There was an article about this in AOPA Pilot a couple years ago. The
airplane was a Cherokee, with the small window, so I don't know how well the
procedure would work in a 172 with the larger hinged window.
Go buy a large oil funnel with a long, flexible spout. Also get a large
sponge (or more than one--enough to fill the window opening). Cut off the
pointy tip at the end of the spout. Repackage the remains into something
easy to pour from.
Fit the sponge into the window. With two people in the airplane, take off.
Close all the vents. While one person flies at a comfortable slow-flight
speed, the other works the spout of the funnel past the sponge (maybe have a
spare sponge?). Open the vents. This should create a suction through the
funnel. Pour the remains into the funnel.
As I stated once before, I'm not a ghoul. This showed up at a time when my
mother was very sick and her wishes are to have her ashes scattered by air.
Dave Reinhart
tony roberts wrote:
I appear to have volunteered for a task I know nothing about.
My friend who owns a Cherokee was approached by a neighbour who advised
him that his father was dying and has asked that his ashes be scattered
from an aircraft over a rural area near where I live. My friend
volunteered, then thought that a high wing may do the job better, and
asked me if I would do it in my 172 - in a weak moment I agreed.
Throwing the sealed bag out of the window is an appealing option for its
simplicity, but I doubt that next of kin or persons on terra firma would
agree.
An open bag, thrown out of the window may or may not empty on its way
down - not reliable enough. Also I don't want the dear departed to hit
the horizontal stab and take us with him.
Tipping the contents of the bag out of the window would, I suspect,
result in the contents siphoning back into the cockpit.
A tube hanging out of the window, with a funnel at the top? That may
create a venturi effect and cleanly suck the ashes out of the plane.
Has anyone ever disposed of cremated remains from a light plane?
How did you do it? All suggestions/ideas gratefully received.
--
Tony Roberts
PP-ASEL
VFR OTT
Night
Almost Instrument 
Cessna 172H C-GICE