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An aircraft is flying eastbound, speaking with a very distant ATC station near its destination airport with the sun setting behind it. 10 minutes later the pilot stops receiving voice communications from the ATC. What has caused this?
  • A
    The aircraft is now inside the skip zone, between the ground wave and the sky wave, and therefore cannot receive the controller.
  • B
    The sky wave and ground wave are fading each other out as they arrive out of phase.
  • C
    The ground wave has been reduced by the night effect and the aircraft is no longer within its range.
  • D
    The aircraft's communication systems have failed and it must divert to the nearest suitable alternate.

Refer to figure.
Note: the feedback for this question is incomplete and therefore 2 of the options are not the same as in the exam. Further feedback is therefore requested, and can be put in the comments or emailed to [email protected], thank you.


Long distance voice communications (outside the line-of-sight) in aviation are achieved by using HF radio. HF (High Frequency) radio waves travel as space waves, but are also low enough frequency to be suitably refracted in the ionosphere, and can return as sky waves. They do also travel as ground (surface) waves, but they do not have a long range in this propagation method, so are not usually used like this. In other words, they can do all 3 propagation types, but we use them as sky waves.

In particular, we use HF radio via sky waves for long distance communication, but with this method comes some limitations and complexity.

The biggest limitation is the fact that sky waves have both a maximum range and a minimum range. The minimum range is that of the "first skip". As sky waves effectively "bounce" off the ionosphere, but cannot bounce at too steep of an angle (think of skimming stones), there is a minimum distance at which each frequency first arrives back at ground level. (For those who are interested, it is closer for low frequencies, and further away for high frequencies). This is called the skip distance, and there is a large distance between the end of the ground waves and the start of the sky waves that we call "dead space" or the "skip zone", where no signal is received.

This "first skip" distance increases massively at night, when ionisation from the Sun is removed, and the ionosphere becomes a lot less "bouncy" to our signals.

Now to our scenario. Not only are we flying towards the ATC transmitter/receiver (and therefore getting closer to the "dead space area"), but the night effect is beginning and our region of dead space is getting further and further away from the ground station. This means that it is incredibly likely that "10 minutes later", we are now inside that first skip distance (but still way further away that space or ground waves), and are therefore in "dead space", sometimes called the "skip zone".


For those who might like to see it: a quick explanation of each option:

  • "The aircraft is now inside the skip zone, between the ground wave and the sky wave, and therefore cannot receive the controller."
    • As described above, not only is the "first skip" distance increasing, but also we are flying towards it, so it is incredibly likely that we are now inside the "skip zone", so this is correct.
  • "The sky wave and ground wave are fading each other out as they arrive out of phase."
    • This can happen intermittently, but only when both the sky and ground waves are being received, which cannot happen in HF radio, due to the short range of the ground wave and long "first skip" of the sky wave.
  • "The ground wave has been reduced by the night effect and the aircraft is no longer within its range."
    • Created by us to fill in the incomplete feedback; we do not really use the ground wave of HF radio (range is too short), and it is not really affected by the night effect.
  • "The aircraft's communication systems have all failed. It must immediately declare emergency and divert to the nearest suitable alternate."
    • Created by us to fill in the incomplete feedback. This is unlikely to have happened (when you understand HF radio you know there is a much better explanation), and the rest is made purposefully ridiculous.

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