The 3 Most Important Things about Airplane
Maneuvering Speed Every Pilot Must Know
Introduction to Maneuvering Speed
Maneuvering speed is a crucial concept for flight safety, as it is associated with the potential of over-stressing the aircraft as a result of an abrupt control input. It is also a somewhat convoluted concept which invites misunderstandings that can lead to structural failure and serious aviation accidents. It is the purpose of this maneuvering speed article to shed light on the matter and clarify the concept of maneuvering speed. We will first look at the regulatory basis of maneuvering speed and then explore its ramifications for the operational pilot.
Summary of the Three Most Important Things about Maneuvering Speed
- Maneuvering speed depends of weight as \(\sqrt{W_t/W_s}\), where \(W_t\) denotes your actual aircraft weight and \(W_s\) is the standard weight (usually max gross weight) with respect to which maneuvering speed has been published in the airplane flight manual. The lighter the aircraft, the lower the maneuvering speed.
- Flying below maneuvering speed provides only protection from structural damage, if you apply the abrupt control input:
1) around one aircraft axis only (i.e. one flight control at a time),
2) in one direction only (no waggling of control back and forth or going from one full stop to the other). - For the above to be true, crucially, you must be below Operating Maneuvering Speed \(V_O\), not Design Maneuvering Speed \(V_A\). Design Maneuvering Speed \(V_A\) does not guarantee any protection from structural damage of the aircraft as a result of an abrupt control input!
Below we elaborate on these three points, in reverse order than they are listed above.
Common Confusions about Maneuvering Speed
Operational pilots often believe that below maneuvering speed, any abrupt control movement will not lead to damage to the aircraft. This is incorrect on three accounts: first of all, there are two different maneuvering speeds, second, there are restrictions on what kind of movements are allowed, and third, maneuvering speed is weight dependent. Please follow along in detail, since the concept of maneuvering speed in aviation is somewhat intricate.
Two Different Definitions of Maneuvering Speed
There are two different types of maneuvering speeds, and one must distinguish between them carefully:
- \(V_A\): Design Maneuvering Speed
- \(V_O\): Operating Maneuvering Speed
Unfortunately, for whatever reasons, when operational pilots speak about maneuvering speed, they always mean the operating maneuvering speed \(V_O\), yet they almost always use the symbol \(V_A\) for it, which stands for design maneuvering speed. This is reinforced by airplane flight manuals typically using \(V_A\) for it as well.
In order to disentangle the confusion, let us dive into the airworthiness regulations and extract the exact definitions of maneuvering speed.
Maneuvering Speed 1: Design Maneuvering Speed \(V_A\)
Design Maneuvering Speed \(V_A\) is based on the structural strength of the elevator, horizontal stabilizer and the ailerons. It does not guarantee in any way that the maximum allowed normal load factor \(n_{max}\) will not be exceeded and that the wings will not come off. This is completely evident already from the old 14 CFR Part 23 before Amendment 23-64 and FAA Advisory Circular AC 23-19A.
14 CFR 23.335(c) specifies that the design maneuvering speed, \(V_A\) must lie between \(V_S\sqrt{n_{max}}\) and design cruising speed \(V_c\), where \(V_S\) denotes the straight-and-level stall speed. Only the lower limit of the above range coincides with what operational pilots generally consider maneuvering speed to be (which coincides with corner speed in the V-n diagram). If the manufacturer chooses any higher value within the above allowed range, then a rapid pull on the elevator can exceed \(n_{max}\) even below design maneuvering speed.
If one reads 14 CFR 23.423, 23.441, and 23.455 carefully, one realizes that design maneuvering speed refers solely to the structural strength of the empennage and the ailerons, and does not refer to \(n_{max}\) of the entire wing of the aircraft itself.
Indeed, Advisory Circular AC 23-19A (see e.g. Paragraph 48, Page 27) states that if \(V_A > V_S \sqrt{n_{max}}\) an abrupt pull maneuver on the elevator by the test pilot must be “checked” by an elevator movement in the opposite direction, before the g-forces have time to build up at the wing and the maximum allowed load factor \(n_{max}\) of the aircraft is exceeded.
Maneuvering Speed 2: Operating Maneuvering Speed \(V_O\)
14 CFR Part 23 Amendment 23-45 added a new definition of maneuvering speed, the so called operating maneuvering speed, \(V_O\). Operating maneuvering speed, \(V_O\), must—by definition—not exceed \(V_S \sqrt{n_{max}}\), which corresponds to what operational pilots generally understand under maneuvering speed.
