Preflight Checks for piston engines
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Jumbodriver, you're still off the reservation. The posts haven't been personal attacks on you. You need to grow up a little, and perhaps realize that everything isn't about you. Considerable information has been presented. If you don't like it, that's tough. If your feelings are hurt, that's tough. You're advocating bad information, and potentially dangerous and damaging information.
Say something dangerous and stupid, you get called on it. It's that simple. If you don't like it...that's just tough.
Perhaps if you have something meaningful to say, you should say it. Perhaps if you have nothing to say but whining about hurt feelings over your own misinterpretations, then you should say nothing. This is a technical forum. You may wish to try to stay on topic. The thread isn't about you, it's about preflighting an engine. Try to keep that in mind.
Say something dangerous and stupid, you get called on it. It's that simple. If you don't like it...that's just tough.
Perhaps if you have something meaningful to say, you should say it. Perhaps if you have nothing to say but whining about hurt feelings over your own misinterpretations, then you should say nothing. This is a technical forum. You may wish to try to stay on topic. The thread isn't about you, it's about preflighting an engine. Try to keep that in mind.
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Most knowledgeable mechanics understand that engine compression means very little, while most laymen place some degree of stockin it it as a symbol of engine health.
With smaller piston engines, both automotive and aircraft, one can feel the relative compressions between cylinders by rotating the engine by hand and feeling the relative effort for each compression stroke. Identifying which cylinder is down is another matter.
Radial engines of course have there own characteristics which would probably prevent one from feeling the compressions, SN3 could probably explain this better than me if he wasn't being so anal.
Cheers
Blackhand
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Props are routinely rotated backwards during maintenance during magneto timing with no adverse effect.
The quickest way to kill a vacuum pump is to let oil/solvents get onto the vanes.
The quickest way to kill a vacuum pump is to let oil/solvents get onto the vanes.
Some engines can be rotated backward, and some can't. If one has condensation inside the vacum pump casing and rotates the engine, it can cause vane breakage and pump failure in short order. This is more likely when rotating the wrong direction, but also possible in the event of rotating in the correct direction...solution there is to ensure the engine is preheated first...though moisture in a dry carbon vane pump can still cause pump failure whether it's frozen or not.
Mike Berry wrote a good article on the subject of pumps, a few years ago, which can be read here:
Vacuum Pumps: Wet vs. Dry
Well Blackhand must fall into the category of UNknowledgeable mechanic because I understand that compression is very important to piston engine performance ( Health).
Even with a calibrated test set and a qualified technician performing an actual compression test, the values received are highly subjective, and generally not repeatable Compression values go up with time, depending on who's doing the test, and they go down. Point is...actual compression doesn't mean all that much. The engine needs it to run, but the values thereof...don't really mean much.
Hearing someone talk about an engine with references such as "it's got great compressions!" is somewhat like hearing them talk about buying a car by saying "it's got cup holders!" A compression test, again with calibrated equipment being performed by a qualified technician, is still subjective to numerous factors previously discussed. When I perform this test, I'm not nearly so interested in the values as I am to listening to the exhuast, and induction, and crankcase through the oil filler cap, for leaks. I couldn't begin to tell you what's wrong with the engine at that point...it's simply an elementary diagnostic tool that gets me started. To suggest that one could do anything by pulling it through by hand on the line with their magic calibrated fingers is...ridiculous.
You're not going to get this information when turning the prop by hand without pressurizing the cylinders. A compression stroke without the addition of external pressurized air is not adequate to determine or diagnose the problem, certainly not as a pilot pulling the prop through by hand.
Pilots have all kinds of excuses for pulling the propeller through by hand. My favorite, usually passed down from one instructor to student who becomes an instructor and passes it to another student ad nauseam, is "I'm limbering up the engine," or "I'm limbering up the oil." This is of course, a statement of ignorance, as one is doing no such thing. The design of the engine doesn't permit such a thing, and all the actor is really doing is scraping protective lubricant off cylinder walls and other such places (valve stems, etc).
