What Maritime Safety Can Learn from Aviation: The Story of TCAS and TAWS

How aviation tackled the challenge of collision avoidance—and what lessons apply to the sea.

When we think about improving safety at sea, we would be foolish not to look skyward. Aviation operates in a far more unforgiving environment than maritime: three dimensions instead of two, speeds measured in hundreds of knots, and margins for error measured in seconds rather than minutes. Yet over the past fifty years, the aviation industry has achieved remarkable safety improvements—particularly in preventing the two most catastrophic types of accidents: mid-air collisions and controlled flight into terrain.

The technologies that emerged from this effort—TCAS (Traffic Collision Avoidance System) and TAWS (Terrain Awareness and Warning System)—offer profound lessons for those of us working to make the seas safer.

The Problem: Human Limitations in a High-Speed Environment

For most of aviation history, collision avoidance relied on the same principle that still governs much of maritime navigation: see and avoid. Pilots were expected to maintain visual separation from other aircraft. Air traffic controllers provided an additional layer of safety, but ultimately, the pilot’s eyes were the last line of defence.

This worked tolerably well in the early days of aviation. But as skies grew crowded and aircraft flew faster, the limitations became deadly apparent. The closure rate between two jets approaching head-on can exceed 1,000 knots—meaning that from the moment a pilot spots a speck on the windscreen to the moment of impact might be mere seconds. Human reaction times are simply inadequate.

Similarly, “controlled flight into terrain” (CFIT)—where a perfectly functioning aircraft is flown into the ground, often in poor visibility—remained a leading cause of fatal accidents well into the 1970s. Pilots, disoriented or distracted, would descend into mountains or terrain they never saw coming.

TCAS: The Electronic Guardian Against Mid-Air Collision

The Traffic Collision Avoidance System emerged from a simple but powerful insight: aircraft already carry transponders that broadcast their identity and altitude. What if those transponders could talk to each other, independently of ground control?

TCAS works by actively interrogating the transponders of nearby aircraft. When your aircraft’s TCAS sends out a radio signal, any transponder within range responds with its identity and altitude. By timing these responses and tracking them over multiple interrogations, TCAS builds a three-dimensional picture of the traffic around you—their distance, relative altitude, and crucially, their rate of closure.

The genius of TCAS lies in its predictions. It doesn’t just tell you where other aircraft are now—it calculates where they will be, and whether your paths will intersect.

When the mathematics indicate danger, TCAS issues alerts in two escalating stages:

What makes TCAS particularly elegant is its coordination capability. When two TCAS-equipped aircraft are on collision course, their systems communicate via data link to ensure complementary manoeuvres. If one aircraft is commanded to climb, the other is commanded to descend. The system prevents the nightmare scenario of both pilots making the same evasive choice.

The Human Interface: How to Command Instant Action

Perhaps the most instructive aspect of TCAS and TAWS for maritime designers is how these systems communicate with pilots. Aviation learned through bitter experience that the interface between machine and human is just as critical as the underlying technology.

Consider what a pilot faces: a cockpit filled with instruments, radio chatter, engine noise, and the cognitive load of managing a complex flight. Into this environment, the safety system must inject a warning that instantly captures attention and triggers the correct response.

Aviation’s answer combines three channels simultaneously:

1. Distinctive Audio

The voice warnings are carefully designed to cut through cockpit noise and radio traffic. TCAS uses phrases like “Traffic, traffic” for advisories and “Climb, climb!” or “Descend, descend!” for commands. TAWS is even more urgent: “Terrain, terrain—pull up!” The phrasing is standardised worldwide—the exact words are mandated, not merely suggested. A pilot trained in Brazil will hear identical commands to one trained in Norway.

Fun fact: Early human factors research suggested female voices were more attention-getting for male pilots—leading to the system being nicknamed “Bitching Betty” in North American military aviation (or “Nagging Nora” in the UK).

2. Visual Display

TCAS overlays its information directly onto the vertical speed indicator (VSI)—an instrument the pilot is already scanning. When a Resolution Advisory is issued, coloured arcs appear on the VSI dial: red shows the vertical speed range that will lead to collision (the “avoid” zone), while green shows the safe range the pilot must fly toward. There is no ambiguity: fly toward the green, away from the red.

