Follows Issuance Of An AD Tied To The GPS Module In The ADS600-B Product

The notice on the NavWorx website was terse. "The ADS600-B Gen 2.0 product utilizes a GPS module from a third-party vendor. Although the vendor represented their GPS module met 14 CFR 91.227, the FAA recently determined the GPS module does not meet 14 CFR 91.227. We are unable to sell the ADS600-B, or provide AD updates, for either certified or experimental aircraft. Therefore, we are not currently conducting any business and have ceased operations. We will provide updates if they become available."

The company's problems stem from an AD that was posted on the Federal Register on June 6, 2017. The FAA proposed to require removing the ADS-B unit before further flight and proposed to prohibit installing the affected ADS-B unit on any aircraft. The NPRM was prompted by a design change that resulted in the ADS-B units broadcasting a Source Integrity Level (SIL) of 3 instead of 0. A broadcast of SIL 0 is required because of the uncertified Global Positioning System (GPS) source included in the unit. The proposed actions were intended to prevent an ADS-B unit from communicating unreliable position information to Air Traffic Control (ATC) and nearby aircraft and a subsequent aircraft collision.

The FAA said its safety concern was primarily that of integrity, and not necessarily accuracy, with respect to the NavWorx ADS-B unit's performance requirements. Accuracy refers to the probability of the unit's true position in relation to its reported position. Integrity refers to the trust that can be placed in the correctness of the information provided by the unit and is specified by a SIL value. The SIL value is set based on design data from the GPS position source manufacturer and reflects the probability that the position source will provide incorrect data without providing an alert. This depends on whether the GPS has fault detection and exclusion, where the equipment will detect a faulty satellite and exclude it from the navigational calculations. If the GPS does not have fault detection and exclusion, the probability that the position source will provide incorrect data increases. Commercial position sources, such as the SiRF IV GPS used in the NavWorx P/N 200-0012, 200-0013, and 200-8013 units, do not have fault detection and exclusion capabilities. This is significant in the event a GPS satellite signal fails or a GPS “signal-in-space error” occurs.

The unsafe condition relates to the potential for the NavWorx unit to incorrectly report its own position to other aircraft and to ATC, by 0.2 nautical miles (NM) or more, without providing an alert.[3] It may also result in ATC providing incorrect and inappropriate separation instructions or traffic advisories to other aircraft for avoidance of the ADS600-B-equipped aircraft, based on the erroneous position being reported by the ADS600-B. In this situation, the pilot of the ADS600-B-equipped aircraft would be unaware that his or her aircraft's ADS-B Out unit is broadcasting an erroneous position (possibly in excess of 0.2 NM), since the ADS600-B would not be providing an alert for this condition. Depending on operating conditions, these effects may occur in instrument meteorological conditions where see-and-avoid is not possible. In view of these factors, this condition poses an unacceptable hazard to other users of the National Airspace System (NAS).

Although there have been no reported cases of a collision or safety incident resulting from an incorrect transmission by a NavWorx ADS-B unit to date, the potential for the unsafe condition exists as long as the units mislead ATC and nearby aircraft by broadcasting a SIL of 3 that they have not been shown to meet. This AD action addresses that potential unsafe condition.

While some commenters on the original NPRM said that they had used the ADS-6000B and found it to be reliable, the FAA said the fact that commenters have made flight tests with satisfactory ADS-B performance monitor reports from the FAA, or that commenters' individual units have been operating successfully, does not negate the existence of an unsafe condition. Flight tests with the ADS-B performance monitor are designed primarily to show that the ADS-B equipment in an individual aircraft performs correctly as installed. These tests are of relatively short duration and occur in fault-free conditions. They are not engineering tests designed to evaluate the unit's ability to handle GPS signal-in-space errors and cannot be used to draw inferences about the unit's position source integrity.

Although the FAA recognizes the benefits of ADS-B equipage and understands that the NavWorx units may work a large percentage of the time for an individual user, the FAA must consider the effect on the entire NAS. Since failure is based on a statistical probability, the odds that a unit will have a failure increase as more units are introduced into the NAS and operate for a longer period of time. The probability of a failure also increases when there is a GPS satellite malfunction, which could affect many aircraft since the information is used by ATC and ADS-B In equipped aircraft for separation. Therefore, despite any benefit to individual owners when the unit works without failure, the FAA has determined that an unsafe condition with the NavWorx units exists and requires corrective action because of the hazard they pose to other users of the NAS.

In light of the development, Dallas Avionics, Inc. says it has discontinued distribution and support of all NavWorx products, effective immediately.

"All pre-orders for NavWorx products, repairs and upgrades received by Dallas Avionics, Inc. will be canceled effective immediately," the company said in a notice posted on its website.

"Over the past year, Dallas Avionics, Inc. has made every attempt to facilitate the success of NavWorx and support of their customers. Unfortunately, under the current conditions and outlook, we can no longer continue to provide this service.

"While we are compassionate to your situation, please direct all further correspondence direct to NavWorx."

(Image from file)

FMI: AD