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1Which two pressure components are measured by the Pitot-Static system to determine Indicated Airspeed (IAS)?

A.Total pressure from the pitot tube and static pressure from the static port

B.Dynamic pressure from the static port and ambient pressure from the pitot tube

C.Manifold pressure from the intake and ambient static pressure

D.Barometric pressure from the altimeter capsule and dynamic pressure from the static port

Explanation: The Airspeed Indicator (ASI) operates by measuring the difference between Total (pitot) pressure and Static pressure. Total pressure is the sum of dynamic pressure and static pressure ($P_{total} = q + P_{static}$), so subtracting static pressure yields dynamic pressure ($q = \frac{1}{2}\rho V^2$), which is converted into airspeed.

2What is the primary internal mechanism inside a traditional mechanical Airspeed Indicator (ASI)?

A.An evacuated sealed aneroid capsule inside an airtight casing

B.A flexible diaphragm receiving pitot pressure inside, surrounded by static pressure inside an airtight casing

C.A Bourdon tube connected directly to the static vent

D.A torque tube driven by a magnetic drag cup

Explanation: In a mechanical ASI, pitot pressure is fed into the interior of a flexible metal capsule/diaphragm, while static pressure fills the airtight instrument casing surrounding the capsule. The expansion or contraction of the capsule represents dynamic pressure, which drives the pointer via linkage.

3An altimeter measures height above a selected pressure datum based on which physical law or principle?

A.Rate of change of static pressure with speed

B.Lapse rate of atmospheric static pressure with increasing altitude

C.Differential dynamic pressure between upper and lower wing surfaces

D.Electromagnetic wave reflection from the Earth surface

Explanation: Atmospheric pressure decreases predictably with increasing altitude in the troposphere (~1 hPa per 30 ft near sea level). An altimeter senses static pressure surrounding its sealed aneroid capsule and converts pressure changes into an altitude display based on standard atmosphere lapse rates.

4How does a standard mechanical Vertical Speed Indicator (VSI) measure rate of climb or descent?

A.By comparing pitot total pressure against cabin static pressure

B.By measuring the rate of change of static pressure via a calibrated leak (capillary tube)

C.By measuring gyro precession forced by vertical acceleration

D.By using an evacuated aneroid capsule connected to the pitot line

Explanation: The VSI contains a flexible capsule connected directly to the static line. The instrument casing around the capsule is also connected to the static line, but through a calibrated leak (capillary tube). During a climb or descent, a pressure differential forms across the capsule due to the restriction in the capillary, deflecting the pointer proportional to rate of altitude change.

5If the pitot tube RAM air inlet is blocked by ice but the drain hole remains open, what will the Airspeed Indicator (ASI) read during level flight?

A.It will overread significantly as speed increases

B.It will drop to zero

C.It will freeze at the speed held prior to the blockage

D.It will act like an altimeter, climbing when speed decreases

Explanation: If the ram air inlet is blocked but the drain hole remains open, trapped air inside the pitot tube escapes through the drain hole into the atmosphere. The pressure inside the pitot tube drops to ambient static pressure, eliminating the pressure differential across the ASI diaphragm, causing the indicator to drop to zero.

6If BOTH the pitot ram air inlet and pitot drain hole become completely blocked in level flight, what will happen to the ASI during a subsequent climb?

A.The ASI will drop to zero

B.The ASI will indicate zero during climb and overread during descent

C.The ASI will overread (indicate an increasing airspeed)

D.The ASI will underread (indicate a decreasing airspeed)

Explanation: When both pitot ram inlet and drain hole are blocked, total pressure $P_{total}$ is trapped inside the pitot capsule. As the aircraft climbs, ambient static pressure in the casing surrounding the capsule decreases. This causes the capsule to expand, making the ASI overread (behave like an altimeter).

7If both the ram inlet and drain hole of a pitot tube are blocked at 8,000 ft and the aircraft descends to 2,000 ft at constant true airspeed, what will the ASI indicate?

A.An erroneously high airspeed (overreading)

B.An erroneously low airspeed (underreading)

C.Correct indicated airspeed

D.Zero airspeed

Explanation: During descent with trapped pitot pressure, ambient static pressure inside the ASI instrument case increases as altitude decreases. This higher static pressure compresses the capsule inward against the fixed trapped pitot pressure, causing the ASI to underread (indicate lower than actual speed).

8What are the indications on the Altimeter, VSI, and ASI if the static port becomes completely blocked during a climb?

A.Altimeter freezes at blockage altitude; VSI drops to zero; ASI underreads in climb

B.Altimeter continues to climb; VSI overreads; ASI overreads in climb

C.Altimeter freezes at blockage altitude; VSI freezes at rate of climb; ASI drops to zero

D.Altimeter drops to sea level; VSI shows maximum descent; ASI overreads in climb

Explanation: When the static port blocks, trapped static pressure stays constant. The Altimeter freezes at the altitude where blockage occurred. The VSI calibrated leak equalises pressure inside casing and capsule, returning VSI to zero. In a climb, as actual pitot pressure decreases with altitude while static casing pressure remains fixed at lower altitude, the ASI underreads.

9If the static vent becomes blocked during cruise and the aircraft subsequently descends, how will the ASI behave?

A.The ASI will underread (show lower speed than actual)

B.The ASI will overread (show higher speed than actual)

C.The ASI will freeze at the cruise airspeed

D.The ASI will drop immediately to zero

Explanation: With trapped static pressure from a higher cruise altitude (low pressure), descending increases actual dynamic pitot pressure inside the ASI capsule. Because static casing pressure remains artificially low, the capsule expands more than normal, causing the ASI to overread during descent.

