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Tuesday, November 29, 2011

Class A / Class B NVGs

Jessie K. Kearby

Question:

What is the difference between Class “A” and Class “B” modifications and Class “A” and Class “B” goggles?

Answer:

Let's begin with discussing the fundamental difference between Class A and Class B NVGs. A Class A NVG refers to a NVG with a visible light sensitivity "cut-off" point of 625nm due to the objective lens filter which has been incorporated into the NVG. A sensitivity cut-off of 625nm means that the NVGs are sensitive to all variations of the color Red and therefore it is strongly recommended from a safety stand point that Class A NVGs not be used in conjunction with any NVIS lighting components emitting light in the Red range. Also, the use of Class A NVGs can lead to serious certification issues when used along with FAA approved aircraft NVIS modifications due to the FAR requirements to maintain the Red Warning indication colors. A Class B NVG refers to a NVG with a visible light sensitivity "cut-off" point of 665nm due to the objective lens filter which has been incorporated into the NVG. A sensitivity cut-off of 665nm means that the NVGs might be sensitive to saturated Red, however, there are many variations of Red, such as NVIS Red, which the NVGs are not sensitive to. Therefore, it is strongly recommended from a safety stand point that Class B NVGS be used if there are any NVIS lighting components emitting light in the Red range. In addition, the use of Class B NVGs is required to meet the specifications defined in RTCA/DO-275 and their use will help avoid any serious certification issues when used with FAA approved aircraft NVIS modifications due to the FAR requirements to maintain the Red Warning indication colors. Now that we have basic understanding of the difference between Class A and Class B NVGs, let's discuss the difference between Class A and Class B aircraft NVIS modifications. A Class A modification is a modification that was designed to filter all NVIS lighting components so that they are compatible with Class A NVGs. Therefore, a Class A modification cannot have any NVIS lighting components which emit light in the Red range. This may be an acceptable modification approach for some military applications, however, this type of modification would not be acceptable to the FAA due to the FAR requirements to maintain the Red warning indications. A Class B modification is a modification that was designed to filter all NVIS lighting components so that they are compatible with Class B NVGs. Therefore, a Class B modification can have NVIS lighting components which emit light in the Red range as is required to meet the specifications defined in RTCA/DO-275 and the FAR requirements. A Class B modification is the standard for FAA certified aircraft NVIS modifications.


Jessie K. Kearby is a Certification Engineer for Aircraft Night Vision System Integration, Modification and Repair Center at Aero Dynamix, Inc.

In addition to his established working relationship with the FAA, Jessie K. Kearby was recently recognized by Transport Canada Civil Aviation (TCCA) as a Subject Matter Expert on Aircraft Night Vision System Modification.


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