Loaded for Air

Keeping the wings on your airplane means flying within the envelope to avoid potentially fatal structural loads


One of the most fatal types of accidents in general aviation is structural failure, in which the chance of death is nearly 100 percent.

Luckily, wings dont come off aircraft very often. But when it happens, it will be catastrophic and probably unsurvivable. Im shocked while listening to some flight instructors who quietly advertise that they would teach aerobatics regardless of the aircrafts certification, citing that the aircraft had a safety margin so it wasnt a big deal.

It is a big deal. In fact, theres a rental/training aircraft at a nearby flight school that is so bent that it wont fly straight and level, and students regularly complain that its stall is so unpredictable and always rolls off. That shouldnt be a surprise. Probably many of you have flown an aircraft like that.

Recently one of my very favorite tail numbers was grounded for many months because of a cracked wing spar.

Some of the pilots who had been flying the aircraft came from the cargo industry and routinely did red line descents regardless of the turbulence, justifying their actions that theres plenty of safety margin at red line. Luckily the wing spar crack was detected before it led to a structural failure.

Every pilot, whether weekend flier or career pilot, must know the operating limits of the aircraft, how to apply the limits properly and how to stay within the limits.

Certainly abuse of the controls and excessive speeds are guaranteed ways to exit the envelope, but many pilots fail to realize that weather phenomena can place them outside the envelope, too.

Thunderstorms, mountain wave rotors, and other forms of turbulence can easily put the aircraft outside the envelope if the pilot does not properly cope with the hazard.

Structural Limits
The velocity and g limits are commonly put into a graph called a V-n diagram, which is generally referred to as the aircrafts flight envelope. Quite simply, flight within the performance envelope of the aircraft is mandatory or else you are exceeding the capabilities of the aircraft.

The horizontal axis is the velocity, and the vertical axis is the load factor. When an engineer first starts to design an aircraft, the aircrafts requirements are stipulated (training, cross country, rural sod unimproved strips, high altitude, etc.) The maximum speed is termed Vd, the maximum speed attained in a dive. At this speed, and in absolutely smooth air, the aircraft should be able to tolerate the high aerodynamic pressures and be free of any serious aero-elastic effects such as flutter, divergence, wing warping, or control reversal.

Next defined is Vne, the never exceed or red line speed. Under FAR Part 23 for light piston planes, Vne is defined as 90 percent of Vd. This limitation is marked on the airspeed indicator with a red line.

This is a small margin of safety. The aircrafts operating margins are very thin when operating at this end of the envelope. Furthermore, the operating margins are only applicable under a very narrow set of conditions.

The low end of the airspeed envelope is also limited, not by the aerodynamic pressures, but rather by the aircrafts ability to produce lift at high angles of attack. During the airplanes initial flight testing, the slowest attainable speeds in the clean and flaps extended configurations are determined, which establish the low end speed limitations.

The bottom of the green arc is Vs1, the minimum steady flight speed, generally in the clean configuration. The bottom of the white arc is Vso, the minimum steady flight speed in the landing configuration. These speeds are established under a certain set of criteria, such as a set atmospheric condition, weight, cg location, load factor, etc.

Furthermore, the way the stall is induced greatly affects the stall speed. Although an aircraft can be made to stall at any pitch attitude and any airspeed, the stall speed labeled on the airspeed indicator will serve as a warning that you will be approaching the low speed side of the aircrafts envelope if youre in normal, 1 g, straight and level flight.

Remember also that the stall markings only apply to clean wings right out of the factory. Dents, airfoil contamination, frost, bugs, weight, c.g. location and technique will all influence the stall characteristics of the aircraft.

Therefore, stall airspeeds should be regarded as advisory numbers rather than absolute truths. It is possible to fly the aircraft at a speed below the white arc, but the airplane cant sustain such a maneuver. Again, the pilot should understand the derivation of these numbers and the limitations of the derivation.

The top of the white arc, Vfe, is the maximum speed at which the flaps should be extended. Pilots should avoid extending the flaps down at speeds close to Vfe on a regular basis because this places an increased stress load on the supporting structures and over time the hinges will weaken due to fatigue.

Since aerodynamic forces increase with the square of the airspeed, even five extra knots drastically increases the aerodynamic load on the flaps at normal speeds for general aviation aircraft. In addition, flap extension often limits the amount of gs the aircraft is allowed to encounter.

