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SESK: Stuff Everyone Should Know · sesk.info/pilot-vor-line-of-sight

Why might a VOR receiver show an OFF flag when you are flying low and far from the station?

VOR signals are line of sight, so terrain and distance block them.

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  1. VORs only work at night
  2. VOR signals travel in a line of sight ✔
  3. The VOR is always under maintenance

Why: VOR uses VHF signals, which travel in straight lines. Too far away or too low, and the curve of the earth or terrain blocks the signal, so the alarm flag appears.

Huh, didn't know that: A Terminal VOR's protected service volume reaches 25 NM from 1,000 through 12,000 feet above transmitter height.

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My VOR and I have a lot in common. If I can't see you, I'm not listening.
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References

Answer: VOR signals travel in a line of sight. VOR uses VHF signals, which travel in straight lines. Too far away or too low, and the curve of the earth or terrain blocks the signal, so the alarm flag appears.

  1. FAA Pilot's Handbook of Aeronautical Knowledge, Chapter 16 www.faa.gov/sites/faa.gov/files/18_phak_ch16.pdf
    This happens if the aircraft is too far from the VOR or the aircraft is too low and, therefore, is out of the line of sight
    Checked 2026-10-09.
  2. FAA AIM 1-1-8, NAVAID Service Volumes www.faa.gov/air_traffic/publications/atpubs/aim_html/chap1_section_1.html
    From 1,000 feet ATH up to and including 12,000 feet ATH at radial distances out to 25 NM.
    Checked 2026-10-09.

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