ARE YOU UPSET?
We almost instinctively slow down when the air becomes uncomfortably bumpy. I think it’s safe to say that most of us slow down when driving a car when the road gets rough, the motivation being not to break something expensive.
The experience of moderate to severe turbulence while flying your airplane could also result in breaking something expensive. However, the expensive and at risk airplane part(s) would likely put you in much greater jeopardy than those in you car should you break them!
So, we reduce the power, slow the rate of descent, extend the landing gear, speed permitting.
We will not extend the flaps, even if below the maximum flap speed. Why? The certificated positive and negative “G” limits stated in the Pilot’s Operating Handbook (POH ) are decreased to some extent when the flaps are deployed. The extent of this reduction may or, likely, may not be a value you can calculate with most light aircraft.
Well great, we will slow down but, to what airspeed? We need to fly at an airspeed which will protect the airplane from being damaged by the effects of excessive structural loading caused by the vertical movement of the air. That airspeed will be slow enough that the wing will reach it’s critical angle attack and stall before any damage occurs. The stall and recovery sequence may happen so quickly that you are not even aware of it.
This magic airspeed may be (VA) the aircraft’s published maneuvering speed(s) found in the POH. Depending on the airplane, there may be more than one VA. As the weight of the airplane changes so does the stall speed, although the critical angle of attack is always the same. VA, which is an airspeed limitation , will decrease when the airplane is light and increase as the airplane has a heavier load to carry. However, the FAA definition of VA is as follows:
VA is the aircraft’s design maneuvering speed. Flying at or below VA, means that the airplane will stall before the structure is damaged by excessive loads. If you encounter a gust that causes a sudden, significant increase in load factor while flying above VA, the aircraft could experience structural failure.
Another important thing to understand is that VA changes with the aircraft weight: VA decreases as weight decreases, and it increases as aircraft weight increases.
It is a mistake to assume that as long as you are at or below VA, you can move the controls from stop to stop repeatedly without damaging the aircraft. To clarify this point, 14 CFR part 25 states that “flying at or below the design maneuvering speed does not allow a pilot to make multiple large control inputs in one airplane axis or single full control inputs in more than one airplane axis at a time without endangering the airplane’s structure.” Although re certificated under 14 CFR part 23, this point is still valid.
There is another consideration here and that’s aircraft controllability. All of the above having to do with potential structural damage is the major concern. The reduction of airspeed also limits the function of flight controls. Larger aircraft have calculated minimum airspeeds, based on the current gross weight. This is known as Rough Air or Gust Penetration Speed (VB). These larger, and faster aircraft have a much wider range between their Clean Stall Speed (VS) and their Design Cruise Speed (VC). It may be practical for pilots to fly at an airspeed below their VA in rough air but not below the VB.
Pilots of light GA aircraft should reduce the airspeed to something below the appropriate VA in rough air but not below the minimum airspeed required to maintain positive aircraft control. That speed, although probably not listed in the POH, becomes the VB.
The following may be a useful guide (From “Mastering the Maze of V-Speeds by FAA’s Susan Parson)
VA and VB are derived as a function of VS, clean stall speed:
VA (gross weight) = 1.95 VS
VB (gross weight) = 1.6 VS