Operating maneuvering speed, \(V_O\), also makes an appearance in ASTM F2245, which is the Part 23 equivalent for light sport aircraft (LSA).
Only operating maneuvering speed guarantees that structural limits will not be exceeded with an abrupt control input, but there are still further constraints on what control inputs are actually allowed in this context. We will look at these next.
Maneuvers Allowed Below Operating Maneuvering Speed
The following constraints are place upon the control inputs allowed below maneuvering speed in order to guarantee structural integrity of the aircraft:
- One control input around one axis at a time (i.e. either aileron, elevator, or rudder input, but not two of these simultaneously),
- The control input must go in on direction only (i.e. no wiggling back and forth of the control).
We will illustrate on two common examples, why the above restrictions are important, and develop physical intuition, what could go wrong. The two examples are common place and the operational pilot and test pilot shall remain very mindful of them.
Example 1: Rolling Pull-up
If the aircraft is upset and goes inadvertently inverted, for instance as a result of wake turbulence, the now will have the tendency to drop towards the earth. By the time the pilot has rolled 90 degrees to vertical, the nose is pointing down quite a bit, and it is very temping—as the pilot continues to roll upright past the vertical—to also start pulling on the elevator.
This is a common scenario, how one can overstress and aircraft very quickly, maybe even without noticing. The resulting dive from the upset is picking up airspeed and the airplane may be close to operational maneuvering speed when rolling upright. If the pilot now inputs back elevator, in addition to the full aileron that they have already been holding during the roll, this becomes a simultaneous two-axis control input. The up-going wing experiences a much higher load factor than the down-going wing, and also than the aircraft itself. This can lead to aircraft overstress.
A simultaneous two-axis control input can be applied, if it is deemed necessary, but the imposed vertical load factor should then be limited only to 2/3 of its maximally allowed published value as a generally accepted rule of thumb.
Example 2: Rudder Waggling Back and Forth
An example of a forbidden back-and-forth single-axis control input is waggling the rudder aggressively. This has happened during the crash of American Airlines Flight 587 in November 2001, when the first officer responded to wake turbulence from a preceding aircraft with aggressive rudder inputs, which led to structural failure and separation of the vertical stabilizer, resulting in the aircraft crashing into Queens in New York.
The above example illustrates how quickly a pilot may find themselves in such a situation. Now for the physics.
The angle of attack, which the vertical stabilizer experiences from a full rudder deflection in one direction is that of the air flowing along the longitudinal axis of the aircraft and the new chord line between the leading edge of the vertical stabilizer and the trailing edge of the rudder. This angle of attack is much smaller than the angle of attack that ensues, if the rudder was previously deflected in the opposite direction and the air is coming in with a sideslip, from the same side as the new rudder input.
This larger angle of attack creates a larger lift force on the vertical stabilizer, which may become so large that it compromises the structural integrity of the vertical stabilizer. A structural failure and separation of the vertical stabilizer as a result has usually disastrous consequences, as both directional stability of the aircraft and directional control with the rudder are lost, and the aircraft generally becomes uncontrollable.
Weight Dependence of Maneuvering Speed
The published maneuvering speed in the aircraft flight manual is given for a standard reference weight \(W_s\), usually the maximal gross weight. If \(W_t\) is the actual weight of the aircraft, then operating maneuvering speed at \(W_t\) becomes
\begin{equation} V_{O_t} = \sqrt{\frac{W_t}{W_s}}V_{O_s}. \end{equation}
This follows directly from stall happening at a value \(C_{L_{max}}\) independent of weight, and from the airspeed equation for straight-and-level flight
\begin{equation} V = \sqrt{\frac{2W}{\rho S C_L}}, \end{equation}
which in turn follows immediately from \(L=W\) and \(L=\frac{1}{2}\rho V^2 S C_L\).
The ramification of this is that if the aircraft is lighter, the operating maneuvering speed is slower. The reason for this is that a lighter aircraft flies at a lower angle of attack at the same airspeed than a heavier aircraft, in order to maintain level flight. The lighter aircraft is therefore farther away from critical angle of attack at the same airspeed than a heavier aircraft. An abrupt pull on the elevator can thus produce a higher normal load factor \(n_z\), before critical angle of attack is exceeded and the aircraft stalls.