Roost Air Lounge => The Classroom => Topic started by: Frank N. O. on October 05, 2008, 04:41:22 AM
Title: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: Frank N. O. on October 05, 2008, 04:41:22 AM
Ok so I'm reading thrue this official danish Aircraft and Engine studybook for a JAA PPL which I loaned at the library and it already leaves me with more questions than answers. Ok I'll skip over complaining about the inaccurate/incorrect engine type description (the basics listed in the book is identical to car-engine principles so I can fairly safely say their description is easily misunderstood by beginners and other descriptions are downright incorrect).
The book lists basics about testing the ignition system before take-off, which they list should/have to be done before each flight. It lists the std. procedure: - Set engine rpm to the prescribed setting (they list a usual value of 1700-1800 rpm) - Switch from Both to L then back to Both. - Switch from Both to R then back to both. - RPM is not allowed to drop more than 125 rpm, and no more than 50 rpm difference between L and R Then they write that if the rpm drop or the rpm difference between L and R are bigger than listed in the manual then flight has to be aborted. My first question is: Then why did the book list those values above? Is that a JAA regulation?
The book also mentions that if the ignition switch accidentally goes to off then the pilot shouldn't give in to tempation and go back to both since this could damage the ignition system and/or exhuast due to the high rpm the engine is running at. Second question: How can that damage anything?
The book then writes that a lack of reaction between L and R could mean a switch failure. My question: Couldn't it also indicate that one ignition system isn't working at all?
The book mentions a technique to test the switch but says that it's controversial and that most instructors advise against it: Have the engine runnin at low rpm (1000 rpm according to the book) and then quickly move the switch to Off and then back to Both before switching the engine off normally with the mixture controls. I don't get this. They just advise against re-activating the ignition at high rpm just a few lines above on the same page and then they list this technique but don't directly say you shouldn't do it. They do list it's impossible to check the function of the switch with the engine off, that's not right to my knowledge, however you would need to be or contact a certified aircraft mechanic I think. Checking an electric switch is really easy (depending on how hard it is to get to that is).
The defect the book lists as a possible reason for the lack of reaction between L and R is that one or both ignition systems aren't short-circuited and that this means that even with the switch in the Off postition and the key out any movement of the propeller could have catastrophic consequences. Now I can only imagine they're saying that with, what I think is called, hot magnetos, any movement of the propeller could act as a starter, but if the key is out, don't you normally have the mixture leaned completely, that should keep the engine from starting shouldn't it?
Frank
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: Rooster Cruiser on October 05, 2008, 07:44:55 AM
Ok so I'm reading thrue this official danish Aircraft and Engine studybook for a JAA PPL which I loaned at the library and it already leaves me with more questions than answers. Ok I'll skip over complaining about the inaccurate/incorrect engine type description (the basics listed in the book is identical to car-engine principles so I can fairly safely say their description is easily misunderstood by beginners and other descriptions are downright incorrect).
That's about par for the course, in my experience.
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The book lists basics about testing the ignition system before take-off, which they list should/have to be done before each flight. It lists the std. procedure: - Set engine rpm to the prescribed setting (they list a usual value of 1700-1800 rpm) - Switch from Both to L then back to Both. - Switch from Both to R then back to both. - RPM is not allowed to drop more than 125 rpm, and no more than 50 rpm difference between L and R Then they write that if the rpm drop or the rpm difference between L and R are bigger than listed in the manual then flight has to be aborted. My first question is: Then why did the book list those values above? Is that a JAA regulation?
No, it is not a regulation. That is very close to being standard for all Lycoming and Continental engines, but there are some exceptions. The Great Lakes I recently flew had a Lycoming IO360 engine, but during runup the checklist called for 2200RPM
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The book also mentions that if the ignition switch accidentally goes to off then the pilot shouldn't give in to tempation and go back to both since this could damage the ignition system and/or exhuast due to the high rpm the engine is running at. Second question: How can that damage anything?
If a pilot accidentally turns off both magnetos and doesn't immediately turn them back on there is the risk of a backfire due to unburned fuel. This could damage a)the Exhaust Manifold if the backfire happens there, b)the Intake Manifold if pre-ignition happens and c)damage to the ignition system from wildly swinging voltages happening. Bottom line is, this could be expensive.
