FLYING IN VOLCANIC CLOUDS
Coen van Wyk
We, the inhabitants of South Africa, have to contend with arms procurement corruption, theft of donor funds, members of parliament chanting "kill the farmer, kill the boer" during their election campaigns, fuel price increases, capital gains tax, pilot licence scams, drivers licence scams, vehicle registration scams, incipient land invasions, record levels of aids, hijackings, nationalisation of mineral resources, and all types of emulations of past leaders of the African continent who successfully lead their countries back into the middle ages, but we do not even have a single active volcano.
It is said that every dark cloud has a silver lining, and the silver lining of volcanic clouds is that they do not gather over South Africa. What follows will give some perspective regarding the "aviational" blessing of a relatively volcano free continent.
I discovered an article tiled, "Historic Incidents", during my visit to the website of the International Aviation Safety Association. The article makes mention of more than 80 reports of aircraft entering ash clouds during the past 20 years and mentions that some of the volcanoes where aircraft encountered ash are: Gallunggung (Indonesia), Pinatubo (Phillipines) Sakurajima (Japan), Pacaya (Guatemala), Galeras (Columbia), Hudson and Lascar (Chile), Mt. Spurr (Alaska), Nyamuragira (Zaire), Sheveluch (Russia), Manam (Papua New Guinea), Redoubt (Alaska), Augustine (Alaska), and Kluchychevskoi, Russia. The article goes on to stress that the list depicts only a small proportion of the incidents where aircraft encountered ash clouds and then deals with an incident concerning Redoubt volcano in Alaska.
The Redoubt volcano incident
The article alleges that the incident at Redoubt volcano is one of the most famous and dramatic accounts of an aircraft entering an ash cloud. It states that, in order to "provide…an idea of the sense of confusion and panic that abounds during such an emergency" it provides the following excerpt of the incident (taken from an article by Ernest E. Campbell titled "Recommended Flight-Crew Procedures if Volcanic Ash is Encountered"):
"Redoubt volcano, near Anchorage Alaska, began erupting on December 14, 1989. On the following day, a 747-400 airplane powered by GE CF6-80C2 engines entered an ash cloud at 25,000 ft. and experienced flameouts on all four engines.
During descent to 25,000 ft., the airplane entered a thin layer of altostratus clouds when it suddenly became very dark outside. The crew also saw lighted particles (St. Elmo's fire) pass over the cockpit windshields. At the same time, brownish dust with a sulfurous smell entered the cockpit. The Captain commanded the Pilot flying to start climbing to attempt to get out of the volcanic ash. One minute into the high-power climb, all four engines flamed out. Due to the volcanic ash and dust in the cockpit, the crew donned oxygen masks.
The Pilot Flying noticed the airspeed descending, initially at a normal rate (given the airplane's altitude) but suddenly very fast. All airspeed indications were then lost due to volcanic dust contamination in the pitot system. At the same time, there was a stall warning and the stick shaker was activated with no signs of buffeting. The Pilot Flying rather firmly put the nose of the aircraft down to avoid a stall and initiated a turn to the left in a further attempt to get out of the volcanic ash.
The crew noticed a "Cargo Fire Forward" warning and deduced that the fire warning was caused by the volcanic ash, so no further action was taken.
As the engine spooled down, the generators tripped off and all instrument were lost except for instruments powered by the batteries.
During the time the engines were inoperative, the cabin pressure remained within limits and no passenger oxygen masks deployed. The crew elected not to deploy the masks because the passenger-oxygen-mask system would have been contaminated by volcanic dust in the cabin air.
An emergency was declared when the airplane passed through approximately 17,000 ft. The crew stated that total of seven or eight restart attempts were made before engines 1 and 2 finally restarted at approximately 17,200 ft. Initially, the crew maintained 13,000 ft. with engine 1 and 2 restarted, and, after several more attempts, engines 3 and 4 also restarted.
After passing abeam and east of Anchorage at 11,000 ft, the airplane was given radar vectors for a wide right-hand pattern to runway 06 and further descend to 2,000 ft. The Captain had the runway continuosly in sight during the approach; however, vision throught the windshields was impaired due to "sandblasting" from the volcanic ash in such a way that the Captain and the First Officer were only able to look forward with their heads positioned well to the side. Finally the airplane did land safely, but approximately 80 million dollars was spent to restore the plane, which included replacing four engines. The in-depth account of this incident helped researchers devise a procedure of what a crew should do when they encounter an ash cloud."
The effects of an ash cloud on aircraft
Micheal G Dunn and Douglas P Wade (Influence of Volcanic Ash on Gas Turbine Engines) conducted various experiments on gas turbine engines and some of their conclusions can be summed up as follows:
· The presence of St. Elmo's glow 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 and the control system of the engine.
· 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 therefore 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.
· Engine operation time in a dust-laden environment should be limited and if entrance into the cloud is unavoidable thrust needs to be reduced.
· 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.
What to do in an emergency
According to Campbell, if an aircraft does enter an ash cloud, there are very specific steps that the flight crew must take in order to increase the chance of making it out of the dangerous area safely. The following are some of his recommendations in dealing with the situation:
"Immediately reduce thrust to idle. This will lower SGR, which in turn will reduce buildup on the turbine blades and hot-section components. The volcanic dust can cause rapid erosion and damage to the internal components of the engines.
Autohrottles off (if engaged). The autothrottles should be turned off to prevent the system from increasing thrust above idle. Due to the reduced surge margins, limit changes with slow and smooth thrust-lever movements.
Exit volcanic cloud as quickly as possible. Volcanic ash may extend for several hundred miles. The shortest distance/time out of the dust may require an immediate, descending 180 degree turn. Setting climb thrust and attempting to climb above the volcanic cloud is not recommended due to accelerated engine damage/flameout at high thrust settings.
Engine and wing anti-ice on.
All air conditioning packs on.
Start the auxiliary power unit (APU), if available. The APU can be used to power the electrical system in the event of a multiple-engine power loss.
Oxygen mask on and 100%, if required.
For systems with autostart, switch to "on" position.
If necessary, shut down and then restart engines to keep from exceeding EGT limits.
Close the outflow valves.
Do not pull the fire switch.
Leave the fuel boost pump switches "on" and open cross-feed valves.
Do not use fuel heat"
A final remark
For those of us who are not old hands at dodging volcanic ash clouds on their overseas flights and who may one day find themselves on the wrong side of Mount Nyamuragira in Zaire, the contents of this article may become relevant.