Mid-Air Aircraft Collision Avoidance Strategies

How do pilots and air traffic controllers make sure aircraft do not make unwanted contact with one other while in flight?



January 11, 2024

DALLAS — Commercial aviation employs various strategies to prevent mid-air collisions, which are accidents that occur when two or more aircraft unintentionally make contact while in flight. These collisions can have dire consequences, including fatalities, damage to aircraft, and destruction of ground infrastructure from debris.

Mid-air collisions can happen due to factors such as miscommunication, navigational errors, lack of situational awareness, deviation from flight plans, lack of trust in the cockpit, and the absence of collision avoidance systems and terrain awareness warning systems. While such collisions are rare due to the vastness of open airspace, they are more likely to occur near airports where aircraft are closely spaced.

To prevent mid-air collisions, protocols and cutting-edge technology have been established in commercial aviation. These measures ensure a respectable level of safety for passengers and crew, making air travel the safest mode of transportation. Here are some strategies employed to prevent mid-air collisions:

  1. Enhanced Communication: Effective communication between pilots and air traffic controllers is crucial to preventing miscommunication and errors.
  2. Improved Navigation: Accurate navigation systems and adherence to flight plans help maintain proper separation between aircraft.
  3. Situational Awareness: Pilots and air traffic controllers must maintain situational awareness to be aware of the position and intentions of other aircraft in their vicinity.
  4. Collision Avoidance Systems: Aircraft are equipped with collision avoidance systems that use radar and other technologies to detect and alert pilots of potential collisions.
  5. Terrain Awareness Warning Systems: These systems provide pilots with information about the terrain and obstacles to avoid collisions with the ground or other structures.
  6. Air Traffic Control: Efficient air traffic control systems and procedures help manage the flow of aircraft and maintain safe distances between them.
  7. Training and Education: Pilots and air traffic controllers undergo rigorous training to enhance their skills and knowledge, including collision avoidance techniques.
  8. Continuous Monitoring and Improvement: Aviation authorities and organizations continually analyze incidents and accidents to identify areas for improvement and implement necessary changes.

In this post, we'll go over some of these strategies, but rest assured that commercial aviation is continuously improving safety measures to minimize the risk of mid-air collisions and ensure the safety of passengers and crew.

Photo: Lorenzo Giacobbo/Airways

1. Proper Pressure Altimeter Sub-Scale Setting

According to the settings made by the flight crew, an aircraft's altimeter is a device that measures altitude above the ground, sea level, or standard pressure datum (1013.25 hPa).

The altimeter's subscale settings for atmospheric pressure at airport elevation (or runway threshold), mean sea level pressure (QNH), and standard pressure (QNE) determine aircraft height above ground level, altitude, and flight levels, respectively.

Failure to set the pressure altimeter to the proper barometric sub-scale pressure setting, as per air traffic control (ATC) instructions, could result in significant deviations from the cleared height, altitude, or flight level. This can lead to level bursts, loss of separation from other traffic, and potential collisions with other aircraft or terrain.

To avoid such scenarios, pilots must correctly set the figure and change the pressure setting at the appropriate points during departure, climb, descent, or approach. ATC, automatic terminal information service (ATIS), or VOLMET, communicate this information.

In uncontrolled airspace, pilots use regional pressure settings (also known as regional QNH) to ensure they maintain a safe altitude for terrain clearance in areas with limited local QNH or radio communication. Regional QNH represents the atmospheric pressure at the highest elevation in the region, and pilots maintain a safe altitude above it.

Regional QNH is calculated as the minimum value of forecasted QNH in a region for a given time.

Pilots usually make the correct pressure setting on the altimeter sub-scale for proper altitude indication: Photo: Jinyuan Liu/Airways

2. Navigation lights and Right-of-way rules

Navigation lights, also known as running or position lights, are essential lights on aircraft that help prevent collisions by providing information about the aircraft's position, heading, and status. These lights are color-coded with red, green, and white.

