Jack wrote:
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.
OK, then both. From my point of view, however, if I had to choose
between 1. point on the ground and ASI and timing to acceleration vs.
2. just look at the engine gauges and interpret them, I'd take #1
first.
http://www.avweb.com/news/safety/182403-1.html
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.
Yep. So add in a calculation for time to airspeed. And add in
a calculation for ground distance to airspeed. If one detects
that the acceleration and airspeed aren't coming up at that point,
abort the takeoff (presumably that point is calculated to be
the point of safe abort).
The simplicity of this is one doesn't need to deal with the complexity
of diagnosing what CAUSED the lack of acceleration. Was it the
RPM or the EPR that was wrong? Is it the RPM or that the prop is
worn thin? Did I leave the hand brake on for takeoff? Is the
nosewheel flat? Did they put a second passenger in the glider and
now it's too heavy? So many factors (some known, some not).
I prefer simplicity...
Of course the simple answer is just to have much more power
available than you could possibly need (RJs with 8000fpm climb,
a nice 400hp stearman) so really I'm talking about using this
as a tool to provide the same level of safety with less training
and less cost, rather than improving safety...
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.
Umm...I didn't say anything yet about visual cues. GPS, a stopwatch,
and marking the point when the takeoff roll starts. For jets,
an automated calculation and "low acceleration" warning
light.
For towplanes, probably a visual cue is easier, though,
and simpler than for an airline, because one tows off the same field
again and again. Another possibility is to simply walk the
field (or taxi up the taxiway) with the GPS and measure the
visual cue distance and the safe takeoff runway.
I've used GPS to measure a short strip before and declined to
takeoff there. Useful things, these little gadgets. Better than
my golfing range estimating eyeball...
If I towed out of a farm field or a "new to me" airstrip, and
I had some questions about whether I could make it over the
power lines, I might use this technique as an "extra tool in my bag"
to improve my confidence and capability. How much does the 1" thick
soft dirt slow me down? Am I light enough on fuel? I can do all
the calculations, but it's never the things I know that hurt me,
it's the things I don't know that I don't know...
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.
-----
Jack
-----
The last time I even dreamed that an airline pilot looked out
the window was at Ontario in low overcast when the tower called
"Delta XYZ traffic 2 O'clock 1 mile below you an experimental
with no transponder tight left base will pass behind you..."
And a real nervous co-pilot "XYZ looking for traffic"
....and I suspect a few expletives from the captain
"Who lets a f***ing no-squawk ragwing fly around in OUR
go***mn airport?"
Again, for any airline guys, I wouldn't DREAM of asking you
to look outside the cockpit for a visual cue of anything.
(Now, now, don't get yer panties in a bunch, I'm jus'
hasslin' ya, with yer umpteen item checklists, and yer
bells and whistles, and yer big cozy pilot chairs, and
yer Datsun 280Z's :PPP)
For you guys, I just thought maybe a nice shiny new
"acceleration" computer might be an extra takeoff tool... :P
Something to go with the pre-calculated trim settings and
"gee ya almost hit the tail" printouts?
But you are right, perhaps try it a while in the pawnee towing
an extra heavy ballasted two seat glider a few times first
and see how it affects the rotation point, and then
come up with some takeoff abort scenarios, and see how
much runway you REALLY have.
I actually really also liked the discussion about how longer
towropes allow an abort where the glider doesn't ram into the
back of the tug, too...