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The book then writes that a lack of reaction between L and R could mean a switch failure. My question: Couldn't it also indicate that one ignition system isn't working at all?
No. If one ignition system was dead, the engine would stop operating on either L or R. Lack of RPM drop when L or R is selected suggests that you have a broken ground wire to one mag. Trust me, I have had this happen!
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The book mentions a technique to test the switch but says that it's controversial and that most instructors advise against it: Have the engine runnin at low rpm (1000 rpm according to the book) and then quickly move the switch to Off and then back to Both before switching the engine off normally with the mixture controls. I don't get this. They just advise against re-activating the ignition at high rpm just a few lines above on the same page and then they list this technique but don't directly say you shouldn't do it. They do list it's impossible to check the function of the switch with the engine off, that's not right to my knowledge, however you would need to be or contact a certified aircraft mechanic I think. Checking an electric switch is really easy (depending on how hard it is to get to that is).
Testing for a hot mag due to broken ground lead is best done at a low power setting in order to avoid damage to the engine. This is not recommended or taught to student pilots as it is unnecessary for them to learn at an early stage. Such a procedure should be done by a mechanic or by an experienced pilot.
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The defect the book lists as a possible reason for the lack of reaction between L and R is that one or both ignition systems aren't short-circuited and that this means that even with the switch in the Off postition and the key out any movement of the propeller could have catastrophic consequences. Now I can only imagine they're saying that with, what I think is called, hot magnetos, any movement of the propeller could act as a starter, but if the key is out, don't you normally have the mixture leaned completely, that should keep the engine from starting shouldn't it?
Frank
Frank, the best analogy for this is to compare an airplane propellor to a car up on jacks with the transmission in gear. If there is some long-haired grease monkey down around the drive wheels and someone accidentally hits the starter, guess what happens? Right, the grease monkey gets his hair caught in the spinning wheel and at the very least he loses his scalp. Usually, he'll lose his head too.
A propeller on an airplane should always be treated like a loaded gun, ready to go off at any moment. It is free to turn, so if there are any fumes left in a cylinder with a hot magneto the engine could fire if the prop is turned by hand. Maybe not enough to start the engine, but it would be enough to at least break someone's shoulder, or put a severe dent in their forehead. ::eek:: The standard practice in aviation is to starve the engine of fuel so there is no leftover fuel to cause a possible kick, and to turn off the ignition. This is known as a double safety precaution.
RC
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: Mike on October 05, 2008, 05:38:12 PM
A propeller on an airplane should always be treated like a loaded gun, ready to go off at any moment. It is free to turn, so if there are any fumes left in a cylinder with a hot magneto the engine could fire if the prop is turned by hand. Maybe not enough to start the engine, but it would be enough to at least break someone's shoulder, or put a severe dent in their forehead. ::eek:: The standard practice in aviation is to starve the engine of fuel so there is no leftover fuel to cause a possible kick, and to turn off the ignition. This is known as a double safety precaution.
RC
That's some really good advice! I'll add that to my repertoire of teachings (and maybe turn it into a strip, hehe).
Had a propeller "go off" once in a hangar. Don't remember the details but I think it was a faulty starter relay and the prop started turning when you turned the master on.....
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: Oddball on October 05, 2008, 06:24:11 PM
Ill add some thing as well, most of our aircraft at the college hangers where insurance write offs mainly C 152's also a whirlwind, a Bell 45, and a C 310, but they all had "live aircraft" tags on them. even though the tanks where drained as much as possible turning of the propeller could in theory start the engine due to the mags sparking, even heard of a "romour" that a student got taken round the side of the hanger and got a kicking by one of the lecturers by turning a propeller.
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: TheSoccerMom on October 06, 2008, 03:10:59 AM
Those are GOOD questions, Frank! (Boy, you've been working!) ::silly::
And yes, props should be considered the potentially dangerous things that they are.... always treat them as "hot"...
A trait which is also a good thing, when you own an airplane without a starter, like the one below..... ::whistle::
:D
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: BrianGMFS on October 07, 2008, 03:09:58 PM
What I was always taught and what I did when I was working line service was if you had to move a prop (To hook up a tow bar for example) is to stay out of the arc and use the palms of your hands to move the prop in the OPPOSITE direction that the engine turned.
