CFII oral exam guide questions?

Robert M. Gary wrote:
1) He showed me disassembled instruments when describing how each works
(how well can most of us explain how an altimeter really works inside,
I thought a mag compass was ball shaped )

I always thought it was worthwhile to show an instrument student a
disassembled compass. Once he sees how the counterweight works to
compensate for the vertical component of the magnetic field, turning
errors are obvious - as well as why there are no turning errors while
taxiing. Accelerationa and deceleration errors are equally obvious.

2) He also explained how VORs really work (I thought they broadcast
actual radials but they actually time the difference between a
reference signal and a rotating sweeping signal).

You know, I once interviewed an electrical engineer who worked for a
major GA autopilot manufacturer who told me he worked on VOR. He
couldn't explain it to me. Of course I didn't hire him.

3) He expects me to memorize the freq of markers and be able to
distinguish a loc freq from vor freq by looking at the freq number.

Well, it can be done - but WHY? There is some value in knowing that
the radio determines whether to use the VOR or LOC circuitry based on
the frequency selected. There is much value to understanding the
difference, especially as it applies to the validity of a VOR check for
LOC/ILS ops. Memorizing what the actual frequencies are seems rather
pointless.

4) Even the stuff I thought I knew was wrong. I thought lots of air was
always running through the pitot tube and out the drain hole.
Apparently only a very, very small amount of air comes out the back of
the drain hole and the actual volume of air running through the pitot
is very small. Looking at all the books, I got the impression air was
just rushing through.

Then you really didn't understand the physics of how a pitot tube
works.

Think of a pitot tube as an energy conversion device. There is energy
in moving air. There is energy in pressurized air. A pitot tube is a
device for converting the former into the latter. An ASI is actually a
pressure gauge (usually a brass bellows that drives the needle) that
measures the difference between the ram pressure and the static
pressure. The ram pressure is always going to be higher, because the
speed of the airplane is forcing air in, and pressurizing it.

All the drain hole does is cause error (non-heated pitot tubes often
don't even have drain holes - the primary function of the drain hole is
to allow the water formed when the pitot heat melts the ice to drain)
by allowing some of the ram pressure to bleed off. If the pitot tube
is to give a reasonably accurate indication, the rate of leakage MUST
be small.

Amazing how much you can learn while working on a rating that
supposedly only allows you to teach what you already know.

Michael

On 06/13/06 12:34, Michael wrote:
Robert M. Gary wrote:
1) He showed me disassembled instruments when describing how each works
(how well can most of us explain how an altimeter really works inside,
I thought a mag compass was ball shaped )

I always thought it was worthwhile to show an instrument student a
disassembled compass. Once he sees how the counterweight works to
compensate for the vertical component of the magnetic field, turning
errors are obvious - as well as why there are no turning errors while
taxiing. Accelerationa and deceleration errors are equally obvious.

2) He also explained how VORs really work (I thought they broadcast
actual radials but they actually time the difference between a
reference signal and a rotating sweeping signal).

You know, I once interviewed an electrical engineer who worked for a
major GA autopilot manufacturer who told me he worked on VOR. He
couldn't explain it to me. Of course I didn't hire him.

3) He expects me to memorize the freq of markers and be able to
distinguish a loc freq from vor freq by looking at the freq number.

Well, it can be done - but WHY? There is some value in knowing that
the radio determines whether to use the VOR or LOC circuitry based on
the frequency selected. There is much value to understanding the
difference, especially as it applies to the validity of a VOR check for
LOC/ILS ops. Memorizing what the actual frequencies are seems rather
pointless.

4) Even the stuff I thought I knew was wrong. I thought lots of air was
always running through the pitot tube and out the drain hole.
Apparently only a very, very small amount of air comes out the back of
the drain hole and the actual volume of air running through the pitot
is very small. Looking at all the books, I got the impression air was
just rushing through.

Then you really didn't understand the physics of how a pitot tube
works.

Think of a pitot tube as an energy conversion device. There is energy
in moving air. There is energy in pressurized air. A pitot tube is a
device for converting the former into the latter. An ASI is actually a
pressure gauge (usually a brass bellows that drives the needle) that
measures the difference between the ram pressure and the static
pressure. The ram pressure is always going to be higher, because the
speed of the airplane is forcing air in, and pressurizing it.

All the drain hole does is cause error (non-heated pitot tubes often
don't even have drain holes - the primary function of the drain hole is
to allow the water formed when the pitot heat melts the ice to drain)
by allowing some of the ram pressure to bleed off. If the pitot tube
is to give a reasonably accurate indication, the rate of leakage MUST
be small.

I thought this allowed rain water to drain as well. Otherwise, couldn't
the water build up and eventually get into the pressure line to the
ASI?

