1Dk4eXg

altimeter fatigue
of climb and descent will give a reading up to 200–300 feet
in arrears; called lag or hysteresis. There are errors in drive
friction and lost motion. Static pressure sensed is subject
to PE(2) and compressibility errors, and to transient
excursions during manoeuvres. Most significant, parameter
measured depends on atmospheric pressure
variation, temperature variation and variation in lapse
rate between departure airfield and aircraft height (see
altimeter setting).
altimeter fatigue Supposed tendency of aneroid system
to become ‘set’ in distorted position in long flight at high
altitude; this error, not confirmed by most authorities, is
called fatigue or, confusingly, hysteresis.
altimeter lag See altimeter errors.
altimeter setting For safe vertical separation all altimeters
in controlled airspace must be set to uniform
datum. Standard is 1013.25 mb (see ISA) throughout
most en route flying. Instrument then registers vertical
separation between aircraft and pressure surface
1013.25 mb, usually below local ground level and may be
below local SL or MSL. Second common setting is QNH,
at which reading is difference between aircraft height and
MSL. Third common setting is QFE, at which reading is
difference between aircraft height and appropriate airfield
height AMSL; thus at that airfield instrument reads zero.
Two other settings, QFF and QNL, seldom necessary. See
next.
altimeter setting region Geographical area for which the
lowest value of QNH is broadcast hourly by control
centres.
altimeter switch Triggered by reaching preset altitude,
one application to trigger explosive charge.
altimetric valve Device sensitive to increasing cabin altitude
(ie, falling pressure) and set to release drop-out
oxygen at given level.
altitude 1 Vertical distance of level, point or object
considered as point, measured from MSL (normally
asociated with QNH) (DTI, UK). In this dictionary
meanings are given for 17 other measures of *.
2 Arc of vertical circle, or corresponding angle at centre
of Earth, intercepted between heavenly body and point
below it where circle cuts celestial horizon.
3 In spaceflight, distance from spacecraft to nearest
point on surface of neighbouring heavenly body (in
contrast to “distance”, measured from body’s centre).
4 In aircraft performance measurement and calculation,
pressure * shown by altimeter set to 1013.25 mb.
altitude acclimatisation Gradual physiological adaptation
to reduced atmospheric pressure.
altitude chamber Airtight volume evacuated and
temperature-controlled to simulate any atmospheric level.
altitude clearance Clearance for VFR flight above
smoke, cloud or other IFR layer.
altitude datum Local horizontal level from which
heights or altitudes are measured (see true altitude, pressure
altitude, height).
altitude delay 1 Deliberate time-lag between emission of
radar pulse and start of indicator trace, to eliminate altitude
hole or slot.
2 See next entry.
altitude-delay parameter Time delay which elapses
between pilot nose-up command and establishment of
positive climb, esp. during landing approach. ADP is
Alumilite
serious in large aircraft with pitch control by tail surfaces,
not canard, and without DLC.
altitude hole Blank area at centre of radial (eg PPI)
display.
altitude line On environmental plot, line joining points
of minimum range at which WX main beam intersects
ground.
altitude power factor In piston engine ratio of power
developed at specified altitude to power at same settings
at ISA SL.
altitude parallax In altitude (2), angle between LOS
from body to observer (assumed on Earth’s surface) and
LOS from body to centre of Earth.
altitude recorder Altigraph.
altitude reservation, ALTRV Airspace utilisation under
prescribed conditions, normally employed for mass movement
or other special-user requirements which cannot
otherwise be accomplished (FAA).
altitude ring Continuous return across WX display at
range equivalent to aircraft’s altitude.
altitude sensing unit Capsule-based unit in engine fuel
system sensing aircraft speed/altitude.
altitude sickness Malaise, nausea, depression, vomiting
and ultimate collapse, caused by exposure to atmospheric
pressure significantly lower than that to which individual
is acclimatised.
altitude signal In airborne radar operating in forward
search mode, unwanted return signal reflected by Earth
directly below.
altitude slot Blank line at origin of SLAR display.
altitude switch Barometric instrument which makes or
breaks electric circuit at preset pressure altitude;
contacting altimeter.
altitude testing Testing an item [e.g., a gas-turbine
engine] in a facility capable of reproducing inlet ram total
temperature and pressure appropriate to the test altitude
and Mach numbers, and also the appropriate exit static
pressure.
altitude tunnel Wind tunnel whose working section can
simulate altitude conditions of pressure, temperature and
humidity. In view of advantages of high pressure
and driest possible air, conditions chosen usually
compromise.
altitude valve In some carburettors, progressively closed
by aneroid to reduce fuel flow at high altitudes.
ALTN, Altin 1 Alternate airfield.
2 Alternating (two-colour light).
altocumulus, Ac Medium cloud, about 12,000 ft in
groups, lines or waves of white globules.
altostratus, As Stratiform veil 6,000–20,000 ft with icecrystal
content of variable thickness (giving mottled
appearance) but usually allowing Sun/Moon to be seen.
ALTP Air Line Transport Pilot licence; confusingly,
now often called ATPL.
ALTR Approach/landing thrust reverser.
ALTRV Altitude reservation.
ALTS Altimeter setting.
ALTV Approach and landing test vehicle.
ALU Arithmetic and logic unit.
Alumel Ni-Al alloy or coating.
Alumigrip Trade name of paint used on airframe
exterior.
Alumilite Trade name for sulphuric-acid anodizing
process for aluminium and alloys.
43