A pilot cannot fly a plane by sight alone. In many conditions, such as flying at night and landing in dense fog, a pilot must use radar, an alternative way of navigating. Since human eyes are not very good at determining speeds of approaching objects, radar can show a pilot how fast nearby planes are moving. The basic principle of radar is exemplified by what happens when one shouts in a cave. The echo of the sounds against the walls helps a person determine the size of the cave. With radar, however, the waves are radio waves instead of sound waves. Radio waves travel at the speed of light, about 300, 000 kilometers in one second. A radar set sends out a short burst of radio waves: Then it receives the echoes produced when the waves bounce off objects. By determining the time it takes for the echoes to return to the radar set, a trained technici
A. The nature of radar.
B. Types of ranging.
C. Alternatives to radars.
D. History of radar.
更多"A pilot cannot fly a plane by sight"的相关试题:
[单项选择]
A pilot cannot fly a plane by sight alone. In many conditions, such as flying at night and landing in dense fog, a pilot must use radar, an alternative way of navigating. Since human eyes are not very good at determining speeds of approaching objects, radar can show a pilot how fast nearby planes are moving. The basic principle of radar is exemplified by what happens when one shouts in a cave. The echo of the sounds against the walls helps a person determine the size of the cave. With radar, however, the waves are radio waves instead of sound waves. Radio waves travel at the speed of light, about 300, 000 kilometers in one second. A radar set sends out a short burst of radio waves: Then it receives the echoes produced when the waves bounce off objects. By determining the time it takes for the echoes to return to the radar set, a trained technici
A. Size.
B. Weight.
C. Speed.
D. Shape
[单项选择]
Many people who fly at least occasionally have come down with a cold or the flu shortly after disembarking. Is the air in airborne commercial jets (21) The Association of Flight Attendants (AFA), (22) which 42,000 flight attendants with 27 airlines are represented, evidently thinks so. The organization claims that the incidence of air-quality-related diseases has (23) among its members and demands that prompt actions be (24) to improve the conditions in the airplane cabin.
A study the AFA (25) in 1997 uncovered about 1,000 self-reported incidents of headaches, dizziness and memory loss (26) flight attendants and passengers. Some flight attendants were too ill to (27) their safety duties, while others have been permanently disabled.
Because of airlines’ efforts to (28) their expenses, cabin-air filters are not cleaned (29) The complaints of flight attendants do not always give (30) to correct maintenance. Airlines turned to recycled air, (31) that they would
A. confided
B. promised
C. ensured
D. convinced
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