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low, look out below.” A one-inch error
in the altimeter setting equals 1,000
feet of altitude.
The One-Eight-Zero Blues
ASRS receives many reports of altimeter-
missetting incidents that occur
when aircraft are transitioning
through FL180 (see “Transitioning
Through FL180”, beginning page 9). A
flight crew’s failure to reset an altimeter
at FL180 has probably caused an
occasional adrenaline rush among
controllers; the added factor of an extremely
low barometric pressure increases
the potential for large altitude
errors. The following report excerpts
illustrate:
- “Altimeter [at departure field] 28.42.
When…climbing through 18,000 feet,
Captain called, ‘29.92 set’ when in fact he
set 28.92. I did the same. The back lighting
in my altimeter was out, and maintenance
had installed post lights [which left] a dark
shadow…thus my meter was in the dark. I
was careless in not double-checking with a
flashlight.” (#290765)
- “Holding off and on. We neglected to
reset altimeters from 29.92 to 29.20 passing
through FL180. Extremely low pressure
caused us to be at 12,200 feet when we
thought we were at 13,000 feet. We didn’t
accomplish the checklist on descent, which
would have prevented this.” (#289818)
- “Received low altitude warning, pulled
up and discovered altimeter was misset. Altimeter
was set at 29.84, and should have
been set at 28.84. Crew distracted with a
[mechanical problem] about the time of altimeter
transition from flight levels to altitudes.”
(#290122)
The cure for the One-Eight-Zero
Blues is strict adherence to checklists
and procedures (sterile cockpit,
readback of ATC clearances, etc.), and
good CRM techniques for cross-checking
with the other crewmember(s).
“Bar” Exam
Hectopascals, more commonly referred
to as millibars, are used in many
foreign countries instead of inches of
mercury as the unit of measurement of
barometric pressure. (More information
on International Altimetry can be
found in ASRS Directline #2, Fall,
1991). Reporters noted that distractions
or inattention to details were
precursors to many of the incidents
that occurred where millibars are the
status quo. Others pointed to a lack of
clear communication, as the next reporter
suggests:
- “During descent, the altimeters were
incorrectly set at 29.99 instead of 0999
hectopascals, resulting in Approach Control
issuing an altitude alert. I believe the
ATIS was copied by the relief pilot using 3
digits with a decimal point. Since [ATIS]
normally issues both hectopascals and
inches of mercury, I incorrectly assumed
the known 50-foot error in
the pilot’s altimeter—
accounted for the mistaken
belief the helicopter was
higher.” (A81W0134)
For those who have never
used an altitude correction
chart, here is an example of
how the Canadian chart
works. The Whitehorse
airport, in the Yukon
Territory along the Alaska-
Canada highway, is
approximately 2,300 feet
MSL elevation. The approach
plates indicate,
“Mountainous terrain all
quadrants. Apply altitude
corrections for cold temperatures.”
At the 10-mile marker, for
example, the published
crossing altitude is 6,600
feet MSL. On a -30°C day
(-22°F) on the ground,
pilots would add more than
700 feet to that altitude;
that is, they would cross the
fix at an indicated altitude
of 7,300 feet MSL to offset
the error caused by the
cold temperature and to
ensure obstacle clearance.
At the final approach fix,
the published altitude of
4,000 feet MSL would need
to be increased approximately
300 feet; that is,
pilots would cross the fix at
4,300 feet MSL indicated
altitude. In other examples,
at higher published
altitudes and at colder
surface temperatures,
corrections can be more
than 1,000 feet—a potentially
critical difference
between true altitude and
indicated altitude, especially
in IMC. _
8 Issue Number 9
that the decimal denoted the inches-ofmercury
scale and announced ‘29.99,’
and set my altimeter. I recommend the
following phraseology: ‘Altimeter zero
999 millibars’ for hectopascal scale; ‘Altimeter
2999 inches’ for inches-of-mercury
scale.” (#295007)
Even when you know you are working
　
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