab = linked tab that moves opposite to reduce control force; servo tab = aerodynamically driven tab that moves the surface; anti-balance tab = moves same direction to increase feel. Match linkage, not shape.
Defines detonation as 'ignition before the spark plug fires' and pre-ignition as 'explosive burning after normal ignition'.
Both produce roughness, high CHT, and power loss, and the terms are often used together in engine-failure discussions.
What to do instead
Anchor on cause and timing: pre-ignition is ignition before the spark, usually from a hot spot such as carbon or a plug electrode; detonation is spontaneous combustion of the unburned end gas after normal spark, producing pressure spikes. Detonation can raise temperatures enough to cause pre-ignition, but the definitions are not interchangeable.
Reverses the governor response: says an overspeeding propeller is corrected by moving blades to fine pitch, or an underspeeding propeller by moving to coarse pitch.
Fine pitch is associated with high RPM during takeoff, so candidates assume fine pitch always increases RPM-true, but they forget the governor's corrective direction.
What to do instead
Think load: overspeed means the engine is turning the prop too fast, so the governor increases blade angle (coarser) to absorb more power and slow it; underspeed means the engine needs less load, so the governor decreases blade angle (finer). Check the direction of the pitch-change link, not the takeoff setting.
States that true airspeed is always higher than indicated airspeed, or treats indicated airspeed as the speed used for structural limits at all altitudes.
The rule 'TAS increases with altitude' is memorized without the density correction; candidates also confuse IAS with calibrated airspeed.
What to do instead
Separate the chain: IAS is the dial reading; CAS corrects IAS for position/installation error; TAS corrects CAS for density altitude and temperature. In the same CAS, TAS rises with altitude because density falls, but IAS can be higher or lower than CAS depending on the aircraft; use the published limit, such as VNE in IAS, for structural questions.
Says the turn coordinator and the turn-and-slip indicator both show bank angle, or says the attitude indicator is electrically driven while the turn coordinator is vacuum-driven.
The instruments look similar and both use a rate gyro; candidates memorize 'vacuum = attitude and heading' without the exceptions.
What to do instead
Distinguish display and power: turn-and-slip shows rate of turn (needle) plus slip/skid (ball); turn coordinator shows rate of turn plus bank (canted gyro) and is usually electric. Attitude and heading indicators are typically vacuum/pneumatic, but always answer from the aircraft/system description, not a blanket rule.
Says an alternator uses a commutator to produce DC, or that a generator and alternator can be paralleled without matching voltage and frequency.
Both are called 'generators' in everyday speech, and the diode rectifier is invisible in the symbol.
What to do instead
Alternator = AC output rectified by diodes; generator = DC output via commutator and brushes. For paralleling, match voltage and, for AC sources, frequency and phase; a reverse-current relay/cutout prevents battery drain. If the question shows a commutator, it is a DC generator.
Says the magneto needs the aircraft battery to produce spark, or that the ignition switch is turned 'on' to energize the magneto.
Automotive ignition is battery-powered and switch-on; aircraft magnetos are self-contained and the switch normally grounds the primary circuit to stop the engine.
What to do instead
Remember the magneto is a self-excited AC generator with a permanent magnet; the P-lead grounds the primary to kill the spark. 'Both/left/right' checks confirm each magneto independently. If the battery is off, a magneto-equipped engine can still run-hence the danger of moving a propeller.
Treats 2024 and 7075 as interchangeable, or says solution heat treatment alone produces the final strength of a precipitation-hardened alloy.
Both are high-strength aluminum alloys and the heat-treatment steps are taught as a sequence; candidates stop at quenching.
What to do instead
Use alloying element and sequence: 2024 is copper-based, good fatigue and damage tolerance; 7075 is zinc-based, higher strength but lower corrosion resistance. Precipitation hardening requires solution treatment, rapid quench, then aging (natural or artificial) to form precipitates. Quenching alone leaves a supersaturated, not fully strengthened, structure.
Chooses a bolt by diameter and length only, allowing the threaded portion to sit in the shear plane, or measures grip length as the full bolt length.
Grip length and thread length are marked on the bolt but easy to ignore under time pressure; candidates confuse overall length with unthreaded shank.
What to do instead
Grip length is the unthreaded shank between the head and the first thread; it must match the total thickness of the parts plus washers so threads do not bear in the hole. If grip is too long, add washers; if too short, use a longer grip bolt, not a longer thread. Use the AN/MS dash number to confirm grip.