On 2/6/04 1:22 PM, in article 4023f76a$1@darkstar, "Mark James Boyd"
wrote:


I've wondered why airliners don't have some spot on the
ground (GPS) some distance down the runway, and abort
at that spot if they haven't reached a certain airspeed.
Seems simpler than doing all them calculamications for
wind, density altitude, etc. which may have changed since
you did them. Why not observe instead of predicting?

We can't afford not to do both, Mark. The calculations would still be
required in every case, making the "spot" different for each takeoff.

Airlines conduct operations on many different runways under all conditions
of weather, ambient lighting, loading, runway slope, braking coefficients,
etc., and many of these factors will vary from flight to flight even at the
same field, on the same day, in the same aircraft. In some cases a crew
member may fly in as many as three or four variations of a single type
(e.g., DC-9-10, DC-9-30, DC-9-40, DC-9-50), with a significant range of
performance and handling characteristics, within the same 24-hour period. I
think you can see that calculations for each flight with go/no-go decisions
made according to predetermined speeds for the anticipated conditions, with
adjustments when conditions change (there's your "observation") is the only
way we can safely operate.

With hundreds, or thousands, of pilots on the roster, and scores or hundreds
of destinations across the country and around the world, the idea of trying
to train every pilot to the visual cues at every runway would leave the
training department using far more assets than the revenue side of the
operation could likely produce.

I'd much prefer to make split-second decisions based upon the well
understood performance of the aircraft, than on the often vague cues
provided by the environment in which we operate the aircraft. In contrast to
the typical sailplane flight, we operate big fast moving machines often
under conditions in which I would prefer not to have to walk to my car, let
alone drive it to the airport.

If visual cues are to comprise the primary reference for takeoff
performance, by all means let it be at 30 to 50 mph in a 1-26 or in a
Pawnee, on a sunny day from 2500 feet of grass, where I am familiar with
every bump and soft spot, and not from the cockpit of a 180,000 lb
(relatively light, at that) Boeing 757 on a snowy February night from 6500
feet of wet runway at Midway, on the south side of Chicago.

Flying sailplanes well, I suspect, will be complicated enough.


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Jack
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