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3. AIRPRESSURE
AND ALTITUDE
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Air Pressure and Altitude
- Introduction
- Pressure
- Temperature
- Mean Sea Level (MSL)
- Altitude
- Height
- Flight Level
- Transition Level, Altitude and Layer
- Minimum usable flight levels
- Minimal altitudes MOCA, MRVA and MSA
- Minimum En route altitudes (MEA)
- Further reading
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3.1
Introduction
There are a large number of terms concerning the
measurement of altitude within aviation. This chapter will
deal with the most usual terms and explain in what
situations you use them.
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3.2 Pressure [S]
The static pressure is defined as the pressure that
the atmosphere is producing. You’ve probably heard of
high and low pressure, but in aviation you have to be
more accurate than that and measure the pressure with
digits.
In aviation air pressure is measured in hectopascal,
hPa. ICAO has defined a standard atmosphere* in which
the pressure at sea level is 1013.25hPa. This is also
called ‘standard setting’ or QNE.
Since the air gets thinner with increased altitude, the
pressure is reduced. More precisely with 30hPa /
1000feet*. An aircraft altimeter uses this fact by
measuring the pressure around the aircraft and
translating it to an altitude.
The pressure at sea level is however not static and an
altimeter has to compensate for this in order to show
the correct altitude. This compensation is done manually
by the pilot by setting a reference that is calculated
from the present air pressure. There are two commonly
used methods for calculating this reference and they are
abbreviated; QNH and QFE.
QNH is the actual air pressure reduced to sea level in
standard atmosphere*. When setting the correct QNH, the
altimeter will show the airfield’s elevation over MSL
(see below), providing that the aircraft is standing on
the airfield. QNH is the most common setting in civil
and private aircrafts.
QFE is the actual air pressure, not reduced to sea level
i.e. the air pressure at the airport. When setting the
correct QFE, the altimeter will show zero, if the
aircraft is standing on the airfield. QFE is rarely used
in commercial civil aviation. VFR-traffic sometimes uses
QFE and it is common that military aircraft uses QFE
instead of QNH.
To be correct, not only the pressure, but also the
temperature has to be taken in consideration in order to
measure the true altitude. OAT combined with QNH is used
to calculate true altitude.
* In standard atmosphere – as defined by ICAO
- Pressure at sea level: 1013.25 hPa
- Temperature at sea level: +15 degrees Celsius (C)
- Decline in temperature: 2 degrees C / 1000 ft
- Tropopaus altitude: 11km
- Temperature in tropopaus: -56.5 degrees C
- Temperature is constant between 11-20km
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3.3 Temperature [C]
Temperature is a measure of the movement of the
molecules in the air.
It is measured in Celsius (C), Fahrenheit (F) or Kelvin
(K).
In aviation OAT (Outside Air Temperature) is used as the
real out temperature and the unit used is Celsius. (To be
correct TOAT should be used, but OAT is the same as TOAT
if the first letter is omitted).
The temperature drops 2 degrees C / 1000 ft in standard
atmosphere.
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3.4 Mean Sea Level (MSL) [S+]
Mean sea level (MSL) is the average height of the sea,
with reference to a suitable reference surface.
Defining the reference level, however, involves complex
measurement, and accurately determining MSL can prove
difficult.
Lucky for you, this is outside the scope of this manual.
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3.5 Altitude [S]
Altitude is defined as the vertical distance between
mean sea level (MSL) and an aircraft. When the pilot has
set the correct local QNH he will fly on an altitude.
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3.6 Height [S+]
Height is defined as the vertical distance between the
ground (GND) or an airport and an aircraft. Height may be
expressed in charts as number of feet above ground level
(AGL). If an aircraft is flying on altitude 1000ft and the
airports elevation (which means height above MSL) is 300
ft, the aircraft height is 700ft.
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3.7 Flight Level (FL) [S+]
Flight levels are expressed in hundreds of feet of
vertical distance from the pressure reference 1013.25.
Hence, when flying on flight levels, the pilot has to
set the altimeter to the standard pressure 1013.25hPa.
If the QNH is 1013.25 FL 80 would be 8000 ft above MSL
(in a standard atmosphere).
If the QNH is 1003 hPa (i.e. lower then 1013.25) FL 80
would be less then 8000 ft above MSL. (It would be
1013-1003 = 10 x 30ft =300 ft less = 8000-300ft =
7700ft).
Flight levels are used above a certain altitude called
the transition altitude.
This is to avoid that pilots flying en-route have to set
their altimeters to local QNH all the time as they fly
though areas with different pressure.
If two aircraft are at the same location, one at FL160
and the other at FL170 they are still 1000ft apart, as
their altimeters deviate by the same amount from the
true altitude.
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3.8 Transition Level, Altitude and Layer [S+]
Transition altitude (TA) is always the same at an
airport, but differs from airport to airport. In Europe
5000 feet is the most common. When an aircraft is
climbing trough the transition altitude the pilot will
set the altimeter to standard setting (QNE) 1013.25hPa.
