Hang Time - Seat Belt Testing

January 2003

NBA legend Michael Jordan and the occupants of this Piper Pacer have something in common. They have all controlled their Hang Time and they have all been Playing for Keeps. After hitting a rut while landing on an unimproved Idaho landing strip, the pilot and passenger found themselves secure in their harnesses hanging upside-down and supporting the bench seat, which had torn loose. When they were ready, they released the belts and tumbled to the ceiling. There were no injuries and because both walked away, it must have been a good landing. Six months prior to this accident, the owner had me replace and update the old lap belts with a four-point system certified under new Technical Standards Orders (TSO). That’s what I call in-the-field-testing!

In the July, 2002 MAPA magazine, Shop Talk discussed the restraints in your Mooney and the importance of having the latest TSO seat belts and shoulder harness system. This follow-up article will explain how aircraft seat belts and restraint systems are tested to guarantee that they meet the TSO standards. We will also review the FAR appropriate for installing updated restraint systems.

TSO C22g (1993) for pelvic restraint systems and C114 (1987) for torso restraints specify minimum performance standards by referring to National Aircraft Standards (NAS) 802 and Society of Automotive Engineers, Inc. (SAE), Aerospace Standard (AS) 8043 documents. For those interested, all TSOs may be downloaded from http://av-info.faa.gov/tso/. Other approved documents can be found with most seat belt manufactures in their component maintenance manuals. This article will focus on the testing procedures required by NAS 802 and SAE AS 8043. Note that the testing requirements change based on when the restraint system was approved, 1987 being the pivotal year. At the present time, C114 refers only to systems with shoulder restraints. C22g references lap-belt-only systems (such as airline seats).

My shop has assembled a variety of restraint systems complying with C114. After upgrading four aircraft, three original restraint systems and one set of newer lap belts were submitted for testing. You can find the restraint descriptions and test results towards the end of this article, but first here is how the testing is done.

The testing occurs in several stages. Each restraint section is tested individually at a specified load. The three to five parts of the restraint system are assembled and tested as a complete unit and various release scenarios are run. The restraint system is attached to a jig that simulates an aircraft seat with a human body in it. A hydraulic or similar test rig is used and the various restraint configurations are tested as shown in this reprint of SAE AS 8043.

The testing jig is forced against the restraints until the restraints reach the specified rated loop load (see table below). If nothing fails, the loop load is reduced to 170 lbs. and the amount of pull required to open the releases on all the restraints is measured. Imagine the pilot and seat flying forward and upward toward the windshield; the test jig is designed to simulate these conditions. The testing of the release mechanisms at a tension at 170 lbs. is to verify that the occupant can get out of the restraints while hanging upside down in the cockpit (refer to the “Hang Time” photo). This is test 3 mentioned in the next paragraph.

Here are a few of the tests and measurements:

1. Ultimate breaking strength (not a TSO C114 requirement)

2. Adjuster hardware slippage at all loads up to rated strength.

3. Buckle release force is measured after the restraint system is tensioned to the maximum rating and then reduced to 170 pounds.

4. The shoulder harness adjustment must not slip or release until the release mechanism reaches 30 degrees.

5. Webbing stretch (both elastic and permanent) at the maximum tensions.

6. Webbing burn testing.

7. Hardware inspection and testing for proper engagement and operation.

8. Fraying of the belt webbing.

These are just some of the tests required to meet the TSO for aircraft seat belts. This article discusses items 1-5. Items 7 and 8 are also required during annual aircraft inspections.

The table, below, delineates the forces each component or system must handle to be in compliance with the respective TSO.

All values are in pounds	C22g	C114
Individual lap sections – min tension	1500	3000
Assembled lap belt – min tension	3000	5000
Individual shoulder section – min tension	NA	2500
Lap buckle release* – max force	45	45
Shoulder buckle release* – max force	NA	11
*Test 3 as explained above.

Here are the restraint samples sent in for testing with the results:

Sample #1: Davis lap belts, manufactured 1997, TT 300hrs. Removed from Piper Pacer “Hang Time”.

Results: These lap belts held through all the tests for a 1500 lb. belt. Because they did not incorporate any type of upper torso restraint, they would have been useless in keeping the front seat occupants from impacting the controls and instrument panel in the crash.

Sample #2: Cessna 3-point restraint system, manufactured 1978, TT 6400hrs. removed from 1978 Cessna C152.

Results: Both upper torso belts failed the slippage test on the adjusters before the total load was applied to each belt. The lap belt portion held well through all the tests for a 1500 lb. belt. Because the upper restraints failed, they would not have prevented head and chest injury during a crash.

Sample #3: Davis 3-point restraint system, manufactured 1986, TT 1700 hrs. removed from a 1986 Mooney M20K.

Results: Both lap belts failed before they reached their 3,000 lbs. rating. Failure was the webbing at the buckle. Both shoulder harnesses tested OK.

Sample #4: Indiana Mills 3-point restraint system, manufactured 1983, TT 1800 hrs. removed from a 1983 Mooney M20K.

Results: The lap belts held well beyond their 3,000 lbs. rated strength. The shoulder harness barely made the 1500 lbs. before it failed at the connector to the lap belt. The buckle release tension was 38 lbs. This is below of maximum of 45 lbs. but is much higher than any other belts we had tested; the other belts tested from 20 to 27 lbs.

A discussion of applicable FARs is appropriate when upgrading an aircraft’s restraint system. An aircraft owner or co-owner who is a pilot may replace the restraint system, making no modifications to the aircraft. However, if the new restraints have a different TSO from the old ones, a licensed airframe mechanic must make a logbook entry regarding the installation. In many cases, an IA mechanic will have to submit a Form 337 for FAA Field Approval. If there is an STC for the system (rare), the Field Approval is not needed. Much of this is delineated in FAR 43, Appendix A, (c) Preventative Maintenance.

Some of the features of new restraint systems are not strictly dictated by regulation. Polyester replaces nylon as the newer webbing material, providing lighter weight, better abrasion resistance and higher breaking strength. The buckles are stronger, lighter and easier to operate. The angle of the buckle release on the belts I provide has been increased to the maximum of 90 degrees. Many pilots have complained of the seat belt inadvertently unbuckling while moving about during flight, specifically while operating the manual gear handle or trim wheel. The high buckle release angle prevents this from happening.

In the long chain of safety items on an aircraft, the restraint system is the ultimate one. Ironically, it is among the least expensive. The kit for a direct replacement restraint system (no airframe modification) typically costs about $150 per seat. Any mechanic’s time, if necessary, would be extra.

If your restraint system is over ten years old, take an objective assessment of its condition. Consider any complaints you have about its reliability or operation. You can easily do this while you are just “hanging around.”

As always, if you have a question about this article, you may contact me at my aircraft repair shop, 307-789-6866 or via e-mail. Until the next ShopTalk, enjoy flying your Mooney.