Pilot/Controller Glossary B4UFLY- A free downloadable application, which allows operators to check airspace and local advisories before flying. BACK-TAXI- A term used by air traffic controllers to taxi an aircraft on the runway opposite to the traffic flow. The aircraft may be instructed to back-taxi to the beginning of the runway or at some point before reaching the runway end for the purpose of departure or to exit the runway. BASE LEG- (See TRAFFIC PATTERN.) BEACON- (See AERONAUTICAL BEACON.) (See AIRPORT ROTATING BEACON.) (See AIRWAY BEACON.) (See MARKER BEACON.) (See NONDIRECTIONAL BEACON.) (See RADAR.) BEARING- The horizontal direction to or from any point, usually measured clockwise from true north, magnetic north, or some other reference point through 360 degrees. (See NONDIRECTIONAL BEACON.) BELOW MINIMUMS- Weather conditions below the minimums prescribed by regulation for the particular action involved; e.g., landing minimums, takeoff minimums. BEYOND VISUAL LINE OF SIGHT (BVLOS)- The operation of a UAS beyond the visual capability of the flight crew members (i.e., remote pilot in command [RPIC], the person manipulating the controls, and visual observer [VO]), if used to see the aircraft with vision unaided by any device other than corrective lenses, spectacles, and contact lenses. BLAST FENCE- A barrier that is used to divert or dissipate jet or propeller blast. BLAST PAD- A surface adjacent to the ends of a runway provided to reduce the erosive effect of jet blast and propeller wash. BLIND SPEED- The rate of departure or closing of a target relative to the radar antennt which cancellation of the primary radar target by moving target indicator (MTI) circuits in the radar equipment causes a reduction or complete loss of signal. (See ICAO term BLIND VELOCITY.) BLIND SPOT- An area from which radio transmissions and/or radar echoes cannot be received. The term is also used to describe portions of the airport not visible from the control tower. BLIND TRANSMISSION- (See TRANSMITTING IN THE BLIND.) BLIND VELOCITY [ICAO]- The radial velocity of a moving target such that the target is not seen on primary radars fitted with certain forms of fixed echo suppression. BLIND ZONE- (See BLIND SPOT.) BLOCKED- Phraseology used to indicate that a radio transmission has been distorted or interrupted due to multiple simultaneous radio transmissions. BOTTOM ALTITUDE" In reference to published altitude restrictions on a STAR or STAR runway transition, the lowest altitude authorized.