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At the Tenerif’s Accident of 1977 two aircraft collided when one aircraft started the take off run while the other was taxiing onto the runway (taxiway was occupied). The aircraft taking off reported, “we are now at take-off” and the tower replied “OK…, stand by for take-off. I will call you”. Simultaneously, the other aircraft taxiing onto the runway reported, “ah…hu..we are still taxiing down the runway”. What is the reason these last two transmissions might NOT have reached the take-off aircraft?

  • A

    If neither of the two messages was received, there must have been a failure of the receiver of the aircraft taking off. There is NO other possible explanation.

  • B

    A technical reason is NOT to be suspected. Most likely, the crew was absorbed by the takeoff and probably failed to notice the calls.

  • C

    As simultaneous transmissions on one frequency are NOT possible, they can block each other out. NOTHING or just interference noises might have been heard instead.

  • D

    The message of the Tower must have been received. The transmissions of ground stations have a general priority to supersede other simultaneous transmissions.

When two or more transmissions occur simultaneously on the same frequency, depending on relative difference between the strengths and timing of the two incoming signals, the local architecture, etc; the Air Traffic Controller hears one message clearly, some garbling or some partial overlap of the messages.

The absence of a read-back from the pilot should be treated as a blocked transmission and prompt a request to repeat or confirm the message. The absence of response from the controller should be treated as an evidence of blocked transmission and the pilot should repeat the message.

At the Tenerif’s accident, if the two transmissions were stated simultaneously, it was impossible for both of them to be transmitted. The result was that they either cancelled each other out, or some parts of one of the messages were heard without enough strength to be received, or the message was heard as an interference noise.

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