nuts & bolts

maintenance & restoration

Seat Belts and Harnesses Lifesaving equipment right in your lap JEFF SIMON, EA A 478233

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y first car was a 1966 Ford Galaxie 500 convertible. While it was a lot of fun to drive, I can’t say that it was the safest car for a teenager to be driving around in. It had lap belts, but no shoulder belts to protect the front seat occupants from a close encounter with the dash, which was about as hard as the fenders. Many years later, as I climbed into a cockpit for my first flight lesson, I recall reaching for a nonexistent shoulder harness and being immediately struck with the memory of that car. I was surprised to find that, as with other technology advances, light aircraft lagged behind automobiles. By 1971, shoulder harnesses were required equipment on all automobiles in the United States. Aircraft, on the other hand, were not required to have shoulder harnesses on all seats until 1986. The lack of shoulder harnesses on an aircraft is significant because studies have shown that proper use of shoulder harnesses would reduce major injuries in aircraft accidents by 88 percent and reduce fatalities by 20 percent. The lack of shoulder harnesses on some aircraft is only part of the problem with aircraft restraint systems in the general aviation (GA) fleet. Other safety issues include worn or damaged harnesses, faulty latches, improper harnesses and improper harness use.

Adding Shoulder Harnesses If you are flying a certificated aircraft, the FAA has gone to great lengths to make it easy to retrofit a shoulder harness for your aircraft. According to the FAA’s policy statement on the topic: Installation of shoulder harnesses in aircraft is a MINOR change if: 1. The aircraft was manufactured before 19 July 1978 for front seats and 12 December 1986 for rear seats. 94

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2. TSO-C114 belts are used. 3. No drilling or welding has been performed. 4. The mechanic doing the install consults AC43.13-2A, Chapter 9, for information on restraint systems, effective restraint angles, attachment methods, and other details of installation. 5. The installing mechanic makes an entry in the maintenance log of the aircraft.

Aircraft, on the other hand, were not required to have shoulder harnesses on all seats until 1986. Classifying the addition of shoulder harnesses as a minor alteration is a significant statement. It means that no FAA supplemental type certificate (STC) or field approval is required to make the modification. However, unless shoulder harnesses were optional equipment on an aircraft, it may not be possible to install them without modifying the aircraft structure, thereby requiring an STC or field approval for the modification. Fortunately, there are a number of companies that sell STC’d shoulder harness kits for a variety of older aircraft. Many of these kits are for the installation of dual shoulder harnesses, which provide even more safety in the event of a crash than a single shoulder harness can. Companies such as B.A.S. Inc. (www.Basinc-Aeromod. com), Alpha Aviation (www.Alpha-Aviation.com), and others sell shoulder harness STCs for a variety of Cessna, Piper, and Beech aircraft. Most kits include harnesses for both front seats and cost from about $600 for single shoulder kits to $1,000 for double shoulder harnesses, including airframe installation hardware.

Maintaining Existing Harnesses

Harness Options and Fitting

It’s easy to overlook seat belts and harnesses during routine aircraft maintenance. However, they play a crucial role in aircraft safety and are designed to withstand tremendous loads in an accident. Any damage or wear can compromise these capabilities significantly. When evaluating a harness, begin at the buckle or latch mechanism. It should operate smoothly and not show significant signs of wear. When the belt is under tension, the buckle should still be able to release without significant force. If the mechanism is loose or does not operate smoothly, it should be replaced. The harness webbing is the next thing to inspect. Over time, the webbing can fray or deteriorate due to mechanical wear, UV exposure, or chemical damage. Fraying is easy to inspect for, but other types of damage can be more difficult to identify. Look for a fuzzy layer on the surface of the fabric or a warp in the weave. If one of these issues is present, the harness material needs to be repaired or replaced. Fortunately, re-webbing seat belts and harnesses is a straightforward and relatively inexpensive process. There are a number of excellent repair facilities that can replace the latches, re-web the belt, and even switch the belt out for a new color that will match your aircraft’s current interior. Finally, inspect the mounting points themselves. The mounting bolts and spacers should be secure and not show signs of wear.

Seat belts and harnesses are typically categorized by the number of straps, or attachment “points,” in the harness. For example, a two-point system is a simple lap belt system. This design is less than ideal because, while it prevents the occupant from being thrown from the vehicle, it creates a pitching moment for the entire, unsupported upper torso that can result in significant head trauma.

