Checking the compression before a short field takeoff

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woxof

Pre-takeoff checklist
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woxof
Just finished reading an accident report from over a decade ago where the highly experienced pilot took of from a 1900' strip with fairly tall tress to climb over. The performance charts said he could do it, but he didn't make it over the trees and crashed. It was discovered that he had low compression on one engine(20/80). The pilot had flown into this airstrip the same day. It is possible that the aircraft had this compression issue prior to departing to the short airstrip.

This got me thinking about what a person might do to prevent this accident. Would it make sense to pull the prop through(with all reasonable safety precautions taken) several times before the flight into the short strip to see if he could notice low compression on one cylinder. I could see this happening on a rental aircraft where one is not particularly familiar with it.

Thanks
 
unless you were really good at sensing these things I have no idea how somebody would be able to determine a low compression cylinder by just pulling the prop through.. can the average human discern that?

It is also not clear when you pull the prop through which stroke a given cylinder is on.. you might think you feel a low compression (gee that pulled through easily) but really it was on intake/exhaust and not compression

I appreciate the desire to check this and someone else may have a different opinion but I think unfortunately this guy just had bad luck that day..

Also a good reminder to not push the aircraft to the edge of its performance envelope, those charts are written in a near perfect new airplane assumed to be flown by an ideal pilot.. that 40 year old POS rental should have a comfortable 50% margin built-in for both pilot and aircraft performance deficiencies..
 
It seems to me a better indicator of proper power development prior to takeoff is static rpm or initial takeoff power rpm. Initial takeoff rpm should be on everyone's takeoff roll checklist. Low compression may or may not cause significant power loss, depending on the cause. The "armstrong" method is not likely to detect small losses of compression, and has the additional risk of handling the prop. My guess is that it is very unlikely that the compression went to 20/80 suddenly without noticeable changes in engine operation, e.g. oil consumption, oil blackening, engine roughness, or power deficiencies.
 
Tantalum said:
unless you were really good at sensing these things I have no idea how somebody would be able to determine a low compression cylinder by just pulling the prop through.. can the average human discern that?
...
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If it's a serious compression issue (burnt exhaust valve for example) you can detect it pulling the prop through. It's a "poor man's" compression test. I have done this on every airplane I've purchased as one of the quick initial checks when I look over the plane for the first time if I am considering purchase. You're looking for a difference in the amount of effort to pull through TDC on the compression stroke between each cylinder.

Agree with what others have posted. Problems like this don't normally develop "instantly". Its indicative of a pilot not paying attention to his airplane, and likely not maintaining other things also.
 
I seriously doubt you'll be able to tell much if a difference of its not something you do every time you fly with the same aircraft. It's also more likely that you end up hurting yourself from a prop kicking back than if you ever identifying a really low cylinder. Juice ain't worth the squeeze, imo. Better to have a hard abort point and watch static rpm for signs of an issue.
 
Better to figure out if your airplane is performing properly during the roll by calculating take off roll and determining a rejection point where if you don't hit your expected airspeed, you abort. Like having 70% of your take off speed by the time 50% of the runway is gone or abort. Things like compression can be a slow change item, so calculating take off distance and comparing to where you actually lift off are good things to routinely check. If you are consistently off the ground much further than you expected it may be time to investigate.
 
If you've got one cylinder that's 20/80 yes, you're going to feel that difference when pulling it through. Not sure how many people are gonna scrub their flight based on that perception though.
 
20/80 is a maintenance deficiency. Cylinders seldom go that flat suddenly, and if they do the engine won't be happy. Rough idle, uneven cranking, stuff like that, but it's stuff that a lot of folks won't notice these days. It doesn't mean anything to them. They don't have any idea what's going on under the cowl. They hear noise and feel vibration but can't read it.

The average dude doing a pull-through check really doesn't know what he's looking for, and he's risking injury or worse. He'd be better off hiring better mechanics.
 
I like to do a "pull through" quick compression test before the first flight of the day. If you are experienced enough you can tell when all the compressions are equal or nearly so. Sometimes you may feel a cylinder that is ever so slightly softer than the rest. A differential compression test may show that cylinder to be a 70/80 or so compared to a 78/80 for the others. Many times a short flight will clear it up.

