L’Empennage

A Guided Tour of Your Plane’s Tail Feathers

By Tom Hoffmann

Source: FAA Safety Briefing, Jan/Feb 2020

I consider myself to be a decent dart thrower. It helped being able to live in London for a year where the game is known to have its medieval roots. As my interest in the sport grew, so did my curiosity about the types of darts I used, as well as understanding how all the different parts and pieces worked. I was intrigued when learning how critical “flights” are to the stability of a dart’s trajectory, and how design variations can cater to an individual’s throwing style. Flights are the tail part of the dart, typically made of plastic, paper, or even feathers. If you’ve ever tried tossing a flightless dart, you’ll see exactly how critical they are.

The stability-inducing properties of flights work similarly for arrows and, by extension, aircraft. In fact, the word empennage, which describes the tail assembly of an aircraft, derives from the French term empenner, to feather an arrow. The empennage is considered one of the five major components of an aircraft. It includes the entire tail assembly, comprising both fixed surfaces (vertical and horizontal stabilizers) and moveable control surfaces (rudder, elevator, and trim tabs). These components work together to provide stability and control around an aircraft’s yaw and pitch axes.

Doing It the Wright Way

Look no further than the Wright Brothers’ body of work for evidence of how important yaw and pitch control was in finalizing a successful Wright Flyer design. In their early glider concepts, the Wrights struggled mightily with adverse yaw. When they raised a wing after a turn, the aircraft sometimes slipped in the opposite direction and spun into the ground — a result notoriously dubbed “well-digging.”

It took some tweaking and scores of test flights, but they soon realized the key to controlled flight was adjustable rudder control. After a significant “aha” moment, they cleverly combined their wing-warping mechanism for aileron control to include the rudder, introducing a moveable tail that provided additional stability. While their initial design schemes called for the elevator to be forward of the pilot and separate from the rudder, experiments with their later Model AB design proved that the elevator controls worked best when located at the back of the aircraft. Shortly thereafter, the Wrights ushered in the Model B, or the “Headless Wright,” the first of their aircraft to use the conventional style empennage we predominantly see in today’s aircraft.

Birds of a Different (Tail) Feather

Just as in dart flight variations, there are variations of aircraft tail assemblies, each with its own unique aeronautical benefits. Although it has evolved slightly from the Model B, the most common arrangement is the tried-and-true conventional tail design, which consists of a single vertical stabilizer that intersects with a two-part horizontal stabilizer at the end of the fuselage. You’ll see this century-old configuration on everything from a Boeing 787 to a Cessna 152.

The T-tail design is a popular variation of the conventional style, but with the horizontal stabilizer positioned on top of the vertical fin in the shape of the letter “T.” This configuration increases the efficiency of the horizontal stabilizer since it remains outside the effects of prop wash and wing wake. However, due to their placement, T-tails impose a greater bending and twisting load on the vertical stabilizer, requiring a stronger and often heavier structure. The configuration also requires a more complex elevator cable run and makes preflight inspection of the elevator more difficult. Be sure to consult your Airplane Flight Manual for any specific preflight inspection procedures, as well as any inflight loading limitations.

Some common GA examples of the T-tail design include the Piper Tomahawk, the Beechcraft Skipper and gliders like the Grob 103. This empennage style more safely accommodates parachute and cargo operations for aircraft like the Boeing C-17. Although commonly reserved for aft-podded military or business jet aircraft, a further variation of the T-tail is the cruciform tail in which the horizontal stabilizer is moved part way up the vertical stabilizer. A good GA example of the cruciform tail design is the Rockwell Commander 112/114 series.

That brings us further down the alphabet to the V-tail, the claim-to-fame design for the Beechcraft Bonanza V35. The intent of this design was to consolidate the control surface functions of the rudder and elevator with two slanted tail surfaces to reduce weight and drag. These “ruddervators” would deflect in the same direction when used to control pitch, and in opposite directions when used to control yaw. One drawback is that V-tail pilots have to contend with an undesirable rolling motion (the “Bonanza Boogie”) that occurs away from a turn and which must be countered with aileron input. This adverse coupling is one reason why the manufacturer switched to a conventional tail in later Bonanza models.

