Volcanic ash and aviation safety
Plumes of
In 1991, the aviation industry decided to set up Volcanic Ash Advisory Centers (VAACs) for liaison between meteorologists, volcanologists, and the aviation industry.[3] Before 2010, aircraft engine manufacturers had not defined specific particle levels above which they considered engines at risk. Airspace regulators took the general approach that if ash concentration rose above zero, they considered airspace unsafe, and consequently closed it.[4]
The costs of air travel disruption in Europe after a volcanic eruption in 2010 forced aircraft manufacturers to specify limits on how much ash they considered acceptable for a jet engine to ingest without damage. In April, the UK CAA, in conjunction with engine manufacturers, set the safe upper limit of ash density at 2 mg per cubic metre of air space.[5] From May 2010, the CAA revised the safe limit upwards to 4 mg per cubic metre of air space.[6]
To minimise further disruption that this and other volcanic eruptions could cause, the CAA created a new category of restricted airspace called a Time Limited Zone.[7] Airspace categorised as TLZ is similar to airspace under severe weather conditions, in that restrictions should be of a short duration. However, a key difference with TLZ airspace is that airlines must produce certificates of compliance for aircraft they want to enter these areas. Any airspace where ash density exceeds 4 mg per cubic metre is prohibited airspace.[citation needed]
Volcanic ash in the immediate vicinity of the eruption plume is different in particle size range and density than that in downwind dispersal clouds, which contain only the finest particle sizes of ash. Experts have not established the ash loading that affects normal engine operation (other than engine lifetime and maintenance costs). Whether this silica-melt risk remains at the much lower ash densities characteristic of downstream ash clouds is currently unclear.[citation needed]
Experts recognised that there was an issue following
Volcanic hazards to aviation
Volcanic ash consists of small tephra, which are bits of pulverized rock and glass less than 2 millimetres (0.079 in) in diameter created by volcanic eruptions.[10] The ash enters the atmosphere from the force of the eruption and convection currents from the heated air, and is then carried away from the volcano by winds. The ash with the smallest size can remain in the atmosphere for a considerable period of time, and can drift away from the eruption point. The ash cloud can be dangerous to aviation if it reaches the heights of aircraft flight paths.
Pilots can't see ash clouds at night. Also, ash particles are too small to return an echo to on-board weather radars on commercial airliners. Even when flying in daylight, pilots may interpret a visible ash cloud as a normal cloud of
Volcanic ash has a melting point of approximately 1,100 °C (2,010 °F), which is below the operating temperature of modern commercial jet engines, about 1,400 °C (2,550 °F). Volcanic ash can damage gas turbines in a number of ways. These can be categorised into those that pose an immediate hazard to the engines and those that present a maintenance problem.
Immediate hazards to aircraft
Volcanic ash is composed of fragments of rock, crystalline material, and volcanic glass. The glass component has the lowest melting temperature—lower than temperatures inside the combustor of a gas turbine engine. Ash that finds its way into the combustor may melt. Combustor and turbine components are cooled, as the metals they are made of have lower melting temperatures than the gas temperature inside the engine core. Molten ash that touches these surfaces is likely to freeze, and accrete on the metal surface.
The most sensitive surface is the high-pressure turbine nozzle guide vanes (NGVs), situated immediately downstream of the combustor. The gas flow is choked through the NGVs, and so the flow area through the NGVs is a controlling area for the engine. If this area is reduced due to an accretion of ash, a smaller mass flow rate of gas passes through the engine core. Reduced mass flow leads to the
Volcanic ash carries significant electrostatic charge. Fine ash that enters electronic components within the engine or airframe can cause electrical failure—which poses an immediate hazard to the aircraft. [12]
Ash-induced problems requiring increased maintenance
- Volcanic ash, as a hard substance, damages gas turbine compressors. It erodes by impacting compressor blades and vanes and removing material—and abrades by three body interactions between the rotating blade, ash particle, and compressor annulus. Changing the shapes of the blades and vanes and increasing gaps between blades and annuli both help reduce engine fuel efficiency and operability.
- Molten ash that sticks to cooled surfaces can block cooling holes. This stops cooling air flow and heats surrounding metal, leading to accelerated thermal fatigue. This process affects combustor and turbine components.
- Ash can accumulate and partially block fuel thermal fatigue rate.[12]
Sulfur dioxide clouds
Sulfur dioxide—another product of volcanoes that is carried in ash clouds after an eruption—is corrosive to aircraft that fly through it.[8]
There has been an attempt to prove that the sulphur dioxide usually accompanying a volcanic eruption is indeed a good indication of the presence of ash clouds such as to facilitate avoidance of ash clouds in aviation.
However, it has been found that the two species of clouds tend to separate due to windshear. Additionally, the detection methods have limitations, as both species have the potential to be masked by other types of aerosol, such as water or ice; this contributes to great variability in the data.
Therefore, as there is no consistent overlap between SO2 and the ash, SO2 is not a reliable indicator for ash clouds.[13]
Accidents and incidents
In 1982,
In 1989,
References
- ^ "USGS: Volcano Hazards Program". volcanoes.usgs.gov.
- ^ "Volcanic Ash - SKYbrary Aviation Safety". www.skybrary.aero.
- ^ "Volcanic Ash–Danger to Aircraft in the North Pacific, USGS Fact Sheet 030-97". pubs.usgs.gov.
- ^ "Can we fly safely through volcanic ash?".
- ^ Marks, Paul (2010-04-21). "Engine strip-downs establish safe volcanic ash levels". New Scientist. Retrieved 2019-11-12.
- ^ "UK ash cloud restrictions lifted". BBC News. May 17, 2010.
- ^ "Changes to the operating procedures in the vicinity of high ash concentration areas" (PDF). Archived from the original (PDF) on 2010-05-22. Retrieved 2010-05-18.
- ^ a b c "Overview of VAAC SACS workshop October 2006".[permanent dead link]
- ^ "International Airways Volcano Watch Programme".
- ^ "USGS: Volcano Hazards Program". volcanoes.usgs.gov.
- ^ Video on Dangers of Volcanic Ash by International Federation of Airline Pilots Associations
- ^ a b Institute of Mechanical Engineers Symposium: Aviation Safety in Volcanic Ash Clouds: Progress since E15. Nov 2013
- .
External links
- Foord, Colin (2010). "Planes and Volcanic Ash". Sixty Symbols. Brady Haran for the University of Nottingham.