Pilots have differing attitudes about safety. You can usually board a
low wing twin with engines running, but it's difficult to hold the
door, etc. The slipstream from even an idling prop is very strong.
Since props in motion tend to be invisible in peripheral vision, it's
easy for even informed airport personnel to accidentally walk into
them, as is reported with distressing frequency. A compromise is to
shut down the boarding side. A very careful pilot will always shut
down both for boarding and accept the time delay. I never leave my
single running to board or exchange passengers.

Even when the saftey problems are adiquately addressed, an idling engine on
the far side of a twin moves a lot of air and can be disruptive to the
operation. I don't know, but suspect that part of the decision might
involve the availability of a jumper cart--just in case.


On a related note, how long can you safely leave an aircraft (engines
off)
with just the parking brake set, and when do you normally put chocks
under the
wheels?


Depends on wind and ground slope. Parking brakes on cars use a cable;
parking brakes on many small aircraft instead lock the brakes with
hydraulic pressure. Since this may expand as temperatures warm up,
most pilots don't leave parking brakes on more than temporarily,
usually only until the chocks are in place. Bigger airports always
have chocks available. I carry a small aluminum pair when flying
cross country.

In my part of the country, parking brakes on light aircraft are used rarely,
if at all. I have always presumed this was to facilitate the towing of
aircraft by ramp service personnel.

Transport aircraft are another matter, on which I have no recent
knowledge--however the hierarchy of ownership and operation, as well as the
mechanical features of the brakes, are completely different--so parking
brake use could be nearly universal.

In any case, chocks are frequently used as well and some careful light plane
owners always tie down their aircraft--even for a few minutes. That
minimizes the hazard of the propeller blast from another aircraft, and is
excellent and inexpensive insurance which I plan to use when I resume
flying.

Peter