1

For the landing performance calculations, I want to calculate the headwind component. I can do this with the runway heading and the reported wind direction and speed. But do I need to make any adjustments for true vs. magnetic north?

Is the reported wind direction towards true or magnetic north? Is the runway heading towards true or magnetic north?

Pondlife
  • 71,714
  • 21
  • 214
  • 410
JM93
  • 263
  • 3
  • 11
  • Related: https://aviation.stackexchange.com/questions/60/when-are-winds-given-with-respect-to-true-vs-magnetic-north – assylias Nov 29 '17 at 14:43
  • 2
    See this question for how runways are numbered, and this one for how winds are reported. Wind directions are where the wind is coming from, not towards, by the way. – Pondlife Nov 29 '17 at 15:10
  • tl;dr It depends. Runways are magnetic and reported winds can be either true or magnetic. If you are planning you flight and get your winds from a METAR or TAF, then you need to make a correction to the numbers. If you are getting your winds from the ATIS or ASOS, they are magnetic and no correction is needed. – JScarry Nov 29 '17 at 16:28
  • @JScarry I agree with you, except that there are some nuances. For example ASOS and AWOS are true, and sent to NWS as true, but are converted to magnetic for computer generated voice broadcast. – mongo Nov 29 '17 at 18:20
  • @mongo I’m not sure I follow your comment. If you are listening to the one-minute weather from an ASOS or AWOS then the wind direction you hear is magnetic. – JScarry Nov 29 '17 at 21:17

1 Answers1

1

Surface winds, as reported by the tower are magnetic. Runway headings are magnetic. However, METARs are reported true, according to Aviation Weather Services and ICAO. Therefore one has to perform a heading correction in comparing runway headings to METARs.

Wind directions are normally rounded to 10 degrees, as are runway headings. However, sometimes runway numbering lags isogonic shifts, so care should be used utilizing runway headings.

It is worth noting that the expression, "If it is written it is true, and if it is spoken it is magnetic," may be misleading. As an example ASOS and AWOS data is recorded as true, and transmitted as true, but is converted to magnetic prior to being broadcast on computer generated voice messages.

Furthermore TWEB are true, as the underlying reports they are generated from are true (winds aloft, TAF, etc.).

To summarize, runways are magnetic, tower wind checks are magnetic, but ASOS, AWOS and METAR are true, unless transcribed to computer generated voice.

mongo
  • 17,746
  • 31
  • 82
  • @mongo The one exception to the written/spoken rule is PIREPs. The winds reported in them are magnetic. – JScarry Nov 29 '17 at 21:15