Re: airplane watching questions

Date:         18 Mar 2001 12:06:18 
From:         James Robinson <>
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Jeff wrote:
> 1.  Most of the airliners seemed to follow an identical path down to the
> runway but every so often one would tip its wings back and forth a little
> bit on the approach. Were the planes that were making these adjustments
> making manual landings and the others making computerized landings?

The adjustments are in reaction to various cross winds encountered on
the approach.  It wouldn't matter if the landing was automatic or
manual, the aircraft would still react the same way.

> What proportion of airliner landings at an airport like Pearson
> International are flown automatically by a computer locked onto a beam?

Virtually no landings are automatic.  The pilot will usually take manual
control from the autopilot on the final approach to landing.

> 2.  It was a cool afternoon (temp had dropped to about 12 C) and very humid
> (raining on and off and very low cloud cover).  Most of the planes landing
> had a streamer of water vapor trailing from the wingtips like smoke
> streamers and some had a cloud of vapor on the upper surface of the wings.
> Someone once explained this to me and I forget what it is.  Something to do
> with low pressure around the wing?

Exactly.  The low pressure will cause the moisture in the air to
condense into a fog when conditions are right.  Lower pressure air will
hold less moisture than higher pressure air, so when the relative
humidity of ambient air is high enough, the sudden lowering of pressure
will cause some of the moisture to condense and become visible.  This is
the case at wingtips, at the tips of propellers at times, behind
canopies on military aircraft, and over the top of wings.

> 3.  I've always noted that the larger the airliner, the slower it appears to
> be moving in the air.  Today, the big planes seemd to be slowly settling out
> of the sky and the occasional executive-type jet appeared to be 'whistling'
> down out of the sky.  Is this an optical illusion?  I am assuming it is and
> may have to do with the fact that a long plane takes longer to travel its
> own length than a shorter one?

It's just an optical illusion.  Large aircraft don't seem to be
travelling as fast as smaller aircraft, but they are in reality.