There was a song that used to play all the time on a children’s educational television show in the U.S. … “One of these things is not like the others – One of these things just doesn’t belong.” When you boil it all down, the success or failure of most NVG panel modifications comes down to just that – a painstakingly tedious process of identifying each little element of light within your panel that deviates in color or intensity from its brethren, and modifying it in some manner so that they all perfectly match – every green matches every other green, every yellow matches every yellow and every red matches every red, in hue, as well as intensity. If the light emitted by every switch and every instrument is not in alignment in both color and intensity in relation to the rest of the panel, the end result is possible confusion. And confusion in the cockpit is never a good thing, least of all when the aircraft is attempting to warn the pilot about some imminent problem or danger.
The FAA takes a “whole system” approach to night vision modifications. That is why there is no such thing as an off-the-shelf NVG-certified instrument, much to the surprise and frustration of many. While you can buy instruments and switches that are “NVG compatible,” or “NVG ready,” that only means that the light source within that product falls within the acceptable range for NVG operations as defined by the appropriate governing authority such as EASA or FAA. But there is no such thing as an STC or PMA that can automatically guarantee any single instrument or switch for NVG operations. Chances are your shiny new NVG-compatible device is still going to have to have its lighting source modified in some manner so that it matches and meshes within all the other lighting sources surrounding it in your cockpit. Only a complete and specific night vision imaging system (NVIS) can be certified, and that system includes a radar altimeter and every light source both internal and external, as well as the cockpit fixtures and the goggles you will be using … well, sort of, when it comes to the goggles.
For all the time and attention that is devoted to managing the light within and around the cockpit, it only seems logical that equal attention to detail should be paid to what type of goggles are being used and how those goggles perceive that light and relay it to the pilot’s eyes. But surprisingly, that is not always the case, even within the regulating agencies involved.
Wait a minute – I can use any approved aviation goggle that is out there - all aviation goggles are basically the same, right?
Well, not exactly. If you are operating under FAA guidelines, the answer is “Yes” to the former, I can use any aviation goggle, and “No” to the latter, all aviation goggles are basically the same – at least as far as the regulations are currently being interpreted and enforced. But technically both EASA and FAA say “No” across the board. You see, all applicable regulations for non-military use of night vision goggles ensure that your choice of goggles will be limited to what are known as AN/AVS 9 goggles. We could spend a lot of time focusing on exactly what that means, but it doesn’t really matter, because those are the only goggles you are going to be offered.
But it is very important to understand the distinctions that exist within the AN/AVS 9 category of goggles in order to accurately answer the questions above. There are two basic variations of goggles approved for aviation use – Class A and Class B, and then there is a modified version of one of those that is alternately referred to as Modified Class B or Class C. So what’s the difference? The U.S. Army uses Class A goggles, so the initial temptation is to think the A-B distinction must be based on the power of the image intensifier which must deliver a higher level of performance than is available to the commercial market. But that is not it at all. While the U.S. Army does use a more powerful image intensifier, that difference is noted by the designation of their goggles as AN/AVS 6, which oddly enough, offer a higher level of performance than AN/AVS 9. The difference between Class A and Class B is actually in the way the incoming light is filtered. Class A goggles were originally developed for use within traditional U.S. Army cockpits, and the traditional U.S. Army cockpit simply does not use the color red. Cautions are green, Master Cautions are yellow and Warnings are orange. No red to be found. But when the U.S. Air Force began to develop their own NVIS standards, they were already well on their way toward utilizing multi-function/multi-colored EFIS displays and a much more broad range of color within their cockpits, so they needed a totally different kind of filter within their goggles which led to the development of the Class B goggle. The filters within Class B goggles are designed to be used with yellows, oranges and reds (all goggles are essentially the same when it comes to whites and greens). When the FAA began to develop a standard for the non-military marketplace in the early 90’s, it was only logical to emulate and adopt that Class B standard, for all the same reasons. The Class C variation simply denotes Class B goggles that have been modified slightly so that they can be used with a head-up display.
But if you are really in the mood to expand the envelope on the definition of the word “frustrating,” just wait until you find out exactly how to determine if you are operating with Class A, Class B or even Class C goggles. Are you ready?
You probably can’t.
No kidding. The vast majority of goggles out there do not have a single external mark identifying whether it is an NVG-A or NVG-B goggle. You can’t even look at a serial number and refer to a chart. It matters not whether ITT Exelis, L-3 or one of their predecessor companies manufactured your goggles. The FAA addressed this oversight for any newly designed goggles via TSO-C164 in 2004 with very specific marking and identification requirements, which is why your NIVISYS goggles do have a serial number stamp.