This is crucial. The system doesn’t merely tell the pilot what to do—it shows them, on an instrument they’re already watching, in colours that require no interpretation.

3. Priority Hierarchy

When multiple systems compete for attention, aviation has established clear precedence. TAWS warnings (terrain) take priority over TCAS (traffic), because hitting the ground is more immediately fatal than a potential collision that might still be avoided. The systems are designed not to conflict: if TCAS commands a descent but terrain lies below, TAWS will override with “Pull up!”

Compare this to typical maritime alarms: a beep, perhaps a flashing light, maybe a text message on a screen that the skipper must read and interpret while simultaneously managing the helm, the sails, and the crew. The contrast is stark.

Compare This to COLREGs: The Maritime Ambiguity Problem

Now consider how maritime collision avoidance works. The International Regulations for Preventing Collisions at Sea—COLREGs—are a masterpiece of nineteenth-century jurisprudence. First codified in 1972, they establish a framework of rules that determine which vessel must give way in any encounter.

The problem? They require interpretation.

Take a simple crossing situation under Rule 15. A power vessel approaching from your starboard side has right of way; you must keep clear. Clear enough. But what constitutes “crossing”? At what angle does an overtaking situation become a crossing situation? The Rules say a vessel coming from more than 22.5 degrees abaft the beam is overtaking—but how precisely does a skipper measure that angle at night, in rough seas, with an approaching light that might be one mile away or three?

It gets worse when sail meets sail. Rule 12 governs sailing vessels: when both have the wind on different sides, the vessel with the wind on the port side keeps clear. When both have the wind on the same side, the windward vessel keeps clear. But here is where the seamanlike mind begins to spin: what if I cannot determine which side the other vessel has the wind? Rule 12(a)(iii) instructs the port-tack vessel to keep clear—but this presumes knowledge the skipper may not possess, especially at night or at distance.

Then consider the hierarchy of Rule 18. Power-driven vessels must keep clear of sailing vessels. Except when the sailing vessel is overtaking. Except in narrow channels under Rule 9, where a sailing vessel “shall not impede” a vessel that can safely navigate only within the channel. What constitutes “impeding”? The Rules do not say. How narrow must a channel be? The skipper must judge.

Rule 17(a)(ii) permits the stand-on vessel to “take action to avoid collision by her manoeuvre alone” when it becomes apparent the give-way vessel is not taking appropriate action—but when does this become apparent? The Rules offer no numerical answer. Meanwhile, closure rates mount.

The fundamental difference from aviation is stark: TCAS removes interpretation entirely. “Climb, climb!” is not subject to debate. The skipper of a sailing yacht, by contrast, may find herself running through a decision tree of extraordinary complexity: Is that a sailing vessel or power? Which tack are they on? Are we crossing or overtaking? Is this a narrow channel? Am I impeding or do I have right of way? What do they think the situation is?

The COLREGs assume that both parties will correctly analyse the situation and reach the same conclusion. When they don’t—and they often don’t—the result is confusion, near-misses, and sometimes tragedy.

TAWS: Seeing the Ground Before You Hit It

The Terrain Awareness and Warning System addresses a different but equally lethal threat. The original Ground Proximity Warning System (GPWS), developed by C. Donald Bateman at Honeywell in the 1960s, used a radar altimeter to measure the aircraft’s height above the ground directly below. If the aircraft was descending too rapidly toward terrain, alarms would sound.

But early GPWS had a critical limitation: it could only see straight down. If an aircraft was flying level toward a rising mountain slope, the system wouldn’t detect the threat until it was too late.

Modern TAWS (also called Enhanced GPWS or EGPWS) solves this through a combination of GPS positioning and a worldwide digital terrain database. The system knows where every mountain, hill, and obstacle is located. It continuously compares the aircraft’s current position and trajectory against this database, providing advance warning of terrain conflicts—even terrain that lies ahead rather than below.

When TAWS detects a threat, pilots hear unmistakable warnings: “Terrain, terrain—pull up!” or “Too low—terrain!” The system has been credited with virtually eliminating CFIT accidents among aircraft so equipped.

The Human Factor: When Technology Meets Culture

Technology alone cannot guarantee safety. The tragic 2002 Überlingen mid-air collision demonstrated this with devastating clarity.

