Volcanic Ash Clouds and Heavy Precipitation

Volcanic Ash Clouds

Refer to OM Part C.
Flying through an ash cloud should be avoided by all means because of extreme hazard for the engines and the aeroplane. Volcanic ash may extend for several hundred miles, and eruptions may send ash plumes up to 40.000 ft.

Do not rely on weather radar to detect ash clouds or volcanic dust as they cannot be detected by the weather radar.

If the aeroplane enters an ash cloud the flight crew can expect:

• Smoke, dust or acrid odours similar to electric sparks in the cockpit;
• At night heavy static discharges around the windshield;
• A decrease in engine rpm, a decrease in fuel flow, and an increase in EGT;
• Engine surge or overheat;
• Engine flame-out;
• Unreliable airspeed indication;
• Pressurisation and electrical systems may be affected;
• At night, St.Elmo’s fire or other static discharges accompanied by a bright orange glow in the engine inlets.

When encountering ash cloud the following procedures are recommended:

• Fly on the upwind side;
• Turn on continuous ignition;
• Declare an emergency;
• Reduce power to idle to provide additional engine stall margin and lower turbine temperature;
• Try to escape the ash cloud by descending and flying an 180° turn (if terrain clearance permits);
• Check the applicable procedure in QRH / FCOM;
• Do not climb in order to overfly the ash cloud;
• Monitor attitude versus airspeed;
• Keep as many engines running as possible;
• Restart any engine which was shut down due to EGT exceeding the limits. If an engine fails to restart repeated attempts should be made immediately to avoid solidification of molten ash on the turbine blades;
• Turn on total anti-ice (engine and wing) as this will provide an additional engine stall margin by reducing engine pressure.

Following an encounter with an ash cloud, a landing at the next suitable airport is recommended. If visibility through the windshield is impaired a runway with autoland capability should be considered. Flight crews are requested to make PIREP when the volcanic eruption is observed or when volcanic ash cloud is observed or encountered.

Information on volcanic activities, including pre-eruption activity, volcanic eruption and volcanic ash clouds is transmitted to aeroplanes operating on routes that could be affected. This information is also published by NOTAMS and SIGMETS up to a 12-hour validity period and, if necessary, recommendations on re-routeings will be included.

Heavy Precipitation

Heavy precipitation in flight can be in the form of rain, snow or hail. It is usually encountered in the vicinity of storm clouds and/or frontal edges. Extremely heavy precipitation has been known to cause jet engines to flame out, therefore under these conditions, the engine ignition should be switched on. In combination with low-temperature, severe airframe and engine icing can occur therefore the aeroplane anti-icing system should be activated and monitored. The weather radar will be showing strong returns and should be used to select the safest routing through the storm activity.

There is likely to be associated turbulence with the heavy precipitation and so prompt action to secure the flight must be taken. Loss of orientation should be guarded against during the change over from instruments to visual flying during the approach, especially in snow showers and blowing snow. In falling snow and blowing snow, landing lights should be used with caution as the reflected light may actually reduce the effective visibility and even cause a false impression of drift during flare and roll-out. On the ground, contaminated runways may influence the performance, crosswind limitations and give a risk of aquaplaning. The respective procedures in OM Part B must be followed.