Another empennage style worth mentioning is the dual or twin-tail design, which consists of two small vertical stabilizers on either side of the horizontal stabilizer. A prime example is the ERCO Ercoupe, which has its share of faithful followers. One of those Ercoupe enthusiasts happens to be author, race pilot, and frequent contributor to this magazine, William Dubois, who is quick to point out its unique safety features and handling prowess. “The rudders are inter-connected to the yoke and the nose wheel on most ‘Coupes, so there are no rudder pedals in the cockpit, and no ball to step on,” says Dubois. This design keeps the plane in coordinated flight at all times. Dubois adds that if the plane is rigged properly, it can’t spin, since you can’t enter a stall uncoordinated. However, this configuration also pre-vents you from being able to slip the aircraft and requires you to crab to the runway during a crosswind landing.

It’s Under Control

Within each of the tail assemblies discussed here are the moveable control surfaces — the rudder and elevator — which allow the pilot to control yaw and pitch respectively. Some empennage designs incorporate the elevator function into a one-piece horizontal stabilizer, known as a stabilator, which pivots from a central hinge point. Although stabilators are more commonly found on military jets (their design aids with efficiency of pitch control at supersonic speeds), airplanes like the Piper Cherokee and the Cessna 177 do sport this design.

A key benefit of a stabilator is that it generates a large pitching moment with minimal control force effort, but that can also make the elevator a bit touchy and easy to over control. This design includes an anti-servo tab, which deflects further and in the same direction as the stabilator to increase the control force feel for the pilot. Anti-servo tabs can also be found on aircraft with more traditional elevator designs, like the sprightly AA-1 Yankee, to provide additional stability.

Staying Fit and Trim

Since we’re on the subject of trim controls, let’s run through the other types of tabs you might come across. A balance tab works in the opposite manner of the anti-servo tab, in that it moves in the opposite direction of the control surface changing its relative position. This allows the pilot to reduce or eliminate control pressure requirements during steady state flight conditions. More common is the single trim tab system attached to the trailing edge of the elevator or rudder. These small mechanically or electronically adjustable airfoils are used to eliminate an unbalanced pitch or roll condition during flight without having to move the primary flight controls. Some multi-engine airplanes use rudder trim tabs to aid with yaw control during single engine operations.

Yet another type of tab you’ll often see on rudders are the ground adjustable tabs. As their name implies, these stationary metal tabs are designed to be bent on the ground in order to correct for left or right skidding tendencies in cruise flight.

On Closer Inspection

It’s important to always be familiar with what type of control and trim surfaces are installed on your aircraft so they can be properly inspected during preflight. Are all surfaces properly installed and secured, including any hardware, fasteners, hinges, and counterweights? Have you checked for normal directional movement, including trim controls? Was there any maintenance performed recently? Discovering an improperly rigged flight control in flight can get ugly fast.

When preflighting your empennage, also check the condition of any antennae, lights, cargo doors, and if pos-sible, your Emergency Locator Transmitter (ELT). Keep an eye out for loose or missing rivets, wrinkled or damaged skin, and any bouts of hangar rash, to which the tail is particularly prone. Oh yeah, you might want to remove that tail tie down too!

While differences in empennage styles abound, its core function across all aircraft remains vitally important. Hopefully this information has introduced and/or reinforced some helpful pointers on better understanding your bird’s tail feathers. Keeping this important part of your aircraft regularly groomed and maintained will help you stay safe and get the most out of your aircraft.

Learn More

Pilot’s Handbook of Aeronautical Knowledge, Chap. 6, Flight Controls - go.usa.gov/xpKBR

Tom Hoffmann is the managing editor of FAA Safety Briefing. He is a commercial pilot and holds an A&P certificate.