Click here to upgrade to a newer version of Internet Explorer or Microsoft Edge.
The Importance of the Shoulder Harness
By Tony Bingelis (originally published in EAA Experimenter, March 1998)
At an altitude of 200 feet, soon after take-off, the engine lost power. The biplane was observed gliding in a shallow left turn until it disappeared from view behind a knoll. The aircraft impacted in a soybean field, in level flight attitude hard enough to fail both main gears, and slid to a stop in approximately 45 to 50 feet.
The seatbelt held, but with no shoulder harness to restrain his upper torso, the pilot’s head struck the top of the instrument panel, displacing it forward.
An extruded aluminum section used to support the top of the instrument panel and secure the coaming, pierced the pilot’s head, causing a fatal injury, (see Figure 1). If this extruded aluminum had not been in this location, the pilot would probably have ended up with nothing worse than a headache.
Immediate Action is Recommended
There was no reason for the spear-like extrusion or angle reinforcement to be positioned as shown in the drawing. If such a reinforcement is installed in aircraft, it should be removed or at least modified so that it does not extend beyond the fuel tank. This kind of reinforcement is more likely to be found in biplanes as the presence of center section brace wires, in some designs, makes the installation of the instrument panel hood in one piece very difficult, if not impossible. Nevertheless, I would suggest that perhaps no reinforcement at all is needed here. That metal hood or coaming merely has a fairing and streamlining function in most cases.
By using nut plates, even a two-piece coaming could be installed with no need for reinforcement strip.
Equally dangerous are protruding knobs, switches and ignition keys in the central portion of the instrument panels of single seaters. In the case of two-seater aircraft, you should avoid the placement of such protrusions in the areas directly in front of either the pilot or the passenger.
Remember the Pledge
The critical need for the installation and use of pilot and passenger restraint systems in sport aircraft has been recognized and supported by EAA for more than 40 years. EAA membership applications long contained the Shoulder Harness Pledge — "I hereby promise to install and wear shoulder harness and safety belts in my private-built aircraft to protect myself, passenger and the good name of the association. Air Force and Navy tests have proven that a 20-G harness will eliminate 90 percent of aircraft accident injuries."
The necessity for pilot and passenger restraints is well documented with numerous examples of serious or fatal injuries sustained in potentially survivable accidents. Potentially survivable accidents are those in which the surrounding cockpit or cabin structure remains intact without serious deformations, and where the level of impact forces is below expected human tolerances.
Time and again it is proven that one of the prime factors contributing to head injuries is the widespread reliance on only a seatbelt to restrain the body. A seatbelt alone cannot provide the needed protection.
The installation and proper use of a shoulder harness will, however, help eliminate the jackknifing action of the body in the event of sudden impact forces (see Figure 2).
Selecting Attachment Points
Each builder should make a critical analysis of his aircraft to establish the best location for the seatbelt and the shoulder harness attachment points. If the belt is to be attached to the seat, the structural integrity should carry through to the aircraft structure.
If the aircraft is being built from plans, the seatbelt attachments and shoulder harness attachments will probably have been worked out and detailed on the plans. Unfortunately, a few of the aircraft designs have what I consider to be inferior shoulder harness installations.
This basic fault is as common to all-wood aircraft as it is to the tube and rag types. The particular deficiency I have in mind is the use of the unreinforced seatback or fuselage cross members as an attachment or loop-over point for the shoulder harness. This method does provide some protection, but not much. Such structural members will fail long before the belts themselves fail. The completed installation might give you a nice moral feeling but very little protection. Aircraft with such installations should perhaps have a section of 1/8-inch flexible control cable running aft to some solid point on the aircraft as an additional restraining point. This would serve to increase the strength of the restraint system considerably.
The builder should also be careful not to weaken the basic structure of his aircraft by drilling holes through unreinforced portions of his longerons just to find a good place for the belt and harness attachments.
Not only must the points of attachment be strong enough to develop the full potential of the restraining belts, but the attachment ends of the belts must present and maintain certain relative positions or angles as they pass over the lap and shoulders.
The shoulder harness must also be positioned in such a way as to permit limited freedom to accommodate normal body movements of the pilot and passenger without head-neck contact or interference with vision.
Typical good and poor restraint configurations are illustrated in Figures 3 and 4.
The installation of shoulder harnesses is really uncomplicated and at worst would require but little change or modification of the structure.
Undoubtedly the four-place aircraft causes the greatest difficulty when it comes to finding a suitable attachment location for the shoulder harness belts. Part of this is due to the likelihood that the front seat harness might unduly restrict the vision of the rear seat passengers. This sort of inconvenience can be minimized, however, by making use of suitable attachment points located in the upper-side areas of the fuselage structure.
A Good Installation Can Still Be Bad
All right, so your seat belt and shoulder harness installation is a pretty good one and should be effective. Well, if you are over that hurdle, this might also be a good time to think about the physical condition of your belts and harnesses. Were they new when you installed them? What sort of condition are they in now? The sun is pretty rough on most material, especially fabrics. Take a good look, for after all, when sudden stoppage of an aircraft occurs, the difference between little or no injury and disaster often hinges on the condition of the belts and their attachment to the aircraft structure. Replacement of your seatbelts and harnesses is recommended when:
1. Frayed belt edges have become apparent.
2. There are indications that the stitching is deteriorating.
3. The buckle serrations have become worn to the point where slippage is possible.
4. The belt shows other signs of deterioration.
Think About It …
It is natural enough for many builders to overrate the protective benefits of the safety belt and as a result underrate the value of the shoulder harness and the need for building crashworthiness into the cockpit and cabin installations. I’d say these gents are dead wrong, and no pun was intended. I still recall early military training reports which the Air Force used to convince us of the value of these safety precautions. Later, studies by the Cornell University Medical College in New York, the FAA and others, all strengthen the case for adequate body restraints and improved cockpit design to reduce bodily injury in survivable accidents.
All this is not new. In a report during the 1950s, the Cornell Medical College Study Group found that almost 80 percent of the survivors of light aircraft accidents, in which seatbelts were the only restraints used, sustained head injuries.
Any time you have a situation where the head is injured three times as often as parts of the body which are adjacent to the safety belt, it is mighty hard not to believe that some sort of restraints should be provided for the upper torso. The simplest means for this sort of protection is a shoulder harness. Not just any shoulder harness but one that is sufficiently strong, properly anchored and used. Take another look at Figure 1. It also makes a good case for the use of crash helmets, doesn’t it?