Load Factors
Airspeeds form the two sides of the flight envelope, and the top and bottom are formed by load factors, or gs. There are positive gs, such as are evident while doing a steep turn, and there are negative gs, which you feel if you push the stick and put your stomach into your mouth.

To the airplane, a g is the ratio of lift to weight. In straight and level unaccelerated flight, the lift equals the weight, so the aircraft is at 1 g. In a constant altitude turn, the wings total lift must increase in order to increase the vertical component enough to offset the weight. The increase in lift can be produced by an increase in angle of attack (back pressure) and/or an increase in velocity.

When the bank exceeds about 30 degrees, a substantial increase in back pressure is required to produce the lift to keep the aircraft at a constant altitude.

Any aircraft in a 60-degree bank constant-altitude turn, whether a 747 or an F-16, is pulling 2 gs. The pilot will feel this increase in gs as the eyelids and cheeks are pulled down, the body settles more firmly into the seat and the blood starts to pool in the legs. If the aircraft is banked to 72 degrees, the load factor now becomes 3.8 gs, which is the maximum load factor for aircraft certified in the normal category. From that point on, a small increase in bank angle greatly increases the load factor if you want to hold altitude.

When engineers design the airplane, they must consider the load factors it will carry in flight. A mechanical engineer with a structural emphasis would prefer to design the wing like an I-beam. I-beams are not very efficient at creating lift, however.

The aerodynamicist would prefer a long, slender wing with a thin airfoil cross section for maximum efficiency in high speed cruise flight. A performance engineer worried about takeoff and landing will prefer having a nice, thick airfoil. The manufacturing engineer would prefer to have a rectangular wing with a box airfoil shape for ease of building.

A balance must be found among the competing interests. Normally performance is the overriding consideration. Sailplanes are an excellent example, as the wings are very long, slender and thin, producing excellent lift to drag characteristics, but load carrying capacity is substantially reduced.

Federal regulations outline the structural requirements for aircraft. FAR Part 23 governs the certification of Normal, Utility and Aerobatic aircraft, while FAR Part 25 deals with transport aircraft. Normal category aircraft must handle +3.8 gs and -1.52 gs. Utility aircraft need +4.4 and -1.76 and aerobatic airplanes are +6.0 and -3.0. In the normal and utility categories, the negative g limit is 40 percent of the positive g limit.

The utility category is for airplanes required to perform mild aerobatic maneuvers such as chandelles and lazy eights. Many general aviation aircraft can only perform these maneuvers within restrictive weight, balance and speed ranges. The POH and a cockpit placard will outline the operating limits of the airplane, as well as any warnings about utility or aerobatic-category maneuvers.

Safety Margins
Many pilots are aware that aircraft are designed with certain safety margins and some rely heavily on what those margins are. As a rule in aeronautical engineering, a safety factor of 1.2 to 1.5 is utilized. That is, a part will be designed to handle 1.2 to 1.5 times the load permitted by the approved flight envelope. Any greater safety factor would increase the structural weight of the aircraft, which would decrease the payload or reduce range.

Just because an aircraft has a safety factor, it doesnt mean it is acceptable to stress the aircraft to the point of the design load factor. The design load factor just means that no permanent deformation will occur on a new airplane stressed to that limit. No tolerance is made for fatigue stresses that will shorten the life of the aircraft and structure.

There are many airworthiness directives on aircraft that limit the maneuvers and lifetime of crucial parts due to fatigue.

The wing struts of Decathlons, the V-tail on Bonanzas, the Piper PA-28 wing spar are all parts that have been shown to suffer from the effects of fatigue as originally designed. For a more graphic reminder, recall the Aloha Airlines Boeing 737 on which the forward portion of the cabin was torn from the aircraft.

Turbulence-Induced Stresses
The pilots operating technique isnt the only variable that puts stress on the airplane. The ever-moving atmosphere can induce stresses on the aircraft that result in deadly outcomes. Rapid changes in the freestream flow in both direction and magnitude will stress the aircraft.

Gusts are separated into vertical, lateral, and longitudinal components.

