Mixture question

small penguin · Post by **small penguin** » Fri May 30, 2008 4:20 pm

I did mean the bypass (cant recall the actual name) idle valve you mentioned? which ... bypasses ... the butterfly valve in the carb.

iflyforpie · Post by **iflyforpie** » Fri May 30, 2008 4:36 pm

Ah.....

I don't think it would matter. The amount of fuel that would be trapped would be very small and since it just works off suction (low manifold pressure due to the closed butterfly) it would suck the fuel out no matter what. Also most operations spend very little time at minimum idle.

I don't know too much of what exactly goes on in a carb, but I know that I've never noticed any major difference in shutting down any engine at low idle or 1000-1200 RPM. I usually shut it down at min idle just after the mag check.

Maybe shutting down at 1000-1200 RPM sets the throttle for the next guy for starting or keeps tension off the control linkages? Again, I don't know.

Cat Driver · Post by **Cat Driver** » Fri May 30, 2008 4:50 pm

So if you have a live mag on a bug smasher it could be a danger for someone moving the prop by hand.....

...how about a live mag on something like a PBY where the props are to high off the ground for someone to turn them by hand, is there any other danger a live mag may pose?

iflyforpie · Post by **iflyforpie** » Sat May 31, 2008 8:39 am

Cat Driver wrote:So if you have a live mag on a bug smasher it could be a danger for someone moving the prop by hand.....

...how about a live mag on something like a PBY where the props are to high off the ground for someone to turn them by hand, is there any other danger a live mag may pose?

It depends on why is the mag is live?

Is it because a wire has broken? What if that wire then decides to ground itself on something? All of a sudden you have a dead mag.

What if there is a malfunction with the switch? You might not know what it is going to do. I've got a 172 in my shop with that problem right now. There's also AD76-07-12 for some Bendix switches that say they have to be checked for grounding every 100hours and if they fail the aircraft is unairworthy.

What happens if the mag goes out of time and decides to screw up your engine? In normal circumstances you can switch to the good mag and get home. If that mag is also live you are going to lose an engine.

My point is you don't know why the mag is live and even though it is a 'fail safe' mechanism until it is examined you don't know what it is going to do.

Maybe in a twin engined PBY flying over water and having high mounted engines isn't critical, but me flying single engine over the rocks I like to keep the 'luck' factor to a minimum.

I've never worked on a PBY but I've worked on the Grumman Goose and Lake Buccaneer. Working on a narrow platform or ladder on one of the engines wouldn't exactly thrill me if there was a live mag.

Cat Driver · Post by **Cat Driver** » Sat May 31, 2008 8:52 am

iflyforpie thanks for coming up with all those possible scenarios that " Could " happen but are so rare many never live long enough to see it.

...how about a live mag on something like a PBY where the props are to high off the ground for someone to turn them by hand, is there any other danger a live mag may pose?

However you have missed the most obvious and more likely danger of a live mag in the above question.

iflyforpie · Post by **iflyforpie** » Sat May 31, 2008 9:24 am

Cat Driver wrote:iflyforpie thanks for coming up with all those possible scenarios that " Could " happen but are so rare many never live long enough to see it.

...how about a live mag on something like a PBY where the props are to high off the ground for someone to turn them by hand, is there any other danger a live mag may pose?
However you have missed the most obvious and more likely danger of a live mag in the above question.

Nope, I didn't miss it.

iflyforpie wrote:
I've never worked on a PBY but I've worked on the Grumman Goose and Lake Buccaneer. Working on a narrow platform or ladder on one of the engines wouldn't exactly thrill me if there was a live mag.

Especially a live mag I didn't know about.

Yeah they all far-fetched scenarios. With all the fuel burned out of the cylinders even with both mags turned on and you purposefully hand propping the chance of the engine firing over is quite minimal.

But I still do a live mag check at the end of every flight. It was reinforced all though my AME training (even though we don't usually fly the aircraft) and further through my flight training. And I will still ground an aircraft-any aircraft-that has a live mag since it is unairworthy.

And if it is a case of one of those bush jobs where the only way to fly home is with a live mag I'll cross that bridge when I come to it.

Cat Driver · Post by **Cat Driver** » Sat May 31, 2008 9:38 am

But I still do a live mag check at the end of every flight. It was reinforced all though my AME training (even though we don't usually fly the aircraft) and further through my flight training. And I will still ground an aircraft-any aircraft-that has a live mag since it is unairworthy.

iflyforpie, please quit insinuating that I might not understand the obvious basic issues you have outlined...what I'm trying to determine is why with all your knowledge and superb training as a pilot / mechanic you have not figured out what other danger there is with a live mag on the engine I used as an example.

In fact the danger is far, far more likely than the things you have mentioned.

I'm sure the light will come on now, however you have shown me your limitations on grasping the obvious already.....

iflyforpie · Post by **iflyforpie** » Sat May 31, 2008 9:51 am

What other danger? Getting zapped by a lead? Maybe I don't think as far outside the box as you do Cat. Maybe the PBY is so completely different to any other plane I've worked on that I don't have a clue.

