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lock letter
block letter Suffix to aircraft serial number, equivalent
to USAF block number (USN).
block number See block (1).
block out To move off blocks, esp. at start of scheduled
flight.
block shipment Rule-of-thumb logistic supply to
provide balanced support to round number of troops for
round number of days.
block speed Average speed reckoned as sector distance
divided by block time.
block stowage Loading all cargo for each destination
together, for rapid off-loading without disturbing cargo
for subsequent destinations.
block template Template for making form block.
block time Elapsed period from time aircraft starts to
move at beginning of mission to time it comes to rest at
conclusion. Historically derived from manual removal
and placement of blocks (chocks). Normally used for
scheduled commercial operations, either for intermediate
sectors or end-to-end.
block upgrade plan, programme Introduction of successive
groups of modifications, each in a particular FY or
production block.
Bloctube Patented mechanical control in which
command is transmitted by push/pull action of steel cable
on which are threaded guidance rings running inside a
tube.
blood-albumen glue Adhesive used in aircraft plywood,
made from dry cattle blood albumen.
blood chit 1 Form signed by aircraft passenger before
flight for which no fare has been paid (eg, in military
aircraft) indemnifying operator against claims resulting
from passenger’s injury or death (colloq.).
2 Plastic or cloth message, usually in several languages,
promising reward if bearer is helped to safety. Often
includes representation of bearer’s national flag (colloq.).
blood wagon Ambulance.
bloom 1 Ingot from which slag has been removed, sometimes
after rough rolling to square section.
2 Of ECM chaff, to burst into large-volume cloud after
being dispensed as compact payload.
blooming 1 In surface-coating technology, to coat
optical glass with layer a few molecules thick which
improves optical properties (by changing refractive index
and/or reducing external reflection).
2 In CRT, defocusing effect caused by excessive brightness
and consequent mushrooming of beam.
3 In atmospheric laser operations, defocusing or undesired
focusing of pulses or beam caused by lens-like
properties in atmosphere (see blob).
BLOS Beyond, or below, line of sight.
blossom effect Sudden apparent growth in size of
aircraft on collision course as distance approaches zero.
blow 1 Rupture in case of solid rocket during firing.
2 To activate blowing system.
blowback 1 Type of action of automatic gun in which
bolt, or other sliding breech-closure, is never locked and
is blown back by combustion pressure on its face, or by
rearwards motion of empty case, inertia sufficing to keep
breech closed until projectile has left muzzle.
2 Improper escape of gas through breech during firing
of gun, due to ruptured case, faulty breech mechanism or
other malfunction.
3 Closure, or partial closure, of spoiler or speed brake
blow-off valve
due to aerodynamic load overcoming force exerted by
actuation system.
blowback angle Maximum angle to which a spoiler can
be extended under given q (dynamic pressure) without
blockback (3).
blowby 1 Loss of gas leaking past piston-engine piston
ring.
2 Increasingly used to mean seepage past rings of lubricating
oil.
blowdown 1 Pilot input or pfcu force overcome by aerodynamic
load on control surface, the latter being either
prevented from moving or returned to neutral position.
2 In captive firing of liquid rocket, expulsion of residual
propellant[s] by gas [usually nitrogen] after burnout or
cutoff.
blowdown period Period in cycle of reciprocating IC
engine in which exhaust valve or port is open prior to
BDC.
blowdown tunnel Open-circuit wind tunnel in which gas
stored under pressure escapes to atmosphere, or into
evacuated chamber, through working section.
blowdown turbine Turbine driven by piston engine
exhaust gas in such a way that kinetic energy of discharge
from each cylinder is utilised.
blower 1 Centrifugal compressor with output (from
NTP input) at between 1 and 35 lb/sq in gauge
(6.9–240 kN m –2 ).
2 Centrifugal fan used on piston engine, esp. on radial
engine, to improve distribution of mixture among
cylinders.
blower pipe In an airship, duct through which propeller
slipstream is rammed to pressurize ballonets.
blow-in door Door free to open inwards against spring
upon application of differential pressure (eg, in aircraft
inlet duct).
blowing Provision for discharging high-pressure, highvelocity
bleed air from narrow spanwise slit along wing
leading edge, tail surface or ahead of flap or control
surface. Greatly increases energy in boundary layer,
increases circulation and prevents flow breakaway.
blowing coefficient For jet or blown-flap or blown aerofoil,
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blown 1 Supercharged [piston engine].
2 Failed [electrical fuse].
3 See next.
blown flap Flap to which airflow remains attached, even
at sharp angles, as result of blowing sheet of high-velocity
air across its upper surface (see boundary-layer control,
supercirculation).
blown periphery Parachute in which part of peripheral
hem becomes blown between two rigging lines in another
part of canopy, and attempts to inflate inside-out.
blown primer Percussion cartridge primer which blows
rearward out of its pocket, allowing primer gases to
escape.
blow off 1 Explosive or other enforced separation of
instrument pack or other payload from rocket vehicle or
other carrier. Hence, ** signal.
2 Controlled reduction [if necessary to zero] of deflection
of flap, tab or [rare] control surface because of
aerodynamic load.
blow-off valve 1 Safety valve set to open at predetermined
dP at chosen point in axial compressor casing to
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