Re: parallel runways on airports

Date:         20 Mar 97 02:38:33 
From:         ksaunders@ll.mit.edu (Kenneth W. Saunders)
Organization: Massachvsetts Institvte of Technology
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In article <airliners.1997.657@ohare.Chicago.COM>, "Marc SCHAEFFER"
<marcmsc@hotmail.com> wrote:

> I read in 'Aero international' that the two parallel runways 25L/07R and
> 25R/07L at the Frankurt airport (FRA) can't be used at the same time
> since they are only seperated by 518 meters.
> - What's the minimum distance where two parallel runways can be used
>   at the same time ?
> - Are there other restrictions for these type of runways ?
> - Are the rules the same in Europe and the US ?

In the US, the FAA allows simultaneous independent approaches in IMC when
parallel runways are spaced at least 4300 ft apart.  Independent approaches
means that the aircraft on one approach DO NOT have to spaced in front of or
behind the aircraft on the other approach.  You can think of independent
approaches to two runways as being two separate (but closely spaced) airports
with two separate streams of traffic and two separate sets of approach
controllers.  Between 4300 ft and about 2000 ft, parallel approaches in the
US are run as dependent approaches in IMC.  Dependent approaches use a single
set of approach controllers that stagger the positions of the aircraft, so
that if one aircraft deviated from the approach course, it would not come
near the aircraft in the other arrival stream.

INDEPENDENT APPROACHES:  (each - is about 1/2 nmi, each X is an aircraft)

-----X------X------X------X------X-

        >= 4300 ft separation

----X------X-------X------X-------X


DEPENDENT APPROACHES:  (each - is about 1/2 nmi, each X is an aircraft)

-----X--------X--------X--------X--------X

      >= 2000 ft and < 4300 ft separation

-X--------X--------X--------X--------X----

Normally, dependent approaches have the aircraft in each approach stream
spaced farther apart in-trail.  This reduces arrival capacity (obviously).
The above pictorials are a close approximation to reality.

There is a 2000 ft "No Transgression Zone" between parallel runways.  If
and aircraft enters the NTZ, aircraft in the other approach stream are
supposed to be given commands to break off their approach to avoid a near
mid-air collision.

The FAA has an on-going project to use high resolution displays and more
accurate surveillance systems (radar or other means) to reduce the 4300 ft
spacing.  Some studies have shown that 3400 ft spacing is safe with the
Parallel Runway Monitor system.  To my knowledge, there's no operational PRM
controlling traffic in the US.

I don't know how much the FAA relaxes these rules in VMC, although I've flown
into airports (e.g. Denver Stapleton) with closely spaced parallel runways
in VMC where the two aircraft on approach were not staggered.  I'm not sure
if the FAA allows approaches to runways spaced closer than 2000 ft apart in
VMC, but there may be special case airports where this is allowed.

Frankfurt's 518 m spacing is less than the FAA's 2000 ft NTZ rule.  There may
be a German CAA equivalent requirement to the FAA's NTZ that prohibits some
types of parallel approaches to these closely spaced runways.

--
Kenneth W. Saunders
MIT Lincoln Laboratory
ksaunders@LL.MIT.EDU