Watch the video below to learn how to do a compression check on a Piper Archer.
The text below is created from the video with the help of AI. For full steps we recommend watching the video.
Learning to perform a compression check on an aircraft engine is a vital skill for any aviation enthusiast or mechanic. A compression check helps find problems in the engine’s cylinders before they become serious issues.
This test measures how well each cylinder holds pressure, which tells you about the health of key engine parts like piston rings and valves.
The process starts with important safety steps to prevent the engine from starting unexpectedly. You need to disconnect spark plug leads, turn off the fuel supply, and make sure the ignition switches are in the off position.
When done correctly, a compression check gives you clear data about your engine’s condition and helps keep your aircraft running smoothly and safely.
Key Takeaways
- Safety preparations are critical before starting any engine compression test
- Regular spark plug inspections help catch early signs of engine wear
- Proper test readings tell you if cylinder parts need repair or replacement
Working Safely with Aircraft Engines
The first step is to make sure you’re operating under safe conditions.
Checking Your Ignition Settings
You need to turn both left and right magneto switches to the OFF position before starting work.
Put the fuel cutoff in the CUT position too.
For extra safety, locate the fuel selector valve and move it to OFF. These steps stop any fuel or spark from reaching the engine.
Working with Aircraft Fuel
Make sure the fuel system is completely shut off before you begin.
Turn the fuel selector valve to the OFF position to prevent any fuel from flowing to the engine.
Taking Off Spark Plug Wires
Remove each spark plug lead carefully. Place them in a marked tray to keep track of which wire goes to which plug. Label the top and bottom plugs for each cylinder to make reinstallation easier. This prevents any accidental sparks and lets you turn the propeller safely.
Always yell “Hot Prop” before touching the propeller to warn others nearby.
Safety Tips:
- Keep track of removed parts
- Use marked trays
- Stay alert of your surroundings
- Watch for people near the propeller
- Move slowly and carefully
Check Your Spark Plugs
Before starting the compression test, check each spark plug for damage. Focus on the insulator, which is the ceramic part. Look for any cracks, missing pieces, or breaks.
The bottom spark plugs often get dirty because of their location. Make sure they’re clean and ready to use.
Keep Your Plugs In Order
Place each spark plug in a marked tray as you remove it. Use separate spots for top and bottom plugs from each cylinder. This helps you remember which plug goes where when you put them back.
Tips for organizing spark plugs:
- Label spots for each cylinder number
- Keep top and bottom plugs separate
- Clean each plug before storing
- Check plug condition before storing
Safety note: Never mix up which plug came from which cylinder – this can cause engine problems later.
Finding Top Dead Center
Place your finger over the extension hole and turn the propeller. Listen for:
- Air pressure building up
- Hissing sounds
- Click from the magnetos opening
Look for the “TC1” mark on the starter ring. Line this up with the dot on the starter housing – this shows cylinder 1 is at top dead center.
Connect your compression tester to shop air (at least 90 PSI). Test the gauge by:
- Building pressure to 80 PSI on both sides
- Making sure both sides read the same
- Starting with 20 PSI for safety
Important: Always yell “hot prop” before touching the propeller. Stay alert for people walking near the aircraft.
Never stand in the propeller’s arc while doing these checks. Keep pressure balanced to avoid unwanted prop movement.
Performing the Compression Check
Before starting, check that your compression tester shows matching readings on both sides.
Connect it to shop air at 90 PSI and verify it reads 80 PSI on each gauge. A properly working tester will show equal numbers – for example, 80 over 80.
Looking at Pressure Numbers
Start with low pressure (20 PSI) when working near the propeller.
A good cylinder reads close to 80 PSI when tested against 80 PSI shop air. Watch for:
- Good reading: 75-80 PSI
- Warning sign: Below 60 PSI
Two main signs of problems:
- Hissing from the oil filler = worn piston rings
- Hissing from exhaust = bad exhaust valve