On the night of 1 July 2002, a Bashkirian Airlines Tupolev Tu-154 carrying 69 people (including 52 children) and a DHL Boeing 757 cargo aircraft were on collision course over southern Germany. Both aircraft were equipped with TCAS. The system worked exactly as designed—it commanded the Tupolev to climb and the Boeing to descend.

But at almost the same moment, an air traffic controller—working alone and overwhelmed—radioed the Tupolev with instructions to descend. The Russian crew, trained that controller instructions took precedence, obeyed the human rather than the machine. The Boeing crew followed their TCAS and descended. Both aircraft flew into each other. All 71 people aboard both aircraft perished.

This lesson—that safety systems must be used identically by all parties, everywhere—has profound implications for maritime technology. Ambiguity kills.

Data Buses: The Nervous System of Modern Aircraft

For those familiar with NMEA 2000 on boats, aviation’s approach to data communication offers interesting parallels—and contrasts.

The dominant standard in commercial aviation has been ARINC 429, first deployed in the early 1980s on aircraft like the Boeing 757 and Airbus A310. Unlike NMEA 2000’s bidirectional network, ARINC 429 is strictly unidirectional: a single transmitter broadcasts to up to 20 receivers over a twisted pair of wires. This simplicity ensures deterministic timing—you always know when data will arrive—but requires many separate wire runs, adding weight and complexity.

Modern aircraft like the Boeing 787 use a mix of technologies: ARINC 664 (essentially deterministic Ethernet) for main systems, with CAN bus (ARINC 825) handling less critical functions. The overhead panel switches on a 787, for example, communicate via CAN bus—eliminating dozens of individual wire runs.

The maritime world, with NMEA 2000 built on CAN bus technology, has in some ways leapfrogged aviation’s legacy architecture. But aviation’s rigorous approach to redundancy, error detection, and deterministic behaviour offers lessons in how to make such systems truly trustworthy for safety-critical applications.

Regulatory Framework: How Aviation Enforces Safety

Aviation safety is not optional. The regulatory framework, coordinated through ICAO (International Civil Aviation Organization) and enforced by national authorities like the FAA (United States) and EASA (European Union), mandates specific equipment for specific operations.

TCAS II is required on all commercial aircraft with more than 30 seats or maximum take-off mass over 5,700 kg. TAWS is mandatory for turbine-powered aircraft with six or more passenger seats. These are not recommendations—they are legal requirements, and aircraft cannot operate without them.

The maritime world has traditionally taken a lighter regulatory touch. AIS (Automatic Identification System) is mandated for commercial vessels over certain sizes, but recreational craft are largely exempt. Collision avoidance remains fundamentally dependent on the Rules of the Road and human vigilance.

As technology makes sophisticated safety systems more affordable and practical for smaller vessels, the question arises: should maritime regulation follow aviation’s lead?

Lessons for the Maritime World

What can those of us working on maritime safety take from aviation’s experience? Several principles emerge:

• Automate the time-critical decisions. When closure rates exceed human reaction times, technology must intervene. TCAS doesn’t wait for the pilot to calculate trajectories—it does the mathematics continuously and issues commands when necessary.
• Make the warnings unambiguous. “Climb, climb!” leaves no room for interpretation. Maritime alarms too often tell the crew that something is wrong without clearly specifying what to do.
• Standardise globally. Überlingen taught aviation that national variations in safety procedures create deadly confusion. Any maritime collision avoidance technology must work identically worldwide.
• Design for the worst case. TCAS assumes that air traffic control has failed. TAWS assumes the pilot cannot see outside. Effective safety systems must function when everything else has gone wrong.
• Integrate rather than overlay. Modern aircraft systems share data seamlessly through standardised buses. Maritime systems too often remain islands, requiring the crew to mentally integrate information from multiple sources under stress.

The seas will never be as regulated as the skies. Recreational sailors will always enjoy freedoms that private pilots can only envy. But as technology makes sophisticated safety systems smaller, cheaper, and more practical, the opportunity exists to bring aviation-grade collision avoidance to vessels of all sizes. The question is not whether maritime safety technology should learn from aviation—it is how quickly we can apply those lessons to save lives at sea.

This article draws on publicly available information from aviation authorities, accident investigation reports, and industry sources. The author is a former pilot with direct experience of the systems described.