Aerodrome surface wind reports: a summary for pilots

Assessing wind conditions is daily business for pilots. Most of the time, the wind values are easily within limits, but sometimes they get close to the maximum permissible. It is on those occasions that we need to have a clear understanding of the aeronautical wind reports. In this article, we will look at the aerodrome surface wind and how it is reported.

Figure 1: Wind awareness is a key factor for pilots

The typical workflow: Overview of aerodrome surface wind sources

Over the course of a flight, pilots use a variety of wind information sources. The workflow below depicts a typical scenario, with a focus on surface wind data:

Figure 2: Typical surface wind information sources

As pilots, we often “switch” from one source to another. It is crucial to be aware of the differences between the reports. To enhance our understanding of this, we will look at the ICAO and World Meteorological Organisation (WMO) guidelines.

What wind is being reported?

The exact details on how aerodrome surface wind reports are assembled can be found in a plethora of documents, starting with ICAO annex 3 [1], continuing to several WMO reports [2] [3] [4] [5]. To provide the reader with a quick reference, the figures below summarize the key points. We shall start with the aerodrome forecast TAF:

Figure 3: TAF-wind properties [5]

TAF reports can be issued for periods between 6 and 30 hours [1]. They are monitored for changes and amended if required (see below). A TAF that cannot be monitored shall be cancelled [1].

TAF change and amendment criteria

During dispatch, flight crews rely on aerodrome surface wind forecasts, which are included in the TAF. But what happens, if the wind does not behave as predicted? Will the TAF always be amended? Unsurprisingly, there are rules for this...

Below is an excerpt of EASA Part-MET, local state regulations may vary.

Figure 4: TAF amendment rules (EASA Part-MET) [6]

The above list gives an overview of the TAF amendment rules. This can be helpful for contingency planning and enhance the understanding of differences between TAF and METAR/ATIS. Next time you spot a difference between METAR and TAF wind, think about the TAF amendment rules. Chances are that the difference is just not large enough to trigger a TAF amendment.

The key-properties of the remaining wind reports are summarized below:

Figure 5: Summary of aeronautical wind reports [1] [3] [5]

WMO determined long ago, that a height of 10 m AGL is appropriate for aerodrome surface wind measurements [5]. As the summary above indicates, there are differences in orientation and averaging time, which can have significant effects. It shall be noted that an onboard wind indication is very helpful during approach, but may be less useful during landing, due to the time lag and limited accuracy. Knowing these limitations, some operators still use it as a “gross error check”.

Averaging and gust derivation: a science on its own

Measuring wind accurately is indeed a tricky undertaking. As one can easily imagine, many factors affect the result: sensor location, sensor type and data processing are just a few. WMO determined over the years that so-called cup-anemometers with a directional vane work quite well for the purpose of measuring the surface wind at aerodromes [3].

Regarding data processing, the WMO guidance can be summarized as follows [2] [3]:

• A sampling frequency of 4 Hz is appropriate.

• Very short gusts (< 1 s) have limited hazard potential and a limited lateral extent.

• For aviation, averaging over 3 s to establish the gust value has proven adequate.

• An averaging period of at least 2 min for the mean wind should be used to provide a good estimate of the actual conditions at the runway.

  
  

Example data: A closer look at one minute of wind samples

Figure 6: Gust derivation over one minute, credit: NLR [7]

The above Figure shows the relation between measured wind (4 Hz sample rate), 3 s gust magnitude and 10 min average wind. The following observations can be made: In dynamic conditions, the wind magnitude changes rapidly. The 3 s gust magnitude gives a good estimate of significant gusts, the 4 Hz sample alone would include very short gusts, which have limited effects [3]. Furthermore, it is really the mean wind averaged over a longer period that gives a good estimate of the prevailing situation. This fact is supported by WMO guidance to use a minimum of 2 minutes averaging for the mean wind [5].

