What’s in a sock – and yes, size matters!

So often as a trainee pilot we were told “check the windsock”. However, we weren’t always told what to look for beyond wind direction. More detailed information was generally picked up as we continued our training.

Assessing the wind is an important factor for take-off and landing distance calculations, and for assessing the change in wind from when calculations are made. I think it is important to be able to use rules of thumb. and to also take things ‘on the fly’ in GA remaining adaptable to change.

First of all, what is the sock rated for? Yes, size matters!  This is a common problem in New Zealand for many pilots I am hearing, with a lack of understanding of the impact of different sizes. The windsock frame should. also be around 6 - 6.5m from the surface.

If the sock is a burnt orange and meets the size and weight specifications of NZCAA as pictured below, the rated erect windspeed is 25 knots and the tail of the sock will flick around at 35 knots. A full-size sock at 45 degrees to horizon will be 15 knots. The sock below is indication around 8-10 knots.

Full size 25 knot windsock

A bright dayglo orange sock that has the same frame throat size but is significantly shorter is still a 25-knot windsock fully extended, but the tail will not flick around over 25 knots so for winds up to 25 knots they can treated as the same. These shorter socks will be found at many non-certified rural aerodromes.  

Credit:  Mark Woodhouse, Waypoints Aviation, the industry standard for learning to fly in New Zealand.

A small windsock as pictured below is un-rated as such, but could be erect at anything from 8 – 20 knots depending on weight. Most helipads have a lighter weight windsock because of the critical nature of tail wind on helicopter operations.

All Socks Are Not Equal

FAA and European specifications are very different to NZCAA requirements. Most foreign legislators require the windsock to be fully erect at 15 knots. Unfortunately for the New Zealand pilot, this means you need to take into consideration this variability.

If the sock seems to be made from lighter material and is red or striped orange and white it is most probably an imported product and meets the overseas requirements of 15 knots. These socks behave quite differently; they move around a lot more laterally and have more mobility horizontally. Pilots need to be very aware in cross wind situations with these socks, and use other visual and tactile clues to assess wind.  

Cross Wind Assessment

Cross wind assessment is very important when deciding which runway to use and which way to land if direct crosswind - or to land at all, if it comes to that! Some aircraft are particularly limited in cross wind conditions, especially if its gusty. 

I use the clock code. It’s a simple quick way. Parallax is another issue at larger aerodromes. A sock some distance away can be hard to assess, so I look for multiple socks and take the environment into consideration when assessing. 

 

Windsock Location

Visualisation should be used to assess the take-off and landing path to assess the wind in relation to the obstacles and terrain beside the approach and runway. Turbulence, wind shear and the funneling effect may cause issues. The assessment of wind and obstacles can start during the preparation period prior to the flight by looking for trees and obstacles along the approach and runway. The funnelling affect is especially prevalent now due to aerodromes being more built up with hangars.

Gusting Wind

If the windsock is going crazy either laterally or horizontally, the wind is unpredictable in direction (moving horizontally) and/or gusting (moving up and down), so caution should be exercised.  Departure and approach speeds should be modified to meet conditions. If the gusting wind is frontal then waiting some time might be prudent to allow the gust front to pass. 

Stuck Frame

If the frame is pointing in one direction and the sock the other, generally the frame is getting stuck. This can actually be a problem in high rainfall areas like the West Coast of the South Island. Using other visual cues and best guess is the trick here.

Other Visual Cues

Importantly, the other visual cues below that are present are critical to safe operations. What happens above 6m (the height of the windsock) is also the pilot’s responsibility to assess from reports/forecasts, and from local knowledge. Visual cues that should be utilised include these below:

Before Take-off 

• Grass near the aircraft
• Tree tops
• Cloud movement
• Cloud formations
• Anything picked up by the wind
• Fixed objects like flags, washing on a line etc
• Prop Noise
• Fumes entering cabin
• Feel of the aircraft and controls

Airborne 

• Lift/sink
• Groundspeed vs airspeed
• Drift
• Turbulence
• Blowing dust
• Tussock/vegetation movement

Go Around

Remember, a go around is always prudent if a tailwind or cross wind is sensed or if the pilot just feels something is not right. Many landing and take-off accidents are attributed to by a “freak” gust of wind. Good preparation, knowledge and situational awareness will minimise these occurrences.

Safe flying!

References:

• Windspeed and Direction CASR 91.380
• FAA 3.2.2 Dimensions
• CASA 8.102 Standards for wind direction indicators
• ICAO (Annex 14)/UK CAA