Re: Runway length for DC-9?

Date:         29 May 98 02:43:43 
From:         ehahn@mitre.org (Edward Hahn)
Organization: The MITRE Corporation
References:   1
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In article <airliners.1998.783@ohare.Chicago.COM>, jcastleANTISPAM@eden.com (Joe Castleman) wrote:
>The other day as I was driving to work, I watched a TWA DC-9 take off from
>runway 17/35 at AUS.  This runway is only 5006 feet long, and I had
>believed that jet aircraft always used 13R/31L, which is 7269 feet.  Is a
>take-off from such a short runway possible for a DC-9?  Well, evidently it
>is, but I sure wonder what the circumstances might have been...  I can't
>imagine that the pilot tried that with a full load of passengers; not so
>much because of weight, but because of the steep climb, excessive G-force
>etc.  I think a lot of people would get scared and complain.  (I myself
>would have liked to have been on that plane).

The procedure by which pilots decide which runways are acceptable are well
established and generally followed by the book.

The factors involved in which runways are acceptable for use have to do with
the so-called Balanced Field Length.  This number is related to the known
ability of the aircraft to accelerate to V1 speed and then stop without using
thrust reversers, and is based on several factors.  These include:

1) Engine type and power setting
2) Aircraft weight
3) Density altitude (combination of altitude, temperature, and humidity)
4) Runway orientation and slope
5) Winds

These factors are calculated separately for each takeoff specifically for the
available runways, and if a particular runway was shorter than the balanced
field length, it would not be used.

There is absolutely no reason for a crew to accept a runway shorter than the
required balanced field length, and I doubt that any crew would do so.

As for other factors which you mention (steep climb, excessive G-forces),
I'm not aware of anything quantifiable for either one that is related to
safety.  For example, "steep climb" means what exactly?  Also, during a
takeoff scenario, the amount of normal acceleration which the aircraft could
possibly experience is limited due to the low airspeed - the aircraft would
stall well before an "excessive" G-force (say, +4G which is the limit load of
the aircraft) would be experienced.  (Aircraft generally climb at a
predetermined airspeed which is nowhere near the stall speed, BTW.)

Also, since the passengers are belted in, generally feel a smooth acceleration
through rotation into climb, I can't imagine that anyone who has flown before
would not expect this sequence of events through takeoff.

Finally, I'd like to mention that gaining altitude as quickly as possible in
the first minutes of a flight is probably the single most important safety
factor if, for some reason, there were an emergency (engine shutdown, etc.)
The extra altitude translates into additional range, and that can mean all the
difference in being able to return to an airfield vs. landing off the airport.

ed

>>>>  Ed Hahn    |    ehahn@mitre.org    |    (703) 883-5988  <<<<
The above statement is the opinion of the author.  No endorsement
or warranty by the MITRE Corporation is expressed or implied.
Really, I wouldn't kid you about a thing like this.