Measuring Voltage, Resistance and Voltage Drop for the A&P Practical

October 20, 2026 · Primary sources checked October 20, 2026

For the Airframe and Powerplant (A&P) oral and practical (O&P), our A&P practical multimeter rehearsal separates three jobs: voltage, resistance, and resistor voltage drop. For each job, identify the meter setting, connection points, circuit power state, and meaning of the reading before moving a probe.

What does this O&P project evaluate?

The Federal Aviation Administration (FAA) Airman Certification Standards (ACS) skills below provide the targets for our meter rehearsal.

One skill element and two practical questions make up a practical project.

FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929, printed 16, PDF 16, 5-5-11-7C3)c)

Reference use is permitted for both parts of that practical project.

FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929, printed 16, PDF 16, 5-5-11-7C3)c)

The evaluation standard for an individual project is pass/fail.

FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929, printed 16, PDF 16, 5-5-11-7C3)f)

If any portion of the project is not properly completed, the applicant fails the project: the skill demonstration must be accomplished successfully on the first attempt, and the two associated practical questions must be answered successfully.

FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929, printed 16, PDF 16, 5-5-11-7C3)f)

The applicant must complete the project to the required performance standard to pass, and the examiner can find that standard in the downloaded test. The Mechanic Test Generator (MTG) FAQ treats each practical project as a standalone task.

FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929, printed 16, PDF 16, 5-5-11-7C3)g) · FAA Mechanic Test Generator FAQs, updated May 22, 2023, unnumbered page, PDF 4

The FAA’s sample project sheet is explicitly for practice purposes only. Our practice advice: say which quantity you intend to measure before connecting a lead.

FAA sample oral and practical test, unnumbered page, PDF 6

FAA-S-ACS-1, printed 2, PDF 7

Risk list before connecting the multimeter

The ACS safety appendix includes use of current maintenance and overhaul publications and procedures.

FAA-S-ACS-1, printed A-3, PDF 67

The ACS safety precautions include, as examples, wearing safety glasses, hearing protection, and any other required personal protective equipment (PPE).

FAA-S-ACS-1, printed A-3, PDF 67

Under the ACS safety appendix, any disregard for safety is not tolerated and will result in a failure. A portion that would jeopardize safety will be simulated at the evaluator’s direction.

FAA-S-ACS-1, printed A-3, PDF 67

• The ACS measurement risk covers voltage, current, resistance, and capacitance measurements.
• High-voltage circuits, such as strobe lighting, are another listed electrical risk.
• Electricity applied to the body can cause severe burns where it enters and exits.
• When electricity is applied to the body, the nervous system is affected and can be damaged or destroyed.
• Our setup advice: use the current aircraft and meter instructions, the correct lead sockets and ratings, and the specified protective equipment for the assigned circuit.
• In a strobe light, the Airframe handbook says the flash is produced by a high voltage discharge of a capacitor housed in a dedicated power pack.
• Our rehearsal boundary: this guide gives no procedure for approaching or discharging a high-voltage strobe system; use the manufacturer’s instructions for that.

FAA-S-ACS-1, printed 2, PDF 7, AM.I.A.R1 · FAA-S-ACS-1, printed 2, PDF 7, AM.I.A.R3 · FAA-H-8083-30B, General, printed 1-1, PDF 22 · FAA-H-8083-31B, Airframe, printed 9-101, PDF 517

Measure voltage across the component

Use the voltmeter function across the component being checked. Select the correct alternating current (AC) or direct current (DC) voltage function before measurement. The probes go across the component in parallel, with polarity matched to that component.

FAA-H-8083-30B, General, printed 12-79, PDF 520

Standard practice connects the red lead to the positive meter jack and the black lead to the negative jack. The red probe goes on the component’s positive side and the black probe on its negative side to prevent meter damage or incorrect readings.

FAA-H-8083-30B, General, printed 12-79, PDF 520 · FAA-H-8083-30B, General, printed 12-79–12-80, PDF 520, 521

If the voltage magnitude is uncertain, the meter should be set to its highest voltage range first. Starting on the highest range prevents the meter from “pegging” and possible damage. Then select lower ranges until the voltage reads at mid-scale deflection; the handbook says readings taken at mid-scale are the most accurate.

FAA-H-8083-30B, General, printed 12-80, PDF 521

If the voltmeter has low resistance, it draws off a significant amount of current, which lowers the effective resistance of the circuit and changes the voltage readings. Use a high-resistance voltmeter to prevent shunting the circuit during a voltage measurement.

