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The A & B of V-Speeds for Turbulence

You don’t see it listed in too many spec sheets for aircraft on any given flight line but, alphabetically, it comes after one that always appears within any given V-Speed table. VB is the middle-child of V-Speeds, often getting pushed into the background by its more commonly recognized recommended numerical siblings. What is it, and how does it relate to VA , the speed that most likely precedes it in a list, and to which it is closely linked if not slightly confused? Let’s have a look.

VB doesn’t appear on your airspeed indicator, but then again, neither does VA, so don’t go looking for either figure there. You’ll find VA, your aircraft’s ‘design manoeuvring speed’, in your aircraft’s Pilot Operating Handbook.. VB, your aircraft’s ‘design speed for maximum gust intensity’ is a bit harder to find. If it’s not in your POH, a call to your aircraft’s manufacturer might solicit an answer, but only so long as the engineer who worked it out can be found to pass it along. Otherwise, it’s just lost in the shuffle of V-Speeds.

VB is developed by an aircraft designer as a suggested speed for penetrating turbulent air when that turbulence is particularly severe – that is, the turbulence has varying vertical gust components in the order of 50 feet/second (30 knots) or more. There is no regulatory requirement that VB be published for light aircraft and, generally, it is quite close to VA anyway. Hence, light aircraft pilots do with VA, and rarely give cause to consider the specifics of VB which, in any case, tends to apply more to larger commercial aircraft.

Your aircraft’s VA may also be referred to as ‘the speed for maximum control deflection’ or, ‘the rough air airspeed,’ this latter definition giving rise to some overlap with that for VB. Of critical importance when contemplating VA is this: if you are flying at a speed greater than VA, any full and abrupt applications of the primary flight controls could exceed the structural limitations of your aircraft.

What are those limitations? For a normal category aircraft – most small single-engine aircraft fall into this category – the certified vertical load limit factor is +3.8g. That is, if the load imposed on the aircraft exceeds 3.8 times the gross weight of the aircraft, it could come apart. For a utility category aircraft, the certified vertical load limit factor is +4.4g.

VA is a fixed theoretical number within each category of aircraft, determined by a formula that multiplies the flaps-up, power-off stall speed (VS) at gross weight by the square root of the design limit load factor for that aircraft’s category. Thus, for a normal category aircraft, multiply the aircraft’s VS by the square root of the category’s load limit factor (i.e. 3.8) and you get VA for that aircraft. VA is slightly less than twice VS for normal category aircraft, while for utility category aircraft, VA is about twice VS..

For ‘turbulence penetration airspeed,’ Transport Canada’s AIM recommends setting power settings as recommended in your aircraft manual. Depending on the severity of the turbulence, this speed could be VB – good luck finding it. You will have to do with VA, but with a few considerations. The best airspeed at which to penetrate turbulence should be about 10 knots below VA to compensate for the stall delaying effects of power (because power-on stall speed is lower than power-off stall speed) and the airspeed fluctuating effects of wind shear (which is, effectively, what turbulence is). As the math in the previous paragraph attests to, as your aircraft’s stall speed goes down, so too does its VA.

Consider also that published VA is valid at the aircraft’s gross weight. Since stall speed decreases as weight decreases, and since VA is a function of stall speed, a lighter aircraft should be flown at a slower airspeed in turbulence than a heavier one since a lightly loaded aeroplane is accelerated more easily by gusts. A pilot, reacting abruptly with the controls to the sudden movement of the aircraft as it enters turbulence, may inadvertently exceed the aircraft’s VA which is already lower than normal because of the lighter load.

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