Brian
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: Rooster Cruiser on October 07, 2008, 04:01:07 PM
What I always taught my students was, "When moving a prop, never stick any part of your anatomy into the arc of the prop that you would care to lose." :o
After 20 years and 2400 hours of instruction given, I have yet to lose a student to such an accident. I like to think that I made a difference by giving my students such little quips that were easily remembered, but I'll never know for sure.
RC
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: leiafee on October 07, 2008, 06:37:30 PM
There's a chap at our field missing three fingers from a prop "incident" -- does focus the mind!
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: Rooster Cruiser on October 08, 2008, 02:21:54 AM
There's a chap at our field missing three fingers from a prop "incident" -- does focus the mind!
Lemme guess... he was hand propping? ::whistle::
A good friend of mine explained it when showing me how to hand prop his J-3 Cub. You place your entire fingers on the flat surface of the prop, and pull through that way. Most people naturally want to hook the trailing edge of the prop by their first knuckle on their index, middle, and ring fingers. When the engine kicks backwards from a pre-ignition, the propper will lose the digits of those three fingers right at the first knuckle!
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: Frank N. O. on October 09, 2008, 03:12:26 PM
That's about par for the course, in my experience.
Ah so that's a general thing among student pilot material and not just a danish thing.
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No, it is not a regulation. That is very close to being standard for all Lycoming and Continental engines, but there are some exceptions. The Great Lakes I recently flew had a Lycoming IO360 engine, but during runup the checklist called for 2200RPM
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If a pilot accidentally turns off both magnetos and doesn't immediately turn them back on there is the risk of a backfire due to unburned fuel. This could damage a)the Exhaust Manifold if the backfire happens there, b)the Intake Manifold if pre-ignition happens and c)damage to the ignition system from wildly swinging voltages happening. Bottom line is, this could be expensive.
Thank you very much for the info |:)\
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No. If one ignition system was dead, the engine would stop operating on either L or R. Lack of RPM drop when L or R is selected suggests that you have a broken ground wire to one mag. Trust me, I have had this happen!
Oh man, I just knew I'd ask a dumb question and here it was. I guess I'm really out of practice with engine stuff. Thanks for the polite answer though, I appreciate it!
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Testing for a hot mag due to broken ground lead is best done at a low power setting in order to avoid damage to the engine. This is not recommended or taught to student pilots as it is unnecessary for them to learn at an early stage. Such a procedure should be done by a mechanic or by an experienced pilot.
Again a very educational response |:)\
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Frank, the best analogy for this is to compare an airplane propellor to a car up on jacks with the transmission in gear. If there is some long-haired grease monkey down around the drive wheels and someone accidentally hits the starter, guess what happens? Right, the grease monkey gets his hair caught in the spinning wheel and at the very least he loses his scalp. Usually, he'll lose his head too.
A propeller on an airplane should always be treated like a loaded gun, ready to go off at any moment. It is free to turn, so if there are any fumes left in a cylinder with a hot magneto the engine could fire if the prop is turned by hand. Maybe not enough to start the engine, but it would be enough to at least break someone's shoulder, or put a severe dent in their forehead. ::eek:: The standard practice in aviation is to starve the engine of fuel so there is no leftover fuel to cause a possible kick, and to turn off the ignition. This is known as a double safety precaution.
RC
Thank you very much that is really a very very good reply and a great technique that I should really remember! |:)\
Mike: Wow, that must've been scary! Maybe one should make sure the prop is clear right from you start turning on power in the aircraft.
Ian: Wow, tough school! But then again, doing something so dangerous directly against warnings is just plain unacceptable. Any word if that person graduated?
Mary: Me, work hard? It's just a few pages. I did actually already read some more and got more questions again (this book continues to seriously lack background info and proper descriptions which seriously makes it necessary to know basic information about engines etc. beforehand).
Brian: Why the opposite direction? Is that since if it started it wouldn't "hit" as much as push the hand away?
Leia: Ouch! I'd agree with that, although I personally would remember it anyway.