10In an unpressurised aircraft, selecting an alternate static source located inside the cockpit will typically cause which instrument errors?

A.Altimeter underreads, ASI underreads, VSI momentarily indicates descent

B.Altimeter overreads, ASI overreads, VSI momentarily indicates climb

C.Altimeter reads correctly, ASI drops to zero, VSI freezes

D.Altimeter overreads, ASI underreads, VSI remains unaffected

Explanation: Airflow over the fuselage creates a slight suction inside an unpressurised cabin, making cabin pressure slightly lower than ambient external static pressure. Exposing instruments to lower pressure causes the Altimeter to overread (show higher altitude), the ASI to overread (larger differential), and the VSI to show a momentary climb.

About the SACAA CPL Instruments Exam

The SACAA CPL Instruments & Electronics examination is a mandatory theoretical knowledge subject for obtaining a Commercial Pilot Licence (Aeroplane) in South Africa. It covers pitot-static flight instruments, gyroscopic instruments, magnetic compasses, EFIS displays (AHRS/ADC), engine power and engine monitoring instruments, stall warning devices, and autopilot flight guidance systems.

Exam sponsor: South African Civil Aviation Authority (SACAA). The requirements and fees below concern the certification or admission exam, separate from our free practice resources.

Questions

35 questions

Time Limit

90 minutes

Passing Score

75%

Exam / Certification Fees

R450 per subject sitting under SACAA Part 187 user fees

Exam sponsor website

Fees, eligibility, and exam policies can change. Confirm them with the exam sponsor before applying or paying.

Official sources

Our practice resources: topics covered

We aim to reflect publicly available exam outlines and topic information in our study resources. Coverage, format, and difficulty may differ from the actual exam, and we cannot guarantee that every detail is accurate or current. Confirm exam requirements, fees, and policies with the official exam sponsor.

25%

Pitot-Static System & Pressure Instruments

Operation, calibrations, errors (position, maneuver, compressibility, density), blockages, alternate static sources, pitot heating, and ISA calculations.

25%

Gyroscopic Principles & Gyroscopic Instruments

Rigidity in space, precession, power sources (vacuum/pneumatic, electric), attitude indicator, directional gyro, turn coordinator/turn & slip indicator, and gyro transport drift.

15%

Magnetic Compasses & Direct/Remote Indicating Systems

Earth magnetic field, variation, deviation, acceleration/turning errors (ANDS/UNOS), direct-reading compasses, flux valves, and slaved gyro-magnetic compass systems.

15%

Electronic Flight Instrument Systems (EFIS) & Avionics

Primary Flight Display (PFD), Multi-Function Display (MFD), Air Data Computer (ADC), Attitude & Heading Reference System (AHRS), flux gates, and reversionary modes.

10%

Engine Power & Monitoring Instrumentation

Direct and remote sensors, tachometer, manifold pressure gauge, EGT, CHT, oil temperature/pressure, Bourdon tube, ratiometer, and thermocouple principles.

10%

Stall Warning Systems & Flight Guidance / Autopilot

Stall warning detectors, angle-of-attack indicators, autopilot control loops, servomotors, flight director guidance, auto-trim, and automatic disengagement criteria.

Preparing for the SACAA CPL Instruments Exam

What You Need to Know

Passing score: 75%
Exam length: 35 questions
Time limit: 90 minutes
Exam / certification fees: R450 per subject sitting under SACAA Part 187 user fees Official sources

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SACAA CPL Instruments: Suggested Study Strategy

1Master pitot and static blockage scenarios for both climbs and descents (remember pitot blockage acts like an altimeter, static blockage freezes altimeter and VSI).

2Understand Southern Hemisphere magnetic compass errors: acceleration errors on East/West headings (SAND: Accelerate South, Decelerate North) and turning errors on North/South headings (UNOS/ONUS).

3Learn calculations for Pressure Altitude, Density Altitude, ISA temperature deviation, TAS from CAS/altitude, and Gyro Drift rate (15° × sin Latitude per hour).

4Study EFIS architecture and failure modes: know what red 'X' flags on PFD signify (ADC vs AHRS failure) and how reversionary mode works.

5Review engine instrument principles: Bourdon tubes for pressure, thermocouples for EGT/CHT, and ratiometers for temperature.

Frequently Asked Questions

What is the pass mark for the SACAA CPL Instruments exam?

Under CAR 61.01.10, the pass mark for all SACAA theoretical knowledge examinations is 75%. There is no negative marking, so candidates should ensure all 35 questions are answered before the 90-minute timer expires.

How many questions and how much time is allowed for SACAA CPL Instruments?

The official exam consists of 35 multiple-choice questions delivered via the PEXO computer testing system, with a total time allowance of 90 minutes.

What is the fee for the SACAA CPL Instruments exam?

The exam fee is R450 per subject sitting under SACAA Part 187 user fees. Additional ATO examination invigilation or facility charges may apply depending on the testing center.

Which subjects are emphasized in SACAA CPL Instruments & Electronics?

Key topics include pitot-static system blockages and errors, gyroscopic principles (precession, rigidity, drift), direct and remote magnetic compass errors, EFIS architectures (PFD, MFD, ADC, AHRS), engine sensors, stall warning devices, and flight director/autopilot loops.

How long are CPL theory passes valid under SACAA Part 61?

All required CPL theoretical subjects must be passed within an 18-month window from the first successful subject pass. Candidates then have 36 months from the date of the final theory pass to complete the CPL practical flight test.

Free Practice Questions for SACAA CPL Instruments

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