Why is AM modulation still used in aviation? A technical look at an unlikely survivor

In an age of digital communication, error correction, and smart radios, it might seem outdated—even strange—that the global aviation industry still relies on amplitude modulation (AM) for voice communication. Yet, every commercial pilot, air traffic controller, and military aircraft across the world continues to use AM radios in the VHF airband (118–137 MHz) as the primary means of real-time communication.

So why hasn’t aviation moved on to FM or digital modulation? The answer lies in a combination of historical momentum, safety-critical design priorities, and some surprising technical advantages of AM that are uniquely suited to the airspace environment.

Let’s dive into the reasons behind this longstanding choice and see why AM remains not just a legacy system, but a deliberate, mission-critical technology.

What is the aviation VHF band and why does it use AM?

The VHF (Very High Frequency) airband spans 118 to 137 MHz and is globally allocated for civil aviation voice communication. Within this range, each communication channel uses amplitude modulation (AM)—specifically, A3E double-sideband full carrier AM, the same fundamental method used in early 20th-century AM radio.

Unlike other services that use frequency modulation (FM) or modern digital schemes (like DMR, QAM, PSK), the aviation world remains firmly anchored in AM.

This isn’t just a holdover from the past—it’s a carefully preserved standard that persists for good reasons.

Historical roots: why AM was adopted in the first place

AM became the dominant voice communication technology during the early years of radio, primarily because:

• It was easy to implement with the analog electronics of the time.

• Receivers could be built with simple circuits.

• It didn’t require digital decoding or frequency synthesizers.

• There were no practical alternatives in the early 20th century.

As aviation grew, so did reliance on AM. By the time FM and digital modulation matured, aviation already had a robust, standardized global network based on AM.

Changing such a deeply embedded system—especially one tied directly to human safety—is no simple matter. In aviation, reliability and backward compatibility always trump innovation for innovation’s sake.

Key technical advantages of AM in aviation

Simultaneous transmission detection (the hidden safety feature)

One of the strongest technical reasons for AM’s continued use is how it handles simultaneous transmissions.

In FM systems, if two people transmit at the same time on the same frequency, only the strongest signal is heard due to what’s called the capture effect. The weaker transmission is entirely suppressed—possibly without anyone noticing.

With AM, however, both signals are superimposed and can be heard simultaneously, albeit distorted. This distortion immediately alerts controllers that multiple aircraft are trying to transmit at once, prompting them to ask for a repeat:

“Two transmissions at once—say again.”

This tiny detail can prevent critical misunderstandings and save lives. In aviation, missing a call is often more dangerous than hearing a garbled one.

Simplicity and robustness

AM radios are relatively simple and resilient:

• Fewer components, less complexity.

• Easier to maintain, especially in older aircraft.

• Minimal processing needed—no digital codecs or error correction.

This simplicity is especially useful in:

• Harsh military or remote environments.

• Older aircraft with legacy systems.

• Redundant or backup systems that must “just work.”

Narrowband spectrum efficiency

The VHF airband is crowded. To maximize spectrum usage, aviation regulators have narrowed channel spacing over time:

• Originally 25 kHz channel spacing.

• Now moving toward 8.33 kHz spacing in Europe and elsewhere.

AM’s narrow bandwidth and analog nature allow it to function effectively in these tight frequency allocations. Many digital or FM systems require more bandwidth and couldn’t fit into the current VHF band plan.

Audible signal degradation

AM is more prone to noise and interference—and that’s actually a feature in aviation.

With AM:

• Interference is audible and obvious.

• Signal degradation (static, fade, distortion) gives feedback to pilots and controllers.

• It’s easy to tell whether a frequency is “live” or if someone is transmitting—even faintly.

In FM or digital systems, signals might cut out completely or drop without warning.

Why not switch to FM or digital?

It’s not that FM or digital systems aren’t good. In fact, they’re used widely in public safety, broadcasting, and industry. But they come with trade-offs that make them unsuitable for primary aviation communication:

• FM’s capture effect makes weak or overlapping signals disappear—unacceptable in a safety-critical environment.

• Digital voice systems add latency and complexity. Glitches, buffering, and codec artifacts can confuse voice clarity.

• Switching would require a global infrastructure upgrade, including all aircraft, airports, and control towers.

The cost, logistics, and risk involved in replacing AM globally are staggering. No international aviation body is seriously pursuing such a shift anytime soon.

Global standards and regulatory mandates

Aviation communication standards are enforced by international bodies:

• ICAO (International Civil Aviation Organization) – UN-level global standard-setter.

• FAA (Federal Aviation Administration) – U.S. civil aviation authority.

• EASA (European Union Aviation Safety Agency) – Regulatory agency in the EU.

All of them mandate:

• VHF airband use (118–137 MHz)

• 8.33 or 25 kHz channel spacing

• AM modulation (A3E)

• Standard phraseology, readbacks, and procedures

These standards ensure global interoperability. When a French pilot flies into U.S. airspace, or a Boeing jet lands in Tokyo, everyone speaks the same “radio language”—technically and procedurally.

What about digital alternatives?

There are digital systems in aviation, but they supplement AM—not replace it:

• CPDLC (Controller–Pilot Data Link Communications) – Used for non-urgent instructions on long-haul flights.

• ACARS (Aircraft Communications Addressing and Reporting System) – Text-based messaging between aircraft and operations.

• ADS-B (Automatic Dependent Surveillance–Broadcast) – Transmits aircraft position, but not voice.

All of these systems reduce controller workload and improve situational awareness—but when it comes to real-time voice communication, AM still rules.

Emergency frequency: 121.5 MHz

The internationally recognized emergency frequency (121.5 MHz) is also AM.

• Every aircraft monitors it.

• Automatic emergency locator transmitters (ELTs) use it.

• Search and rescue (SAR) operations depend on it.

Changing this frequency, or its modulation type, would disrupt decades of procedures and equipment compatibility.

Audio quality: a compromise that works

While FM or digital voice can sound clearer, AM is “good enough” for the frequency range of human speech (~300 to 3000 Hz):

• Pilots and controllers use concise, standardized phraseology to reduce confusion.

• Distortion is tolerable if the core meaning is clear.

• Slight background noise can even help users stay alert and confirm the system is working.

In a cockpit, absolute audio fidelity matters far less than clarity and consistency.

Maintenance and troubleshooting

AM systems are:

• Easier to test and calibrate.

• More predictable under interference.

• Less prone to sudden failure or silent data loss.

If something goes wrong, the symptoms are immediately audible. That speeds up diagnosis and fixes.

Has AM ever been seriously challenged?

Yes. Over the decades, several attempts have been made to replace AM:

• FM trials in general aviation failed due to the capture effect.

• Digital voice systems (e.g., TETRA) were considered but lacked reliability and global standardization.

• IP-based communication systems introduced latency and error-handling issues.

In every case, AM’s transparency, simplicity, and safety-critical performance kept it ahead.

Lessons from the ham radio world

Interestingly, amateur radio (ham radio) enthusiasts also debate AM vs. FM:

• AM has a warm, natural tone, but is more sensitive to noise.

• FM is cleaner, but can completely drop weak signals.

Hams understand the same trade-offs aviation engineers face—making the aviation choice all the more understandable.

Despite being nearly a century old, AM modulation remains a cornerstone of aviation communication. Its quirks and limitations are well-known—and in many cases, intentionally embraced. Until a future system can match its transparency, simplicity, safety, and global compatibility, AM isn’t going anywhere.

Image(s) used in this article are either AI-generated or sourced from royalty-free platforms like Pixabay or Pexels.