Aircraft Engine Borescope

How To Perform An Aircraft Engine Borescope

An aircraft engine borescope is arguably one of the most valuable inspections performed on a piston aircraft engine. Why? It doesn’t require a lot of labor (remove spark plugs), and it sheds insight on how your piston engine operating.

We’re about results at Phalanx Aviation. In this article we’ll discribe how to perform an aircraft engine borescope inspection, and give you some borescope picture examples we’ve seen in the field. Then we’ll cover what to look for when performing an aircraft engine borescope inspection. Alright let’s jump in!

Aircraft Engine Borescope
Valve Exposed To High Temperatures
Table Of Contents

How To Perform An Aircraft Engine Borescope

Step 1: Gather tools & materials needed for the job. We created a list of tools and where to get them under the resources section below

Step 2: Gain access to the aircraft engine spark plugs. You’ll only have to remove one set of plugs (top or bottom). We usually remove the ones that are easiest to get to. 

Step 3: Borescope the cylinders. The three major areas to inspect are: Exhaust valve, intake valve, and cylinder side wall. This will take a little practice getting the best pictures. We usually use the 90 deg camera feature with this borescope. You’ll have to slightly bend the camera end to take a good shot of the valves.

Step 4: Upload images to a computer/phone and label the pictures. Simply write down the picture file name and cylinder # next to each other on a piece of paper. When your done taking pictures relabel the picture files to correspond to the correct cylinder.

Example: IMG001 = Cyl #1 Exhaust Valve, IMG002 = Cyl #1 Intake Valve, IMG003 = Cyl #1 Side Wall.

Step 5: Reinstall engine spark plugs. Use aviation anti-seize and the correct torque when installing spark plugs. See our reference section for the different engine torques, and anti-seize recommendation. 

Step 6: Re-install removed items to gain access to spark plugs. Make a logbook entry in the aircrafts engine log. “Removed top spark plugs, performed engine borescope inspection, pictures on file with aircraft owner, re-installed top spark plugs IAW engine manufactures instructions (insert torque and page reference from manual)” Make sure to insert a date, tach time, and your pilot number with signature. Performing an engine borescope is within the capabilities of an aircraft owner/operator (preventitive maintenance).

Pilot Aircraft Logbook Entry Example

If your in need of an A&P to perform an aircraft engine borescope inspection, contact one of our mobile aircraft mechaincs. We provide mobile aircraft maintenane in all 50 US states. If we don’t have a mechanic near you we’ll look for one at NO COST.

"If we do not have a mechanic near you we'll look for one at NO COST"

Aircraft Engine Borescope Pictures

Normal Exhaust Valve Flawed Exhaust Valve
Normal Aircraft Valve Aircraft Engine Borescope
No Action Needed. Simply inspect at next oil change or annual inspection. Valve will need replaced. Remove engine cylinder and send to engine shop for repairs.

Flawed Exhaust Valve:  Notice the green coloring. Green coloring is an indication of abnormally high valve temp. Consider changing your mixture or engine operating procedures if you continue to see green coloring on the valves. 

Normal Engine Side Wall Flawed Engine Side Wall
Aircraft Cylinder Side Wall Side_wall_bad
No major scoring from piston. Cylinder cross hatching still showing (good). Side wall scoring. Symtoms: Excessive oil consumption, aluminum piston pin wear, or low compression

What To Look For In An Aircraft Engine Borescope

Over Temped Valves

As discussed earlier valve coloring will provide insight as to how hot the cylinder is operating. Brown or gray colored valves are normal. Green or yellowish color is a sign of high temperatures. The next stage after a valve gets green in color is typically a mechanical failure (bend, or crack). In the flawed valve pictured above you’ll notice the valve is still rotating (the burn pattern is symetrical), but the valve has been over temped, and will need to be replaced. A valve that isn’t turning will typically have a moon shaped burn pattern on one of the valve’s sides. See the resource section below for an article that shows improper burn patterns.

"The next stage after a valve is green in color is typically a mechanical failure (bend, or crack)"

Exhaust valves endure the higher operating temperatures. This is why you’ll find exhaust valves manufacutured with a hollowed core filled with metalic sodium. This design helps dissipate heat. Learn more about aircraft engine valve design in the resource section below.

Valves Not Rotating

Aircraft piston engine valves are designed to rotate as they’re opening and closing. This prevents carbon build-up on the stem and valve seat area. Rotating valves can be identified by the symmetrical burn pattern on the valve face. Once a valve stops rotating an irregular burn partern will show. This will lead to early carbon build up on the valve and valve seat area.

"Rotating valves can be identified by the symmetrical burn pattern on the valve face"

Improper Valve Seating

Valves are designed to seat in one position on the valve seat area. The valve seat area on an aircraft piston engine cylinder is typcally a machined insert that is pressed into the cylinder. Improper valve seating can lead to low compression readings, and lower engine power output. A common reason for this is a worn valve guide. A worn valve guide will allow the valve to have side to side movement as it’s traveling up and down the valve guide. This causes improper seating of the valve which will eventually need repaired.

Cylinder wall flaws

Common cylinder wall issues include pitting from corrosion, and scoring from the piston or piston pin plug. Piston pin plug wear on the cylinder side wall is a common aircraft engine issue. The first indication of this is  excessive aluminum found in the oil filter. A quick borescope in the cylinders can show which cylinder has scoring on the side walls from the piston pin plug.

How To Repair Aircraft Piston Engine Valves

Most internal cylinder repairs require removal of the cylinder. We recommend sending the cylinder to an aircraft engine shop. See some piston aircraft engine shops listed below. These shops have the experience of repairing cylinders that typical A&P mechanics don’t have. When you ship the cylinder out for repairs, provide a sheet of paper discribing the issue with the cylinder along with your contact information.

In some cases if the valve seat area needs cleaned you can use valve grinding compond and the rope trick. This saves you from having to remove the cylinder.  This trick is only used to clean minor carbon build up on the valve and valve seat area. The rope trick really deserves its own write up… We’ll cover that seperately. 

In some circumstances all the aircraft engine needs is additional flight time. Before releasing an aircraft for flight ensure it’s safe. Perform a borescope inspection for over temping or mechanical failure. Engine compressions are supposed to be accomplished while the engine hot or warm.  A trip around the patch before an engine compression reading can save you a lot of headaches. 

"Aircraft Engine compressions are supposed to be accomplished with the engine hot or warm"

Summary

An aircraft engine boresope is a routine maintenance item that can be accomplish in one or two hours.  It’s one of lowest cost ways to check your engine’s health. We recommend borescopeing your aircraft engine annually or every 100 flight hours. This gives you the opportunity to catch and even adjust your operating procedures to prevent perminate damage.

If your in need of an A&P to perform an aircraft engine borescope inspection, contact one of our mobile aircraft mechaincs. We provide mobile aircraft maintenane in all 50 US states. If we don’t have a mechanic near you we’ll look for one at NO COST.

"If we do not have a mechanic near you we'll look for one at NO COST"

Aircraft Engine Borescope Resources