A longitudinal gust is a change in the speed, but not direction, of the freestream flow. With an instantaneous change in the freestream velocity, the angle of attack does not change, but the increased velocity increases lift. A 10 percent increase in velocity increases lift by 21 percent, and thus increases the load factor by 21 percent

Vertical gusts, like a thermal, can also induce stress. As the aircraft moves from still air into an updraft, the angle of attack changes, sometimes enough to induce a temporary stall. The upward gust produces an added load factor on the aircraft, which most pilots have uncomfortably felt and referred to as an air pocket.

Just how much the load is induced depends on such factors as weight, speed, wing loading, gust strength, air density, and the lift curve slope of the aircraft.

A heavier aircraft is more resistant to disturbances than a light aircraft. This is fairly intuitive. A blitzing linebacker can easily knock a wide receiver whos trying to block him out of the way. That same linebacker up against an offensive guard will have a more difficult task.

Closely related to weight is the wing loading. High wing loading means a greater amount of weight is lifted by each square foot of wing area. With more weight to push (or a smaller surface to act upon), the gust has less effect than on an aircraft with lower wing loading.

The lift curve of the aircraft depends on the airfoil it employs, the aspect ratio of the wing and the wing sweep. The slope of the lift curve is a measure of how much the angle of attack must change in order to change the lift. The thicker airfoils common to older, low performance aircraft typically have higher lift curve slopes than thinner, high performance airfoils. High aspect ratio wings, such as those found on sailplanes, are much more efficient at creating lift, and therefore have greater lift curve slopes than low aspect ratios. Finally, wing sweep decreases the lift curve slope.

The more effective the wing is at converting angle of attack to lift (high lift curve slope), the more susceptible it is to turbulence-induced stress. The long, slender, high-aspect-ratio straight wings common to sailplanes converts turbulence into increased lift more effectively than swept, short, low-aspect-ratio wings of transport aircraft.

Another factor in the creation of additional lift is the airspeed. Imagine a car driving over a speed bump. The bump feels less severe when the car drives over at a slower airspeed than during faster speeds. This also applies to aircraft. Aircraft encountering turbulence at faster airspeeds are subjected to greater stresses than at slower speeds.

Since the loading increases predictably with airspeed, engineers have added this information to the V-n diagram. This information is not readily available to the pilot in its raw form because it wouldnt be terribly useful. Instead, the operational ramifications are used to create the markings on the airspeed indicator.

For standardization, FAR Part 23 defines a strong gust at sea level as 50 fps and a weak gust as 25 fps. At high airspeeds, even a weak gust could overstress the aircraft. If the aircraft is already pulling a couple gs in a turn or chandelle, the combined effects could be a permanent deformation of the worst kind.

The FAA has therefore designated some standardized airspeed indicator markings to help the pilot keep the plane from being overstressed by turbulent weather. These markings are calculated for a 1 g, cruise-flight condition. Since even a weak gust could result in an overstress at higher airspeeds, the pilot should fly at these speeds only the air is smooth.

Marked for Trouble
This caution area is marked on the airspeed indicator in yellow, with the high end of it defined as the maximum structural cruising speed, Vno, or the red-line speed.

The yellow arc should be avoided unless the air is very smooth. When you venture into that realm, realize that gust tolerance is substantially decreased when flying in the yellow arc or beyond Vno.

Even within the green arc, the aircraft structural limits can be exceeded. Any pilot who has had a close encounter with a mountain wave knows how severe the turbulence from rotor clouds can be.

Investigators at the National Center for Atmospheric Research (NCAR) have recorded average gusts within mountain rotors of two to four gs, with an occasional seven-g reading. Updrafts and downdrafts of 2,000 to 3,000 fpm are not uncommon.

Certainly in that kind of environment an aircraft flying faster than maneuvering speed can be overstressed. Experiencing a severe gust at speeds less than maneuvering speed will result in a stall. The pilot must avoid abrupt control inputs if turbulence is already stressing the aircraft that much. If you find yourself in such strong turbulence, position the power to a setting that will reduce speed to maneuvering speed or below. Then hold the pitch at a constant level attitude. Resist the temptation to chase altitude and airspeed.

In many cases, you will be bounced around so much that you cant read the instruments, and at times you wont be able to keep your hands and feet on the flight controls. It will be all you can do to keep the aircraft near a level pitch attitude. At that point the only alternative is to ride it out.

