16 |flight safety foundation | AeroSAfetyWorld | november 2011
flightopSflightops
the 3-Kelvin-spread rule, an aviation
rule of thumb for winter opera-
tions, can enhance pilots’ sense of
the actual landing risk level by
calling attention to the likelihood that
aircraft braking performance will not
meet pre-landing calculations. The rule
takes advantage of information readily
available from aviation routine weather
reports (METARs), and its potential value
has been recognized in the recent report
of an investigation of winter operations1
by the Accident Investigation Board Nor-
way (AIBN; AS W, 10/08, p. 14 and 11/10,
p. 30). Nevertheless, its ease of adoption
comes at the expense of absolute validity.
The rule emerged from several years
of scientific research and analysis in Nor-
way, including my contributions in the
report’s Appendix J, which explain the mi-
crometeorology of the natural processes
involved in water vapor saturation and ice
on runways (see “Can You Stop?” p. 12).
The impetus was that about two-thirds
of the air transport accidents and serious
incidents studied by the AIBN (21 of 30)
occurred when a difference between air
temperature and dew point (or spread)
of 3 kelvin (K)2 or less was reported by
METARs (see Table 1, p. 17, and Table 2,
p. 18 for Kelvin-scale conversions). The
statistica l probability of that proport ion
occurring by chance is 0.023 (2.3 percent).
Such a spread means that air
containing water vapor is not very far
from being saturated. The appendix
discusses many factors involved; this
article focuses only on saturated water
vapor at freezing temperatures.
The rule states, “When the spread
(air temperature minus dew point …
read at 2-m [6.6-ft] level) is less than 3 K,
compacted snow or ice may constitute
slippery conditions.” The rule has proved
especially valuable when air temperatures
are lower than 3 degrees C (37 degrees F),
in cases of recent or current precipitation,
when snow contains liquid water and/or
when the surface of frozen contaminants
is considerably colder than the METAR
air temperatures collected 2 m above the
runway indicate.
The rule’s underlying research was
first reported by an AIBN official in
2007.3 Before then, insufficient friction
on the runway during line operations in
Norway often had been attributed only
to precipitation or to liquid or frozen
water deposited on the runway.
A spread of 3 K or less means the
relative humidity is high — often 80
percent or higher, indicating likely pre-
cipitation. Often, such a spread occurs in
precipitation, intermittent precipitation
and precipitation in the vicinity of fog or
in conditions conducive to fog.
Valuable IntelligenceValuable Intelligence
By ReinhaRd Mook
Temperature–dew point spread helps pilots
anticipate slippery winter landing conditions.
