Interpretation Response #25-0035
Below is the interpretation response detail and a list of regulations sections applicable to this response.
Interpretation Response Details
Response Publish Date:
Company Name: Fleener Consulting LLC
Individual Name: Art Fleener
Location State: IA Country: US
View the Interpretation Document
Response text:
January 14, 2026
Art Fleener
Fleener Consulting LLC
3741 Mathews Road
Ames, Iowa 50014
Reference No. 25-0035
Dear Mr. Fleener:
This letter is in response to your March 27, 2025 email requesting clarification of the Hazardous Materials Regulations (HMR; 49 CFR Parts 171-180) applicable to cargo tanks. Specifically, your inquiry is related to the required frequency of the external and internal visual inspections of insulated and non-insulated MC 331 cargo tanks.
We have paraphrased and answered your questions as follows:
Q1. If you are unable to conduct the annual external visual inspection on an insulated MC 331 cargo tank, are you required to conduct an internal visual inspection once a year?
A1. Yes, for those areas of the cargo tank unable to be externally inspected. Each specification cargo tank must be tested and inspected as specified in § 180.407(c) and in accordance with § 180.407(d)(1), where insulation precludes a complete external visual inspection, the cargo tank must also (emphasis added) be given an internal visual inspection in accordance with § 180.407(d)(1).
Q2. If you are unable to conduct the annual external visual inspection on a non-insulated MC 331 cargo tank that has a coating, or vinyl wrap applied to the external shell, are you required to conduct an internal visual inspection once a year?
A2. Yes. If external visual inspection is precluded because any part of the cargo tank wall is externally lined, coated, or designed, to prevent an external visual inspection, those areas of the cargo tank must be visually inspected in accordance with § 180.407(d)(1).
Q3. Historically, it is your understanding that PHMSA has required an internal visual inspection once every 5 years for insulated MC 331 cargo tanks and an internal visual inspection annually for non-insulated MC 331 cargo tanks that have wraps or other external coverings of the tank. If this understanding is still accurate, why are non-insulated MC 331 cargo tanks with a coating or vinyl wrap required to conduct an internal visual inspection every year?
A3. Insulated and non-insulated MC 331 cargo tanks are not treated differently with regard to internal visual inspections. In both cases, a complete internal visual inspection is required once every five years as reflected in the table of § 180.407(c). The external visual inspection is required annually for MC 331 cargo tanks. For both cases in accordance with § 180.407(d)(1), if insulation precludes complete external visual inspection or if any portion of a cargo tank is externally covered (i.e., lined, coated, or designed to prevent external visual inspection), then an internal visual inspection of those areas affected by the covering is required in conjunction with the external visual inspection for the remaining areas of the cargo tank wall. Because the interval for external visual inspection is annually, the interval for the partial internal visual inspection is also annually.
I hope this information is helpful. Please contact us if we can be of further assistance.
Sincerely,
Dirk DerKinderen
Chief, Standards and Development Branch
Standards and Rulemaking Division
§ 180.407(c), § 180.407(d)(1)