Matt: Interesting technique, definately safe sounding. Didn't someone here describe a technique for propping where you stood so you took a step back when turning the prop?
Ahhh, so much learning, so much lack of an actual aircraft for me :( I even drove past the airport today just to take a look (Caravan, PC12 and a couple other planes outside, and some more inside open hangars and yet another new building had been made).
Frank
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: Oddball on October 09, 2008, 04:03:44 PM
I don't think he did Frank, before we go up to the hangers we get a hanger safety lecture and one of them is about the signs and turning of props (also fire training where the fire exits are and extinguishers etc) so i think he was pretty dense to do that when he was up there.
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: Mike on October 09, 2008, 04:36:57 PM
Mike: Wow, that must've been scary! Maybe one should make sure the prop is clear right from you start turning on power in the aircraft.
It's considered good form to always look around before you turn on the master switch, even on a helicopter. ESPECIALLY if there are more people working on the same aircraft! You should always ask everybody if you can turn the master on because you never know what the other guy could have been up to. On the helos we usually also pull a few circuit breakers when they are in the hangar to keep systems from accidently starting up....
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: Rooster Cruiser on October 10, 2008, 05:16:02 AM
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Matt: Interesting technique, definately safe sounding. Didn't someone here describe a technique for propping where you stood so you took a step back when turning the prop?
Yes Frank, that is the correct technique. I just didn't get into it as it was late when I wrote that post and I was a bit tired. ::sleep::
RC
Title: More Dumb Question: Now about fuel
Post by: Frank N. O. on October 12, 2008, 08:24:55 PM
Ok, this isn't exactly pre-flight stuff but I'm not really in a good mind to organize a new topic atm (I'm really stressed and worried about my mom and my future) but I'll try to post it here in an attempt to force my mind along.
In the above mentioned book I'm now on to the fuel system, and again it's not that good in descriptions imho. For instance it says that the throttle-lever for an engine with a fixed-pitch propeller controls the rpm while the same for an engine with a constant-speed propeller controls the intake-pressure, this again sounds really confusion, if not directly incorrect for me so a person learning to fly without any previous knowledge about reciprocating engines etc. could imho easily get the wrong idea from this book. Anyway, to my question. The book writes that high-wing airplanes with carburators don't use fuel-pumps while fuel-injection models do and low-wing aircraft have an electric so-called Boost Pump used during starting whereafter the engine-power pump takes over. Now here's the interesting part: The book says it's normal procedure to have the boost on during take-off and landing, which I can understand, however also during low flight. What about low flight would require the boost pump? And now flying with a low power setting perhaps, or at high altitude?
The book also writes that if an engine delivers 100hp @ 2700 rpm at sea level at std. conditions then at 10000ft it would only deliver 50hp @ 2700. My question is then, how come superchargers aren't used on more planes? For a car engine, I once read/saw listed that a mechanical twin-screw type supercharger can help an engine maintain full output even at high altitude (high for cars anyway) whereas a turbocharger can't quite match it, which I assume is due to the lower pressure in the exhuast? Most aircraft I see with supercharged engines use turbos, however I've heard some planes are normalized, where the pressure never exceeds sea level, is it still then called turbo-charging? For instance the Beech Baron 58TC, is that turbo-normalized or charged (if charged means above sea-level pressure)?
The book continues to write about fuel-tanks and fuel-selector and lists that high-wing aircraft usually have a both-setting for the tanks whereas low-wings don't, but I'm sure I've seen the Commander 112/114 have a both-setting as well, how can it have that?
Frank
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: Oddball on October 13, 2008, 09:21:21 AM
If you sent me your address frank ill dig out my old college books about props and power plants for you and send them on. lots of pretty pictures and explanations and maybe some of my own hand written notes in them.
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: Frank N. O. on October 15, 2008, 09:29:47 AM
I've sent you a pm Ian.
Frank
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: Baradium on January 14, 2009, 07:18:35 PM
Hey Matt, when I started flight training they had us do a ground check before shutdown every flight. Guess they figured they'd rather have to teach students how to do it rather than increase the risk of a hot mag.
