How does volcanic ash affect airplanes?

 

Volcanic ash in the atmosphere may pose a serious hazard to aircraft in flight. That is why the aircraft should avoid volcanic ash encounters. It comprises minerals unique to the volcanic eruption. Minerals common to most volcanic ash are silica together with smaller amounts of the oxides of aluminum, iron, calcium, and sodium. The glassy silicate material is hard and extremely abrasive. Its melting point is below jet engine burner temperature that introducing additional hazards.

Furthermore, a volcanic ash cloud may be accompanied by gaseous solutions of sulfur dioxide (when combined with water create sulphuric acid), chlorine (when combined with water create hydrochloric acid), and other chemicals which are corrosive to the airframe and are hazardous to health.

Another term you may need to know it, the volcanic cloud, which is the sum of the material ejected from a volcano into the atmosphere and transported by winds aloft. It comprises volcanic ash, gases, and chemicals.

Before I go deeper by my explanation, you need to have an idea about the following two terms:

Affected area :

A volume of airspace, an aerodrome, or another area on the ground, identified by VAA / VAG and/or SIGMET as being affected by known or forecast volcanic cloud contamination.

 

Note :

VAA - Volcanic Ash Advisory

VAG - Volcanic Ash Advisory in graphical form

SIGMET- information concerning en-route weather phenomena which may affect the safety of aircraft operations. 

Danger area :

A volume of airspace identified by NOTAM as being affected by levels of known or forecast volcanic cloud contamination which States judge merit publication to operators.

Note :

NOTAM - A notice distributed through telecommunication containing information concerning the establishment, condition, or change in any aeronautical facility, service, procedure, or hazard, the timely knowledge of which is essential to personnel concerned with flight operations.

 

Effect of Volcanic Ash on Aircraft

It is easy to imagine the hazard that volcanic ash poses to an aircraft that encounters it in the atmosphere. The effects are:

1-   Effect on Jet Engines

Volcanic ash mixtures are introduced through the fans. Three effects contribute to the overall engine damage.

The first (see number 1 in the below image)

It is the most critical, is the fact that volcanic ash has a melting point below jet engine operating temperatures with thrust settings above idle. The ash melts in the hot section of the engine and fuses on the high-pressure nozzle guide vanes and turbine blades. This drastically reduces the high-pressure turbine inlet guide-vane throat area causing the static burner pressure and compressor discharge pressure to increase rapidly which, in turn, causes engine surge. This effect alone can cause immediate thrust loss and possible engine flame-out.

The second (see number 2 in the below image)

The volcanic ash being abrasive also erodes compressor rotor paths and rotor blade tips (mostly high-pressure section), causing loss of high-pressure turbine efficiency and engine thrust. The erosion also results in a decrease in the engine stall margin.

The third (see number 3 in the below image)

The ash can clog flow holes in the fuel and cooling systems. Such a condition would render engine restart very difficult. 

 

2-   Effect on Airframe and Equipment

In addition to engine abrasion, volcanic ash abrades cockpit windows, the leading edges of the flight surfaces, and the tailfin and can "sandblast" the paint from the airframe. Any parts protruding from the airframes such as antennas, probes, ice detectors, and angle of attack vanes can be damaged and may be rendered inoperable. From the safety standpoint, the abrasion of the cockpit windows reduces the cockpit crew forward visibility and can present a serious problem during landing. Damage to the antennas can lead to a complete loss of high frequency (HF) communications and a degradation of very high frequency (VHF) communications. Damage to the various sensors can seriously degrade the information available to the pilot through the cockpit instruments. Thus, rendering control of the aircraft difficult.

 

Note :

British Airways B747 made an emergency landing at Jakarta in 1982 following its encounter with volcanic ash.

 

Now, you may ask, is there any aircraft accident due to flying aircraft through volcanic ash or ash clouds?

There have been no known crashes as a result of volcanic ash ingestion. A total of 253 encounters with volcanic ash or gas clouds between 1953 and 2016 were identified, of which 122 occurred in the period 2010-2016. Significant effects worth mentioning include:

-  blocked pitot tubes

-  replacement of damaged engines

-  multiple engine failure

-  electrical and computer failure

-  physical restrictions (breathing, speech, nausea, eye irritation)

 

Summary

Volcanic ash poses to aircraft, like with most threats in aviation, the best course of action is to avoid it in the first place.

As part of the preflight planning, if a flight is going to fly through an area that could be affected by volcanic ash.  Airline flight planners can adapt the route to avoid the worst of it.

 

Volcanic ash damaged the jet turbine engines, abrades cockpit windows, airframe, and flight surfaces, clogs the pitot-static system, penetrates air conditioning and equipment cooling systems, and contaminates electrical and avionics units, fuel and hydraulic systems, and cargo-hold smoke-detection systems.

 

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The references used in writing this article are:

1-   ICAO DOC 9974, Flight Safety and Volcanic Ash, Risk management of flight operations with known or forecast volcanic ash contamination.

2-   ICAO DOC 9691, Manual on Volcanic Ash, Radioactive Material and Toxic Chemical Clouds.

3-   German Aerospace Center (DLR), Flying into Volcanic Ash Clouds: An Evaluation of Hazard Potential.