High-Pressure Gas Cylinder Inspections

July 13, 2006
There are misunderstandings surrounding the inspection requirements for high-pressure gas cylinders used as part of aircraft standard and supplementary systems.

There seem to be misunderstandings surrounding the inspection requirements for high-pressure gas cylinders used as part of aircraft standard and supplementary systems. We would like to discuss where the inspection and maintenance requirements come from and what recent changes the government has made relative to those requirements. Maintenance of high-pressure gas cylinders has been a confusing issue for a long time and rightly so since it is not very clearly addressed either by the manufacturers or the FAA.

In August 2002, the Department of Transportation (DOT) issued new regulations covering the handling of hazardous materials including the use and testing of high-pressure gas cylinders for the purpose of transporting hazardous materials. We will discuss the new regulation in depth, but first a look at how the lesser-known DOT regulations fit into the aircraft inspection and maintenance requirements set forth by the FAA.

We are taught that it is the manufacturer and the aircraft’s type certificate data sheet that drive all of our inspection and maintenance requirements. Let’s briefly review what makes up the inspection and maintenance requirements for an aircraft.


First of all, aircraft inspection and maintenance requirements are designed with one primary purpose, to ensure the aircraft’s continued airworthiness. Although the definition of airworthiness is not specifically contained in the regulations, it can be found on the aircraft airworthiness certificate. “Airworthiness” is defined as “conforms to type design and is in a condition for safe operation.” When we as maintenance technicians make a statement that an aircraft or the maintenance we have performed is airworthy we are saying that the aircraft, or the piece of the aircraft maintained, conforms to its original or properly altered (STC, field approval, etc.) type design.

“Conforming to type design” means conforming to how that aircraft was originally built and configured, i.e. how the engines are installed, the way the structure is put together, how the equipment is installed, the configuration of the interior, etc. The type certificate also defines and/or refers to “Airworthiness Limitations” for that aircraft. These limitations are contained either in the type certificate itself or in another FAA approved document referred to by that type certificate data sheet. The Airworthiness Limitations consist of specific time change requirements for parts that are critical to the safe operation of the aircraft, i.e. engine components, landing gear components, specific bolts, etc., and in some cases, the structural inspections of critical safety of flight components on the aircraft. The list is as different for each aircraft and its engines as the type certificate itself. In addition, aircraft that are altered from their original type design are considered to have received a “major alteration.” Major alterations must be accomplished in accordance with data that has been FAA approved (STC, field approval, etc.). The data used to support a major alteration may also contain its own “Airworthiness Limitations” that must be adhered to.

The “condition for safe operation” portion of the definition of airworthiness means just what it says, the condition of the aircraft. In order to keep the aircraft in top condition we are required to inspect and maintain the aircraft in accordance with the regulations contained in operational rules Part 91, 135, 121, etc., and maintenance rules Part 43. The operational rules (relative to maintenance) tell us where to find the inspection requirements for an aircraft, like what to inspect and maintain and at what frequencies, while the maintenance rules tell us where to find information on how to perform inspections and maintenance on an aircraft as well as record the maintenance.

Inspection and testing requirements

Now, how do we get to the inspection and testing requirements for gas cylinders? Well, the type design of an aircraft may include certain cylinders installed in the aircraft as equipment. For example, oxygen cylinders for crew and passengers, cylinders that contain pressurized gas as a secondary means to extend or “blow down” the retractable landing gear, and cylinders that carry fire extinguishing agents for engine fires and cabin fires. The aircraft manufacturer is responsible for issuing a maintenance manual as well as an inspection and maintenance program for the aircraft. In general, the maintenance manual will include some reference to the Title 49 CFR inspection requirements for these cylinders; however, these cylinders are one case where the maintenance manual does not hold the final authority for what maintenance to perform and when to perform it. The final authority is really traced back to Title 49 CFR, the basis of the aircraft manufacturer’s maintenance manual requirement. More specifically, the new Title 49 CFR Part 180.

Title 49 CFR Part 180

The reason Title 49 CFR Part 180 takes precedence over the maintenance manual is because it is the actual regulation rather than an FAA accepted maintenance manual. An example of a regulatory vs. manufacturer maintenance requirement relationship more familiar to us is the relationship between an aircraft inspection program and an Airworthiness Directive (AD).

A specific example is that of the nose landing gear fork inspection requirements for the Raytheon Super King Air B200. The inspection program for the King Air was once at a frequency interval of 150 hours. A problem was found in specific part number nose gear forks and the FAA issued an AD requiring an inspection each 150 hours until the part was replaced. The inspection is to be performed in accordance with a service bulletin, also calling the inspection out at 150 hours. A few years later Raytheon revised the inspection program and extended the look-phase inspection frequency to 200 hours. Shortly thereafter the service bulletin calling for the nose gear fork inspection was extended to 200 hours as well. The problem is, the AD continues to refer to the original revision of the service bulletin (in this case Revision III) and continues to require the inspection be performed each 150 hours. Now logic might say that the manufacturer has analyzed the situation and determined that a less frequent inspection is adequate to maintain a high level of safety and through issuance of the revised service bulletin has authorized the interval of the inspection to be extended. This however is not the case. Why? Because ADs are regulatory and accepted maintenance manuals and service bulletins are not.

