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Airline Pilots Worry About HD Radio Power Proposal

Pilots union says move could cause interference with aviation band

The largest airline pilots union in the world says it has potential concerns about an FCC proposal that would allow more FM stations in the United States to increase digital power levels.

The Air Line Pilots Association (ALPA), which represents the safety interests of 75,000 pilots flying for 43 airlines in the United States and Canada, believes an increase could threaten the adjacent Aeronautical Radio Navigation Service (ARNS) band from 108.0—117.975 MHz.  ALPA is asking the government at least to look into the issue more.

The FCC’s proposal, if adopted, would allow use of a new formula to determine which radio stations can use the highest allowable power levels for their digital FM stations. A companion proposal in the NPRM seeks to allow FM digital stations to operate with asymmetric power on digital sidebands.

[Related: “FCC Proposal to Boost HD Power Receives Strong Support“]

The pilot union says the ARNS band is occupied by navigation and landing systems essential for safety of flight. Those systems include Instrument Flight Rules (IFR), which are needed for landing during low-visibility conditions at night and during inclement weather.  “Interference from FM digital broadcasting at the upper end of the 88—108 MHz band that affects the operations of these aviation systems would present a significant safety hazard and would disrupt aviation operations by delaying, diverting and canceling potentially hundreds or thousands of flights,” ALPA told the FCC.

ALPA says that, after consulting aviation experts including those at the Federal Aviation Administration, it is unsure whether existing rules governing FM broadcast in conjunction with the rules proposed are sufficient to protect aviation use of the adjacent ARNS band. The group is urging the FCC to “reanalyze the compatibility of FM digital broadcasting with the ARNS band.” 

“The analysis and discussion of the proposed increase in digital sideband power is entirely concerned with inter-channel interference with nearby adjacent-channel FM broadcast stations. However, the proposal to allow increases in digital sideband power without formal commission approval in the DAB NRPM needs to be examined for any significant impacts on the adjacent ARNS band starting at 108.0 MHz.”

ALPA went on to detail the navigation systems that could be affected. It says the ARNS band is used in three systems on all classes of airplanes: VHF Omnidirectional Range (“VOR”), Instrument Landing System (“ILS”) Localizer and Ground-Based Augmentation System VHF Data Broadcast.  The three VHF ARNS band systems provide some of the en route navigation and all of the high-precision approach capabilities in the U.S., and therefore are critical to maintaining safety and efficiency during poor weather conditions, ALPA says.

“If an aircraft on approach were to experience loss of the navigation signal due to interference from FM broadcast during an approach, unless the pilots can already see the landing runway, the approach must be abandoned, and the aircraft must go-around. If a loss of signal affecting multiple airplanes persists, and the weather is not suitable for using alternate approaches, passengers and shippers can expect mass delays, diversions and cancellations until the weather improves, leading to significant economic losses for both the passengers and the airline or operator,” ALPA told the FCC in its comments.

ALPA is asking the commission to work with the National Telecommunications and Information Administration and the Federal Aviation Administration to fully analyze the risk to aviation, as the assigned user of the band, from relaxed rules as described in the NPRM, and to develop appropriate mitigations as needed.

Specifically, if necessary, ALPA asks the FCC to impose limitations on the upper part of the 88—108 FM broadcast band, and consider limitations in the use of digital upper sideband of stations at 107.9 MHz and limitations in the total emitted power of stations at 107.9 MHz.

Broadcasters have given their full support to the FCC’s proposal giving more digital FM stations the ability to increase their power. Comments on the FCC’s “Modifying Rules for FM Terrestrial Digital Audio Broadcasting Systems” (Docket 22-405) can be read online at the FCC’s website.

[Related: “For HD Radio, Use the Maximum Authorized Power“]