Compression Benefits Study

**Pure Compression vs. Substitution Benefits**

**Lara Shisler**

**Metron, Inc.**

One of the key elements under CDM/GDPE is compression. Compression is a
process used to ensure that no valuable arrival slots at an airport go
unused during a ground delay program. The algorithm identifies open
arrival slots due to flight cancellations and delays. It then moves
other flights up, reducing their delays, to fill the vacated slots.
Compression always attempts to fill an open slot by moving up a flight
which belongs to the same carrier as the open slot. If that is not
possible, it then tries to find a CDM-participating airline that can
benefit from the slot. Finally, the slot is made available to all
flights.

Since an open slot is always made available to the airline which owns
the slot first, some of the delay reductions achieved from compression
are due to moving flights up into slots they have vacated due to flight
cancellations. Airlines have already had that capability provided to
them prior to CDM. It is called the substitution process. Any airline
can cancel a flight which has been assigned an arrival slot in a GDP and
move another one of their flights up to fill the open slot.

This study was an attempt to identify what component of the benefits
achieved through compression could have been achieved by the airlines
via the substitution process. Utilizing FSM in historical mode,
compression can be re-modeled and the results analyzed. FSM also
includes a substitution algorithm which models how delays can be reduced
by the airlines using the substitution process. The methodology used for
this study was the following:

1.  Utilizing FSM in historical mode, set the time to match the model
    time of the compression.

2.  Use the "Subs" algorithm in FSM to model the delay reductions which
    can be achieve from substitution.

3.  Do not reset the data.

4.  Run the "Compression" algorithm in FSM using the same parameters
    that were used initially.

5.  The minutes reduced due to this compression are the pure compression
    benefits.

6.  The difference between the total reduction and the pure compression
    reduction is the subs reduction.

By running the substitution algorithm prior to running compression, all
benefits which could have been achieved by the airlines alone using the
substitution process are removed. The resulting benefits are delay
savings that could never have been achieved prior to CDM/GDPE.

This methodology of identifying pure compression savings is actually a
conservative one. This is due to a number of reasons. Even though an
airline could have achieved the benefits using the substitution process,
some airlines have not implemented this capability and have no way to do
substitutions. Also, even though an airline may use the substitution
process, it can be difficult to stay on top of every opportunity for a
sub. In the past, arrival slots frequently went unused because the
airline that owned that slot did not substitute one of their flights up
to fill it. It can become especially difficult during the more severe
weather events. The more an airline's schedule is being disrupted, the
harder it is to stay on top of substitutions. In these cases,
compression has provided a tool for the ATCSCC to do their substitutions
for them, the results being overall delay reductions across all of their
flights.

This methodology was applied over all compression cycles between
September 8, 1998 and March 17, 1999. This included 585 compressions
over 246 ground delay programs at 15 airports. The average breakout
between subs and pure compression over all of these compression cycles
was 66% of the compression benefits could have been achieved
theoretically via the substitution process and 34% could never have been
achieved without compression.

The following table contains the results by each of the 15 airports.

![](media/image1.wmf){width="5.995833333333334in"
height="2.7159722222222222in"}

The results are also displayed in the graph below. The airports are
sorted from highest minutes of compression savings to lowest. The total
height of each bar is the total percent reduction due to compression.
Each bar is broken into 2 sections; red for pure compression savings and
blue for substitution savings.

![](media/image2.wmf){width="6.2in" height="3.3819444444444446in"}

The final graph below normalizes the results. Each airport's total
compression savings is set at 100%. Each bar is then broken into 2
percents; red for pure compression savings and blue for substitution
savings. This gives a graphic representation of what percent of the
total savings are due to pure compression. The airports are sorted by
total minutes reduced, the highest at the bottom.

![](media/image3.wmf){width="6.3in" height="4.875694444444444in"}

The average results over all airports was roughly 1/3 savings due to
pure compression, 2/3 savings due to theoretical substitution. We
believe that a big enough data set was used to establish this trend.
Thus, breaking out compression benefits between pure compression and
substitution was not continued after March 17, 1999.