I have an immense fear of flying, and I try to understand all I can
about it. What I am particularly interested in is how much "chop" are
commercial jets built to handle, and what is "chop"'s affect on the
aircraft itself? I recently flew back to the US from Ireland and the
747 I was on dropped quite a long distance and it felt like it was
getting torn apart by wind, but all the staff seemed quite calm about
it, as if it happens all the time. So I'd like to understand the
fundamentals of "chop" and its effect on aircraft as well as the
flying experience.

Aniligital --

I'm a pilot with thousands of hours of flight time.  I fly out of
Seattle's Paine Field, where Boeing builds all of its wide-body
aircraft, and a number of my neighbors are Boeing engineers.  That,
plus the experience of flying in the mountains here, means that I've
seen more than a little of what's technically called "clear air
turbulence."

The "chop" that you're experiencing has three basic causes -- down low
it's usually from heat waves coming off roads and fields (a pilot with
an option might fly over water because it's generally less bumpy). 
The second cause is in clouds, particularly when thunderstorms are
building.  This is the type of "chop" that can be dangerous but pilots
are taught to avoid flying into thunderstorms.  The third type is what
you likely experienced coming back from Ireland: air turbulence, often
due to the jet stream.

I'll come back to all three but first should spend a little time on
aircraft construction.  All airplanes, from the smallest Cessnas to
the largest Boeing jets, are built to withstand heavy loads.  They're
built to be able to handle everything but the worst thunderstorm
conditions and frequent inspections (and standards for aging aircraft)
keep them capable.


AIRCRAFT CONSTRUCTION
========================

In normal operation, aircraft undergo considerable stress, so they're
designed for high "g loads" or gravity loads.  A tight turn in an
aircraft actually doubles the weight that the wings are carrying.

Boeing's Commercial Aircraft division, in its section on jetliner
safety has quite a bit of detail.  It notes that its aircraft are
built "to withstand 150 percent of the greatest load an airplane might
encounter."  What it doesn't say is that the FAA already requires that
the aircraft be built to handle loads and stresses of almost 4 times
normal flight - before the added design factor.

You may find the Boeing safety presentation interesting:
Boeing Aircraft
"Jetliner Safety"
http://www.boeing.com/commercial/safety/flash.html

Boeing's attention to wing stress is common across the industry, which
is why there is rare a failure of any of the wing or any of the flight
controls.  Mooney Aircraft Corp. builds a single-engine plane in
Kerrville, TX and in the early 1990s published this picture of one of
its planes with 30 employees (about 5,250 pounds on a 2,100 pound
airplane).  Note that engineers testing the wing weren't able to break
the wing with a 9.3G load -- they broke the test fixture instead.  The
spar and aluminum skin structure of the Mooney is very similar to most
modern jetliners built to-date:
Mooney Owner Events
"Famous Photo," (December, 1991)
http://www.mooneyevents.com/wingld.htm

Except in the most-severe turbulence or wind conditions, the wing
loads don't even approach the 2G or 3G levels.  Pilots normally only
run into them in thunderstorms.

There are some rare conditions where they can be found.  Even then the
pilots are trained to do several things to ease the stress on the
aircraft: keep it straight-and-level and slow down.

Now, let's get back to the subject of turbulence.


TURBULENCE
============

The Australian government's Civil Aviation Safety Authority has an
excellent page detailing the types of turbulence.  Pilots have 6
levels of turbulence that they report to controllers: it can be
helpful in alerting other aircraft to problems.  But, as you'll see in
the Australian page, the hazard isn't to the integrity of the
aircraft, it's due to things flying about the cabin and to passengers
not having a seatbelt attached:
Australia CASA
"Turbulence" (undated)
http://www.casa.gov.au/airsafe/trip/turbulen.htm

At the altitudes that you're flying trans-Atlantic, virtually all of
the turbulence is due to the jet stream.  Many researchers equate this
mild turbulence to being like normal disruptions that you see in a
river or stream.  We also know several things about the jet stream's
turbulence:
*  it's more noticeable at the edge of a jet stream due to boundary effects
*  it's stronger in winter in the Northern Hemisphere because the
speed of the jet stream is stronger
*  it's generally felt above 30,000'

There are some other causes of clear-air turbulence, including
following another aircraft too close (a situation controllers take
care to avoid); mountain air turbulence (usually worse in the
altitudes 2,000' to 5,000' and downwind of the mountain range); and
left-over thunderstorm air currents.

NCARS
"Turbulence in the Air," (2004)
http://www.ucar.edu/research/society/turbulence.shtml

For an excellent discussion of the attitude of an airline captain on
the topic of turbulence, you may wish to see Capt. K.H Lim's website
talking about the topic.  Lim flies a Boeing 777 internationally:
K.H. Lim Home Page on Aviation
"Air Turbulence"
http://www.geocities.com/khlim777_my/asturbulence.htm



Google search strategy:
Boeing construction wing loading
causes of clear air turbulence


Knowing that your concerns are as emotional as they are rational,
please don't hesitate to ask any additional questions with a
clarification request.

Best regards,

Omnivorous-GA

thank you very much!! that was my main concern, that the turbulence
could bring the plane down, I guess it's all in my mind :)