Special VFR

[mhcasey]

Can you get a special VFR clearance to an airport with overlying controlled airspace above the surface? The regs seem to indicate the airport must have controlled airspace all the way down to the surface.

Another random question: How do you get an engine to backfire? My understanding is this could be useful information should I get myself into an icy situation someday...

[jrh]

Originally Posted by mhcasey

Can you get a special VFR clearance to an airport with overlying controlled airspace above the surface? The regs seem to indicate the airport must have controlled airspace all the way down to the surface.

You won't ever need one.

Think about this...what does a Special VFR clearance give you? The right to fly around in 1 mile visibility / clear of clouds, right? Ok, now look up VFR minimums for Class G airspace. Get the connection?

Originally Posted by mhcasey

Another random question: How do you get an engine to backfire? My understanding is this could be useful information should I get myself into an icy situation someday...

A lot of times it'll backfire if you flip the mags to "off," wait a second, then flip them back to "both."

But the bigger question is, why would you want to? An engine backfiring shouldn't have anything to do with icing. That's what carb heat is for.

[mojo6911]

For Special VFR, the intent is to allow you to leave an airport and get to better conditions, which is why it must go down to the surface. If you are leaving a Class G airport, it isn't needed.

To backfire the engine, you can turn the mags off and then back to both quickly, and sometimes yanking the throttle all the back and jamming it back in does the trick.

Edit: jrh beat me to it!

[mojo6911]

Originally Posted by jrh

That's what carb heat is for.

I read in AOPA one time about a pilot that was forced to do it. I think he waited too long to use the carb heat, and there wasn't enough heat to break the ice. That, or the carb heat failed in flight. The carb heat just goes to the air intake box and controls a vent. It seems to me like that could ice up and not work.

[jrh]

Originally Posted by mojo6911

For Special VFR, the intent is to allow you to leave an airport and get to better conditions...

Or get *in* to an airport that has crappy conditions. I've used it going both ways.

[mhcasey]

Originally Posted by jrh

You won't ever need one.

Think about this...what does a Special VFR clearance give you? The right to fly around in 1 mile visibility / clear of clouds, right? Ok, now look up VFR minimums for Class G airspace. Get the connection?



A lot of times it'll backfire if you flip the mags to "off," wait a second, then flip them back to "both."

But the bigger question is, why would you want to? An engine backfiring shouldn't have anything to do with icing. That's what carb heat is for.

Special VFR could get you an extra couple hundred feet on the way in, which would be pretty valuable if you've got obstructions in the area at 700agl or above (aka, protruding all the way through your class G).

As for the backfiring, I'm not convinced that carb heat would prevent/remove most ice formed on the intake. I guess I'll emphasize anyway that I'm looking at this as an emergency procedure, as in I either forgot the carb heat or carb heat wasn't doing the trick.

[tgrayson]

Originally Posted by mhcasey

Can you get a special VFR clearance to an airport with overlying controlled airspace above the surface?

No

Another random question: How do you get an engine to backfire? My understanding is this could be useful information should I get myself into an icy situation someday...

The process jrh refers to is an afterfire, not a backfire. A backfire is an explosion of the intake manifold, whereas an afterfire is an explosion in the exhaust pipes.

Technically, a backfire happens when the mixture is overly lean. I haven't ever had to generate one, but I suppose you could just start leaning the engine. I vaguely recall reading somewhere about using this to break up induction ice. Bob Buck maybe? Where did you see it?

[tgrayson]

Originally Posted by tgrayson

I vaguely recall reading somewhere about using this to break up induction ice. Bob Buck maybe?

Yes, it was Bob Buck, on p. 216 of "Weather Flying":

In the awful condition where there isn't enough heat to clear out the ice, a desperate trick might help. The trick is to try to make the engine backfire in the hope that the backfire will clear out the ice. Generally this can be done by leaning the mixture until the engine runs rough and backfires. If it's successful, the engine will come back in with a great roar as you enrich the mixture again.

[VDEE7]

Originally Posted by mojo6911

and sometimes yanking the throttle all the back and jamming it back in does the trick.

Why does it backfire when you do that?

[mojo6911]

Originally Posted by VDEE7

Why does it backfire when you do that?

I have no idea, just something I noticed. It could be an afterfire, something I wasn't aware of, that tgrayson pointed out.

[mhcasey]

Originally Posted by tgrayson

Yes, it was Bob Buck, on p. 216 of "Weather Flying":

In the awful condition where there isn't enough heat to clear out the ice, a desperate trick might help. The trick is to try to make the engine backfire in the hope that the backfire will clear out the ice. Generally this can be done by leaning the mixture until the engine runs rough and backfires. If it's successful, the engine will come back in with a great roar as you enrich the mixture again.

Great thanks Tgrayson. I was thinking Fate of the Hunter as well.

[tgrayson]

Originally Posted by VDEE7

Why does it backfire when you do that?

I suspect this is an afterfire as well. When you close the throttle abruptly, you cut off the supply of air. The fuel flow doesn't adjust as quickly and some fuel doesn't burn in the cylinders, due to lack of air. It makes its way into the exhaust, where it ignites.

Perhaps Berkut could shed some light.

[cre8flyer]

Does a backfire sound any different than an afterfire? How can the pilot tell which is which / what just happened?

What does the backfire / afterfire do that resolves the icing issue? Is it the mechanical jolt from the explosive ignition of fuel? Or is it the additional heat that comes from this explosion?

[Berkut]

Originally Posted by tgrayson

The fuel flow doesn't adjust as quickly and some fuel doesn't burn in the cylinders, due to lack of air. It makes its way into the exhaust, where it ignites.

It's been a while since I've thought about this and my books are in storage right now, but that sounds right to me as far as afterfire is concerned.

I can't think of any way to make an aircraft backfire in flight. If I remember correctly, backfire occurs at relatively low RPM, like during starting. There's a short period of valve overlap during which both the intake and exhaust valves are open at the same time. At low RPM, some of the air/fuel mix can flow back out through the intake valve during this overlap and cause a backfire.

Causing a backfire at cruise just by leaning sounds unlikely to me. Maybe I'm missing something.