Aircraft Instrument Systems Oral Exam Questions and Answers
Practice tracing an instrument indication from its input to its display. These questions follow Airframe subject AM.II.H, Aircraft Instrument Systems, covering pressure instruments, temperature sensing, the magnetic compass, gyros, and annunciators.
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What should my answer explain?
For each instrument, say what is sensed, how that input moves or changes something, and how the result appears to the pilot. Separate the analog mechanism in a question from a digital system that may calculate the indication.
For maintenance prompts, say which component could be damaged and the precaution that protects it. Use the aircraft manufacturer's procedures when moving from study to an actual instrument-system check.
FAA-S-ACS-1, p. 32, AM.II.H
Use these prompts alongside the ACS codes on your Airman Knowledge Test Report (AKTR); the ACS says deficient knowledge areas shown by those codes are retested during the oral. Rehearse an answer before opening the cited source to check it. For the other Airframe subjects, see the Airframe oral questions overview and the ACS codes guide.
Which ACS elements do these questions practice?
The table identifies the elements used below. It is not a prediction of your assigned questions.
| ACS element | Study focus |
|---|---|
| AM.II.H.K10 | Pitot-static system. |
| AM.II.H.K4 | Pressure indicating instruments. |
| AM.II.H.K2 | Magnetic compass inspection and operation. |
| AM.II.H.K5 | Temperature indicating instruments. |
| AM.II.H.K7 | Gyroscopic instruments. |
| AM.II.H.K1 | Annunciator indicating systems and the meaning of warning, caution, and advisory lights. |
| AM.II.H.R1 | Use of pressurized air and water during maintenance or cleaning of aircraft instrument systems. |
Which oral exam questions can I practice?
These practice questions are built from ACS elements; they are not actual FAA test questions. The short explanations are study answers, not complete aircraft maintenance procedures.
1. Which three common instruments use the pitot-static system?
ACS: AM.II.H.K10
The altimeter, airspeed indicator, and vertical speed indicator are the three most common pitot-static instruments. All three connect to static pressure. The airspeed indicator also connects to pitot pressure.
FAA-H-8083-31B, Airframe, p. 10-12
2. How does an analog pressure altimeter respond to a climb?
ACS: AM.II.H.K4
Static pressure enters the instrument case and surrounds a sealed aneroid. As the aircraft climbs and pressure falls, the aneroid expands. Linkages and gears translate that movement into pointer movement to indicate altitude.
FAA-H-8083-31B, Airframe, p. 10-16
3. How does an analog vertical speed indicator sense a climb?
ACS: AM.II.H.K4
Static pressure reaches the diaphragm without restriction, while case pressure changes through a calibrated orifice. During a climb, diaphragm pressure falls faster than case pressure, causing the diaphragm to contract and the pointer to indicate a climb. This basic mechanism has a six-to-nine-second lag before a stable differential pressure is established. An IVSI reduces the lag with a small accelerometer that pumps air into or out of the diaphragm.
FAA-H-8083-31B, Airframe, p. 10-21 · FAA-H-8083-31B, Airframe, p. 10-22
4. What pressures does an analog airspeed indicator compare?
ACS: AM.II.H.K10
The airspeed indicator compares pitot ram-air pressure in its diaphragm with static pressure in the surrounding instrument case. Changes in that difference expand or contract the diaphragm and move the pointer through a linkage. The instrument face is calibrated in knots or miles per hour.
FAA-H-8083-31B, Airframe, p. 10-23
5. Why use an air data computer instead of compensating each instrument separately?
ACS: AM.II.H.K10
On high-performance aircraft, the many air temperature and density factors must be compensated for, and an air data computer is commonly used for this. One-time compensation calculations can be done inside the computer, so compensating devices do not have to be built into numerous individual instruments. Using an ADC also simplifies the pitot-static plumbing into a lighter, simpler system with fewer connections, so it is less prone to leaks and easier to maintain. Digital air data computers can also check the plausibility of the data they receive and automatically alert the crew to a parameter that is out of the ordinary.
FAA-H-8083-31B, Airframe, p. 10-15
6. What causes magnetic compass deviation?
ACS: AM.II.H.K2
The compass magnets align with the composite field made up of all magnetic influences around them, including local influence from metallic structures near the compass and operation of the aircraft's electrical system. That is magnetic deviation, and it alters the magnets' alignment with the earth's magnetic field. To correct it, the N-S and E-W compensating screws on the face of the instrument are turned to move small permanent magnets that counterbalance those local influences. The process for knowing how to adjust for deviation is called swinging the compass. Deviation should never be more than 10 degrees; nonferrous mounting screws and shielding or twisting the wire to the compass lamp help keep it to a minimum.
