Light Aircraft Association

Warning to others,
I expected not to have to worry about this one, but I have a split in my forehead from something I thought I knew about, I was doing a differential pressure test on a rebuilt engine . I held on tight to the test prop and added pressure, as it went up an I noted the very good pressure figure, turned off the air an left the pressure to drop, after a time I moved away and KAPOW Wallop! the prop came down on my head as I turned away causing a split on the hairline that sent me down, pushed by the prop with a lot of push, I thought the air had gone but it had not, I should know by now BUT!!! So beware it might be some time later but the pressure was still there. I know people will say it was stupid and obvious but it was stupid but NOT obvious, I thought it was long down but instead it was gently pushing off TDC and once it went passed by a small amount, down it came. I am very thankful it was a wooden test prop and not a sharp metal one, also if it had gone backwards it would be the balls..... REMEMBER THE BETTER THE FIGURES, THE SLOWER IT WILL LEAK AWAY

Oh dear, hope the prop isn't damaged

A friend of mine while turning his Bulldog prop, allowed it to flip over TDC. It didn't fire, but went over smartly and whacked him on the bonce. Lots of blood and a nice scar was the result.

Sorry to hear about your misfortune. Why Differential test? Surely you mean Compression Test.?

why do aero engines use the differential pressure test when everything else seems to use a simple compression test done with a gauge in a spark plug hole? The whole procedure seems to be a bit close to the danger zone. I have seen it done with a couple of guys struggling to hold TDC while a 3rd plays with the compressed air supply. Too risky.

Rans6....

rans6andrew wrote:why do aero engines use the differential pressure test when everything else seems to use a simple compression test done with a gauge in a spark plug hole?

I think the differential pressure / leak down test provides more information, if done properly.

Somebody explained it to me once but I got very lost in the detail. Between thee, me and the gatepost, I think the somebody was somewhat confused themselves.

I would think using compressed air negates the need to spin the engine over as required by a compression test, as well. Standing by a Lycoming like the little Dutch boy with his finger in the dam while the big mincer on the front thrashes past is not something I would care to try.

Can anybody enlighten us how the test is done and what it tells us?

Hi Rob, a compression test as per the type you do in a car where you spin the engine over will only tell you that there is some compression, at best you can compare the different cylinders but how much compression should there be? Are four pots with roughly the same compression all good, all bad or all averagely OK?
The differential compression check uses a standard baseline - 80psi - so each time you check you can see how a particular cylinder measures up to that baseline and whether it is improving, or getting worse (it can improve as a new engine beds in) since the last time you checked it. If it is low on compression - say it is holding only 65 psi, you can listen to find out where the 15 psi is leaking from - if you hear it coming from the exhaust then you have a duff exhaust valve, if you hear it from the engine breather then it is the rings that are at fault.
When doing a differential compression test it is very important to remove all the plug leads and make sure the ignition is OFF. As you open the tap to let in air, make sure somebody is holding the propeller, or it will turn and hit somebody!

It is possible to make your own Dif Pressure device. I have the drawings somewhere. I agree with Brian it is a two person job to do it safely and to get the best out of the data.

Thanks for that, Brian.

Anyway Dawes, where is this hairline - on the front or the back of yer 'ead !

YEAH YEAH!!
I still have some on my head not like some others, it was across the top of the forehead. I looked around just as it moved. It might be described as an hairline crack.
Just hope that Karma doesn't really exist. I could do with a laugh now.

Well last post by Mr McArthy made me laugh

I would also point out that I'm entitled to laugh as I've suffered misfortunes, through misadventure, at the hands of machinery.
I have one finger that is somewhat shorter as the result of one cock up

Whilst chuckling through this thread I have learnt a little more. I had never actually taken the time to understand what on earth a leak down test was.

So your suffering has not been in vain

when you use the "spin it over" and get the peak reading method you know if it is good because it reads the value given in your Haynes manual.

May not work well for hand prop installations though.

Rans6.........

With all due respect, the "spin over" test will give you a duff indication of combustion chamber leak integrity.