Horizon Airlines and Republic Airlines have asked the FAA for three more months to comply with new filtered flight data requirements, because challenges encountered by supplier Honeywell mean necessary upgrades can’t be completed on their Bombardier Q400s by the rule’s April 2014 deadline.

The rule, finalized four years ago in response to frustrated accident investigators hampered by inaccurate data, requires operators to ensure that certain flight data parameters are not being filtered between their origination and the digital flight data recorder (DFDR). Operators have until April 21 to upgrade non-compliant recorders or show they can accurately reconstruct filtered data streams.

After several false starts and technical challenges, Q400 DFDR supplier Honeywell has developed a plan that calls for upgrading Flight Data Recorder Signal Conditioning Unit (FSCU) software on at least 83 in-service, U.S.-based Q400s—51 at Horizon and 32 at Republic. The plan involves rotating 10 compliant FSCUs through the fleet—six at Horizon and four at Republic—and shipping in-service units to Toronto for modification. The upgraded units will then be shipped back to the carriers and installed.

Honeywell is developing a service bulletin that details the changes and expects to issue it in the next few weeks. Once the bulletin is out, the upgrades can start. Bombardier says the first modified units will be ready for installation in mid-February. Based on a projected upgrade rate of five aircraft per week, the work should be done in about four months, or by mid-July.

“While the compliance plan includes certification of hardware prior to the regulatory deadline, there will be insufficient parts availability for operators to install the new hardware on the entire fleet prior to the regulatory deadline,” Republic explains in its request to have until July 21 to demonstrate compliance.

Soon after the rule was finalized in 2010, Bombardier and Honeywell analyzed the Q400 DFDR for compliance with the new rule. They found that five parameters were being averaged by a software-based sampling algorithm within the FSCU.

“This averaging was determined to be contrary to the new requirements,” Republic explains.

Bombardier and Honeywell explored three possible fixes: develop a new FSCU, modify the existing FSCU software, or change the way certain data is fed to the recorder, documents filed to support the exemption request show.

Bombardier rejected the last option as too costly. Honeywell’s initial choice was a new FSCU because it felt that modifying the existing one “would be problematic,” the company explained in a September 2013 Quality Assurance Coordination Memo. However, Honeywell soon determined that the time needed to develop and certify the new FSCU was “well in excess of the April 2014 mandated date of compliance,” sending it back to the drawing board.

Honeywell settled on a FSCU software modification plan with a schedule that is “compressed where possible,” the company’s memo explains. “However, the overall program lead-time currently exceeds the April 2014 incorporation date with current program completion by July 2014.”

Part of the problem: developing an in-field software loading tool “has proven to be difficult to implement by Honeywell,” Bombardier explains in a letter to FAA, forcing the suppliers to rely on the more time-consuming rotable exchange process.

The change, once approved, will be incorporated into the Q400 production line, Honeywell notes.

The FAA, in letters to each carrier, says it is “processing” the requests and will respond with a decision “as soon as possible.”