But most likely, the only way you will be able to tell if your goggles are Class A or Class B is to send them to an authorized service and repair center and ask them to tell you what you have. And believe it or not, until very recently, the only way the repair station could tell you the difference was to take your goggles apart and look at each individual lens tube. Recognizing this as a serious flaw in the system, Hoffman Engineering developed the ANV 450, a piece of test equipment that hit the market in September 2011 which is able to determine whether a goggle has a Class A or Class B filter without taking the whole thing apart. Hoffman Engineering uses the equipment for its own customers at their facility in Stamford, Connecticut, but there are presently only two or three others out in the field. Night Flight Concepts utilizes one of the machines for their customers at its repair station in Bedford, Texas, as does Aero Dynamix in Euless, Texas.
But if FAA and the goggle manufacturers themselves don’t even think it is important enough to make the clear distinction between Class A and Class B goggles, then it can’t possibly be that important, right? I don’t know … how important could it possibly be to be able to quickly distinguish between different colors of light in a cockpit?
Not surprisingly, EASA has determined that the distinction between an orange light and a red light is pretty darned important, and EASA regs only allow the use of Class B goggles in commercial use. In theory, FAA thinks it is a pretty important distinction as well. The operative document the FAA uses regarding Minimum Operational Performance Standards for night vision imaging systems is RTCA DO-275, which specifically calls for NVG Class-B goggles.
But remember – just as there is no such thing as any single “NVG certified” instrument or switch, neither is there such a thing as any specifically “NVG certified” goggle that is automatically approved for use. Even a TSO’d goggle still has to pass the “whole system” test along with all of the other lighting and cabin elements, proving that it meets the minimum performance standard as specified.
The answer is that it would be quite valuable as a guideline listing all the specifications a manufacturer would need to meet and follow in order to produce a new goggle for the commercial market from scratch. Prior to TSO-C164, there simply was no guidance. On a practical level, once you have met the terms of the TSO in your design and production lines proving that your goggles will allow an operator to get NVIS certification from the proper authorities, then the value of that TSO is greatly diminished. And there are significant and ongoing reporting requirements that must be continuously met in order to keep a TSO in force. So it is no surprise that NIVISYS, the one company that did go through the extensive TSO process to design a new set of goggles for commercial operators, chose to later surrender that TSO. But the quality of the goggles produced by NIVISYS is still enforced in the same manner it is enforced upon the other goggle manufacturers – as part of the NVIS STC approval process.
But here is where the whole topic begins to get rather fuzzy with gray areas. Goggles are obviously required to view the panel in order to certify the STC, but specific goggles are not generally included as a part of that STC. (However, a specific goggle or type is usually called out in some form within the flight operations manual of the operator.)
In fact, in a rather odd and head-scratching lapse in the whole system approval approach, the FAA evaluation guidelines do not even require that the NVIS system evaluation be performed with the same type of goggles that will actually be used in operation. In a document titled, “Night NVG Aided Readability Ground Evaluation,” which is the FAA’s internal guidance to its own design certification inspectors, under section 184.108.40.206 “Required Items,” item number 6 simply states that the evaluation requires:
“A minimum of one set of NVG for each front seat evaluator that meets TSOC164 or RTCA/DO-275 specifications.
a) Use the same type/model of NVG that operator will use in flight if possible.” (emphasis added by R&W)
The implication then is that one goggle is as good as another, as long as it allows the NVIS modification to meet the minimum specified requirements. This seems to be a bit contradictory on the part of the FAA given that their own requirements all call for Class B goggles. But there is wording within TSO-C164 that was clearly designed to allow for this contradiction.
f. Deviations. We provide for alternative or equivalent means of compliance to the MPS (Minimum Performance Standards) of this TSO. If you invoke these provisions, you must do so in accordance with the provisions outlined in 14 CFR § 21.609.
So here is what it all comes down to – it doesn’t seem to matter to the FAA whether you are using Class A or Class B goggles, as long as they meet the minimum performance standards. EASA still requires Class B.
But should it matter to you? Absolutely. There are very real differences in the manner in which each class of goggle perceives colors. To be clear, we are not proposing that there is absolutely a right goggle or a wrong goggle for all NVIS modified aircraft – we are simply stating that for every NVIS modified aircraft, there will absolutely be a best goggle for that particular modification and operating environment. It is possible that the other class of goggle will also provide acceptable performance, but certainly is not guaranteed. And there will absolutely be times when the other class of goggle would fail if it had to be resubmitted for the certification process.
As a practical matter, many operators may have no idea that their aircraft was certified using one class of goggle, while their pilots are flying it using the other, or as in some cases, their pilots are using a mixture of the two. The difference may be perfectly acceptable, but it is something certainly worth knowing and understanding.