Re: airplane watching questions

Date:         18 Mar 2001 12:06:10 
From:         don@news.daedalus.co.nz (Don Stokes)
Organization: Daedalus Consulting
References:   1
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In article <airliners.2001.27@ditka.Chicago.COM>,
Jeff <jcbowen.nospam@home.com> 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?  What
>proportion of airliner landings at an airport like Pearson International are
>flown automatically by a computer locked onto a beam?

I'm not sure how gross autoland corrections can be, but note that most
autoland systems don't autopilot down to the deck -- the flare and
rollout are controlled by the crew.  The next level up has the autopilot
working until touchdown, and only the most advanced (expensive) include
control of rollout.

I've heard lots of anecdotes to the effect that pilots very much prefer
to hand-fly the landings, leaving the autoland only for really marginal
conditions.

Given that, it seems likely that most of the approaches you see are manual,
and if the crew got the approach vector right (and the wind didn't upset
things), you don't see gross corrections, and if they didn't, you do.

>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?

High pressure air can hold more water vapour (ie water in its colourless
gaseous state) than low pressure air.  If the air is close to saturation
(ie the maximum amount of water vapour it can hold for the given
pressure), lowering the pressure causes water to condense into tiny water
droplets, which form the vapour trails.  Wings have lower pressure on top
(higher underneath), especially over the flaps -- you often see vapour
trails coming off the outboard corners of the flaps on approach in humid
weather, as this is a point on the wing which is at particularly low
pressure.

>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?

You got it exactly.  Jets from bizjets to widebody airliners tend to fly
at speeds pretty much in the same ballpark, so smaller planes seem to
fly faster because they cover their own length more quickly.

-- don