Explainer: 5G, Radio Altimeters & Flight Safety

New Delhi: Major US airlines including American, Delta, United and Southwest are concerned. Several foreign carriers from Asia, Middle East and Europe have cancelled US-bound flights. Air India cancelled over a dozen US flights on Wednesday.

Why has the 5G rollout in the US sparked flight safety concerns among airlines and US aviation regulator FAA?

The primary concern is that signals from 5G telecom towers located near airports could interfere with the radio altimeter of certain aircraft types, affecting flight safety.

The 5G spectrum will operate in the 3.7-3.98 GHz bandwidth. Radio altimeters of certain aircraft types operate in the 4.2-4.4 GHz range. The gap in both bandwidths is quite close, triggering fears that 5G signals could interfere with radio altimeters signals.

Modern aircraft are fitted with two types of altimeters:

Barometric or pressure altimeter

It works on the principle that atmospheric pressure decreases with altitude. It can be calibrated by the flight crew to indicate altitude above sea level or the ground over which the aircraft is flying. For most phases of flight, the crew refers to it to maintain altitude.

Radio altimeters

The radio altimeter transmits radio waves to the ground, then determining an aircraft’s height by measuring the time a radio signal takes to travel to the ground and return. The radio altimeter indicates height of an aircraft at 2,500 ft or below. This is the instrument whose accuracy could be compromised by 5G signals.

How critical is a radio altimeter?

A radio altimeter measures the distance between the aircraft and the terrain directly below it, making it a critical navigation equipment during the landing phase of a flight — during instrument approaches, when an aircraft is close to the ground in low or poor visibility and windshear encounters, when an aircraft sinks rapidly.

The Ground Proximity Warning System (GPWS), which alerts crew if an aircraft is dangerously close to terrain, also draws readings from the radio altimeter.

Several computerised automatic flight guidance systems which help descend, decelerate and stop an aircraft are also coupled with radio altimeter data, such as the autothrottle, which automatically adjusts engine power without any manual inputs from the pilot, and thrust reversers, which slow down an aircraft after landing.

For example, even if an aircraft has touched down but a faulty radio altimeter indicates that it’s still airborne, then the automatic flight guidance systems may prevent the speed brake and reverse thrust to deploy and safely decelerate the aircraft to a stop. Fed faulty radio altimeter data, the flight computer may prevent the aircraft from transitioning to ‘ground’ mode after touchdown.

“5G interference can result in degraded deceleration, increased landing distance and runway excursion,” the FAA has warned.

Turkish Airlines flight 1951 crash

A faulty radio altimeter, compounded by errors on the part of the flight crew, caused the crash of a Boeing 737 on February 25, 2009 during final approach to Amsterdam’s Schiphol Airport.

At 1,950 ft above ground level, the radio altimeter erroneously showed -8 feet. The altimeter had malfunctioned several times in the days before the accident and engineers had not been able to fix it.

Because of the faulty data, the flight computer sensed that the aircraft was about to touch down. Still descending, a landing gear warning went off as the computer believed the plane was near the ground without its landing gears down.

The flight computer then set the autothrottle to idle, which went unnoticed by the pilots. When the crew finally caught the error and took corrective action, it was too late. Still airborne, the aircraft lost speed, stalled and crashed from a height of 450 ft, killing 128 passengers and seven crew members.