What Happens to the Plane?

In order to understand the danger associated with encountering a volcanic ash cloud it is important to comprehend the damage that the ash causes to the aircraft. Different levels of damage can occur depending on various parameters like, the density and size of the ash cloud, the components of the cloud, and the melting temperature of the different constituents. In addition, the type of aircraft and the instruments on it is also important. In order to determine the effects of entering an ash cloud has on an aircraft Gunn and Wade conducted various experiments on gas turbine engines. They concluded:

  • The presence of St. Elmo's glow (which results from a discharge of electrostatic charge that has built up because of dust particles striking the moving metallic surfaces), at the engine face is indicative of dust in the environment.

  • The manner in which the engine will behave in a dust environment is dependent upon the dust concentration, the dust constituents the operating turbine-inlet temperature of the engine, and the engine control system.

  • The turbine inlet temperature required to cause material deposition on the hot-section components is approximately 2,000 degrees F (1,094 degrees C). Many older engines, which operate at lower temperature, will not experience deposition but will experience compression system erosion, The newer engines will experience both deposition and erosion unless the thrust level is reduced in order to lower the turbine inlet temperature.

  • It must be emphasized that engine operation time in a dust-laden environment should be limited. If entrance into the cloud is unavoidable then the air crew is advised to reduce thrust, exit the cloud, and carefully monitor engine diagnostics (especially the EPR, EGT, fuel flow, core speed, and fan speed). This combination of parameters will help determine the degree of engine damage and how the control system is handling the problem.

  • Prolonged operation in the dust-laden environment may result in permanent engine damage. If the aircraft is operating at high altitude, the surging associated with this type of damage may cause engine flame-out. If the fuel nozzles have not become covered with carbon, then it should be possible to restart the engine at lower altitudes. Some of the deposited material will have blown off during the period of time that the engine was inoperable.

  • A significant amount of material will make its way through the environmental control system and into the electronics cabinets and the cabin itself. The particle size of this material is on the order of 6 micrometers.

    The following photographs illustrate the damage that is done to specific parts of the engine:

    High-pressure nozzle guide vane assembly removed from Rolls-Royce RB211-524C2-19 after volcanic ash encounter shows heavy deposits of resolidified ash on nozzle guide vane leading edges. This assemblage was removed from the British Airways B-747 that suffered a temporary four-engine flameout after flying through an ash plume from Indonesia's Mt. Galunggung on June 24, 1982.

    Photo and description provided by Airline Pilot

    This photograph shows the fuel nozzles with the swirl vanes, center hole and carbon like deposits labeled. The center hole of the nozzle, from which the fuel is sprayed, was opened and capable of passing fuel at the design flow rate however, the swirl vanes were plugged, thus inhibiting atomization of the fuel.

    Photo and description provided by Dunn and Wade

    The lower blade of the second-stage fan is new, while the upper blades show erosion caused by ash. Throughout the compressor, tip-region erosion occured on almost every stage.

    Photo and description provided by Dunn and Wade

    Photograph of high-pressure compressor ninth-stage rotor. This shows an example of the ninth-stage compressor blade row. The trailing edge of the airfoil in the tip region became so thin that the material folded away from the pressure surface.

    Photo and description provided by Dunn and Wade

    Photograph of environmental control system plumbing showing erosion of duct walls. Scale in inches.

    Photo and description provided by Dunn and Wade

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