What is maneuvering speed, and why should you care?
Perhaps more importantly, what should you know to maximize your flying safety?
The widely accepted definition of maneuvering speed (Va) is the speed at which an airplane will stall before exceeding its maximum load limit.
But, wait! There’s more…to the story.
Maneuvering speed has been masquerading as the magic speed to protect you from structural damage in turbulence. It’s important, but it’s not the panacea of protection we were trained to believe. Before explaining, however, let’s review maneuvering speed and how and why it changes with weight.
Airplanes are designed to withstand varying stress loads based on their intended purpose. The FAA certifies aircraft in one of three categories: normal, utility and aerobatic. The maximum load limits for each category, are:
Normal - +3.8Gs and -1.52Gs;
Utility - +4.4Gs and -1.76Gs;
Aerobatic - +6Gs and -3Gs.
An aircraft flying straight and level at a constant speed is in a steady state where the aircraft's lift is equal to its weight. Since G-force is calculated by dividing lift by weight, the load, or G-force in this condition is equal to one (2,500 lbs. lift/2,500 lbs. weight = 1G). Exactly the same G-force as sitting in the hangar.
If, however, the airplane's lift doubles, either from a turbulent gust or the pilot pulling back on the elevator, the load on the aircraft doubles (5,000 lbs. lift/2500 lbs. weight = 2Gs).
If lift continues to increase, the load or G-force continues to increase proportionally; i.e., 7,500 pounds of lift would put a 3G load on the aircraft (7,500 lbs. lift/2,500 lbs. weight = 3Gs) and 10,000 pounds of lift would produce 4Gs of load (10,000 lbs. lift/2,500 lbs. weight = 4Gs of load).
Lift is increased when the angle of attack (AOA) increases.
AOA is the angle between the chord line and the direction of flight. For general aviation aircraft, there is a direct, almost one-to-one relationship between lift and AOA until the critical angle of attack is reached.
The critical angle of attack, typically between 15 and 20 degrees in general aviation aircraft, is the AOA that produces the maximum lift. Any increase beyond the critical angle of attack results in a stall.
For the purposes of calculating maneuvering speed in the examples below, we will use a critical angle of attack of 18 degrees. Our examples will assume a linear or one to one relationship between lift and angle of attack as the graph below depicts; whereas, a doubling of the AOA would produce a doubling of lift.
Assume an airplane at gross weight is flying straight and level at 140 knots. At that weight and speed, this aircraft requires an AOA of 3 degrees to produce sufficient lift to maintain level flight.
If turbulence or manual input increases the AOA from 3 degrees to 6 degrees, the lift doubles and the load factor doubles to 2Gs. The load continues to increase as the AOA increases until its critical angle of attack of 18 degrees is reached.
Under these conditions, this aircraft's maximum load would be 6Gs because the AOA was 3 degrees in level flight and could only increase six times before reaching its critical angle of attack of 18 degrees (18 degrees is six times its level cruise AOA of 3 degrees).
As stated previously, the maximum load (G-force) is reached at the critical angle of attack of 18 degrees because any further increase in the AOA would result in a stall; thus, eliminating the load altogether.
If the airplane in this example was certified in the normal category with a maximum load of +3.8Gs it could experience structural damage at this speed in strong turbulence.
In this case you should slow down because flying slower will require a higher AOA to maintain level flight. A higher AOA in level flight would put you closer to your critical angle of attack. For example, if a slower speed requires an AOA of 5 degrees to maintain level flight, the load would max out at 3.6Gs (18 divided by 5 = 3.6 Gs).
Another determining factor is the aircraft weight. The Pilot’s Operating Handbook (POH) specifies the aircraft’s maneuvering speed at gross weight. If you’re flying below gross weight, the maneuvering speed is reduced because a lower AOA is required to produce the lift necessary to maintain level flight.
For example, an airplane at 2,500 pounds may require a 4.5-degree AOA at 110 knots to generate 2,500 pounds of lift for level flight; whereas, the same aircraft at the same speed weighing only 2,200 pounds may be able to maintain level flight with only a 3-degree AOA.
Again, with a reduced AOA of 3 degrees, a 6G load could be generated by a strong gust of turbulence before stalling at the 18-degree critical angle of attack.
In turbulence you want a high AOA in level flight to reduce the multiplier before reaching your critical angle of attack. You can’t change your critical angle of attack but you can increase your cruise AOA by slowing down.
Now, for the rest of the story.
Traditionally we were taught that flying at or below maneuvering speed would protect us from structural damage during turbulence or from a rapid control deflection from one extreme to another.
We now know there are exceptions.
One practical exception is when two or more forces are at play simultaneously.
To better understand it may be helpful to know how flight tests are conducted on new aircraft testing maneuvering speeds.
In smooth air, the test pilot rapidly moves a flight control to its positive design limit and then returns it to its neutral position. After pausing, he repeats the process to the control’s negative limit before returning to the neutral position. This test is repeated for each axis separately; pitch, roll, yaw.
So maneuvering speed may protect the wings from structural failure if the loads are vertical like choppy air; however, gusts often introduce stresses in more than one axis simultaneously. Pitching, banking and yawing occurring at the same timestress the aircraft in multiple directions.
Another exception is the rapid and repeated deflection of the control of one axis.
As so eloquently stated by Boldmethod, “In 2001, American Airlines, Flight 587 encountered wake turbulence during climb-out from JFK. The pilot moved the rudder pedals nearly full right, then full left, then full right, then full left, then full right in 6.5 seconds. The resulting stress sheared off the vertical fin, and the aircraft lost control and crashed causing the death of all soles (sic) onboard. The entire event happened below design maneuvering speed.”
It is important to understand maneuvering speed is an approximate number that does not protect your airplane from structural failure in all situations.
In 2010, TheFederal Aviation Administration amended the airworthiness standards applicable to transport category (not aerobatic) airplanes to clarify 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. The FAA issued this final rule to prevent pilots from misunderstanding the meaning of an airplane's maneuvering speed, which could cause or contribute to a future accident.
Docket No. FAA-2009-0810
Amendment No. 25-130, effective October 15, 2010
If you can't count on flying at or below maneuvering speed to protect your aircraft's structural integrity, what should you do when you encounter turbulence?
Clearly the best option is to avoid turbulence whenever possible. Familiarizing yourself with the sources of turbulence will help you avoid them. Refer to Turbulence, an article on Turbulence to learn, among other things, to:
1. Fly behind and not in front of storms,
2. Fly above mountain waves if possible. If not, fly perpendicular to mountain waves or parallel on the windward side.
3. Remain above and/or upwind of another aircraft’s flight path to avoid their wake turbulence.
When turbulence cannot be avoided, slow down to maneuvering speed and fly smoothly.
It's more important to maintain attitude than altitude; and, unless you are very close to the ground, it is not critical to react quickly. Take your time and make control adjustments smoothly.
This article is scheduled to appear in November, 2019 issue of Plane & Pilot Magazine.