Differential Compression Test for the A&P Practical
A differential compression test on the Airframe and Powerplant (A&P) practical calls for a safe setup and a correct interpretation of the readings. This oral and practical (O&P) guide explains the Federal Aviation Administration (FAA) handbook’s general differential pressure method. Engine-specific steps and limits are per the engine manufacturer’s instructions.
On this page
- What does the O&P practical evaluate?
- Start with the assigned data and a safe setup
- What does the test measure?
- Which ignition and propeller hazards come first?
- What is the handbook’s general workflow?
- When does AC 43.13-1B apply?
- Where does the calibrated orifice fit into the check?
- How do you locate a leak without inventing a repair?
- Which mistakes should you catch during rehearsal?
What does the O&P practical evaluate?
The FAA Airman Certification Standards (ACS) list the hands-on skills below. A Designated Mechanic Examiner (DME) evaluates the assigned project against the performance standard in the downloaded test.
FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929, printed 16, PDF 16, 5-5-11-7C3)g)
Each practical project contains a skill element and two practical questions. You may use applicable reference material for both the skill and the questions.
FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929, printed 16, PDF 16, 5-5-11-7C3)c)
A project is pass/fail. Passing requires a successful skill demonstration on the first attempt and successful answers to both associated practical questions.
FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929, printed 16, PDF 16, 5-5-11-7C3)f)
The FAA Mechanic Test Generator (MTG) FAQ says projects are standalone. The two practical questions are asked at a safe and appropriate time during the project.
FAA Mechanic Test Generator FAQs, updated May 22, 2023, unnumbered page, PDF 4
The FAA sample test shows the project format: description, given equipment or information, performance standard, ACS code, references, and associated questions. It is marked for practice purposes only.
FAA sample oral and practical test, unnumbered page, PDF 6
Our study advice: use this guide to rehearse the sequence and explain your decisions. A published skill is a study target, not a promise that your downloaded test will assign this exact task.
| ACS skill | Exact published skill |
|---|---|
| AM.III.C.S1 | Perform a compression check on a cylinder. |
| AM.III.C.S2 | Evaluate powerplant for compliance with FAA-approved or manufacturer data. |
FAA-S-ACS-1, printed 46, PDF 51
Start with the assigned data and a safe setup
Use current maintenance and overhaul publications and procedures. The ACS safety appendix includes proper information and tools, equipment preparation, skill in handling tools, and required personal protective equipment (PPE), including safety glasses and hearing protection.
FAA-S-ACS-1, printed A-3, PDF 67
Disregard for safety will result in failure. If a project requires an action that would jeopardize safety, the evaluator will ask you to simulate that portion.
FAA-S-ACS-1, printed A-3, PDF 67
Our study advice: identify the applicable manufacturer’s instructions and any required approved data before starting. These handbook summaries do not replace the procedure for the assigned aircraft, engine, component, or equipment.
What does the test measure?
A cylinder compression test checks whether the valves, piston rings, and pistons adequately seal the combustion chamber. Excessive pressure leakage prevents the cylinder from developing its full power.
FAA-H-8083-32B, Powerplant, Chapter 10, printed 10-39, PDF 39
The differential pressure tester measures leakage through the cylinder. With both valves closed and the piston at top-center compression stroke, air is applied through the tester’s fixed orifice. The handbook identifies that piston position for this test.
FAA-H-8083-32B, Powerplant, Chapter 10, printed 10-39, PDF 39
Our study advice: distinguish the regulated-pressure gauge from the cylinder-pressure gauge. Record both sides as the applicable procedure requires. A reading alone is not a diagnosis or an engine-specific acceptance decision.
Which ignition and propeller hazards come first?
Be sure the ignition switch is OFF so there is no accidental firing of the engine. The handbook directs this before removing the accessible spark plugs for the compression test.
FAA-H-8083-32B, Powerplant, Chapter 10, printed 10-39, PDF 39
Keep the propeller path clear of all objects. Cylinder air pressure is sufficient to rotate the propeller if the piston is not at top dead center.
FAA-H-8083-32B, Powerplant, Chapter 10, printed 10-40, PDF 40
Our study advice: treat control of propeller movement, safe positioning, and the applicable ignition precautions as prerequisites. If the assigned setup cannot be demonstrated safely, stop and use the evaluator’s direction on simulation. This guide is not a stand-alone procedure for pressurizing a cylinder.
What is the handbook’s general workflow?
Review the engine’s maintenance records and earlier compression results. The handbook says these help identify progressive wear and establish necessary maintenance actions. Identify removed spark plugs with their cylinders so their condition can assist diagnosis.
FAA-H-8083-32B, Powerplant, Chapter 10, printed 10-39, PDF 39
The handbook calls for testing as soon as possible after shutdown while the parts are freshly lubricated. It gives a different preparation case during engine buildup or for individually replaced cylinders: running the engine first is not necessary, and lubrication is addressed before testing.
FAA-H-8083-32B, Powerplant, Chapter 10, printed 10-39, PDF 39
Our study outline: obtain the correct engine and tester instructions; prepare the engine and identify the plugs; install the specified adapter and tester; position the piston at top dead center on its compression stroke; apply pressure and obtain readings using the prescribed control sequence. Maintain control of the propeller throughout. This outline deliberately leaves valve operations, pressure settings, and acceptance limits to the applicable instructions.
Check all cylinders and record the readings. The handbook compares each reading with the minimum specified for the engine being tested; it gives no single universal rejection limit.
