Some 5 days before the beacons would have stopped emitting signals, the search team finally found EgyptAir MS804 cockpit voice and flight data recorders at the bottom of the Mediterranean Sea. Why the search of the flight recorders is so difficult and what are other ways to improve it?


90-day rule

Each of the black boxes is mounted with a beacon that emits a “ping” at 37.5kHz once per second as soon as it comes into contact with water. The signals can be detected by sonar receivers up to 3,000 m to 5,000 m deep. However, if the aircraft sinks to the bottom of the sea, the search vessels have to be almost directly under the beacon to hear the ping. To make matters more complicated, the beacon might be shielded by wreckage or ocean topography.

“The signal may also get refracted by layers of water at different temperatures, making the source very difficult to pinpoint,” explains David Barry, Senior Lecturer of Aviation Safety at Cranfield University.

In order to get closer to the signal, hydrophones can be lied down under the water, but this requires a special equipment, that may take weeks to arrive at the search zone. 

According to FAA’s Technical Standard Order that was released in 2012, the batteries that power underwater beacons should last for thirty days. That is why time plays such a crucial part in locating the black boxes. In most cases the period has been sufficient, but after the challenging search for Air France Flight 447 recorders that took nearly two years, there has been an international effort to take additional safety precautions.

EASA enacted a new rule for mandatory 90-days underwater beacons transmitting time to be effective from 2021. In addition, airliners will have to carry another beacon that emits a signal at 8kz frequency, increasing the speed of recorders’ retrieval.

“This lower [8kz] frequency has a greater range and can be detected by military equipment which might reach the scene faster,” comments David Barry. 


Deployable recorders

As an aftermath of EgyptAir crash, some experts argued that the black boxes could eject from the tail of the aircraft before the accident removing the need for similar searches altogether.

“If we have a deployable recorder it will be much easier to find,” said Charles Champion, Airbus Executive Vice-President for Engineering.

Allegedly, deployable recorders could be installed outside the aircraft and the small explosion at the beggining of the accident would eject them from the plane. What is more, the deployable recorders could be mounted with special die, that would paint the surface of the water, making it easier to locate them. Similar technology is used on military aircraft, however, safety experts note that the drawbacks of the solution could be secondary damage after being ejected and harm to the other people.

”The deployment leads to the possibility of an accidental deployment in normal operating circumstances that could in turn cause an incident or accident,” says Scott Montgomery, Engineering Product Manager at Universal Avionics Systems Corporation. 


Triggered transmissions

Another possible solution is making a satellite connection that can send data from cockpit voice and flight data recorders to air traffic controllers. Cranfield University researchers are working on algorithms which could detect the early stages of an unusual situation and start transmitting data via satellite or trigger deployable recorders in potentially dangerous situations. At the moment, Scott Montgomery remains critical on the cost-effectiveness of the solution.

“The bandwidth required to send large amount of data from the aircraft to a ground station would be cost-prohibitive,” says Scott Montgomery.

However, David Barry is looking at the future for 10-20 years and hopes similar solutions will be viable, and will provide data to investigators to detect the crash site faster. 



Prospects for the future

Generally, it is a rare occurrence when the black boxes from a crash site are not located. Only around twenty times investigators have not been able to locate the flight recorders over the last fifty years. The most notable case was MH370 flight from Malaysia to Beijing in 2014. The plane disappeared from air traffic controllers‘ radars over South China Sea. In 2015 it was confirmed that the battery of an underwater locator beacon had run out more than a year before the accident.

90-day beacon battery requirement, as well additional 8kz frequency and constant checks on batteries are steps in the right direction when searching for the black boxes. However, until the method of data streaming becomes cost-effective, maintaining continuous position of the aircraft by ground stations is all we have to hope for faster retrieval of flight recorders.