As far as feeling "relative compressions," what is it that magically makes the engine fail by hand that doesn't reveal itself in operation? The engine ran fine on shutdown, but while sitting idle, begins to stick and have other such problems? Ever stop to think that perhaps by pulling it through without adequate pressurized lubrication, you're creating the problems you're trying to find in the first place? We've been told by one offended poster that he learns things by pulling the engine through that can't be found when it's running...a truly amazing concept...as that's the best time to find the problem.
Pulling an engine through to check for hydraulic lock...valid in some cases, but not all...because even in cases where the check is necessary, often the starter motor provides more protection for the engine by employing a clutch...whereas one can bend a rod pulling on the propeller for leverage.
Pulling an engine through to find the compression stroke prior to hand propping...legitimate, though one certainly won't be able to tell the compression of the engine while doing this, or be able to diagnose it.
Pulling an engine through while performing maintenance...also legitimate under the proper circumstance.
Pulling an engine through to limber it up, to magically feel compressions and to hear and diagnose as a pilot on the line...not legitimate, and not very smart, either. Especially with a charge of fuel in the cylinders which can wash off lubricants on the cylinder walls before engine start (when the most wear occurs), cause the engine to fire with greater power inadvertently, leak into the exhaust and cause an exhaust (or induction) fire on start, and which is wasteful.
So far as pulling backward, it's not only a potential issue with accessories like the vacum pump (circumstance and pump dependent), but also with generator brushes, which can be chipped or broken by turning them the wrong way. It can be inappropriate for certain gear trains in accessory drives, or even in propeller reduction drives.
So far as preflighting the engine, again, one should make as thorough an inspection as possible. This includes tugging on everything that can be touched or felt, and considering even small oil leaks for their source. If the "checklist" says "Engine, Check," then this isn't adequate. No law or principle of reason suggests that one should do the bare minimum; check everything. This may include pulling the propeller through...just not for the reason that some enthusiastic, but misinformed posters have advocated.
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Mr SNS3 retorted
You are committed to being anal aren't you??
You will find that I am talking about relative compressions, not high verses low.
But then again it is probably not something a boggie pilot would pick up anyway.
Personally I just make sure the compression is above 55 and not leaking from any place that it should not be, thats for lycoming and continental opposed engines, radial engines are your forte - so will not comment.
Cheers
Blackhand
Your comments would indicate this to be the case; most people tend to blindly believe that high compression numbers are good, lower compression numbers are bad, and take a fairly generic view toward what compression really is.
You will find that I am talking about relative compressions, not high verses low.
But then again it is probably not something a boggie pilot would pick up anyway.
Personally I just make sure the compression is above 55 and not leaking from any place that it should not be, thats for lycoming and continental opposed engines, radial engines are your forte - so will not comment.
Cheers
Blackhand
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What exactly is a "boggie pilot?"
They are. But then so are horizontally opposed lycomings, continentals, franklins, etc, too. Garrrett/honeywell turboprops, pratt & whitney turboprops, allison turboprops, and garrett/honewell, GE, and pratt turbojets, too...
This isn't really relevant to preflighting a powerplant, per the original poster's request. Perhaps you might stay on track.
radial engines are your forte - so will not comment.
This isn't really relevant to preflighting a powerplant, per the original poster's request. Perhaps you might stay on track.
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I'm a low time pilot? I guess that's relative, depending on your perspective, and very possibly true...but not really relevant to the conversation. I've got experience on over 70 different types of aircraft at this point as a pilot and a lot more as a mechanic...but that's also not really relevant. Boggie, huh? Do you perchance speak English?
Must be that calibrated wrist of yours that can sense 55 differential psi...without the cylinder having been pressurized, without a gauge or compressor, without a test set. All with your hand as you pull it through, then. You're truly a wonder to behold...yet full of dangerous advice to those who might not know better.
Personally I just make sure the compression is above 55
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SNS3
You misunderstand, I was talking about when one carries out a differential compression test using the correct tooling - but you knew that did you not?