FAA-H-8083-32B, Powerplant, Chapter 10, printed 10-40, PDF 40
When does AC 43.13-1B apply?
Advisory Circular (AC) 43.13-1B is listed as Active. The AC’s methods concern inspection and repair of nonpressurized areas of civil aircraft only when no manufacturer repair or maintenance instructions exist; its data generally concerns minor repairs.
FAA AC 43.13-1B: Active status and applicability, retrieved October 20, 2026
For major repairs, the AC’s identified repairs may only be used as a basis for FAA approval. The repair data may also be used as approved data, with chapter, page, and paragraph listed in block 8 of FAA Form 337, when appropriate to the product, directly applicable to the repair, and not contrary to the manufacturer’s data.
FAA AC 43.13-1B: Active status and applicability, retrieved October 20, 2026
Where does the calibrated orifice fit into the check?
The FAA descriptions use the terms fixed orifice and restrictor orifice. AC 43.13-1B says the restrictor dimensions should be sized for the particular engine; its general procedure supplements the particular tester manufacturer’s instructions.
AC 43.13-1B with Change 1, editorial update, printed 8-8, PDF 446 · AC 43.13-1B with Change 1, editorial update, printed 8-9, PDF 447
Our study advice: when the assigned engine’s instructions call for a calibrated orifice or a tester calibration check, use those instructions and the specified test apparatus. Do not invent a master-orifice procedure, change the restrictor, or transfer a limit from a different tester or engine. The FAA’s general description does not supply a universal engine-specific calibration check.
How do you locate a leak without inventing a repair?
For a cylinder below the specified minimum, the handbook calls for further checks to determine whether leakage is past the exhaust valve, intake valve, or piston. Listen at the exhaust outlet for exhaust-valve leakage, the air intake for intake-valve leakage, and engine breather outlets for leakage past the piston rings.
FAA-H-8083-32B, Powerplant, Chapter 10, printed 10-40–10-41, PDF 40,41
Our study advice: report the observed location, readings, and applicable next instruction. Do not turn this general guide into permission to stake a valve, run an engine, or replace a cylinder without the manufacturer’s applicable procedure and limits.
The handbook’s valve-staking discussion expressly limits that procedure to manufacturer approval. Our study advice: if you mention valve staking during the test, state that condition.
FAA-H-8083-32B, Powerplant, Chapter 10, printed 10-40, PDF 40
Which mistakes should you catch during rehearsal?
Our checklist: do not confuse differential leakage with compression ratio; do not quote a universal pass number; do not read the wrong gauge; and do not ignore prior results or propeller movement. Explain the safety setup before explaining a low reading.
Frequently asked questions
Can carbon contamination affect a compression result?
Yes. The handbook lists carbon particles between a valve face and seat among the conditions affecting engine compression. Because it lists several such conditions, a low reading alone does not show which one is present; follow the applicable diagnostic procedure.
FAA-H-8083-32B, Powerplant, Chapter 10, printed 10-39, PDF 39
Is cylinder leakage percentage the same as lost engine power?
Not in the FAA handbook. It describes the tester as measuring cylinder leakage and says excessive leakage prevents full cylinder power, but it gives no formula converting a differential reading into a percentage of lost engine power.
FAA-H-8083-32B, Powerplant, Chapter 10, printed 10-39, PDF 39
Can I stop the O&P practical once enough projects have passed?
No. The current Order says the applicant must complete the entire practical test generated by the MTG to achieve a passing result. The Order’s note adds that the examiner must not stop the test at the point a passing score is guaranteed.
FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929, printed 15–16, PDF 15,16, 5-5-11-7C3)b)
Rehearse the spoken answer with DMEReady
DMEReady is an AI voice examiner for the A&P oral. You answer out loud in your phone or computer browser, and it asks follow-up questions the way a DME would. You can rehearse an oral built from the ACS codes on your Airman Knowledge Test Report (AKTR). It rehearses the oral; it does not train hands-on practical skills. The first drill is free, with no sign-up. DMEReady is an AI examiner, not a human DME. It is independent and is not affiliated with or endorsed by the FAA. It does not predict or guarantee results.
Start your free drillFor examiner fees and planning, read the DME exam cost guide.
Oral coming up soon? Try the full thing free for 30 days
We are running a small pilot before launch. If your oral and practical is in the next few weeks, you get every section of the voice mock oral free for 30 days, no card. In return, tell us honestly what helped and what didn’t.
Start the free pilotLimited spots. The pilot closes on November 30.
Passed a written? Get your oral study plan by email
Paste the ACS codes on your Airman Knowledge Test Report (AKTR). We’ll email you a study plan now, then two short tasks built from your codes: one on day 3, one on day 7. That’s all.
Related guides
- Engine Inspection Oral Exam Questions and Answers
- Reciprocating Engine Oral Exam Questions and Answers
- Ignition and Starting Systems Oral Exam Questions and Answers
- A&P Practical Projects: What the DME Evaluates on the O&P
Sources
Primary sources checked October 20, 2026.
- FAA Order 8900.1, Vol. 5, Ch. 5, Sec. 11, CHG 929
- FAA Mechanic Test Generator FAQs, updated May 22, 2023
- FAA sample oral and practical test
- FAA-S-ACS-1
- FAA-H-8083-32B, Powerplant, Chapter 10
- FAA AC 43.13-1B: Active status and applicability
- AC 43.13-1B with Change 1, editorial update
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
Report an error or ask a question
Email hello@dmeready.com.