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areas and in Eastern Canada.
The fierce cold fronts that race
through central Canada and the
north-central and northeast sections
of the United States often have steep
frontal slopes, resulting in rapid lifting
movement of air. This movement
causes sudden drops in barometric
pressure. After frontal passage, the
barometric pressure rises again. During
these rapid ups and downs, a
number of pilots found themselves
missetting their altimeters. A General
Aviation pilot reports:
- “I set the field elevation on the
altimeter…and departed VFR, [then] contacted
Center and received my clearance.
In the climb I encountered icing conditions
and…I must have misset the altimeter.
A frontal passage was in progress
and the barometer was changing rapidly.
Center told me to stop squawking altitude,
as my transponder and altimeter
did not agree. Then I suddenly was surprised
to realize I had set my altimeter
wrong at 30.82 instead of 29.82.”
(#293162)
A helicopter accident resulting in
four fatalities was attributed at least in
part to an incorrectly set altimeter during
a period of known low barometric
pressure. The report from the Canadian
Aviation Safety Board states:
- “The helicopter was being used to
transport personnel to work sites across a
large frozen lake. An approaching low
pressure area with snow and high
winds…reduced visibility to near zero in
some areas. The pilot most certainly encountered
adverse conditions and altered
course to circumvent the worst areas. The
aircraft was later found…wreckage was
widely scattered. The altimeter showed a
setting on impact of 30.05; the correct
setting would be about 29.22, causing the
altimeter to read about 800-850 feet
high. The altimeter had obviously been
set two days previously [apparently during
a time of high barometric pressure–Ed.].”
(A80C0002)
Weather or Not you are flying near
significant weather activity, ensure
that you have the most up-to-date
weather reports, including complete
information on frontal movement.
Barometer Surprise
A number of reporters indicated
that the low altimeter setting they
encountered was unusual for the
area or unexpected for the current
weather conditions. In many of these
cases, the crew subconsciously ignored
the correct setting in favor of a setting
that seemed more appropriate. Some
examples:
- “The altimeter setting I wrote down
was 28.85, but we had both set 29.85. I
did not recognize the unusual nature of
the setting, and reverted to more familiar
settings during the checklist.” (#97654)
- “I read back the clearance, understanding
the altimeter to be 30.37. Factors
in this incident include my hearing
“.37” and assuming it was the more normal
30.37 rather than the [actual] low
reading of 29.37.” (#287167)
6 Issue Number 9
- “Altimeter was 29.32. First Officer
set his altimeter to 29.32; I did not check
mine. Weather was such that a 29.32
altimeter setting was not expected-winds
were calm and a clearing trend was in the
works.” (#295359)
Crews can avoid a Barometer Surprise
by listening carefully to ATIS and
ATC broadcasts, especially before, during
and after significant weather,
when the altimeter setting may be an
unusual number.
Et cetera
Our research produced one other
oddity associated with a low altimeter
setting:
- “The temperature was 53 degrees
with an altimeter setting of 29.34. I advised
the Captain that we were over our
allowable takeoff gross weight for that
runway. The low altimeter setting required
a further reduction of 270 pounds
in allowable takeoff weight. The Captain
said he was not aware of correction due
to altimeter settings. The procedures for
adjusting weights…had been in effect for
only a few months. Before this, altimeter
settings had only affected takeoff power,
not takeoff weights. A test or some classroom
work should be sufficient to correct
the situation.” (#145629)
This report serves as a reminder for
all pilots to review flight and operations
manuals frequently for changes
or additions to out-of-the-ordinary
procedures.
The Far Side
FAR 91.121 states that, when operating
an aircraft below 18,000 feet
MSL, pilots will maintain altitude by
reference to an altimeter that is set to:
1) the current reported altimeter setting
　
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