Close monitoring of engine health is something that may be more normal for us experimental airplane builders that perform our own conditions inspections than pilots of certified airplanes that have their mechanic do all the grunt work under the cowling.

I've been quite amazed at what some pilots will ignore in their run-up and taxi before flight: https://planecrashmap.com/plane/tn/N198PC/

There are a number of folks flying these experimental slide carbs that are famous for getting hot and having a stumble or "burp" on take-off and they get used to the engine running horribly for a few seconds on the take-off roll before the engine gets enough cooling air to run correctly. Perhaps that is what caused the pilot in the above story to continue his take-off when an abort was clearly what should have happened. It is said that "ignorance is bliss" but in aircraft that rule don't apply very well ... just sayin'.
 
chemgeek said:
It seems to me a better indicator of proper power development prior to takeoff is static rpm or initial takeoff power rpm.
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Being as this was a Bonanza with a CS prop, I’d think it’d have to be pretty bad to not make static RPM at full throttle. I know the R985 has some kind of power check that I can’t remember at the moment, but is there something similar for a flat engine with a CS prop?
 
I agree with what many here have said - the average Joe isn’t going to be able to detect a loss of compression like that. I do think that it’s wise to have a compression test performed more often than at each annual. One time per year isn’t enough imho, especially for those who don’t fly all that often.
 
On my last annual I was surprised to hear that I had a cylinder that failed the compression check. I had just flown it 5.5 hours and never had a hiccup. Prop got the static RPM that it was supposed to. No longer take off roll. No decrease in climb.

My A&P said I most likely would not have ever known that I had a bad piston unless we did the check. That's the reason for an annual.
 
The best way to keep tabs on engine health is full engine monitoring and using it to observe trends. I would not own a aircraft where I can’t see all engine parameters.
 
The unfortunate accident seems like a strong argument for a full engine monitor.

With big engine displacements it's easy to miss a single poor performing cylinder in normal ops.
 
k9medic said:
On my last annual I was surprised to hear that I had a cylinder that failed the compression check. I had just flown it 5.5 hours and never had a hiccup. Prop got the static RPM that it was supposed to. No longer take off roll. No decrease in climb.

My A&P said I most likely would not have ever known that I had a bad piston unless we did the check. That's the reason for an annual.
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Or your A&P needed some extra $$$$
 
Another thing not mentioned is that to get full performance is that you need to fly the profile correctly... and precisely.

I have two airplanes with NO electrical system. A weak cylinder is something like this: thump, thump, thump, swish... lather rinse repeat! Still, quite able to make static rpm. One out of six cylinders weak? I think it was likely more than just that.

performance tables are complicated. Are we sure all was computed correctly? All notes and corrections done accurately, completely and correctly?

food for thought.
 
You can notice if a cylinder is low enough when pulling the prop through to cause a noticeable loss in performance. Seeing as how you would hold your brakes to develop max rpm for a short field takeoff, the tack reading will be the primary indication of something wrong.
 
Static RPM will tell you but I would think that with a jug that sick, you would notice it while cranking.
 
woxof said:
Just finished reading an accident report from over a decade ago where the highly experienced pilot took of from a 1900' strip with fairly tall tress to climb over. The performance charts said he could do it, but he didn't make it over the trees and crashed. It was discovered that he had low compression on one engine(20/80). The pilot had flown into this airstrip the same day. It is possible that the aircraft had this compression issue prior to departing to the short airstrip.

This got me thinking about what a person might do to prevent this accident. Would it make sense to pull the prop through(with all reasonable safety precautions taken) several times before the flight into the short strip to see if he could notice low compression on one cylinder. I could see this happening on a rental aircraft where one is not particularly familiar with it.

Thanks
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Without reading the remainder of the thread, here is my immediate reaction to the original post:

My thought is that the accident pilot was comfortable with cutting things very close. I am somewhere closer to the other end of the scale. I like having safety margin. If the book and even my personal experience said I could get out in 1,900’, I would 2,500’. If I need four hours of fuel for my destination, I want to have 5 1/2 or 6 hours fuel on board.

The only way I would be going to such lengths as checking compression before take off would be if the plane were stranded in that field and I was pulling out all the stops to get it out. In such a case I would have already gotten every bit of weight out of it that I possibly could, and would have been doing things like pushing the plane as far rearward on the end of the runway as possible. I would have checked the air in the tires, made sure there wasn’t a brake dragging. And done a careful engine run up.

