Seat Belts and Restraint Systems

Oct. 1, 2002

Reducing major injuries

By Fred Workley

We are generally very diligent about adhering to checklists used for flying and maintaining the airplane. One area that needs constant vigilance is seats and restraint systems.

How many people use the seat belt and shoulder harness every time they get in the airplane? I vividly remember sitting in a second floor classroom at Oakland Airport and watching a Cessna 150 stopped on the ramp, flip over. It ended up with the aircraft upside down with the gear pointing up. The airplane got caught in the jet blast of a DC-8 turning on to the taxiway in front of where the C-150 was stopped. Both pilots in the C-150 had shoulder harnesses on and they were very suppressed but uninjured. You cannot predict when seat belts and restraint systems become a very important safety system.

Studies of "serious" accidents, over time, have shown that the proper use of seat belts and shoulder harnesses reduces major injuries by 88 percent and reduces fatalities by 20 percent. Some aircraft incidents and accidents result in minor or no significant risk to the airplane or its occupants. And since Dec. 12, 1986, all small aircraft manufactured must have shoulder harnesses for all seats.

Proper restraint

FAA regulations require the seat belts and shoulder harnesses (when installed) be properly worn during takeoffs and landings. A slack restraint system can’t protect you in a serious impact. The reason: the body keeps moving forward until the slack is taken up. At some point your body must abruptly stop to "catch up" with the airplane. In other words, the restraint system should be adjusted as tight as possible within reasonable comfort levels. The goal is to minimize potential injuries.

The seat belt needs to be placed low on your hipbones. This permits the belt loads to be taken by the skeleton strength of your body. If the belt is on your thighs it cannot limit your body’s forward motion. Internal injuries can be caused if the seat belt is too high on the abdomen.

Shoulder harnesses

Shoulder harness systems can either be a single diagonal belt or dual shoulder belts. The design of the belts should avoid rubbing the head or neck to preclude neck injuries during an impact. A single diagonal shoulder harness needs to be positioned so that the body’s upper torso center of gravity is within the angle formed by the seat belt and the shoulder harness. The shoulder belt has to be tight enough to prevent the torso from slipping out.

The lower end of the shoulder harness is usually fastened to the safety belt buckle or the buckle insert. Thus the safety belt buckle for a single diagonal strap should be positioned on the side of your hip. This differs from the central location of the buckle that is common when only the safety belt is installed. As the single diagonal shoulder harness is so low on the hip, it has to be unlatched without interference from the interior wall of the airplane, the seat armrest, or aircraft controls.

A dual shoulder harness installation is usually fastened to the safety belt near the center of the torso and the shoulder belts will tend to pull the safety belt up off your hipbones. This may cause internal injuries during an impact. To prevent this, when the safety belt is tightened about the hips the seat belt should be positioned so that it makes an angle of about 55 degrees with the centerline of the airplane. Some dual shoulder harness installations have a tie-down strap from the buckle to the center, forward edge of the seat or floor structure. This tie-down resists the upward pull of the shoulder harness. The tie-down strap needs to be adjustable to remove slack. A properly installed and adjusted tie-down strap should be comfortable.

Only provide safety when used

Unless restraint systems are used they can’t perform their safety function. Accident injuries, in general aviation, have occurred because occupants were not wearing seat belts or shoulder harnesses or the seat belt was loose. If the restraint system is unbuckled there is no time to fasten it in a sudden impact. Seat belts alone will only protect you in minor impacts.

Preflight briefings by pilots should emphasize the importance of wearing the restraints properly all the time. Pilots need to take responsibility that all passengers have their belts snug and that no one has hard or sharp items in their pockets that could cause injury. Passengers also need briefings on how to release the restraint system in case of the need for a quick egress from the airplane. All belts are now required to have a metal-to-metal buckle and can be released by pulling up on the bucket top.

What to look for

During aircraft inspections you need to look closely at the belts. Are they frayed? Is there any broken or torn stitching? Are the Technical Standard Order tags still on the belts? A TSO covers all belts. Will the buckles release easily? Is the belt webbing clean and undamaged? Does the restraint system pass a simple functional check? Restraint attach points need to be inspected for cracks and, where a nylon bushing is used where the shoulder harness attaches, check to see that it is not cracked, broken, or missing.

Floor-mounted seats and seats on roller tracks need visual inspections that include the aircraft interior and restraint systems. Roller assemblies wear out and jam. The nuts on the roller bolts need cotter pins. Are there any gouges in the tracks? Are the seat tracks and rollers clean with all the lock pinholes clean. Excessive lubrication sometimes collects dirt and lint. The linkage from levers to the locking pins becomes worn and the spring action of the actuating arm gets weak.

Mechanical seat stops need to be installed with the correct hardware, both forward and aft, so that they wouldn’t break out if the seat lock doesn’t hold during sudden acceleration and deceleration because of sudden impact. During the flight profile the initial climb is where a lot of seats become unlocked. Are the lock pinholes in floor tracks elongated or cracked and the tracks worn in spots? Is the floor around the seat tracks cracked or broken? Is there any corrosion evident on the tracks or the floor structure?

Is the seat itself structurally sound? Look closely at the welds for cracks. If cracks are suspected do a dye-penetrant inspection. If installed, make sure that the locks to keep the seat back from breaking forward in an impact are not cracked and are properly adjusted. Look at the aircraft manufacturer’s instructions for adjustable seat assemblies. For example, Cessna’s Secondary Seat Stop Mandatory Service Bulletin SE89-32. Seat installation inspection criteria can be found in service and maintenance documents as well as Airworthiness Directives and Service Bulletins.

Profit potential

Don’t pass up an opportunity to install shoulder harnesses. Kits are readily available. It is a profit maker with a true safety benefit. Also, there are STCs and modifications that remove the seat belt anchor points from the seat to the floor. In an aircraft where there is no structural floor like the Piper J-3 Cub, the seat belt attach points for the pilot can be moved on to the tubular aircraft frame. F. Atlee Dodge of Anchorage, Alaska, has a modification, Drawing 3246, that is used to weld tabs to the fuselage tubs for seat belt anchors. Anchor points for above the head shoulder harnesses can be installed in most aircraft.

If anyone is carrying children in the aircraft, longer seat belts or seat belt extensions may have to be installed to accommodate child safety seats. Usually the child seat is in a rear airplane seat but not near an entry door or emergency exit. If a child seat is in a front seat it must be assured that the seat cannot interfere with the airplane controls or limit pilot access in flight. Safety seats for small infants are often installed in a rear-facing position and should be installed in the airplane in that direction. Sometimes the angle of the seat bottom will not let the child seat sit level. The bottom line according to FAR 91.107 is that small children must be secured in approved child restraint systems. The FAR says "Use of safety belts, shoulder harnesses, and child restraint systems" are required. Read AC 91-62A, "Use of Child Seats In Aircraft" for additional information. Let’s stay safe! Keep ’em Flying!

Fred Workley is the president of Workley Aircraft and Maintenance Inc. in Alexandria, VA, Benton City, WA, and Indianapolis, IN. He holds an A&P certificate with an Inspection Authorization, general radio telephone license, a technician plus license, ATP, FE, CFI-I, and advance and instrument ground instructor licenses.