The Art of the Helicopter John Watkinson - Karatunov.net

is set by the use of a control knob. Turning the knob will simultaneously change the
reference pressure displayed on the subscale and the aneroid capsule spring tension.
If, for example, the reference pressure is to be raised, the act of setting the higher pressure
on the subscale slightly extends the spring and the capsule will reach equilibrium
showing a higher altitude. Conversely if a lower reference pressure is to be used, the
spring is allowed to contract slightly and the capsule reaches equilibrium with a lower
altitude reading. Throughout this process the absolute pressure at the altimeter did not
change, but the pressure difference and the altitude did because the subscale setting
was changed. Clearly without an appropriate subscale setting an altimeter reading is
meaningless. It should also be clear that, in general, height and altitude are different.
There are three main subscale settings used with an altimeter and these are shown
in Figure 7.10(a). If the subscale is set to the ground pressure at the elevation of the
airfield, the altimeter will read zero prior to take-off and in flight will measure height
above the airfield. The barometric pressure at an airfield is called QFE. The ‘Q’ dates
from the days of carrier wave radio and Morse code and indicates a question. An
aircraft arriving at an airfield may ask for QFE in order to set the altimeter subscale
so that the altimeter reads height above the field. QFE is easily remembered as ‘field
elevation’. In some parts of the world this technique is not used. For example, the
QFE of a mountain airfield may be so low that it is beyond the end of the altimeter
subscale.
During a flight at moderate altitude the overriding concern is clearance of terrain and
structures. These are quoted on maps as height above mean sea level (AMSL). In order
to establish the clear height above such obstacles the altimeter reference is set to QNH;
the barometric pressure in the area ‘reduced to mean sea level’. This means that the
actual barometric pressure is measured at a known elevation and the value the pressure
would have had at sea level is computed from the height and temperature. With QNH
on the subscale, the altimeter reads feet AMSL and subtracting the height of obstacles
in feet AMSL gives the clearance. QNH is easily remembered as ‘nautical height’.
Figure 7.10(b) shows a typical low-level flight. The machine takes off with QFE in
the subscale to show height above the airfield. Once away from the field the altimeter
is set to regional QNH and the machine is trimmed to cruise at 2000 feet AMSL. On
the chart is a hill rising to 1200 feet AMSL and the machine will clear it by 800 feet.
On approaching the destination airfield, the pilot asks for QFE and sets this on the
altimeter. This should read zero as he touches down.
Each country is divided up into altimeter setting regions which each have their own
QNH value as a function of the weather system across the country. The QNH quoted
takes into account the rate at which barometric pressure is changing. If pressure is
falling, a machine flying at constant pressure altitude based on QNH would actually
descend and not have the expected clearance. QNH is adjusted upwards by the authorities
to ensure that clearance is still achieved at the end of the period for which a setting
is valid even if barometric pressure falls. On a long flight an altimeter setting region
boundary may be crossed. The pilot must obtain the new QNH for the area to ensure
his altimeter still reads feet AMSL. The subscale is generally calibrated in millibars
(mb) or, more recently, hectoPascals (hP) that are numerically identical. In America,
scales calibrated in inches of mercury (inHg) will be found.
For flight in airways, the overriding concern is not terrain clearance, but vertical
separation between aircraft. For this reason the same reference pressure is always used
in airways. This is the pressure of the International Standard Atmosphere (ISA) at
Mean Sea Level (MSL): 1013.25 mb. Older texts may use the term QNE to describe
this pressure. Aircraft flying with ISA on the subscale are flying at pressure altitude.
The flight level is the pressure altitude with the last two digits removed. For example,
Control 273