Typically, red and green navigation lights are located on the left and right wingtips of an aircraft, respectively. The white navigation light is usually placed as far back as possible on the tail or vertical stabilizer. These navigation lights must be turned on from dusk till dawn or as directed by the authorities.

To ensure safety and avoid potential disasters, pilots use right-of-way rules in conjunction with navigation lights when aircraft are on a collision course. In a head-on scenario, where both aircraft see each other's red and green lights, neither aircraft has the right of way. In this situation, pilots must immediately change course to the right to avoid a collision.

When aircraft are on a crossing course, the pilot in the aircraft on the left sees the red navigation light of the other aircraft, while the pilot in the aircraft on the right sees the green navigation light. Regardless of their speed, the pilot on the left, seeing the red light, must give way by veering to the right, while the pilot on the right, seeing the green light, has the right of way and maintains course and speed.

If a faster aircraft is approaching from behind and on a collision course with another aircraft, the pilot of the aircraft in front may not be aware of the danger. However, the pilots of the aircraft behind can see the situation. In this case, the aircraft in front has the right of way and maintains speed and heading. The pilots of the overtaking aircraft must give way by veering to the right to avoid a collision.

Image: The Navigators. Original file: Eurocontrol - Vector redrawing and enhancement of, Public Domain


Traffic Collision Avoidance System (TCAS), also referred to as Airborne Collision Avoidance System (ACAS), is a system that utilizes secondary surveillance radar (SSR) transponder signals to enhance awareness of surrounding aircraft and reduce the risk of mid-air collisions, independent of Air Traffic Control (ATC).

According to the International Civil Aviation Organization (ICAO), any aircraft with a maximum take-off mass (MTOM) exceeding 5700 kg (12600 lb) or authorized to carry more than 19 passengers must be equipped with TCAS/ACAS. The technology is divided into two generations: TCAS I and TCAS II.

TCAS I, the first-generation technology, can monitor the flow of traffic around an aircraft within a range of approximately 26 NM (48 KM) and provide information regarding the bearing and altitude of the surrounding aircraft. It is capable of issuing "Traffic Advisory" (TA) alerts, which notify the pilot of potential collision risks. However, it is the pilot's responsibility to determine how to avoid the potential collision based on the TA alert.

On the other hand, TCAS II, the second-generation technology, can simultaneously track up to 30 aircraft within ranges of 14 NM (26 KM) and 30 NM (55 KM) for mode A/C targets and mode S targets, respectively. It provides the pilot with specific instructions known as "Resolution Advisory" (RA) on how to maneuver the aircraft to avoid a collision with the detected traffic. The RA may instruct the pilot to descend, climb, or adjust vertical speed.

By utilizing TCAS/ACAS, aircraft operators can enhance situational awareness and take proactive measures to prevent mid-air collisions, ensuring the safety of the aircraft and its occupants.

TCAS helps pilots ensure sufficient spacing between their aircraft: Photo: Michael Rodeback/Airways


A ground proximity warning system, commonly abbreviated as GPWS, is a system designed to alert pilots when an aircraft is at imminent risk of colliding with the ground or other obstacles. The system issues advisory alerts and mandatory response warnings to the flight crew when the aircraft is near the terrain. The crew must then initiate a terrain avoidance maneuver by increasing engine thrust and climbing away from the terrain.

The GPWS tracks patterns in the radar (radio) altimeter readings of the aircraft between 50 and 2450 feet above the ground. If the aircraft is in a configuration that could potentially result in a collision with the terrain or an obstruction, the system provides visual and audible alerts to the crew.

There is a more advanced version of the GPWS called the Enhanced Ground Proximity Warning System (EGPWS), also known as the Terrain Awareness Warning System (TAWS).

The EGPWS combines a radio altimeter with a GPS to accurately determine the height of the ground directly in front of the aircraft. This is achieved by comparing the aircraft's precise location, determined by GPS technology, with a comprehensive global digital terrain database. The cockpit terrain display provides pilots with visual orientation, highlighting high and low regions close to the aircraft. This helps pilots make informed decisions to prevent collisions with terrain or objects.

