As an aircraft mechanic, I work on aircraft. Go figure.
That's right, you do. Most mechanics are not aircraft mechanics. Most work on other things, but the commonalities are the basic physics and knowlege base of internal combustion engines. Mechanics are more alike than they are different, IMHO.
Go figure?
Actually, it really was related to testing and running checks
As you later said:
There was nothing wrong with the maintenance program; the work card had been falsified (as is often the case, unfortunately), and the wrong grease had been used. If the work had been done properly, there would never have been an issue. The proceedure was fine. Company issues and personnel issues caused the failure.
The company "issue" was the use of a "procedure" of falsification. Of course, falsification was not the "official" procedure, but it was the one being used, apparently. The
de facto procedure, so to speak.
Other maintenance procedures which are modified on aircraft in general happen because we are fallible humans, and we are unable to make a "perfect" device, or even to service that device perfectly, 100% of the time. All of the information on the CD-roms that mechanics use point to this design ability deficiency on the part of humans, the thousands of AD's, the modified procedures, the limited service life values. Think Parker Hannifin. They get a lot of blame when pilots can't fly partial panel, or haven't a good standby electric gyro on board. We are fallible humans, always looking to blame someone else for our faults.
So, we have a jackscrew that was not even being maintained to accepted standards, nor was the approved lubricant being used. If your argument is that the valve could be worn out by this moving and checking, as was the improperly maintianed jackscrew, that might be possible in a very extreme case. I'd imagine most cases are far from extreme. I'd move that valve rather gingerly, actually, because it was expected to function on almost every training flight. It had done so (functioned well) for several years, and since it is a checklist item, I expect it to do so for years to come. The mechanics who work on the aircraft are generally pretty sharp, and they no doubt check that valve themselves.
No mechanic with any understanding or experience believes a compression test says anything about engine life, because it doesn't.
It does not say EVERYTHING about engine life, but it does speak to the condition of the engine, which is a large component of how we might judge the egine's "life." You cannot return an engine to service if the compression is not up to snuff. How many cylinders and and other parts such as valves and rings, are serviced or replaced every day due to loss of proper compression? Hundreds? Thousands? Why? Because compression is an essential element of an engine in good condition. A condition that lends itself to service life. If wear or damage makes compression too low, you have effectively run out of service life. If it is getting lower over time, you can see a trend that means you will soon be out of service life.
General standards are provided stipulating that 75% of the input pressure should remain during a compression test, but that provides NO insight into the life of an engine.
If the engine compression does not meet the standard, can it be considered for continued service, ie: life? No, it cannot. It is then deficient, and cannot be used in that condition. A part that cannot be used can certainly be considered "dead," or at the end of its life. It can resurrected, yes, but until then, it's over. The Easter theme is not intentional, for those about to accuse me of thread drift.
Laymen and the uninformed see it as some great panacea of engine information; the compression test means almost nothing.
A "panacea?" I don't know ANY mechanics that see it that way. In fact, even when I had tens of thousands of dollars worth of specialized electronic equipment at my disposal, I never though of
anything, not any tool, as a panacea. Okay, you were being scarcastic. That's cool.
At it's most simplistic, you can count on compression decreasing as the service life is spanned, and it is a reliable tool whether you use a leak down set or a classic compression gauge. When troubleshooting an engine performance problem, for example, a reasonable level of compression is considered fundamental by all manufacturers of engines. The purpose of the engine is to provide power, whether you are talking about a G-V engine or a Briggs and Stratton atop your backyard lawnmower. Without compression, you have no ability to produce power using current technology.
Oil analysis is a wonderful tool for judging engine wear, including bearings, rings, almost anything that rubs against something else with oil. In liquid cooled engines, the oil lab routinely looks for evidence of coolant. At one point, I had a fleet to maintain from Ford Tempos to LN9000 dumps and IH TT's, and every kind of implement and bulldozer, loader, and backhoe in between. Oil analysis was a teriific tool. It no doubt saved us a lot of time and trouble, to say nothing of tens of thousands of dollars.
Absolutely not...not unless you attempt to take off with the crossfeed in use. Is there a place on your checklist where you are directed to do so, or is it prohibited? Case in point. You won't have an engine failure on takeoff related to moving the selector valve during taxi...unless you move the selector valve during taxi. Exactly my point; thanks for making it.
I read this a couple of times and I'm not sure of what you might have intended to say. I can observe that the valve being returned to "on" before even beginning the runup IS a checklist item, and that all of the items are intended to be performed as required.
Are you saying that following the checklist might lead to an inadvertent takeoff in crossfeed? Perhaps, but not if you are exercising due dilligence.
But with more experience, you'll learn that some components have limited life in them. Published, or not. They'll only work through so many cycles and tests. Running exhaustive tests may mean that you're going to have component failure in use. Think about it; it will either fail when you're testing it, or using it in practice. If you test it more than you ever use it, you're probably going to cause it to fail eventually during testing...but if not, then you've put yourself in the position of wearing it out in testing, and not having it when you really need it. Sometimes it's far better to just leave alone until you need it.
Good. I had no problem at all with that.
In fact, I agree with your principle. There is, however, a corollary. That is that something which is not checked or exercised, like a muscle or joint, or a valve, cannot be considered to provide reliable service when called to operate. You're a jumper, aren't you? Isn't there some sort of limit date on a packed chute, beyond which it must be opened and repacked before it can be used? It seems I read someone mentioning this here at some point. One day I'll take the jump course and go up. Some of the posters here no doubt would rather I skipped the use of the parachute.
In November, they may volunteer to jump without one.
At any rate, I have to have some level of faith in the record of use of this valve in the many Senecas still in service, along with the example of Piper being able to design and manufacture a reliable valve in the singles (not that electric thing that fails in the Aerostar) as a basis for following the checklist and an expectation of a reasonable number of life cycles for the valve. If this moving during taxi has caused these valves to fail in the past few decades, I haven't seen a report that suggests that is the case, or an AD on the valve that suggests that Piper has found this valve to hold a design defect.
Nothing is perfect, except for, well, you know.