Whenever a new AD is issued it is issued as an amendment to 14 CFR Part 39. So in the case of the King Air nose gear fork inspection, until the AD is revised, the applicable forks must be inspected at 150 hours.

The DOT regulation found in 49 CFR Part 180 governing the inspection, testing, and replacement of cylinders is really the higher authority over the aircraft manufacturer’s maintenance manual. There are, however, a couple of ways that an operator might have a different inspection requirement than that contained in Title 49 CFR for gas cylinders. The manufacturer of the aircraft could place the requirement for inspections testing and replacement of cylinders in an FAA approved Airworthiness Limitation section of a maintenance manual, or in any other FAA approved maintenance requirement such as an air carrier’s operation specification. In those cases, the FAA is very careful to make reference to Title 49 CFR in an effort to keep the requirements the same. If by chance the requirements contained in an FAA approved document such as an Airworthiness Limitations section of a maintenance manual or an Air Carrier Operation Specification are in conflict with those requirements contained in 49 CFR Part 180, the most restrictive requirement must be applied.

Now for the Title 49 change. The DOT issued final ruling on, Aug. 8, 2002 covering the inspection, testing, and replacement of cylinders, however, many are still not aware of the change. The change to Title 49 reorganized and moved the cylinder requirements from the old familiar 173.34 to

180.205 — General requirements for re-qualification of cylinders;

180.209 — Requirements for re-qualification of specification cylinders; and

180.213 — Re-qualification markings.

These new regulations can be found in various places on the Internet. Although there was no substantial change to the requirements contained in the new rule, the new organization makes reading and determining the requirements for cylinders manufactured to DOT specifications much easier.

New rule Following is an outline of the new rule:

For our purposes, cylinder inspection requirements include all cylinders used to transport compressed gas in aircraft.

49 CFR Part 173.1 (a)(2) specifically includes air transportation — “Requirements to be observed in preparing hazardous materials for shipment by air, highway, rail, or water, or any combination thereof…”

49 CFR Part 173.1 (a)(3) refers to the inclusion of inspection and testing requirements — “Inspection, testing, and retesting responsibilities for persons who retest, recondition, maintain, repair, and rebuild containers used or intended for use in the transportation of hazardous materials.”

49 CFR Part 173.301 (a)(1) describes the general requirements for shipment of compressed gases in cylinders and spherical pressure vessels and specifically states that — “Compressed gases must be in metal cylinders and containers built in accordance with the DOT and ICC specifications…”

49 CFR Part 173.301 (a)(2) further requires that “A cylinder may be repaired and requalified only as prescribed in subpart C of Part 180 of this subchapter.”

49 CFR Part 180.205 titled “General requirements for re-qualification of cylinders” defines who can perform the required inspections and tests:

“(b) Persons performing re-qualification functions. No person may represent that a repair or re-qualification of a cylinder has been performed in accordance with the requirements in this subchapter unless that person holds a current approval issued under the procedural requirements prescribed in subpart I of Part 107 of this chapter. No person may mark a cylinder with a RIN and a re-qualification date or otherwise represent that a DOT specification or exemption cylinder has been re-qualified unless all applicable requirements of this subpart have been met. A person who re-qualifies cylinders must maintain the records prescribed in § 180.215 at each location at which it inspects, tests, or marks cylinders.”

49 CFR Part 180.205 also provides a detailed description of visual inspection requirements for cylinders:

“(f) Visual inspection. Except as otherwise provided in this subpart, each time a cylinder is pressure tested, it must be given an internal and external visual inspection…”

49 CFR Part 180.205 also describes the pressure test requirements for cylinders:

“(g) Pressure test. (1) Unless otherwise provided, each cylinder required to be retested under this subpart must be retested by means suitable for measuring the expansion of the cylinder under pressure. Bands and other removable attachments must be loosened or removed before testing so that the cylinder is free to expand in all directions.”

It is under “Cylinder Condemnation” that we find the familiar replacement requirement for 3HT bottles:

“(i) Cylinder condemnation. (1) A cylinder must be condemned when — (v) For a DOT 3HT cylinder — (C) The cylinder bears a manufacture or an original test date older than 24 years or after 4,380 pressurizations, whichever occurs first. If a cylinder is refilled, on average, more than once every other day, an accurate record of the number of rechargings must be maintained by the cylinder owner or the owner’s agent.”

The listing of bottles and their test period or intervals is contained under 49 CFR Part 180.209 titled “Requirements for re-qualification of specification cylinders.” The common test periods we’re familiar with haven’t changed but it is easier to find and understand what needs to be done and how often.

The requirement for hand-held fire extinguishers is found in 49 CFR Part 180.209 “(j) Cylinder used as a fire extinguisher.”

The reorganization of 49 CFR for DOT cylinder re-qualification requirements provides all interested parties with a much simpler rule. All of us will have an easier time finding and understanding the requirements applicable to the aircraft equipment. I suspect that it will be years before all of the aircraft manufacturer maintenance manuals are revised to include the new regulatory reference, but remember, just because the reference is wrong in the maintenance manual doesn’t mean that you are not responsible for the new requirements.

About the Author

Joe Hertzler