FAA-H-8083-30B, General, printed 12-71, PDF 512

Measure resistance with power removed and the part isolated

When using an ohmmeter, the tested component must be isolated and the circuit power source removed. To measure a resistor’s value, the resistor must be disconnected from the circuit. Disconnection prevents possible meter damage and inaccurate readings from the parallel circuit.

FAA-H-8083-30B, General, printed 12-82, PDF 523 · FAA-H-8083-30B, General, printed 12-81, PDF 522

Connect the ohmmeter leads across the external resistance to be measured. The ohmmeter’s internal battery supplies the measuring current. The analog ohmmeter should be zeroed before each use. For that analog adjustment, short the leads and adjust the pointer to zero resistance.

FAA-H-8083-30B, General, printed 12-71, PDF 512 · FAA-H-8083-30B, General, printed 12-81, PDF 522

Avoid contact with the circuit or probes while taking any measurement. Such contact can introduce a parallel path and misleading indications. A near-zero resistance indicates a good connection in the handbook’s continuity-check example. An open or very poor connection produces infinity or a very high resistance in that example.

FAA-H-8083-30B, General, printed 12-81, PDF 522

Current and capacitance cautions

The handbook’s ammeter precautions are to always connect the ammeter in series with the element through which the current flow is to be measured, and never to connect an ammeter across a source of voltage, such as a battery or generator. The resistance of an ammeter, particularly on the higher ranges, is extremely low, so even a volt or so can cause very high current to flow through the meter, causing damage to it. Before measuring a current, form some idea of its magnitude, then use a large enough scale or start with the highest range and work down.

FAA-H-8083-30B, General, printed 12-69–12-70, PDF 510, 511

For a suspected capacitor problem, the handbook’s basic ohmmeter test removes the capacitor from the circuit and first shorts its two leads to ensure that it is entirely discharged.

FAA-H-8083-30B, General, printed 12-81, PDF 522

Measure voltage drop across a resistor

Our practice sequence: after restoring the assigned circuit to its authorized voltage-test condition, select voltage and place the probes across the resistor’s two ends. In the handbook’s series-circuit troubleshooting example, a voltmeter across a resistor that is not shorted indicates part of the applied voltage. Across the shorted resistor in that example, the voltmeter reads zero. Across an open component in the handbook’s series circuit, the voltage drop equals the total series voltage. In a parallel circuit, the handbook says a voltmeter cannot be used to find an open, because when it is placed across an open resistor it reads the voltage drop in a parallel branch.

FAA-H-8083-30B, General, printed 12-84, PDF 525 · FAA-H-8083-30B, General, printed 12-82, PDF 523

Illustration, not an aircraft limit: at 0.20 ampere through a 20-ohm resistor, Ohm’s law gives a 4.0-volt drop, because 0.20 × 20 = 4.0.

FAA-H-8083-30B, General, printed 12-17, PDF 458

Common connection mistakes to catch

Our review prompts: distinguish voltage from resistance mode, identify the component’s actual terminals, remove power and isolate the part for resistance, and keep analog zeroing separate from a digital meter’s setup. Record the quantity, units, connection points, and circuit condition with the reading so your explanation can be checked.

Rehearse the meter explanation

DMEReady’s artificial intelligence (AI) examiner lets you rehearse the A&P oral out loud in a browser on a phone or computer. Follow-up prompts use the style of a Designated Mechanic Examiner (DME). The rehearsal covers oral responses; it does not provide hands-on practical instruction. The free first drill can be started without signing up. DMEReady is an AI examiner, not a human DME, and it is independent and not affiliated with or endorsed by the FAA.

Start your free drill

For examiner fees and planning, read the DME exam cost guide.

Related guides

• Electricity principles and oral questions
• Aircraft electrical system questions
• The O&P practical project standard
• Aircraft Battery Inspection and Servicing for the A&P Practical
• Wire Terminals and Splices for the A&P Practical

Sources

Primary sources checked October 20, 2026.

• FAA-S-ACS-1
• FAA-H-8083-30B, General
• FAA Mechanic Test Generator FAQs, updated May 22, 2023
• FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929
• FAA sample oral and practical test

About this guide: drafted with AI from the FAA sources listed on this page and checked claim by claim against them. Not reviewed by an A&P mechanic or DME. How we make our guides