<s>
The	O
heading	B-Device
indicator	I-Device
(	O
HI	O
)	O
,	O
also	O
known	O
as	O
a	O
directional	B-Device
gyro	I-Device
(	O
DG	O
)	O
or	O
direction	B-Device
indicator	I-Device
(	O
DI	O
)	O
,	O
is	O
a	O
flight	O
instrument	O
used	O
in	O
an	O
aircraft	O
to	O
inform	O
the	O
pilot	O
of	O
the	O
aircraft	O
's	O
heading	O
.	O
</s>
<s>
The	O
primary	O
means	O
of	O
establishing	O
the	O
heading	O
in	O
most	O
small	O
aircraft	O
is	O
the	O
magnetic	O
compass	B-Language
,	O
which	O
,	O
however	O
,	O
suffers	O
from	O
several	O
types	O
of	O
errors	O
,	O
including	O
that	O
created	O
by	O
the	O
"	O
dip	O
"	O
or	O
downward	O
slope	O
of	O
the	O
Earth	O
's	O
magnetic	O
field	O
.	O
</s>
<s>
Dip	O
error	O
causes	O
the	O
magnetic	O
compass	B-Language
to	O
read	O
incorrectly	O
whenever	O
the	O
aircraft	O
is	O
in	O
a	O
bank	O
,	O
or	O
during	O
acceleration	O
or	O
deceleration	O
,	O
making	O
it	O
difficult	O
to	O
use	O
in	O
any	O
flight	O
condition	O
other	O
than	O
unaccelerated	O
,	O
perfectly	O
straight	O
and	O
level	O
.	O
</s>
<s>
To	O
remedy	O
this	O
,	O
the	O
pilot	O
will	O
typically	O
maneuver	O
the	O
airplane	O
with	O
reference	O
to	O
the	O
heading	B-Device
indicator	I-Device
,	O
as	O
the	O
gyroscopic	O
heading	B-Device
indicator	I-Device
is	O
unaffected	O
by	O
dip	O
and	O
acceleration	O
errors	O
.	O
</s>
<s>
The	O
pilot	O
will	O
periodically	O
reset	O
the	O
heading	B-Device
indicator	I-Device
to	O
the	O
heading	O
shown	O
on	O
the	O
magnetic	O
compass	B-Language
.	O
</s>
<s>
The	O
heading	B-Device
indicator	I-Device
works	O
using	O
a	O
gyroscope	B-Application
,	O
tied	O
by	O
an	O
erection	O
mechanism	O
to	O
the	O
aircraft	O
yawing	O
plane	O
,	O
i	O
.	O
e	O
.	O
the	O
plane	O
defined	O
by	O
the	O
longitudinal	O
and	O
the	O
horizontal	O
axis	O
of	O
the	O
aircraft	O
.	O
</s>
<s>
The	O
heading	B-Device
indicator	I-Device
is	O
arranged	O
such	O
that	O
the	O
gyro	O
axis	O
is	O
used	O
to	O
drive	O
the	O
display	O
,	O
which	O
consists	O
of	O
a	O
circular	O
compass	B-Language
card	O
calibrated	O
in	O
degrees	O
.	O
</s>
<s>
The	O
gyroscope	B-Application
is	O
spun	O
either	O
electrically	O
,	O
or	O
using	O
filtered	O
air	O
flow	O
from	O
a	O
suction	O
pump	O
(	O
sometimes	O
a	O
pressure	O
pump	O
in	O
high	O
altitude	O
aircraft	O
)	O
driven	O
from	O
the	O
aircraft	O
's	O
engine	O
.	O
</s>
<s>
Because	O
the	O
Earth	O
rotates	O
( ω	O
,	O
15°	O
per	O
hour	O
,	O
apparent	O
drift	O
)	O
,	O
and	O
because	O
of	O
small	O
accumulated	O
errors	O
caused	O
by	O
imperfect	O
balancing	O
of	O
the	O
gyro	O
,	O
the	O
heading	B-Device
indicator	I-Device
will	O
drift	O
over	O
time	O
(	O
real	O
drift	O
)	O
,	O
and	O
must	O
be	O
reset	O
using	O
a	O
magnetic	O
compass	B-Language
periodically	O
.	O
</s>
<s>
To	O
counter	O
for	O
the	O
effect	O
of	O
Earth	O
rate	O
drift	O
a	O
latitude	O
nut	O
can	O
be	O
set	O
(	O
on	O
the	O
ground	O
only	O
)	O
which	O
induces	O
a	O
(	O
hopefully	O
equal	O
and	O
opposite	O
)	O
real	O
wander	O
in	O
the	O
gyroscope	B-Application
.	O
</s>
<s>
Otherwise	O
it	O
would	O
be	O
necessary	O
to	O
manually	O
realign	O
the	O
direction	B-Device
indicator	I-Device
once	O
each	O
ten	O
to	O
fifteen	O
minutes	O
during	O
routine	O
in-flight	O
checks	O
.	O
</s>
<s>
Some	O
more	O
expensive	O
heading	B-Device
indicators	I-Device
are	O
"	O
slaved	O
"	O
to	O
a	O
magnetic	O
sensor	O
,	O
called	O
a	O
flux	O
gate	O
.	O
</s>
<s>
The	O
flux	O
gate	O
continuously	O
senses	O
the	O
Earth	O
's	O
magnetic	O
field	O
,	O
and	O
a	O
servo	O
mechanism	O
constantly	O
corrects	O
the	O
heading	B-Device
indicator	I-Device
.	O
</s>
<s>
The	O
table	O
however	O
allows	O
one	O
to	O
gauge	O
whether	O
an	O
indicator	O
is	O
behaving	O
as	O
expected	O
,	O
and	O
as	O
such	O
,	O
is	O
compared	O
with	O
the	O
realignment	O
corrections	O
made	O
with	O
reference	O
to	O
the	O
magnetic	O
compass	B-Language
.	O
</s>
