WEATHER BRIEF

Volcanic ash

Often invisible, fallout from volcanic eruptions poses risks to aircraft at any altitude. The ash can cause widespread flight ops disruption.


Hysplit numerical dispersion model output for an eruption of Mt Redoubt in Alaska. Hysplit output is one of several sources the VAACs use in issuing volcanic ash advisories and SIGMETS.

Volcanic ash SIGMETs are issued every 6 hours and updated every 6 hours. In general, a volcanic ash SIGMET will provide the relevant flight information region (FIR), latitude and longitude of the valid area, the valid times, the name of the volcano of interest, the vertical extent of the ash cloud, and the cloud's estimated movement.

Dealing with volcanic ash

Because the actual extent of the invisible edge of a volcanic ash cloud can only be estimated, there have been numerous instances of aircraft inadvertently penetrating ash clouds and being affected. In most cases, crew or passengers will only notice a weak sulfur smell. In other cases they could notice a general haze to the air.

In more concentrated amounts, ash may clog pitot tubes, which will show up as airspeed changes or a disconnection of autopilot. Communications also may appear garbled due to the conductivity of the ash, and St Elmo's fire may appear around the windscreen. Clogging of ports can also affect pressurization systems, causing a reduction or loss of cabin pressure. At worst, higher concentrations can have the aforementioned effects on the powerplants.

If operating in areas of suspected volcanic ash, follow your aircraft or company operating procedures. AIM 7-5-9, Flight Operations in Volcanic Ash, also addresses this situation. If ash cloud penetration is suspected, the best immediate action is a course reversal away from the plume. And if you know the relative location of the volcano that produced the cloud, such as from volcanic ash advisories and SIGMETS, you can probably determine where the nearest clear air resides. If not, a descending 180° turn is advised.

Pilots should also reduce power to lower hot section temperatures below the ash melting point of around 1100° C and prepare any essential aircraft systems for a possible engine failure. The crew should also go on oxygen in case of cabin depressurization. Importantly, pilots should monitor their airspeed, altitude and engine readings carefully for any trends away from what is expected.

Once in the clear, filing a pirep using the categories on the Volcanic Activity Reporting (VAR) form (AIM 7-5-9, Appendix 2) will help refine the ash advisories and SIGMETS and keep other aircraft in the clear. It is also requested that pilots file the VAR after landing. On the ground, the aircraft should be inspected by an A&P before any further flight to ensure that it is still airworthy.

Ash avoidance

Of course the best course of action is to remain clear of any ash cloud in the first place. After the eruption of Eyjafjallajökull in 2010, much of the European airspace closed for a week at a cost to aviation of around $2.6 billion.

EasyJet, Airbus and Nicarnica Aviation developed and tested an instrument called the Airborne Volcanic Object Imaging Detector (AVOID) that employs infrared cameras mounted on an aircraft to "see" ash up to 100 km away, even when it is invisible to the pilots' eyes. Such technology holds great promise for helping pilots avoid future ash cloud encounters altogether.

Karsten Shein is a climatologist with NOAA in Ashe­ville NC. He formerly served as an assistant professor at Shippensburg University. Shein holds a commercial license with instrument rating.


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