CNA Explains: What happens when a plane flies into a volcanic ash cloud?

SINGAPORE: Hundreds of travellers were stranded or had their Bali vacations cancelled when airlines on Wednesday (Nov 13) halted flights after a nearby volcano erupted, spewing a 9km ash tower into the sky.
The next morning, however, several airlines were quick to resume flights to the Indonesian holiday destination, with some carriers noting “improved” conditions.
Volcanic ash contains fine ash and sharp glass-like particles. When that enters a jet engine, it can lead to severe damage and potentially cause the engine to malfunction.
Jet engines draw in air, compress it, mix it with fuel and ignite it. This process generates high-pressure exhaust gases that propel the engine and the aircraft forward.
To function properly, engines rely on a precise balance of fuel and airflow. Disrupting this airflow, as volcanic ash does, can create problems.
Aviation safety consultant Michael Daniel told CNA that in the worst-case scenario, the abrasiveness of volcanic ash on the engine blades can cause an engine “flame-out”, or shutdown.
And yes, there have been instances where engines flamed out due to flying through volcanic ash clouds, he added.
Flame-outs can then lead to loss of thrust and propulsion and altitude drops among others.
Additionally, the high temperatures inside the engine also melt the volcanic glass in the ash, causing it to stick to and clog engine parts, according to the UK Civil Aviation Authority.
The International Civil Aviation Organization (ICAO) meanwhile wrote in a publication how volcanic ash can block plane sensors, resulting in unreliable airspeed indications and erroneous warnings.
Ash can also render windscreens partially or completely opaque – an immediate visibility hazard – and even contaminate cabin air.
Volcanic ash can drift in dangerous concentrations hundreds of kilometres downwind from an erupting volcano. This means early detection and avoidance are crucial for flight safety.
According to ICAO, the aircraft operator is typically responsible for monitoring volcanic activity and flight operations, under the oversight of their respective state regulatory authority.
Before flying in airspace forecasted to be contaminated by volcanic ash, operators must complete a safety risk assessment, which must be evaluated and approved by the relevant civil aviation authority.
Globally, meteorological agencies also work closely with aviation authorities to quickly disseminate information about eruptions.
Nine Volcanic Ash Advisory Centers (VAAC) are tasked by the ICAO with monitoring the movement of volcanic ash within their assigned airspace.
Satellite technology these days is “quite good” as it shows both the magnitude and density of ash clouds, said Mr Daniel, who sits on the panel of experts for the Association of Aerospace Industries (Singapore).
This then helps airlines and authorities make decisions on avoiding them.
He added that air traffic controllers also play a key role in directing flights to prevent aircraft from flying through ash clouds.
While ash clouds might be visible during the day and in good weather, it gets harder to see at night.
Plus, radar cannot detect small particles of volcanic ash, according to an article by SKYbrary, a portal set up by ICAO and other aviation safety organisations.
Former airline engineering executive Chow Kok Wah concurred, saying volcanic ash cannot be detected by commercial airplane radar.
“So there is no way to divert until the airplane is inside the ash cloud. Once inside the ash cloud, the smell of ash will be apparent inside the airplane.”
“Visual glow from static discharges will be observed on the cockpit windshields and the engine inlets,” said Mr Chow, who has more than 30 years of experience in aerospace.
Mr Daniel said, however, that depending on the concentration of ash particles, pilots may still be able to visually spot ash clouds.
If dense enough, pilots can rely on various radar systems and satellite imaging to avoid them just as they would with thunderstorms, he said, suggesting that an up to 20-mile deviation around a cloud would suffice to avoid problems.
In the case of the Bali volcano eruption, however, the ash clouds were considered too severe to fly around or over safely.
The flight cancellations indicate that authorities were unable to provide alternative routes, which could also be due to air traffic congestion, Mr Daniel added.
According to the SKYbrary article, there are several actions crew can take to mitigate the danger.
First, the pilot should reduce engine thrust to lower the core temperature of the jet engine. It should be lowered below the point at which silicate ash particles melt, which is around 1,100 degrees Celsius.
“If this action is not taken at or before the first signs of engine malfunction then flame-out is likely to follow if clear air cannot be regained,” the article said.
“Terrain permitting, a descending 180-degree turn is likely to be the fastest way out of the contaminated air,” it added.
“Under no circumstances should a climb be attempted as an escape option.”
Monitoring the engine is also critical during these manoeuvres, particularly the exhaust gas temperature, which could rise dangerously if significant ash contamination occurs, said SKYbrary.
The portal noted ways to recover aircraft systems from potential engine failure, including increasing the flow of bleed air to prevent the engine from stalling.
Bleed air is compressed air taken from the engines of an aircraft, which helps keep the engine running.
The auxiliary power unit, which serves as an independent power source for the aircraft, should also be operated to maintain a source of bleed air and electrical power, said SKYbrary.