Live mag=unairworthy aircraft and to use caution when around the prop. That should cover all the bases.

Now enlighten me on the detail I missed.

Or is it that you can't shut the engine down right away when docking or reduce power when sailing? I've got only a dozen hours on the water and that was a few years ago, I had to dig that out of the dregs.

Cat Driver · Post by **Cat Driver** » Sat May 31, 2008 10:32 am

Now, now, now iflyforpie don't let your emotions run away on you to the point that you fail to grasp the obvious.

What other danger? Getting zapped by a lead? Maybe I don't think as far outside the box as you do Cat. Maybe the PBY is so completely different to any other plane I've worked on that I don't have a clue.

There is a vast chasm between thinking outside of the box and being exposed to these basic mechanical issues that different engines pose.

I have no idea if you have a clue or not, that is what we are trying to determine here. The PBY is only the airplane the engines are hung on. It is an engine issue we are examining here.

Live mag=unairworthy aircraft and to use caution when around the prop. That should cover all the bases.

It should in your mind but there is a far more important issue that can bite you in the ass if you don't understand the subject.

Now enlighten me on the detail I missed.

Radial engines are subject to hydraulic lock and one is risking blowing a cylinder or several off the engine if you have a live mag and you attempt to turn the prop...especially with the starter.....

........here again was the question.

...how about a live mag on something like a PBY where the props are to high off the ground for someone to turn them by hand, is there any other danger a live mag may pose

?

iflyforpie · Post by **iflyforpie** » Sat May 31, 2008 11:59 am

What, that's it? Hydraulic lock?

Cat, it seems like if I offer too much information I'm insinuating that you might not understand the obvious basic issues. If I offer too little you can't figure out why I don't understand the issue.

I've never flown an aircraft with a round engine and worked on only a handful. But I've never started one without first pulling through the prop to check for hydraulic lock. On the Goose, I used a step ladder to do it.

Your scenario with the PBY is quite unique and you shouldn't be surprised that even an experienced person (without experience on that specific aircraft) wouldn't get what you are trying to say given the limited context of the situation and the inherent limitations of this form of communication.

That being said I do appreciate the information. I'm here to learn.

But my original course of action still applies whether I understand the consequences in detail or not: An aircraft with a live mag is unairworthy. As pilot I will snag it in the logbook and alert the ground crew. As a mechanic I will not dispatch the aircraft until the problem is found and fixed. A pilot who decides to turn the engine over anyways is going to be in big trouble.

All that aside, do you really use the starter to turn over the engine to check for hydraulic lock? How violent is it when there is a hydraulic lock?

Cat Driver · Post by **Cat Driver** » Sat May 31, 2008 12:54 pm

All that aside, do you really use the starter to turn over the engine to check for hydraulic lock? How violent is it when there is a hydraulic lock?

That is a reasonable question and here is the answer.

When starting the P&W R1830's, especially on the PBY where it is very difficult to hand turn the prop to cycle every cylinder through it's compression cycle we count the blades during the initial starting process to ensure that there are no cylinders that are hydraulic locked before turning on the mags to start the firing process.

If a cylinder does lock there is a slip clutch in the starter that will slip before exerting enough torque to break the cylinder.

The bad part for me was in the business that I was in I had to get the sea stands or a portable stand and get up there and pull the plugs and pump the oil out by hand turning the engine.....

.....that is another reason that I can't figure out why mechanics are so poorly paid compared to pilots.

Now just to sum up here iflyforpie, these discussions are more than a way to pass time they can be informative.....

You wouldn't believe the things I have learned on Avcanada to add to the things I have learned in the industry.

iflyforpie · Post by **iflyforpie** » Sat May 31, 2008 1:02 pm

Appreciated. Thanks a lot!

1340 is as big as I've gone and on the Harvard you can't really turn it over with the inertia starter.

I'm still waiting for my big pilot paycheque, if I had to live on flying alone I would starve.

Cat Driver · Post by **Cat Driver** » Sat May 31, 2008 1:13 pm

I'm still waiting for my big pilot paycheque, if I had to live on flying alone I would starve.

Aviation is a lot like sex, not only does it have it's ups and downs the quality of the ride depends on how selective one is when choosing what you are about to get into.

Siddley Hawker · Post by **Siddley Hawker** » Sat May 31, 2008 1:44 pm

I always thought the way to lean a 1340 was to switch one mag off, lean until she began to kick a little, then switch the mags back to both.

Cat Driver · Post by **Cat Driver** » Sat May 31, 2008 2:41 pm

I was lucky and never had a 1340 quit on me, mainly because I managed to avoid flying them as much as possible.

The 1830 was idiot proof with only four selection indents .... I.C.O. // Auto lean // Auto rich// Full emergency rich.

If only they could design a turbine that sounds like a big radial that would be the ultimate power plant.

iflyforpie · Post by **iflyforpie** » Sat May 31, 2008 3:52 pm

The thing I remember about the 1340 on the Harvard was the mixture control was backwards. Forward for lean, back for rich. I believe it was normal on the Otter but I've only worked on Turbine Otters.