However, the “ultimate” solution is the five-point harness system. A better approach is the common three-point restraint system, which adds a single diagonal shoulder strap to the lap belt. Most aircraft use this design, but have a fixed shoulder strap that is separate from the lap belt and attaches to the male seat belt buckle with a locking clip. This two-piece design is necessary mainly because most aircraft do not have inertial systems. Inertial-reel shoulder harnesses are a significant advantage because they automatically adjust to fit the occupant and remove the slack from the restraint system. Slack is one of the most common reasons that injuries occur, even when occupants are wearing seat belts. During an impact, the occupant’s body continues to move forward until the slack is

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maintenance & restoration Designing Harness Systems in Experimental Aircraft If you are designing a harness system for your own experimental aircraft, you can mount the harness to either the seats or the aircraft structure. Either way, there are two critical factors to consider: 1.) Mounting Point Strength Since we know that the human body can withstand g loads up to 20g’s, the mounting points for the harness system should be able to withstand more than that 20g force. Whether you’re dealing with a steel, aluminum, or composite structure, it’s critical that the mounting points be securely attached to the aircraft structure and designed to spread the load at the time of impact.

This Mooney aircraft includes an aviation inflatable restraint system from AmSafe using the same technology as automotive air bags, customized for aircraft use. The inflatable portion is contained within the belt itself, and the electronics are contained within a small control box.

taken out of the restraints. However, seat belts do not stretch, and once the slack is taken out, the body will stop abruptly. This sudden stoppage can cause serious internal injury. The human body can withstand decelerations of 20g’s without injury, and aircraft harness systems are designed to handle this load. This means that most aircraft accidents can be survivable assuming that the harness is fitted properly and the occupant is securely restrained. Both the location of belts and the lack of slack are critical to safety. The goal is to locate belts over the strongest skeletal parts of the body. Lap belts that are positioned too high will cause internal injuries to the abdomen. If they are positioned too low, they will not restrict forward movement enough to adequately protect the occupant. While three-point restraints are adequate, four- and especially five-point restraints are especially suited to aircraft use. This is due to the three-dimensional forces common in aircraft accidents. In automobiles, frontal impacts are much more common than rollovers. In aircraft, nose-down impacts are typical. Therefore, it’s essential that the pilot and passengers are protected in all directions. Four-point restraint systems add a second shoulder belt, ensuring that the occupant cannot twist past a single diagonal restraint. However, the “ultimate” solution is the fivepoint harness system. The five-point system has a crotch strap that the four-point system doesn’t have. This strap is critical because it stabilizes and “locks in” the rest of the straps to the seat bottom. While this is less common, and not the most comfortable in your average GA traveling machine, it’s a must for aerobatics. 96

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2.) Location of Mounting Points Without proper locating of the mounting points, you will never be able to properly adjust the harness. The location of the mounting point should allow the lap belt to sit directly across the upper pelvis of the occupant, regardless of the seat position. A mounting point too far back will cause the belt to locate too high; too far forward and the belt will sit too low. The mounting points for the shoulder harnesses may be on a secondary aircraft structure. However, the mounting location should be able to withstand a 500-pound test load. The mounting points should be within an angle of 30 degrees above the horizontal from the shoulder of the seat occupant. It is extremely important that the shoulder straps’ mounting points be outside of the shoulders of the occupant. If they fall too close to the occupant’s neck, the harness could asphyxiate the occupant during an accident.

Air-Bag Systems One of the most significant new products with respect to aircraft crash survivability is the aviation inflatable restraint system (AAIR) from AmSafe. This system incorporates an air bag that is built into the seat belt itself, with a separate inertial sensor and deployment system. AmSafe has received supplemental type certificates for its AAIR harnesses for a variety of general aviation aircraft. They are even standard equipment for many new aircraft from Cessna, Hawker Beechcraft, Cirrus Design, Diamond, Mooney, and more. As pilots, we work hard to avoid accidents at all cost. However, for all the effort that we put into training to avoid accidents, it pays to put some effort into maintaining and improving on the restraint systems that may save our lives, and those of our loved ones, in the event that an accident does happen.

Jeff Simon is the president of Approach Aviation, a provider of educational products, tools, and supplies for aircraft owners. To learn more about aircraft ownership and maintenance, visit www.ApproachAviation.com or call 877-564-4457.