Takeoff minimums

Those of us operating under Part 91 of
the FARs are legally allowed to take off in zero-zero conditions. It's
not particularly smart, but then again it's not prohibited by the regs.
Commercial operators adhering to Parts 121 and 135, on the other hand,
have prescribed takeoff minimums to adhere to. A pilot flying under
Part 91 would be prudent to adhere to these regulations, as well. If
nothing else, the GA pilot should consider having weather decent enough
to allow a return to the airport in case of an emergency after takeoff.
Like alternate minimums, takeoff minimums are listed in the TPP
volumes and on Jeppesen's airport diagram plate. For airports that
don't have specific minimums, the FARs list criteria that must be met
instead. For aircraft, other than helicopters, with two engines or
less, a one-mile visibility is considered minimum. For airplanes with
more than two engines, the minimum is one-half mile.
Instrument departure procedures may also have minimum climb
gradients associated with them. These gradients are usually
runway-specific and are attainable by most general aviation airplanes
in most conditions. However, the pilot of a light single with a big
load on a hot day at a high-altitude airport may find that the airplane
cannot meet the minimum climb gradient. It may be necessary to use
another runway with a lower (or no) minimum climb gradient. If that's
not available, the pilot may have to wait for better weather or cooler
temperatures. Twin pilots need to consider their airplane's engine-out
climb gradient. Most likely, a piston twin flying on one engine isn't
going to perform nearly as well as the climb gradient requires.
Climb gradients are always expressed in feet per nautical
mile, requiring the pilot to translate that figure into a more usable
rate-of-climb figure expressed in feet per minute. Charts are available
in TPP and Jeppesen books but can be figured in your head by
calculating your estimated groundspeed, dividing by 60, and multiplying
by the required climb gradient. For example, 120 knots divided by 60
equals two. Multiply two times 300 (the required foot-per-nautical-mile
gradient) and the result is a required rate of climb of 600 feet per
minute. Other calculations are not nearly that easy and may require you
to fish out the table in your approach books.