Observation period effects

For ATC (tower) and ATIS wind information, the mean wind is calculated from the 2-min moving-average, while the gust is reported for the last 10 min [3] [1]. This leads to an interesting phenomenon: The same gust value may “disappear”, if the mean wind gets closer to the gust magnitude. The process is depicted in Figure 7 below: Both wind traces reach a maximum value of 25 kt. The 2-min average, however, is different. This causes the blue wind trace to be reported as XXX15G25KT, while the orange wind trace is reported as XXX20KT. If noise abatement i.a.w. PANS-ATM is in operation, this may still be reported as XXX20G25KT [8].

Figure 7: Sample wind data and averaging effects, data from [8], adjusted

Wind information application considerations

Once wind information has been obtained, flight crews need to apply it in an operational context. Operators should provide guidance for flight crews on how to apply wind values, especially for variable conditions. The following points can be noted [9]:

• For performance, mean wind is the dominant factor.

• For controllability, gust values may be dominant, especially if crosswind.

• Manufacturer guidance and operating rules should be followed.

  
  

EASA encouraged manufacturers some years ago, to specify if demonstrated crosswind values contain gusts or not [10]. Unfortunately, there are still manufacturers that do not specify this.

What is considered during certification?

Some interesting facts come to light when studying the certification and flight test literature [11]:

• Performance figures account for 150 % tailwind, 50 % headwind.

• Controllability is tested to 150 % of the certified tailwind (steady component)

• Systems are tested at 150 % of the certified tailwind component e.g. engines at zero groundspeed, TAWS warning envelope (above glide recovery) etc.

• Some manufacturers have specific thrust setting procedures for crosswind, which can affect the take-off distance, even if there is no tailwind component.

• Vmcg certification rules do not require any crosswind consideration [12].

  
  

Fun question: What about a wind sensor at the North pole?

Imagine a wind sensor at the geographical North Pole. How would a METAR message represent the wind direction? (The obvious problem being the directional ambiguity) …

It turns out, there is a WMO instruction on this!

“Wind direction at stations within 1° of the North Pole or 1° of the South Pole should be measured so that the azimuth ring should be aligned with its zero coinciding with the Greenwich 0° meridian.”  [3]

References

[1]	ICAO, «Annex 3: Meteorological Service for International Air Navigation, 20th edition,» 2018.
[2]	WMO, «No. 731: Guide to Meteorological Observing and Information Distribution Systems for Aviation Weather Services,» WMO, 2014.
[3]	WMO, «No. 8: Guide to Instruments and Methods of Observation, Volume I –Measurement of Meteorological Variables,» 2024.
[4]	WMO, «No. 8: Guide to Instruments and Methods of Observation Volume V – Quality Assurance and Management of Observing Systems,» WMO, 2023.
[5]	WMO, «No. 49: Technical Regulations, Basic Documents No. 2, Volume II – Meteorological Service for International Air Navigation,» 2021.
[6]	EASA, «Change and amendment criteria for aerodrome forecast (TAF), In accordance with Annex V (Part-MET) to Regulation (EU) 2017/373,» 2020.
[7]	G. van Es, P. van der Geest und T. M. Nieuwpoort, «NLR-TP-2001-217: Safety aspects of aircraft operations in crosswind,» NLR, 2001.
[8]	Flight Safety Foundation, «FSF ALAR Briefing Note: 8.6 - Wind information,» FSF Flight Safety Digest, Nr. August-November, pp. 185-188, 2000.
[9]	EASA, «Research Project EASA.2011/08: NGW - Near Ground Wind Gust Detection,» 2012.
[10]	EASA, «SIB 2014-20: Aeroplane Operations in Crosswind Conditions,» 2014.
[11]	FAA, «AC25-7C: Flight Test Guide For Certification Of Transport Category Airplanes,» 2012.
[12]	E.-J. A. M. Huijbrechts, H. J. Koolstra und J. A. Mulder, «Using Vmcg-Limited V1, Controllability Issues on Contaminated Runways and in Crosswind,» JOURNAL OF AIRCRAFT, Bd. 56, Nr. 4, pp. 1342-1352, 2019.