0
1992-93 (0cm)
1993-94 (0 cm)
4
8
Figure 79. Ice surface temperatures
at 2000-ft marker on the east edge of
12
the runway throughout the 199293
30
10
20
30
10
20
30
and 199394 austral summers.
Nov
Dec
Jan
daily air temperature and comparing this with
past and current trends. For example, during 1992
the temperature profiles in the runway ice (Fig.
93 the ice temperatures were too high for flight
79, App. D), we set the latest time to begin depos-
operations to begin until around 21 January. Dur-
iting snow onto the Pegasus runway to prevent
ing the 199394 season (thermocouple strings in-
melting and, after the peak of summer, the time
stalled at 2000 ft, 5000 ft, and 8000 ft) the ice
to remove snow from the runway in order to start
temperatures were lower and operations could
air operations. At Pegasus, our basic guidelines
have begun as early as 17 January (see Fig. 79 and
were as follows: placement of snow cover should
App. D). Also, the ambient and ice temperatures
be completed by the time air temperature has
can be as much as 2 degrees warmer on the north
risen to an average daily value of 10C (for a
(340) end of the runway. As landing and refuel-
span of three or more days) or ice temperature
ing take place on the north end, having different
(the highest recorded value in a 1-m vertical pro-
locations along the runway for monitoring tem-
file) has risen to 15C, whichever occurs first.
peratures is important. It is not sufficient to
The beginning of snow removal activities is trig-
"guess" that the season had been colder than nor-
gered, after the seasonal cooling trend is clearly
mal. Although the ice temperature does respond
established, by the average daily air temperature
to ambient temperatures by conduction, predict-
reaching a value less than the highest ice tem-
ing the ice surface temperature is impossible, as
perature (measured that day in the ice column).
convection and radiation also play dominant roles
Characteristically, this translates to placement of
in the heating/cooling of the ice surface. The over-
the snow cover by about 15 November and re-
all surface heat budget determines the ice tem-
moval about 7 January (Fig. 78 and 79). However,
perature and an approximate date for the heat
the decision to strip the runway of its snow cover
budget becoming neutral and then negative is 21
and commence/cease flight operations must be
January. There is no way to assure safe flight
determined by the ice surface temperature and
operations without this information as the run-
temperature profile in the ice (App. D).
way has only been proofed for "cold" ice. The
Thermocouple strings were installed during the
ultimate strength of ice is reduced drastically at
season of 199293 at the 2000-ft, 4000-ft, 6000-ft,
higher temperatures.
and 8000-ft positions along the runway. These
Furthermore, these guidelines for snow cover
should be connected to some type of datalogging
and removal are based solely on the simplified
equipment. Reading thermocouples on a daily
estimation of solar radiation penetration as pre-
basis is insufficient as the ice surface temperature
sented above. Spectral detail has also not been
fluctuates quite dramatically. The average daily
considered. Snow grain size is also a factor in
ice surface temperature must be less than or equal
finding the penetration depth of solar radiation
to 5C. Retaining a record of the ice temperature
(Brandt and Warren 1993). Conduction by the
profile is critical as decisions must be based on
snow to the ice surface and convection at the
74
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