• As
altitude
increases,
the
weight
of
the
• Poles:
Negligible
• K
=
Mean
temperature
in
Kelvin
atmosphere
above
you
will
decrease.
• P
=
Mean
pressure
• Pressure
at
MSL
is
1000
hPa
and
temperature
PRESSURE
VARIATION
WITH
is
+20
0C.
An
aircraft
is
flying
at
the
910
hPa
TEMPERATURE
level
where
temperature
is
-‐6
0C.
What
height
is
it
flying
at?
• Cold
air
is
more
dense
so
more
rapid
• Mean
pressure
=
(1000-‐910)/2
=
955
hPa
decrease
in
pressure
with
height.
• Mean
temperature
=
[(20)+(-‐6)]/2
=
+7
=
• Warm
air
is
less
dense
so
slower
decrease
280
K
PRESSURE
MEASUREMENTS
in
pressure
with
height
(compared
to
ISA).
• APLR
=
[(96x280)/955]
=
28.15
ft
/
hPa
• When
flying
from
ISA
to
colder
air,
aircraft
• Pressure
difference
is
(1000
–
910)
=
90
hPa
• QFE:
Pressure
observed
at
a
specific
will
descend
but
still
show
18,000ft.
High
to
• 90
x
28.15
=
2533.5
ft
AMSL
location
with
a
datum
of
0
ft.
low,
watch
out
below!
• QFF:
QFE
changed
to
MSL
pressure
using
• The
pressure
in
a
column
of
warm
air
is
• An
APLR
<
27’
/
hPa
=
Cold
Air
APLR.
Used
on
synoptic
chart.
likely
to
be
greater
than
the
pressure
at
the
• An
APLR
>
27’
/
hPa
=
Warm
Air
!"#$"%&' !"#$!!
o +/ − 𝑄𝐹𝐸 = 𝑄𝐹𝐹
same
height
in
a
cold
column
of
air.
!"#$
o 𝐴dd
if
airfield
is
above
MSL
• Shortest
distance
between
two
pressure
• QNH:
QFE
changed
to
MSL
pressure
using
levels
occurs
when
pressure
increases
and
ISA
lapse
rates.
temperature
decreases.
!"#$"%&' !"#$!!
o !"
+/ − 𝑄𝐹𝐸 = 𝑄𝑁𝐻
o 𝐴dd
if
airfield
is
above
MSL
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