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of a station along the route of
flight and within 100 nautical miles of
the aircraft; 2) the current reported altimeter
setting of an appropriate available
station; or, 3) in aircraft without a
radio, the elevation of the departure
airport or an appropriate setting available
before departure.
This is not a problem on most
flights. However, some routes or
operations may take a pilot far from
an altimeter reporting station, as was
the case with this General Aviation
pilot:
- “Southeast bound [on airway] at
17,000 feet indicated altitude, Controller
reported my altitude encoder indicated
16,000 feet on the readout. I had departed
VFR and picked up my IFR clearance
at about 4,000 feet… I had set the
barometric pressure as provided by Center
when clearance was provided. I was approaching
a cold front which was lying
north to south over Lake Michigan. I
asked for an altimeter setting. The setting
provided was 1 inch lower than the previous
provided setting (about 100 nm earlier).
I reset my altimeter… After the reset,
my altimeter now indicated 16,000
feet… The problem was evidently a very
steep pressure gradient behind the cold
front.” (#190851)
Large portions of the Canadian
provinces and territories are remote,
making aircraft an ideal form of transportation
to and from these far-off areas.
However, flying in such remote
locations is not without some hazards.
Two incident reports from Canada
provide graphic examples of why an accurate
altimeter setting can be critical:
High to Low
Hot to Cold
Flying into cold air
has the same effect
as flying into a low
pressure area, that is,
the aircraft is lower than
the altimeter indicates.
Unfortunately, altimeters
cannot be corrected for
temperature-related errors.
However, pilots can adjust
their minimum procedure
altitudes to compensate for
temperature errors.
Canadian pilots consult a
government-provided chart
to determine how much
altitude to add to the
published procedure
altitudes listed on approach
charts, thereby ensuring
obstacle clearance when
temperatures are extremely
low. The U.S. Defense
Mapping Agency also
publishes an altitude
correction table, which is
available to military pilots.
Another helicopter accident
report from the Canadian
Aviation Safety Board points
out the hazards of failing to
correct for temperature.
Fortunately, there were no
fatalities in this incident:
-“The helicopter was
dispatched [to an offshore oil
rig inside the Arctic Circle] at
night, in IFR conditions…
The crew descended on the
inbound leg to 150 feet, with
reference to the pilot’s
altimeter. The helicopter
struck the sea ice and was
destroyed by post-impact
fire. The crew had not
applied a temperature
correction to the minimum
descent altitude [approximately
40 feet to as much as
100 feet–Ed.], and this
omission—combined with
Issue Number 9 7
- “The pilot stated that he was lined
up [on approach] for the runway and that
the altimeter was reading 300 feet when
the nose wheel struck the ice. The pilot
applied full power…and flew back to [his
departure airport over 100 nm south–
Ed.], where the landing was uneventful.
The pressure in the area was lower than
the point of departure, sufficient to make
the altimeter read 250-300 feet high if
not properly reset.” (A80W0001)
- “On a night VFR flight, the pilot encountered
deteriorating weather as he approached
his destination. He received an
IFR clearance…During a procedure turn,
the aircraft started to strike the tree tops.
The aircraft stalled and crashed into the
trees. Because the airport had been closed
for the night, no altimeter setting was
available. The FSS operator gave the pilot
the setting for XYZ (29.68) and for ABC
(29.87) [approximately 90 nm south and
90 nm east, respectively–Ed.]. The aircraft
altimeter was set at 29.94. Both pilots
had been without proper rest for approximately
20 hours.” (A80C0079)
To avoid being left on the Far Side,
obtain altimeter settings from the
nearest FSS or ATC facility. Then give
yourself an extra margin for error
when flying or landing in areas far
from the altimeter-reporting station.
Remember the old adage, “High to
　
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