QJB
From an old hand, keep up your current practices, your engineer will appreciate that you are interested in your machinery. If you are working your way up the food chain, you will at times find yourself working alone and away from resources.You will need to make decisions based on your own understanding of the machines you are flying
Cheers
Blackhand
Must be that calibrated wrist of yours that can sense 55 differential psi.
This isn't really relevant to preflighting a powerplant, per the original poster's request. Perhaps you might stay on track.
From an old hand, keep up your current practices, your engineer will appreciate that you are interested in your machinery. If you are working your way up the food chain, you will at times find yourself working alone and away from resources.You will need to make decisions based on your own understanding of the machines you are flying
Cheers
Blackhand
Last edited by blackhand; 3rd Oct 2009 at 10:41. Reason: verbosity
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Bitter experience taught me always to check much more carefully if the aircraft was directly off maintenance; check that no nuts are loose, and wirelocked properly where required; check that any belts are correctly fitted, check that the prop is not on backwards if it's possible to fit it backwards (yes, it's been done), check that all wires are secured properly; in short check everything that a rushed mechanic might have missed or not reassembled correctly, as well as checking what the handbook lists, as you would at all other times.
It follows that you have to be very familiar with what's under the hood, and how it looks when everything's present and correct.
Draw fuel off the water trap until it's clear of water. You need your personal baby food jar for this in your nav bag.
It follows that you have to be very familiar with what's under the hood, and how it looks when everything's present and correct.
Draw fuel off the water trap until it's clear of water. You need your personal baby food jar for this in your nav bag.
Now that we have gotten the pull the prop through thing off our chest I am going to answer the original post
IMO practical things to look for:
1) What is on the belly of the airplane will tell you alot. Fresh oil that is more than a light smear should be investigated by an AME. This once saved me from taking off in an airplane that had developed a crack in the oil filter and was probably only minuites away from loosing all its engine oil.
2) The exhaust pipe should be checked to make sure it is firm when you give it a shake. If it moves and makes a clunking noise get an engineer as there is likely a cracked /broken exhaust pipe or muffler. Also look nside of the tail pipe (ie the part that sticks out from the cowl). It should have a light coating of grey exhaust residue. If it is oily or covered in thick black soot , get an engineer to look at it. (oily means potential cylinder issues and black soot means a too rich mixture)
3) Check the shimmy dampner a notorious weak spot on light cessnas. If you see any drops of red hydraulic fluid on it than the unit is leaking and will probably not function properly leading to a violent nose wheel shimmy. Again get an engineer to look at it.
IMO practical things to look for:
1) What is on the belly of the airplane will tell you alot. Fresh oil that is more than a light smear should be investigated by an AME. This once saved me from taking off in an airplane that had developed a crack in the oil filter and was probably only minuites away from loosing all its engine oil.
2) The exhaust pipe should be checked to make sure it is firm when you give it a shake. If it moves and makes a clunking noise get an engineer as there is likely a cracked /broken exhaust pipe or muffler. Also look nside of the tail pipe (ie the part that sticks out from the cowl). It should have a light coating of grey exhaust residue. If it is oily or covered in thick black soot , get an engineer to look at it. (oily means potential cylinder issues and black soot means a too rich mixture)
3) Check the shimmy dampner a notorious weak spot on light cessnas. If you see any drops of red hydraulic fluid on it than the unit is leaking and will probably not function properly leading to a violent nose wheel shimmy. Again get an engineer to look at it.
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pre flight
if the flying school can only tell you a few basics and not show you how to do it properly suggest you look at another school, after all if this is the level of knowledge of pre-flights what else are they missing elling you
Cutlass engine inspection - as I recall, you cannot open the cover so your engine check is limited to dip stick at the top, fuel sample at the bottom and feel for the belt behind the prop. For what is worth in the wake of the expert dicussions above, I was taught NEVER try to pull the prop through.
I was taught to sample all 10 under wing fuel drain points (5 each side) as apparently the Cutlass has prior convictions for localised condensate lurking in the tanks. Sounds a lot but it is easy and very quick.
My flying school insisted on dipping the tanks - you need a small ladder, unless your Cutlass has the conversion which entails fitting foot rests to climb up. But I think this applies to all high wing Cessnas(?)
