During the months of February and March we realize that it’s a time typically associated with very windy and warmer weather. The average rainfall for February in the Cape Town Region according to the SA Weather Service is only about 20mm, making it a very dry month. With the lower possibility of cloud formation during this time, it is great for us VFR pilots, but it comes at the price of turbulence that is significantly more than in other months.
We are interested Low Level Turbulence that is operationally defined as turbulence experienced in the atmospheric boundary layer below 15,000 feet, where surface heating and friction is significant. We are exploring four (and there are more) reasons why you may experience a bumpy departure or arrival at FASH.
A) Thermal Turbulence – The sun heats the surface of the earth unevenly by Radiation. This heat is not retained and heats the immediate air, a process called Conduction. This warm air rises further through the process of Convection. We can consider the air above the open barren piece of land as one “pocket” of air, and the same for one above a vineyard. Since these two pockets are warming at different rates, their actual temperature is also different. This differential causes isolated convective currents and will shake you around a little bit as the aircraft flies through them. The greater the difference between the cool and hot “pockets”, the greater the effect on the aircraft. This difference will be at its peak for about one hour before noon and two hours after noon.
B) Obstruction Convective Turbulence – FASH is surrounded by beautiful valleys created by the natural contours of the countryside. On these waving slopes are buildings, roads, big trees, greenhouses, golf courses and settlements. Although this causes a slight directional change (see point C) in a prevailing wind, like the South Easter, these man-made obstacles are contributing to further convective currents bringing us back to the same result as per point A. This is just slightly more pronounced and quite dramatic at very low levels in the Up-Wind or Short-Final.
C) Descending Currents – With the two bodies of water on final approach for RW 01 and 19, the pilot may experience a need to adjust the flight path. Again, this is due to convective currents as the body of water are significantly cooler than the dry and barren land right next to it. The dry area produces more lift (strong upward convection) than the cooler area and small adjustments in the aircraft becomes necessary. This kind of turbulence might then be pilot-induced… Be careful and keep an eye on your reference points for a safe landing and prevent undershooting or overshooting.
D) Mechanical Turbulence – Surrounding the stunning airfield are some significant hills and mountains – Stellenbosch Mountain, Helshoogte, Helderberg, Botterlary Hills and Klapmuts. On your way to your destination you’ll be flying close to Sir Lowry’s, Paarl Mountain, Paardeberg or through the Franschhoek Valley. The Windward side of the mountain or slope is the side that is facing the wind, or the up-wind side. The Leeward side is the “roll-over” or down-wind side that is away from the direction of the wind. With the typical strong winds we have in February or early March, these rolling winds causes a tumble-dryer effect and may cause the aircraft to be shaken around quite dramatically as this air wants to flow downward on the other side and becomes turbulent. This is especially rough when the strong South Easter or Cape Doctor is blowing. Something to really watch out for at the airfield is the small valley formed between the golf course and short final of RW19. Also, be aware of the Noise Abatement sand-wall that is built along the Eastern side of the runway. The wind hits this wall from the East and rolls over toward the clubhouse causing a circular motion of air making it slightly more difficult for a greaser landing.
Some tips for you to consider:
- Plan to fly early mornings or late afternoons when the convective differentials are reduced.
- Stay away from the slopes of the surrounding mountains.
- When the TMA of FACT allows it, climb above the lower turbulent levels.
- Stay ahead of the aircraft by expecting some bumps when flying over dry patches.
- Know your aircraft V speeds. You do not want to get close to Vno (top of the green arc) when flying in turbulent air. Rather slow the aircraft down to about Vfe + 10% as a reference.
- When taking passengers with you, let them know it’ll be bumpy. They might reconsider the flight, or, make sure you have at least one important little bag for each passenger.
- If it is too rough, turn back or land at one of the many options available in the area.
- Going around is always an option, always.
Thank you for your time, see you at the club real soon.
Cheers, SFC Blogger.