Re: airplane watching questions

Date:         21 Mar 2001 18:33:29 
From:         Carl Peters <cpeters@davesworld.net>
References:   1
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Jeff wrote:

> Hello folks.
>
> I indulged myself today in driving out to Pearson airport here in Toronto
> Canada to watch planes land.
>
> I had a fun time but came up with a few questions I hope someone (or
> someone_s_) can answer.
>
> 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?

1. Regarding landings, pilots will do both - use autopilots at times, and hand
fly. Regarding what you saw, either 'mode' could have been in effect, and the
planes that "would tip its wings" were probably responding to some cross winds.
A wind coming in from one side will generate extra lift on the same side wing,
with a mild decrease in lift to the opposite wing, as it is partially in the
wind 'shadow' created by the interfering fuselage. Ailerons will be used,
whether by the pilot or autopilot to bring the wings back level.

> 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?

2. The vapor you saw is usually seen in humid conditions as you describe, and is
the result of water condensing into a cloud by the lowered pressure over the top
of a wing, and in the center of a vortex, as seen emanating from wing tips or
the flap edges (I have seen many off the far flap outside trailing edge on the
737 while flying, many off the wingtip, and several times a vertical vortex
below the leading lower edge of a wing mounted engine - L1011 and 767). For the
same reasons, the effect is also noted by survivors of sudden cabin
depressurization, where a sudden fog develops in the cabin.

> 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?

3. The short answer is that you are being fooled by an illusion. A 747,
depending on many variables, will land at a similar or slightly higher landing
speed than a 737, for example.

Carl Peters