FAA-H-8083-31B, Airframe, p. 10-42 · FAA-H-8083-31B, Airframe, p. 10-43
7. How does a thermocouple produce a temperature indication?
ACS: AM.II.H.K5
A thermocouple is a circuit or connection of two unlike metals. The metals touch at two separate junctions. Heating one junction above the other produces an electromotive force; measuring that voltage gives a temperature indication. Common uses include reciprocating-engine cylinder head temperature and turbine-engine exhaust gas temperature.
FAA-H-8083-31B, Airframe, p. 10-37
8. What are rigidity in space and precession in a mechanical gyro?
ACS: AM.II.H.K7
Rigidity in space means that the rotor of a spinning free gyro keeps pointing in the same direction no matter which way its base is positioned; attitude and directional instruments that use gyros rely on this as their reference. Precession is how the gyro reacts when a force is applied to the axis of the spinning gyro: the axis does not tilt, and the gyro responds as though the force were applied 90 degrees around in the direction of rotor rotation. This predictable, controlled precession is used in a turn-and-bank instrument.
FAA-H-8083-31B, Airframe, p. 10-51 · FAA-H-8083-31B, Airframe, p. 10-52
9. What do green, yellow, and red annunciator indications mean?
ACS: AM.II.H.K1
Green indicates a satisfactory condition. Yellow cautions of a serious condition requiring further monitoring. Red identifies an unsatisfactory condition needing immediate attention or action. Each light is identified by the name of the system it represents and is usually covered with a colored lens that shows the meaning of the condition it announces.
FAA-H-8083-31B, Airframe, p. 10-73
10. What precautions protect instruments when clearing pitot-static lines with air?
ACS: AM.II.H.R1
Water trapped in a pitot-static system may cause inaccurate or intermittent indications. Many systems have drains at the low points to remove moisture during maintenance. Lacking drains, dry compressed air or nitrogen may be blown through the lines. Always disconnect all pitot-static instruments first, and always blow from the instrument end of the system toward the pitot and static ports. That procedure must be followed by a leak check. During any pitot-static check, follow the manufacturer's instructions and apply and release pressure and suction slowly to avoid damaging the instruments.
FAA-H-8083-31B, Airframe, p. 10-81
How do I answer it out loud?
Draw three boxes labeled altimeter, airspeed indicator, and VSI. Connect each box to its pressure source, then explain the two different pressure paths inside the analog VSI. Finish by explaining out loud how you would remove water from the lines and what protects the instruments while you do it.
How should I check my practice answer?
After speaking, compare your explanation with the answer and the cited FAA page. Check the reason, the conditions, and any limitation stated in the source. Repeat the question without looking until you can give a connected spoken answer.
Use references during study, then close them for the spoken rehearsal. The ACS states that reference material is not allowed for answering oral questions.
What else should I know?
Does the altimeter need a pitot-pressure connection?
No. The altimeter is connected to the static pressure system. Of the three most common pitot-static instruments, the airspeed indicator is the one that is additionally connected to the pitot pressure system.
FAA-H-8083-31B, Airframe, p. 10-12
Why is an analog VSI slower to settle than an instantaneous VSI?
Because the case pressure has to pass through the restriction of a calibrated orifice, the basic VSI mechanism has a lag of six to nine seconds before a stable differential pressure is established. An IVSI adds a small, lightly sprung dashpot, or piston, that reacts to an abrupt climb or descent and pumps air into or out of the diaphragm, hastening the establishment of the pressure differential that causes the indication.
FAA-H-8083-31B, Airframe, p. 10-21 · FAA-H-8083-31B, Airframe, p. 10-22
What are common thermocouple applications in aircraft?
Two common applications are measuring cylinder head temperature (CHT) in reciprocating engines and exhaust gas temperature (EGT) in turbine engines. Thermocouples are used to measure high temperatures. Iron and constantan, or copper and constantan, are common for CHT, while chromel and alumel are used for turbine EGT. The leads are designed to provide a specific resistance, so their material, length, or cross-sectional size cannot be altered without compensating for the change.
FAA-H-8083-31B, Airframe, p. 10-37 · FAA-H-8083-31B, Airframe, p. 10-38
Can I blow compressed air through connected pitot-static instruments?
No. Always disconnect all pitot-static instruments before blowing dry compressed air or nitrogen through the lines, and always blow from the instrument end of the system toward the pitot and static ports. The procedure must be followed by a leak check.
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