What I was always taught and what I did when I was working line service was if you had to move a prop (To hook up a tow bar for example) is to stay out of the arc and use the palms of your hands to move the prop in the OPPOSITE direction that the engine turned.
Brian
I was told that turning the prop backwards (and in turn turning components backwards) can be especially harmful to some components like vacuum pumps, increasing the risk of breaking a vane. I'd say something if I saw you trying to turn the prop of an airplane I owned backwards... if I owned one anyway...
Also, the method I was taught, and use, is the one I believe Matt was advising. Palm flat on the face of the prop and push it around, keeping clear of the arc.
I heard that with some of the larger radial engines, a way you'd "hand prop" them was to put the blade against your shoulder and run in the direction of engine rotation. If the engine kicked you'd be already getting out of the way. Never seen anyone do it, but had a couple of old timers mention it.
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: ZK Kiwi on January 14, 2009, 08:32:02 PM
Regarding you later batch of questions (which are worth asking) Fixed pitch vs constant speed: fixed pitch is pretty simple, opening the throttle makes the prop go faster, nothing too tricky there. On a constant spped unit, a governor adjusts the prop blades so the prop always maintains the same speed. at low throttle settings the blades go fine to unload the engine and maintain RPM, as throttle settings increase, the governor matches it by coarsening the pitch, loading up the engine and maintaining the same RPM. The Throttle butterfly valve however opens up and more charge is going into the cylinders as this happens, which registers as manifold Pressure (which is confusing in itself as on normally aspirated engines it is actually a vacuum, which decreases as throttle is opened!) hence when you open the throttle, RPM should stay the same but Manifold pressure decreases back towards 30 in Hg.
with regards to low wing vs High wing fuel systems, there is no reason at all to say low wings cant have a "both" position, not any reason why highwings have to have one. it all depends on how the designer decided to plumb everything up. for example, my aircraft has an on - off selection and no individual selection for each wing tank. Both wing tanks gravity feed to a header tank supplying the fuel injection. Cherokees and the like dont have a both selection, but that is more to do with the location of fuel pumps etc in the system - it just happens to be the way Jim Thorp wanted to do it, there are plenty of alternative methods.
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: ZK Kiwi on January 14, 2009, 08:53:00 PM
Further to boost pump questions - my high wing has 2x High pressure pumps as it is fuel injected, generally carbs require lower supply pressure, and the head from the tanks in the wings is enough to provide this. On low wing aircraft the gravity flow doesnt exist so the engine driven pump provides the feed to the carb - at relatively low volume. Switching on the boost pump is a backup to this pump to provide feed in the event of pump failure. it is used at low altitudes mainly because should the mechanical pump fail the engine will stop and there is less time to attempt a restart. In practice, it is good to put the boost pump on any time when an engine failure would ruin your day (e.g steep manouvers at low speed etc). Injected engines (such as mine) require higher fuel pressures than can be obtained from gravity feed or mechanical pumps, hence even high wing injected aircraft such as Cessna 207s have electric pumps operating continuously. in most cases there is a backup pump used in the same way the boost pump is on carb models - whenever you cant afford to have engine failures! as these pumps move huge quantities of fuel (more than the engine uses) the surplus normally goes back into the tanks. On High wing aircraft this causes problems as it is difficult to put it back into the tank it came out of. this is why header tanks are used (as on my aircraft) as the fuel can go back there, and the main tanks just top up what is used. The other thing you will notice is on injected engines, a reliable electrical supply is essential, as without the electric pumps, things go quiet - injected aircraft generally have more complex electrics to ensure relaibale operation.
superchargers etc: Aircraft rarely require the full power available from an engine at altitude - especially light planes. the additional complexity and weight of a supercharger system is rarely justified. Unless you live in Bolivia or the likes, most takeoffs (the only time max power is really needed) will be within the altitude band where adequate power is available. There are lots of other factors, such as the fact propellers are inefficient anyway. designers have generally kept to the "Keep it simple" rule to improve reliability, serviceability and practicality, even if it means they are not as efficient.
Title: Re: Dumb Question 5: Pre-Flight Igntion Check (piston-engine)
Post by: Frank N. O. on January 27, 2009, 07:19:11 AM
Thank you very much for the good and detailed information :)
