Respecting the Laws of Gravity

By Frederick E. Tilton, M.D. FAA Federal Air Surgeon
Reprinted with permission from FAA Safety Breifing

The Next Generation Air Transportation System (NextGen) may be changing the face of aviation technology, but the human body’s reaction to the ever-present, accelerative force of gravity is a constant. Described in units of “G,” this force causes a constant acceleration of 32 feet-per-second squared. A pilot in a steep turn may experience forces of acceleration equivalent to many times the force of gravity. This is especially true in military fighter jets and high-performance aerobatic aircraft where the acceleration forces may be as high as nine Gs. Air race pilots in a tight pylon turn also experience high G-forces, but the important thing to remember is that any aircraft operated in a maximum-performance profile will subject the pilot to acceleration that is greater than the one G acceleration encountered on the ground. Pilots need to understand this to successfully master flying.

Types of Acceleration

There are three types of acceleration:
Linear Acceleration involves a change of speed in a straight line. This type occurs during takeoff, landing, or in level flight when a throttle setting is changed.
Radial Acceleration involves a change in direction, such as a sharp turn.
Angular Acceleration involves a simultaneous change in both speed and direction, such as in spins and climbing turns.

A pilot may experience a combination of these accelerations, categorized as Gx, Gy, and Gz. Gx acts from chest to back. Positive Gx pushes the pilot back as the aircraft accelerates during the takeoff roll. Negative Gx can occur during landing and pushes the pilot forward into the shoulder strap. Gy acts from shoulder to shoulder. It is encountered during aileron rolls. Aerobatic pilots routinely encounter Gy.

Gz acts on the body’s vertical axis. If experienced from head to foot, as in pulling out of a dive, it is positive Gz. Negative Gz travels from foot to head, as when a pilot pushes over into a dive.

Respecting G-Force

Aviators need to understand and respect G-force, because any flight maneuver has the potential to expose the body to more than one positive Gz. When the pilot experiences positive Gz, the cardiovascular system must respond to keep blood flowing to the brain. One of the first indications of trouble may be a progressive loss of vision, because the eyes are extremely sensitive to low blood flow. If the rapid onset of G-force continues and the cardiovascular system does not keep pace, the result may be G-induced loss of consciousness (GLOC). NOTE: In some acrobatic airplanes it may be possible to experience GLOC without experiencing any early visual symptoms.

G tolerance is degraded by alcohol, fatigue, and dehydration. Lack of physical conditioning, a sedentary lifestyle, and smoking can also reduce G tolerance. A well-rested, well-hydrated, and fit aviator will be able to withstand higher G-forces. A regular conditioning program with a mix of aerobic exercise and resistance weight training will increase resistance to the effects of Gs. Regardless; a smart aviator will always include consideration of G-forces when it comes to flight planning.

Dr. Tilton received both an M.S. and a M.D. degree from the University of New Mexico and an M.P.H. from the University of Texas. During a 26-year career with the U.S. Air Force, Dr. Tilton logged more than 4,000 hours as a command pilot and senior flight surgeon flying a variety of aircraft. He currently flies the Cessna Citation 560 XL.