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3He
3He Helium, valency 3.
3LM Third-level maintenance.
three-moment equation For solving bending moments
and other loads at ends of two adjoining spans of continuous
beam.
3-P Planning, production, progress.
three-phase current Alternating electrical current made
up of three phases, each with vector separation of 120°;
carried by triple wire.
three-phase equilibrium See triple point.
3φ Three-phase current [3-phi].
three-pointer altimeter Dial instrument with short
needle for thousands (ft or m), mid-length for hundreds
and longest for tens.
three-point landing Correctly judged landing by
tailwheel-type aeroplane in which main and tail wheels
touch ground simultaneously with wing stalled.
three-point mooring Mooring for aerostat in which three
lines are run (often from single point, eg nose of airship)
to three ground anchors, usually at apices of equilateral
triangle.
three-point tanker Equipped with two outer-wing
HDUs and one at the tail.
3-pole switch Opens and closes three conductors or
circuits.
three-poster STOVL or V/STOL vectored-thrust
propulsion system having three jets; normally two cold
fan jets and one hot core jet, but alternatively two main
(rear) jets plus an auxiliary nose jet fed via a bleed air duct.
three-shaft engine Gas turbine having LP, IP and HP
shaft systems.
three-stream engine 1 Turbofan (HBPR) in which fan
thrust (probably VIGV modulated) and core jet are used
for propulsion and LP compressor (core supercharger) is
used for blowing purposes.
2 Any engine in which fan thrust, core thrust and lift
thrust or bleed are used separately.
3 to 1 rule Air distance 3 n.m. for each 1,000 ft lost in
letdown.
3-view drawing GA drawing, normally showing elevation
(left side), front and plan.
3-way switch Routes input along either of two outputs.
3-wire Target of most carrier arrested landings, No 3
wire; hence ** landing.
3-wire circuit Neutral wire between two outer wires,
latter having potential difference from neutral equal to
half that between them.
threshold 1 Beginning of usable portion of runway, ie
downwind end.
2 In automatic control systems, point at which response
is first noticed, usually defined in terms of input displacement
(see * level).
3 Flight condition when fixed-wing aerodyne is on
point of stall.
4 Point at which sound just becomes audible (* of audibility
or of hearing), normally 2 × 10 -5 N/m 2 .
5 EAS giving lowest comfortable cruising, possibly
higher than that for minimum fuel.
6 See thresholds.
threshold contrast Smallest contrast in luminance visible
under given conditions.
threshold crossing height Height of glideslope above
threshold.
threshold curve Plot of sound frequency against noise
throttle
level in dB (or other noise measure) just audible against
quiet background, eg anechoic chamber.
threshold displacement Linear distance between end of
full-strength runway pavement and displaced threshold,
with latter shown on airfield charts as white bar across
runway crossed by narrow black line, and expressed as
minus quantity in certain navaid figures, eg Vorloc II
= 380 ft.
threshold dose Minimum quantity of radiation
producing detectable biological effect.
threshold illuminance Minimum value of illuminance
eye can detect under given dark adaptation and target
size; also called flux-density threshold.
threshold level Threshold (2), esp. in rate gyro developing
electrical output as function of rate of turn; that
angular rate after rotational acceleration from rest at
which there is first indication of output, or change in
output; normal unit is °/s × 10 -6 .
threshold lights If fitted, bidirectional units, showing
green towards approach and red towards runway, in
continuous row across threshold (rare at displaced
threshold).
threshold limit value Average airborne concentration of
toxic substance[s] normal person can withstand 8 h per
day 5 days per week, usually expressed as ppm or mg/m 3
at 25°C/760 mm Hg.
threshold marking For simple runway, runway number
in white, visible to pilot on approach; if displaced
threshold, preceded by white transverse bar touched by
four arrowheads pointing upwind and preceded by series
of centreline arrows. For instrument runway, four bold
white axial stripes in rectangular group on each side
preceeding runway number.
thresholds Limits on programme monetary changes
imposed by US Defense Secretary.
threshold sampling time Time since overhaul at which
engines are removed and inspected in preparation for
extension in TBO; * may be less or more than new TBO.
threshold speed V T , VAT and VT max.
THR HOLD Thrust, or throttle, hold.
THRFTR Thereafter.
throat 1 Point of smallest cross-section in duct,
especially that in con/di nozzle, supersonic tunnel
upstream of working section, gas-turbine rotor blades and
rocket engine or motor thrust chamber and nozzle.
2 Entry to windsock.
throatable Jet or fluid flow controllable by changing
shape or area of throat (unusual except in tunnels).
throat control In gas turbines, system controlling flow
through nozzle guide vanes upstream of turbine.
throatless chamber Rocket thrust chamber without
throat yet still achieving supersonic expansion, eg multichamber
toroidal type.
throatless shear(s) Power shear for cutting large sheet or
plate which may be rotated during cut to leave curved
edge.
throat microphone Microphone held against skin of
throat; better for deep or guttural voices or languages.
throttle 1 Input control, usually hand lever rotating
through arc, for main vehicle propulsion.
2 System responsible under pilot for varying engine
power.
3 Valve in carburettor or fuel control which governs
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