Unpacking the FAA's Boeing 787 Transponder Directive
As SARC Communicator editor I read a lot of blogs, club websites and other sources of amateur radio news. This one particularly caught my eye.
The source
The ‘click-bait’ headline:
Ham Radio Enthusiasts Land US Airlines With 8 Million Bill To Fix Faulty Equipment On Boeing 787s
Ham radio enthusiasts could be partly responsible for landing U.S. airlines with an $8 million bill to fix faulty equipment on Boeing 787 Dreamliner airplanes after it was discovered that simple radio signals can knock out a faulty transponder on the popular widebody plane used by American, United, and Alaska Airlines.
The issue came to light after the Federal Aviation Administration (FAA) reported “multiple instances of loss of transponder for airplanes entering airspace in the presence of CW interference.”
CW interference refers to continuous-wave radio signals like Morse code, military transmitters, and even amateur ham radio signals, which could interfere with the transponder on some Boeing 787s...
When I saw this story it didn’t seem to add up. After all, Amateurs have been sending CW for a century and there has never been an allegation such as this. Although my own and other readers’ feedback has resulted in an adjustment of the original deceptive headline, the underlying story deserved further investigation.
The actual facts
When the FAA warns of "CW interference," hams think of Morse code. Aviation engineers think of something far more dangerous—a silent, invisible wall of noise that can blind a Dreamliner to oncoming traffic.
In the world of amateur radio, "CW" is a beloved mode—the rhythmic cadence of Morse code cutting through the static, a testament to communication's simplest form. But when the Federal Aviation Administration (FAA) uses the same two-letter abbreviation in an airworthiness directive, it is describing something far more insidious and utterly unrelated to the operator in the shack.
For an avionics engineer, "Continuous Wave (CW) interference" refers to a pure, unmodulated, single-frequency carrier signal that has no business being where it is. It is a rogue tone, a sustained note of radio energy that can overwhelm sensitive aircraft receivers. And according to a new Notice of Proposed Rulemaking (NPRM) from the FAA, this type of interference is posing a direct threat to the Boeing 787 Dreamliner's ability to see and be seen by other aircraft.
The proposed directive, https://www.federalregister.gov/documents/2025/06/13/2025-10759/airworthiness-directives-the-boeing-company-airplanes which would affect 150 U.S.-registered 787-8, -9, and -10 aircraft, mandates a costly hardware replacement to fix a vulnerability that could, quite literally, render an aircraft invisible in busy airspace. But what exactly is this interference, and why is a simple hardware swap estimated to cost U.S. operators nearly $8 million?
The Problem: A Transponder That Won't Talk Back
At the heart of the issue is the 787's Integrated Surveillance System Processor Unit (ISSPU), a critical component that manages the aircraft's transponder. The transponder's job is to listen for interrogations from Air Traffic Control radar and other aircraft's Traffic Alert and Collision Avoidance Systems (TCAS) on 1030 MHz, and reply on 1090 MHz. Note that this is far from the usual HF frequencies that Amateurs normally operate at.
According to the FAA directive (Docket No. FAA-2025-0924), multiple reports have surfaced of 787s entering airspace with active "CW interference" and suffering a specific, dangerous failure: the transponder stops meeting its Minimum Operational Performance Standards (MOPS). Instead of correctly replying to at least 90% of interrogations, the unit becomes desensitized, failing to respond.
This is not a gradual degradation. It is an "unannunciated" loss, meaning the pilots receive no warning light, no aural alert, no indication that their aircraft is no longer replying to ground radar or TCAS inquiries. The first sign of trouble could be a gap in the sky where an airliner used to be, visible to everyone except the pilots of the aircraft that just went silent.
"CW" for the Layman: Not Morse Code, But a Wall of Noise
This is where clarification for the broader technical community is essential. For the amateur radio operator, "CW" (Continuous Wave) is synonymous with Morse code—a carrier wave that is turned on and off to form characters. It is intermittent, intentional, and communicative.
The "CW interference" cited by the FAA is something else entirely. In engineering terms, a "continuous wave" simply means a steady, unmodulated carrier signal. Think of it less as a conversation and more as a sustained, single-frequency tone—a pure, unbroken note of radio energy. If a pulsed radar signal is like a strobe light, CW interference is a laser pointer held steadily on a sensor, blinding it.
For a transponder receiver trying to pick out weak interrogation pulses from the sky, a powerful CW signal on or near its operating frequency (1030 or 1090 MHz) acts as a "jammer." It raises the noise floor, drowning out the very signals it needs to hear.
The Hunt for the Source: Who Is Generating This Noise?
The FAA directive is notably silent on the source of this interference, focusing instead on fixing the aircraft's vulnerability to it. So, who or what is generating these rogue continuous wave signals? The answer is complex and points to a crowded, modern radio spectrum. While the public document does not specify frequencies, the affected systems point squarely at the 1030/1090 MHz bands. Likely culprits for high-power CW interference in or near these frequencies include:
- Ground-Based Military and Civilian Radars: Some radar systems, particularly those used for long-range surveillance or specific military applications, can produce strong continuous or quasi-continuous output that generates harmonics or spurious emissions.
- High-Power Data Links: Terrestrial microwave data links, used for point-to-point communication by telecom companies and utilities, operate in frequency bands that can, with faulty equipment, generate out-of-band emissions that bleed into the aviation surveillance bands.
- The 5G Debate, Revisited: The recent spectrum battles between aviation and 5G carriers centered on the potential for signals from powerful ground-based transmitters to cause interference with radar altimeters. While that specific fight involved different frequencies (3.7-3.98 GHz), it perfectly illustrates the principle: a powerful, continuous transmission on a nearby frequency can overwhelm aircraft receivers if filtering and shielding are insufficient.
The $7.95 Million Fix
Because the sources of interference are myriad and largely outside an airframer's control, Boeing and the FAA have chosen to harden the aircraft itself. The proposed solution is not a software tweak, but a physical replacement of the vulnerable hardware.
While then issue is a worldwide problem, the directive would require US based operators to replace the left and right ISSPU units, swapping out current part numbers (822-2120-101 and -102) with a new, presumably better-shielded or more selective unit (part number 822-2120-113) .
The FAA estimates the parts alone will cost $52,661 per aircraft. With labor, each of the 150 affected U.S. planes will incur a $53,001 expense, bringing the total for U.S. carriers to $7,950,150 .
This is a significant investment for a problem that many in the industry suspect is not going away. As the radio spectrum grows ever more congested with diverse signals, the threat of "CW interference"—in its true engineering sense—will only increase. For the pilots of the Dreamliner, this hardware upgrade can't come soon enough. For the amateur radio operator tuning up on 40 meters, rest assured: your key is not the culprit. The real threat is coming from elsewhere in the increasingly noisy radio spectrum we all share.
73,
~John VE7TI