Or does the air pressure keep/force the water out the front of the
tube somehow?


Amazing how much you can learn while working on a rating that
supposedly only allows you to teach what you already know.

Michael




--
Mark Hansen, PP-ASEL, Instrument Airplane
Cal Aggie Flying Farmers
Sacramento, CA

Mark Hansen wrote:
I thought this allowed rain water to drain as well. Otherwise, couldn't
the water build up and eventually get into the pressure line to the
ASI?

If there is a hole, then yes, it will drain water as well, because
there is flow into the tube. But without a drain hole, there is no air
flow into the pitot tube at all - so how will the rain drops get in? I
suppose if you flew through a heavy enough rain it could happen - but
that implies flying the plane IFR, and it's pretty rare for a plane to
be IFR capable and have no pitot heat. Rare, but not unknown.

Michael

Michael wrote:
Then you really didn't understand the physics of how a pitot tube
works.

Think of a pitot tube as an energy conversion device. There is energy
in moving air. There is energy in pressurized air. A pitot tube is a
device for converting the former into the latter. An ASI is actually a
pressure gauge (usually a brass bellows that drives the needle) that
measures the difference between the ram pressure and the static
pressure. The ram pressure is always going to be higher, because the
speed of the airplane is forcing air in, and pressurizing it.

I understood that. I just way over estimated how much air went through
the pitot tube. Any tube that has a hole in the front and a smaller
hole in the back would do the job. The net pressure to the A/S
indicator would be less but that's just a calibration issue. So I
understood the concept but over estimated the flow.

-Robert

Robert M. Gary wrote:
I understood that. I just way over estimated how much air went through
the pitot tube. Any tube that has a hole in the front and a smaller
hole in the back would do the job. The net pressure to the A/S
indicator would be less but that's just a calibration issue. So I
understood the concept but over estimated the flow.

Mea culpa. There was actually something you didn't know (or didn't
consider) but it was so obvious to me that I never even thought about
it. And really it's much less obvious than the physics - unless you
happen to routinely build and design stuff. I say this by way of
explanation and apology.

You're right - you could in theory have any tube with a bigger hole at
the front and a smaller one at the back, and the indication would be
less but could be calibrated out. There's a reason it's not done that
way, and it has nothing to do with the physics and everything to do
with manufacturing practices and standards.

ASI's are interchangeable, and it would be a maintenance nightmare not
to have them interchangeable. Imagine that every pitot tube had to
have its own matching (calibrated) ASI. Often the pitot tube is made
by the manufacturer. Fortunately they rarely fail, because
replacements are sometimes very expensive or even impossible to find.
Piper made its own pitot tubes for a long time. Go try to find one for
a short winger.

ASI's will all fail eventually. They're made with brass bellows that
are soldered together. The solder joints move and hold (just a little)
pressure. Eventually they are guaranteed to leak. The leak will show
up as a too-low airspeed indication.

So for practical purposes, you really need a pitot tube that supplies
approximately the right ram pressure.

Michael

Michael wrote:
I always thought it was worthwhile to show an instrument student a
disassembled compass. Once he sees how the counterweight works to
compensate for the vertical component of the magnetic field, turning
errors are obvious

Seeing stuff opened up is always useful. My FBO has a sectioned
carburator sitting around in one of the classrooms; I take every
opportunity to show people how it works inside. Some of them actually
pay attention :-)

I also like to pull the cowling off planes and show students what's
underneath. Many have never seen anything more than can be seen from
the oil filler door. It's very instructive when you see things like
the alternate air control opening up a little door on the air filter
housing, or tracing out the wires from the plugs to the magnetos.

Well, it can be done - but WHY? There is some value in knowing that
the radio determines whether to use the VOR or LOC circuitry based on
the frequency selected. There is much value to understanding the
difference, especially as it applies to the validity of a VOR check for
LOC/ILS ops. Memorizing what the actual frequencies are seems rather
pointless.

I agree that knowing which are VOR and which are LOC is not terribly
practical for most pilots, but knowing which are NAV and which are COM
is. I've seen more than one pilot keep spinning the knobs on the COM
radio trying to to figure out why they can't find 116.6 to get the
ATIS.

1) He showed me disassembled instruments when describing how each works
(how well can most of us explain how an altimeter really works inside,
I thought a mag compass was ball shaped )


I always thought it was worthwhile to show an instrument student a
disassembled compass. Once he sees how the counterweight works to
compensate for the vertical component of the magnetic field, turning
errors are obvious - as well as why there are no turning errors while
taxiing. Accelerationa and deceleration errors are equally obvious.

Calling Jay, calling Jay. Disassembled instruments would be way cool in
your aviation themed motel.

Jose
--
The price of freedom is... well... freedom.
for Email, make the obvious change in the address.