The GPWS and EGPWS/TAWS systems play a crucial role in enhancing flight safety by providing pilots with timely and accurate information about potential terrain-related hazards.

GPWS warns pilots on the aircraft's proximity to terrain: Photo: Michael Rodeback/Airways


Reduced Vertical Separation Minimum, commonly known as RVSM, is a regulation that requires a minimum vertical separation of 1000 feet between aircraft up to Flight Level 410 (41,000 feet). Above Flight Level 410, the minimum separation increases to 2000 feet due to the reduced accuracy of the pressure altimeter at higher altitudes.

This regulation ensures that aircraft on nearly opposite magnetic courses, such as those flying at 090 degrees and 270 degrees, will never be assigned the same flight level or altitude, thus eliminating the risk of collision.

To maintain separation and avoid potential collisions, aircraft flying magnetic courses from 000 degrees to 179 degrees are assigned odd numbers in thousands of feet for IFR flights (e.g., 11,000 feet or 13,000 feet). For VFR flights, they are assigned odd numbers in thousands of feet plus 500 feet (e.g., 11,500 feet or 13,500 feet).

Conversely, aircraft flying magnetic courses from 180 degrees to 359 degrees are assigned even numbers in thousands of feet for IFR flights (e.g., 10,000 feet or 12,000 feet). For VFR flights, they are assigned even numbers in thousands of feet plus 500 feet (e.g., 10,500 feet or 12,500 feet).

This system of assigning flight levels and altitudes based on magnetic courses ensures proper separation between aircraft, reducing the risk of mid-air collisions and promoting safe air travel.

Aircraft have pitot tubes for measuring airspeed. Photo: By User: Kolossos - Own work, CC BY-SA 3.0

6. Vx Airspeed

When operating an airplane, Vx is the airspeed commonly used for the best angle of climb. It is typically maintained during the initial climb out after takeoff to safely clear obstacles such as trees, buildings, and hills that may be located close to the end of a short runway.

Flying at Vx airspeed allows the aircraft to climb to a higher altitude while covering the minimum horizontal distance, reducing the risk of collision with obstructions.

The specific Vx airspeed varies depending on the type and model of the airplane, and it is influenced by various factors. These factors include the aircraft's weight, altitude, temperature, air density, wind conditions, and the configuration of the landing gear and flaps.

As altitude, aircraft weight, and temperature increase, more engine thrust is required, resulting in a decrease in the angle of climb and an increase in the Vx airspeed. Conversely, a tailwind decreases the climb angle by increasing the ground speed and extending the horizontal distance traveled. Conversely, a headwind increases the climb angle by reducing the horizontal distance covered by the aircraft.

By carefully managing the Vx airspeed and considering these factors, pilots can ensure a safe climb that allows for obstacle clearance and efficient aircraft performance.

Flights have to initially climb at Vx to clear obstacles near the runway: Photo: Liam Funnell/Airways

7. Communications

To ensure the safe and efficient flow of air traffic, pilots and air traffic controllers (ATC) maintain continuous communication throughout all stages of flight, including preflight, takeoff, departure, en route, descent, approach, and landing. This communication is facilitated through radio transmissions on specific frequencies or the use of light signals.

ATC relies on a comprehensive system of computers, radars, VHF (Very High Frequency) and HF (High Frequency) radios, and reference materials to communicate with pilots effectively. The primary objective of this communication is to prevent mid-air collisions and mitigate the effects of wake turbulence. Radar technology plays a crucial role in ensuring that aircraft maintain a safe distance from one another during the en route phase of flight.

By utilizing these communication and radar systems, ATC can regulate the flow of air traffic, provide guidance on routes and altitudes, and ensure the safety of all aircraft in the airspace. This collaborative effort between pilots and ATC is essential for the overall safety and efficiency of air travel.

Minimum Grid Area Altitude. Image: Extract from Rogers Data GmbH.