Maneuvering speed is a very important operating number to know. However, it is not marked on the airspeed indicator. Maneuvering speeds are established for certain atmospheric and weight conditions. Since a heavier aircraft is harder to disturb, the maneuvering speed is highest at maximum gross weight and decreases as the aircraft becomes lighter.

Yet its important to know what the maneuvering speed is for the aircraft on any given flight. Flying in rough air at speeds over the actual maneuvering speed for the aircrafts current weight is risking overstressing the aircraft.

A recent reinvestigation of an accident involving a C-130 aerial fire fighting aircraft found that the crew was flying at speeds above maneuvering speed in moderate to strong turbulence. The atmospheric turbulence, combined with the high airspeed, produced g-loads on the aircraft that were beyond its structural limits. The wings collapsed, killing the crew.

In light aircraft, adding or subtracting a passenger can have a marked effect on the gross weight of the aircraft. For example, consider the Grob 103 with a published maneuvering speed of 92 knots at maximum weight. If the aircraft is flown solo with an average weight pilot, maneuvering speed drops to 83 knots.

An inexact rule of thumb is to decrease the airspeed by half of the percentage weight change. So if youre flying at 20 percent below max gross, subtract half of that, or 10 percent, from the published maneuvering speed at max gross weight. To be more exact, see the sidebar below.

The Effects of Father Time
Aircraft structures use their shape in order to carry extraordinary loads for such light materials. They are very sensitive to deformation.

As an eye-opening experiment, ask a friend to carefully stand on an empty soda can and observe that a piece of aluminum that weighs mere ounces can support a 150-pound person. However, take a pen or stick or even a pin and slightly touch the side of the can. It will instantly collapse.

The same dynamics apply to aircraft structures. The shape of the structure is very important and cannot be compromised.

The wing root takes the highest bending moments, so fix a wary eye on that joint during preflight. The top part of the main wing spar is subject to compressive loads during erect flight, whereas the bottom of the wing spar is subject to tension forces, which can result in cracks due to stress.

The horizontal stabilizer normally has the opposite loading condition. Tensile stresses are high on the top surface and compressive stresses are present on the bottom. A close examination of the tail surfaces may yield signs of overstress on the aircraft. Avoiding high speeds and high load factors is one way to reduce the stress on aircraft.

Some pilots feel the urge to go out and pull some gs. A pilot intent on this should not rely on the feel of the stick forces for a sense of the load factor, because some aircraft have balanced control assemblies.

Conventional aircraft such as basic Cessnas and Pipers provide a direct feel through the yoke of the loads on the tail, but a balanced control assembly, such as those found on the Citabria and Decathalon, employs a system of pulleys or counterbalances that reduces the stick forces at high load factors.

An accurate accelerometer (g-meter) is the only reliable way to determine loads on aircraft equipped with balanced control assemblies.

Remember that the effects of stress are cumulative. We have focused on treating an airplane properly but assumed the airplane we started with was structurally sound.

However, the aircraft on the flight line are used, maybe abused and definitely no longer new. As aircraft age, their structural integrity can be weakened by time-based failures such as fatigue or corrosion. Aircraft exposed to such factors may have reduced capabilities.

When the engineer designs the aircraft to withstand certain loads, they approximate the number of times it will be exposed to such loads. However, not all aircraft will be flown according to this idealized use pattern. Some aircraft will be used harder than others. Some are high-time trainers. Some are ancient and have wooden spars.

My wife and I were soaring in a club-owned aircraft that is rather aged. It was a great afternoon of soaring and my wife was learning how to master stick and rudder coordination in a sailplane while in thermals. Suddenly we experienced flutter at slow speed. You really dont want to be in the air in an aircraft whose structure may be compromised.

We immediately landed and tried to determine the source of the flutter. Even after disassembly, we could not find the culprit. Had we tried to fly outside of the aircraft envelope, I really dont think the aircraft would have held together. It may not have stayed in one piece even within the aircraft envelope if we had attempted to speed up.

Dont be paranoid, but remember that its your full right and responsibility to know the history of the aircraft, to be thorough during a preflight inspection and to fly within the envelope.

Also With This Article
Click here to view “Aircraft Renters.”
Click here to view “Gross Work.”

-by Patrick Veillette

Patrick Veillette has flown and instructed in aircraft ranging from J-3s to 727s and aerial firefighters.


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