When an aircraft is above the transition altitude it
will fly on flight levels. Upon descent the pilot should
set the altimeter to local QNH (or QFE if appropriate)
when passing the transition level (TL) at the latest.
This means that below the transition level, altitude
will be used.
Transition layer is defined as the airspace between the
TA and the TL. In some countries the transition layer
has to be 1000 feet or more, but in some it can be
thinner.
Whatever the rules, in a specific country are, the TL
will thus vary with the air pressure in order to
maintain a certain minimum thickness. (This will change,
since new rules are soon implemented) You have to look
up the local rules in your vACCs operating manual.
Rule of thumb is that the TL is lower when the pressure
is above standard 1013.25 Hpa, and higher when the
pressure is below standard 1013.25 Hpa.
|
Country |
TA* |
* This is the most common setting,
variations may occur on some airports within the
country; please refer to the AD for specific
settings. |
|
Austria |
by AD |
|
Belgium |
4500 |
|
Denmark |
5000 |
|
France |
4000 |
|
Germany |
5000 |
|
UK |
by AD |
|
Ireland |
5000 |
|
Italy |
by AD |
|
Luxembourg |
4500 |
|
Netherlands |
3000 |
|
Norway |
4000 |
|
Slovenia |
10500 |
|
Spain |
6000 |
|
Sweden |
5000 |
|
Switzerland |
By AD |
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3.9 Minimum usable flight levels [C]
Aircraft that are flying at Transition Altitude
(TA) or above, set their altimeter to a standard pressure
of 1013 Hpa. Because the pressure changes en route, they
are actually flying at different altitudes as they travel
through areas with differing atmospheric pressure. This
works fine for all aircraft at TA or above, because
regardless of their actual altitude above mean sea level,
they are still separated from each other by 2000 feet.
Unfortunately this does not always work between two
aircraft assigned an altitude close to TA, and a
Flightlevel close to Transition Level (TL). If the local
altimeter ever drops below 1013 Hpa, then the 1000 foot
separation required between the two aircraft (called
Transition Layer) is lost. For this reason, the Transition
Level is variable to assure that the Transition Layer is
the minimum 1000 feet.
Rule of thumb is that the TL is lower when the pressure is
above standard 1013 Hpa, and higher when the pressure is
below standard 1013 Hpa (as discussed in previous
paragraph). To have more idea about Transition Level,
please check the below figure.
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3.10 Minimal altitudes MOCA, MRVA and MSA [C+]
MOCA = Minimum Obstacle Clearance Altitude
This is the lowest altitude that an aircraft can fly in
IMC (Instrument Meteorological Conditions) and still keep
safe clearance from terrain and obstacles. MOCA is often
lower then MEA (se below). It is only used in emergencies,
especially to get below icing.
MRVA = Minimum Radar Vectoring Altitude
MRVAs are established where ATC vectors are initiated or
requested frequently. They are often lower than other
applicable minimum IFR altitudes because they can be more
focused in specific areas rather than general like a 25 NM
safe altitude around a final approach fix, for example. As
a result, these are not published on any instrument
approach procedure, but may coincide with published
values.
Since ATC is responsible for obstacle clearance of
an aircraft on radar vectors, the MRVA will, by
definition, provide a number of issues like cold weather
altimeter errors, terrain and obstructions, restricted
airspace, etc. The MANOPS Definition for an MRVA also
includes radio coverage, but oddly enough, not radar
coverage. It is also supposed to consider the base of
controlled airspace in a given area, so the radar
vectoring altitudes are often lower in terminal areas
where the airspace classification is better designated to
include lower transition areas. On this note, ATC may
provide vectors to an aircraft in uncontrolled airspace
(class G) if requested by the pilot, or if ATC suggests it
and the pilot accept it.
MSA = Minimum Safe/Sector Altitude
Minimum Sector Altitude is the minimum altitude that may
be used under emergency conditions which will provide a
minimum clearance of 1000ft above obstacles and terrain
contained within a sector of 25 NM radius centred on a
radio navigational aid. MSA can be given as areas between
radials from a VOR at the airport.
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3.11 Minimum En route altitudes (MEA) [C+]
MEA's takes into account both navigational aid
limitations and obstruction clearance. This is important
to you for two reasons.
First, if you clear an aircraft below the MEA in your
area, and the pilot discovers rocks in the clouds ahead,
he may be cranky.
Second, in case of radio failure, be aware that the pilot
will climb to the MEA if he has not received a new
altitude clearance prior to the radio failure.
You need to become familiar with the MEA's in your VACC
area to the extent that you will not inadvertently clear
an aircraft below them.
The MEA's can be found on the low altitude en route charts
along airways. The number starting with an asterisk is the
MOCA, which can be assigned under certain circumstances,
but will not be useful in our environment (see above).
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3.12 Further reading
MSL :
http://en.wikipedia.org/wiki/Sea_level
Altitude and Flight Level :
http://en.wikipedia.org/wiki/Flight_level
http://www.auf.asn.au/groundschool/umodule3.html
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