I am not a widely experienced aviator as many posters here are but I grew to LOVE the Cutlass - it has a useful combination of weight limits/fuel which enable some quite long flights, with safe margins. And it cruises at a good pace (yeah,yeah, I know thats all relative...)
The comment on the shimmy above interested me - our Cutlass used to shimmy like merry hell on take off and the AMO never seemed to manage to eliminate it. A bit of yoke back-pressure did the trick when taking off. Never did it on landing (thank goodness!)
I was taught to sample all 10 under wing fuel drain points (5 each side) as apparently the Cutlass has prior convictions for localised condensate lurking in the tanks. Sounds a lot but it is easy and very quick.
My flying school insisted on dipping the tanks - you need a small ladder, unless your Cutlass has the conversion which entails fitting foot rests to climb up. But I think this applies to all high wing Cessnas(?)
I am not a widely experienced aviator as many posters here are but I grew to LOVE the Cutlass - it has a useful combination of weight limits/fuel which enable some quite long flights, with safe margins. And it cruises at a good pace (yeah,yeah, I know thats all relative...)
The comment on the shimmy above interested me - our Cutlass used to shimmy like merry hell on take off and the AMO never seemed to manage to eliminate it. A bit of yoke back-pressure did the trick when taking off. Never did it on landing (thank goodness!)
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than the unit is leaking and will probably not function properly leading to a violent nose wheel shimmy. Again get an engineer to look at it.
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if the flying school can only tell you a few basics and not show you how to do it properly suggest you look at another school
Cripes, most of the aero clubs/flying schools I went to showed me to look for leaks, hose and cable condition, cooling baffles, engine mounts, HT leads, mag earth leads connected, etc etc...the point is, they were pretty thorough (I thought.) Is that no longer the case these days?
There are many subjects being discussed here that appear to be ripe for testing on "Mythbusters"...
Let's light the touchpaper with an extract from the O-320 powered PA18-150 POH (SuperCub):
"When the engine is cold, prime three to five strokes after turning the fuel valve to the proper tank. Push mixture control to full rich, carburetor heat off, and open throttle about one-eighth of an inch or until the intake of air at the carburetor can be heard when the engine is pulled through by hand. Engine should be pulled through at least four times."
Gosh!
There are many different opinions in aviation surrounding the serviceability checking and operation of aircraft - some of them might even be correct (excluding the manufacturers', of course
).
I'm not going to offer any opinions, other than the suggestion that a bit of critical thinking goes a long way when assessing some of the 'lore' that gets quoted sometimes, and to treat any powerplant as something that will burst into life as soon as you're within injury range.
Let's light the touchpaper with an extract from the O-320 powered PA18-150 POH (SuperCub):
"When the engine is cold, prime three to five strokes after turning the fuel valve to the proper tank. Push mixture control to full rich, carburetor heat off, and open throttle about one-eighth of an inch or until the intake of air at the carburetor can be heard when the engine is pulled through by hand. Engine should be pulled through at least four times."
Gosh!
There are many different opinions in aviation surrounding the serviceability checking and operation of aircraft - some of them might even be correct (excluding the manufacturers', of course
).I'm not going to offer any opinions, other than the suggestion that a bit of critical thinking goes a long way when assessing some of the 'lore' that gets quoted sometimes, and to treat any powerplant as something that will burst into life as soon as you're within injury range.
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... an extract from the O-320 powered PA18-150 POH (SuperCub):
"When the engine is cold, prime three to five strokes after turning the fuel valve to the proper tank. Push mixture control to full rich, carburetor heat off, and open throttle about one-eighth of an inch or until the intake of air at the carburetor can be heard when the engine is pulled through by hand. Engine should be pulled through at least four times."
"When the engine is cold, prime three to five strokes after turning the fuel valve to the proper tank. Push mixture control to full rich, carburetor heat off, and open throttle about one-eighth of an inch or until the intake of air at the carburetor can be heard when the engine is pulled through by hand. Engine should be pulled through at least four times."
(revolver firmly holstered)
JD
