WX BRIEF

Volcanic ash

Drifting clouds of fine silica-rich particles pose severe danger to all forms of aviation.


Settling ash from Mt Pinatubo, Philippines in Jun 1991 damaged this World Airways DC10 at Cubi Point NAS. Ash mixed with rains from Typhoon Yunya, which struck the island at the same time, creating a highly corrosive slurry that also triggered massive mudslides across the country.

Since ash carries a mild charge, this is quite likely. Once you recognize that you may have penetrated an ash cloud, there are a few things you can do. A safety article from Boeing on volcanic ash (boeing.com/commercial/aeromagazine/aero_09/volcanic_story.html), for example, recommends the following steps.

Since the greatest danger to the aircraft will be the loss of engine power, the advisory suggests reducing throttles to idle and disengaging any autothrottle to ensure lowered engine heat.

Attempt an immediate exit from the ash cloud, preferably by making a descending 180° turn, because there may be no way to know the full extent of the plume.

Engaging anti-ice systems and air conditioning packs will add bleed air that should increase engine stall margins, while starting an APU, if available, will provide an alternative power source for engine restart and critical avionic and control systems.

The crew should also be on supplemental oxygen as the ambient air may become unbreathable. Also, monitor EGT closely. Increasing EGT is a sign of ash congestion, and the engine may need to be shut down and restarted to avoid permanent damage.

Finally, remember to fly the aircraft by reference to pitch and airspeed, keeping in mind that your airspeed indicator may be unreliable.

Avoiding the plume

Major ash plume from the eruption of the Chaiten volcano in the mountains of Chile on May 3, 2008, as seen by a NASA satellite. The plume, which rose to about 56,000 ft, moved quickly on the prevailing winds to cover hundreds of miles of airspace from Chile into the South Atlantic.

Of course, the best course of action is to stay well clear of any volcanic ash plumes. Before you leave the ground, you should look for and carefully read any volcanic ash Sigmets.

Plot their coordinates on a map if possible—they will form a polygon that you can plot relative to your route.

Pay close attention to the upper and lower bounds of the plume forecast. Often, plumes occupy a relative narrow range of altitudes and won’t generally occur much above their forecast ceiling (although falling ash can be a danger all the way to the surface beneath the plume).

Reference pilot reports as well. Reports of no ash at altitudes above the plume can confirm the forecast altitudes. Second, know the prevailing winds around the plume. Any deviation around the plume should be upwind of it and at least a few hundred miles clear of it on the sides.

Also, strong winds will tend to push the plume further from its source, but may also not disperse it as much laterally as more gentle winds might do. And, lastly, do not depend on onboard radar to identify an ash cloud. Keep an eye out for a brownish haze or other indicators instead.

If you do fly inadvertently into an ash cloud, take immediate action to extricate your aircraft with minimal danger. Once you have cleared the cloud, file a Pirep to help the VAAC refine its advisories and your fellow pilots avoid the same situation.

Karsten Shein is a climatologist with the National Climatic Data Center 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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