8. Safety, Minimum Grid Area Altitude

During flight planning, flight dispatchers and pilots collaborate to determine the safest and most cost-effective route and flying altitude. They rely on navigation charts to make these decisions.

The first consideration is the Safety Altitude, which ensures adequate obstacle clearance within a 5 NM (9.26 KM) radius on all sides of the true track. To calculate the Safety Altitude, the highest obstruction within the defined area is identified, and the measurement is rounded to the nearest 100 feet. If the result is 5,000 feet or less, 1,000 feet is added to it. If the result is 5,001 feet or more, 2,000 feet is added.

To minimize the risk of collisions with tall structures, non-mountainous areas are given a 1,000-foot buffer. Mountainous areas, on the other hand, are allocated a 2,000-foot buffer to provide sufficient altitude for recovery in case of hazards associated with mountain flying, such as turbulence, microbursts, aircraft icing, mountain waves, and degraded aircraft performance.

Additionally, to enhance safety during Visual Flight Rules (VFR) flight when pilots may be unsure of their position or lose sight of the ground, the Minimum Grid Area Altitude, also known as the Minimum Off Route Altitude (MORA), is used. MORA is determined by identifying the highest figure within the grids or rectangles passed by the true course, even if it appears that the high ground is located behind the course.

By considering Safety Altitude, MORA, and other factors, flight dispatchers and pilots can plan routes and altitudes that prioritize safety and ensure efficient and smooth flights.

Sun Country Airlines N814SY Boeing 737-800. Photo: Andrew Henderson/Airways

9. Proper Airspace, Procedure Design

Under ICAO Docs 8168 (PANS-OPS) and 4444 (PANS-ATM), careful planning and design of routes, airspace structure, holding patterns, and ATC sectorization in both terminal and en-route airspace are crucial to reducing the potential for level bust incidents, which can lead to mid-air collisions.

Proper airspace design ensures that arriving, departing, and in-flight aircraft can operate without the need to cross paths or pass through each other's flight levels. This helps to maintain clear separation between aircraft and minimizes the risk of conflicts.

Additionally, the design of airspace takes into consideration the clearance of obstructions along approach and takeoff flight paths. By ensuring that obstacles are cleared, the risk of loss of separation between aircraft and the occurrence of Controlled Flight Into Terrain (CFIT) incidents is minimized.

By adhering to the guidelines outlined in ICAO Docs 8168 and 4444, aviation authorities and air traffic controllers can effectively plan and design airspace to promote safe and efficient air travel, reducing the likelihood of level-bust incidents and mid-air collisions.

Flights at LGW will be capped at 825 per day in July and 850 in August. Photo: Gatwick Airport.

10. Short Term Conflict Alert (STCA)

STCA (Short-Term Conflict Alert) is a computer-based safety program specifically designed to assist air traffic controllers in preventing collisions between aircraft. Its primary function is to swiftly alert controllers to any potential or actual violations of separation minima.

The STCA operates as a predictive safety net, utilizing surveillance data from radars, ADS-B (Automatic Dependent Surveillance-Broadcast), or multilateration, along with environmental data and optional flight plan information. As the system operates in the background, air traffic controllers are generally unaware of its functioning unless a potential violation of separation is detected.

Upon identifying a violation that could potentially result in a collision, the STCA promptly issues warnings to the air traffic controller, highlighting the precarious situation and identifying the conflicting aircraft involved. The system is designed to look ahead for approximately two minutes, as extending the look-ahead duration would only result in an increased number of potentially disruptive notifications.

By providing real-time alerts and aiding in the identification of potential conflicts, the STCA plays a crucial role in enhancing air traffic safety. It assists air traffic controllers in proactively maintaining separation between aircraft and mitigating the risk of collisions.

In the wake of the recent events involving Japan Airlines (JL) Flight 516, we have reposted a closer look at how airports avoid aircraft collisions on the ground.

Featured image: United Airlines Boeing Fleet. Photo: Boeing