Cat, have you ever heard a Nene start up on a CT-33? It sounds like the deepest bass note you've ever heard and you can feel it in your chest cavity. They sound pretty sweet to me, but I also saw one blow up on the ramp during a ground run.

square · Post by **square** » Sat May 31, 2008 8:13 pm

Cat Driver wrote:
...how about a live mag on something like a PBY where the props are to high off the ground for someone to turn them by hand, is there any other danger a live mag may pose
?

LOL you both missed it, if the mags are on you locked the keys inside dummy!

Big Pistons Forever · Post by **Big Pistons Forever** » Sat May 31, 2008 8:26 pm

Ah hmmmm..... Mr Square...... I would suggest you be carefull about making comments that could be construed as derogatory without a full understanding of the issue....

square · Post by **square** » Sat May 31, 2008 8:35 pm

Yeah sarcasm's bad at the internet, the guy with the ultralight ATPL question thought I was serious too

Hedley · Post by **Hedley** » Mon Jun 02, 2008 8:38 am

the 1340 on the Harvard was the mixture control was
backwards. Forward for lean, back for rich

Is this supposed to be sarcasm, too? Every T-6/SNJ/Harvard
that I've ever flown had a "normal" mixture (forward for rich).

Didn't the Yale have some engine control backwards?

iflyforpie · Post by **iflyforpie** » Mon Jun 02, 2008 9:34 am

Hedley wrote:
Is this supposed to be sarcasm, too? Every T-6/SNJ/Harvard
that I've ever flown had a "normal" mixture (forward for rich).

Didn't the Yale have some engine control backwards?

Nope, no sarcasm. It was a Harvard Mk 1 made by North American in 1942. We had it as an instructional aircraft for the AME course at SAIT.

I was reading the book 'Behind the Glory' about the BCATP and the students lamented that the controls of the Yale and the Harvard were exactly backwards to each other. On the Yale you pulled the throttle back to open it and mixture forward for rich, while the Harvard was forward for open throttle and backwards for rich mixture.

Later ones were probably modified to have more conventional controls; like Beechcraft with their P-T-M engine control layout being changed to the more common T-P-M.

Strega · Post by **Strega** » Mon Jun 02, 2008 9:35 am

Hedley,

Im sure the only experience hes had with a harvard/T6 has been sitting in SAITs doing a runup. I bet at the time, he really didnt have any idea what he was doing, other than it was cool.

a couple of years back, I made the suggestion to a couple of people at sait, that students, after performing maint on a plane (sait has a couple that could be airworthy) would be required to "go for a ride" in the aircraft they had worked on (with a checkover and signout by a licensed individual) but I was told it was "far too much liability".. makes you wonder.....

iflyforpie · Post by **iflyforpie** » Mon Jun 02, 2008 10:03 am

Strega wrote:Hedley,

Im sure the only experience hes had with a harvard/T6 has been sitting in SAITs doing a runup. I bet at the time, he really didnt have any idea what he was doing, other than it was cool.

a couple of years back, I made the suggestion to a couple of people at sait, that students, after performing maint on a plane (sait has a couple that could be airworthy) would be required to "go for a ride" in the aircraft they had worked on (with a checkover and signout by a licensed individual) but I was told it was "far too much liability".. makes you wonder.....

Who cares? If you read a few posts back you see that I've never flown a round engined aircraft. But I've maintained a few and ran them all. How much experience do you need to tell which way a friggin control goes? I could tell you volumes about planes I've never flown or even run up.

All the ground runs at SAIT were done without an instructor on board; you think they are going to give that much potential carnage (like overboosting and blowing a jug) to someone who has no idea what they are doing? It's a little more involved than a 150, 172, or even a J3 or Tiger Moth that most pilots here soloed on.

As for working on live aircraft I was doing that a week out of school. What possible purpose would what you suggest serve? I don't do my work any differently now that I fly everything I fix.

Hedley · Post by **Hedley** » Mon Jun 02, 2008 10:36 am

Sorry to get inbetween you two.

Round engines are kind of groovy in an antiquish
sort of way.

I've mostly flown the Kinner R-56 (540 ci 5 cyl),
P&W R-985 and P&W R-1340. They're all a bit persnickety
to start, what with the hydraulic lock and the oil - and
you gotta sit there and wait for the loooongest time while
they warm up - and you simply can't bang the throttle back
and forth on the supercharged engines, they way some people
do with flat engines.

But they do sound GREAT!

Big Pistons Forever · Post by **Big Pistons Forever** » Mon Jun 02, 2008 6:56 pm

"Round engines are kind of groovy in an antiquish
sort of way."

Hedly you philistine

that's like saying the mona lisa would be a better painting if only she had a big grin.

I just wish I was flying 60 yrs ago when all you heard at an airport was the basso profundo roar of real engines

. Now all you hear is the whine of a bunch of overgrown vacuum cleaners

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