In one of my favorite movies, “The Spirit of St. Louis” when they tow the extremely heavily loaded plane to the end of the muddy runway they turned around and didn’t position the plane as far rearward as possible. If the facts could be known, I fully expect that the real Lindbergh positioned that plane so as to have use of every possible inch of runway.

My point, safety margins are a good thing. Continually flying with just enough fuel or just enough runway could very well catch up with you at one point or the other.
 
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I like to stare at the airplane, sense its inner feelings, give it a brief snuggle and then I am able to tell compression differences of 1/4 PSI or less......

I can also tell if I need to fill up the blinker fluid.
 
OMG. You Mike Busch fans should know that engines with low compression make full power. Just ask him!

1900' isn't short. If that engine wasn't making good power the pilot should have aborted the initial leg and he had opportunity to abort the accident leg. There has to be more to the story than one low cylinder.
 
" Highly experienced pilot" means nothing. Bad decisions are still bad decisions no matter how many hours are in the log book.

I agree with Sterwartb.
 
Stewartb said:
OMG. You Mike Busch fans should know that engines with low compression make full power. Just ask him!
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Actually, he said that Continental said that. Their report says that an engine with all cylinders at 40/80 will make full power on a dynameter. But then Continental also does not consider anything above 40 to be low. Even at 40, their SB directions are to use a boroscope to determine the cause, not to replace the cylinder. If you want to argue that, argue with Continental.

Therefore, it if was low compression, it should have been worse than that. Low power should have been detectable on a runup or by a long takeoff roll.
 
There is a rumor, either Continental or Lycoming raan an engine without compression rings and it developed either max static RPM, max power, or both.

Anyone know it this is true?

I could not find a reference.
 
Domenick said:
There is a rumor, either Continental or Lycoming raan an engine without compression rings and it developed either max static RPM, max power, or both.

Anyone know it this is true?

I could not find a reference.
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If that did happen, how long did it make power for?
 
Domenick said:
There is a rumor, either Continental or Lycoming raan an engine without compression rings and it developed either max static RPM, max power, or both.

Anyone know it this is true?

I could not find a reference.
Click to expand...

Continental took a good engine and systematically degraded the compression after each test run by widening rig gap enough to reduce compression by 10 psi between runs. The engine made full power until under 40 psi in the leak-down test.

Here's a write-up with regulatory history and today's present state case on Avweb: https://www.avweb.com/ownership/the-savvy-aviator-13-putting-compression-in-context/

We have to remember the 80 psi test standard is just a fraction of the compression in the combustion process. The static 80 psi (SB03-3) test holds pressure for 5 seconds, while actual combustion process is 800-1000 psi lasting only a few milliseconds.

This is predicated largely on compression ring leakage, exhaust valve leakage also need a borescope check for valve abnormalities if air is hissing through the exhaust.

Groundpounder said:
If that did happen, how long did it make power for?
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It made power until a given test cycle was terminated, until the compression ring gap was so large a given cylinder could not make rated power, but the engine still cycled.
 
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NordicDave said:
Continental took a good engine and systematically degraded the compression after each test run by widening rig gap enough to reduce compression by 10 psi between runs. The engine made full power until under 40 psi in the leak-down test.
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I wouldn’t be surprised if it made full power below that threshold but there were other problems that started cropping up.

My friend has a cub powered by a Jabiru 3300 that had stuck rings. Oil consumption was terrible and oil temp was high due to blowby. Cylinder leakage checks were terrible but a cranking compression test was fine. That thing still made full power even with all of its problems and ran a long time like that.
 
mondtster said:
I wouldn’t be surprised if it made full power below that threshold but there were other problems that started cropping up.

My friend has a cub powered by a Jabiru 3300 that had stuck rings. Oil consumption was terrible and oil temp was high due to blowby. Cylinder leakage checks were terrible but a cranking compression test was fine. That thing still made full power even with all of its problems and ran a long time like that.
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Yes, Continental kept increasing compression ring gap down to 20/80 psi. The engine still made rated power, but now had so much oil blow-by the engine would be starved of oil in operation and fail.
 
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