Geoscientist Online

If your flight has been cancelled today because of high level volcanic ash from Iceland, read this. Ted Nield* describes the first time a pilot (just) survived an encounter with volcanic dust, 11km high.

Geoscientist Online 15 April 2010

“Good evening ladies and gentlemen. This is your captain speaking. We have a small problem. All four engines have stopped. We are doing our damnedest to get them going again. I trust you are not in too much distress.”

Captain Eric Moody, flight announcement, 24 June 1982.

On the clear, moonless night of 24 June 1982, Scheduled BA flight 009 took off from sweltering Kuala Lumpur bound for Australia. There were 249 passengers on board the plane, which was laden with 91,000kg of fuel for the five-hour flight to Perth.

At the controls was Captain Eric Moody (pictured recently). As they levelled out at the 747’s cruising altitude of 11,300 metres, the crew ate their evening meal, just as the flight passed south of the city of Jakarta.

Everything was routine; though “routine” for a jumbo jet involves some fairly impressive figures. For example, through each of the aeroplane's four Rolls Royce engines, 150 tonnes of air was being sucked every minute. Think about what volume of air you need – even at sea level - to weigh a hundred and fifty tonnes.

This may be some help. A cubic metre of air at sea level weighs 1.204kg at 20 degrees Celsius, according to the International Standard Atmosphere. But at the cruising altitude of flight BA 009, one cubic metre of air only weighs about 360 grams; so the 600,000kg of air that Speedbird 9 needed every minute occupied a space of about 1.67 million cubic metres. This means that, up there, in a mere 10 hours, one thousand million cubic metres of the Earth’s atmosphere will pass through a 747's engines. On its round trip, the KL to Perth service would gobble up about a cubic kilometre of the Earth's atmosphere. These facts were to prove significant in the tense moments ahead.

The function of all this air entering the maw of the jet’s engines is to undergo compression and mix with some of the 91,000 kg of kerosene that is squirted into it from fine nozzles. These are surrounded by "swirl vanes" that break the fuel into a fine mist - rather like the spiral grooves inside the head of a domestic spray gun. The mixture is then immediately ignited. Oxygen in the air combines with the fuel, at over 700°C. The massively expanding exhaust gases exit the four engines and thrust the aeroplane forwards at 800km per hour.

The speed of the plane was also to be significant in the coming moments. The difference in speed between plane and air means that anything in the air will collide with it at about 222 metres per second. A rifle bullet, by comparison, can travel at anything between 180 and 1220 metres per second. So it is a good thing, then, that under normal conditions there are no rocks in the air, 11,300 metres up.

Unfortunately on that night, there were - but nobody could see them because their average diameter was only seventy five thousandths of a millimetre. Worse still, the temperature in the engines was high enough to melt finely powdered rock.

His dinner eaten, Captain Moody left the cabin and made his way down the spiral stairs to the first class section in search of an unoccupied toilet; but before he could find one he was called back. Moody remembers noticing as he turned some little puffs of what seemed to be smoke issuing from vents on the floor. There was also an odour that reminded him of the smell left behind after electrical sparks have flown.

When Moody reached the cockpit the crew had already switched on the seatbelt signs and the engine igniters, to support the combustion of the fuel, just in case. The windscreens were lit up with the most impressive displays of St Elmo’s fire the captain had ever seen. The weather radar, though, showed nothing unusual. Then the First Officer pointed out that all the engines appeared to be lit from within by electrical discharges. In moments, the St Elmo’s fire changed to something resembling tracer bullets. Almost immediately, Roger the Senior Engineer called: “Engine failure number four”.

There was a pause. Then he announced: “Engine failure number two….three’s gone…. they’ve all gone.”

Moody said: “OK Roger, put out a Mayday”.

13.44 “Jakarta, Jakarta, Mayday, Mayday Speedbird 9. We’ve lost all four engines. We’re leaving 370.”

Captain Moody struggled to understand what was happening, because it contradicted many of the assumptions that underlay the emergency training of every airline pilot. Four engines simply don’t fail, for a start; but when rehearsing for that near-impossible scenario, simulators assume that there would also have been massive electrical failure too, leaving everyone in near darkness. Yet the instruments were all working and the lights all on. The auto-pilot was also in control. The displays, moreover, were contradictory. Airspeed was falling, yet some lights suggested the failed engines were overheating. The Jumbo had now become a glider, but Eric Moody put it into a shallow descent on autopilot and bought some time to think the situation through.

As the other crew members went through the manuals performing the necessary checks, Moody thought about what it could mean. Electrical failure was ruled out. He checked the fuel flow. He checked for icing. Nothing. Everyone on the flight desk remembered thinking one thing: “What have we cocked up?”

As they descended through 7900 metres the cockpit pressure warning horn blew and the crew put their oxygen masks on. Roger, the First Officer, found to his dismay that, for no apparent reason, the mask fell apart in his hands. This forced a decision on the Captain to begin an emergency descent to an altitude where the air was more breathable, rather than risk losing his first officer to anoxia.

They were now heading back to Jakarta, but the safe limit around that mountainous region is about 3200 metres. The crew decided they would turn the plane back to sea and ditch if they fell below 3600 metres. Moody also kept the landing gear up, just in case they had to land on water but found themselves unable to retract it again. At about 6100 metres, Eric slowed the descent; but by this time Roger had been able to put his mask back together.

All during this sudden steep descent, the crew’s continuing attempts to re-light the engines had been visible to the passengers sitting aft of the wings, who now thought that all four were on fire. While the crew wrestled with conflicting airspeed indicators and wondered if they had been trying to re-ignite the engines outside their re-light envelope, the pressure in the passenger cabin fell enough to deploy oxygen masks, which promptly dropped about the passengers’ ears from the overhead lockers. It was at this point that Captain Moody thought it prudent to deliver his reassuring message, quoted in the epigraph.

Moody was now faced with the possibility that he might have to attempt a touchdown on the sea with all engines failed – something discouragingly known in the trade as a “deadstick landing”. He remembers thinking about how with his father, he had watched from Hythe Pier as the flying boats landed in the channel, and reflected that flying boats never flew in darkness because of the near impossibility of judging a plane’s height above water at night. But at this moment engine number four, the first to fail, became the first to re-ignite. Another 90 very long seconds passed before the other three did likewise just as the flight hit 3600 metres – the point of no return.

13.57 “Speedbird 9. We’re back in business. All four running. Level 12,000.”

Fifteen minutes had passed since the Mayday, but the adventure was not quite over. The crew wanted to climb to a safer height to clear the mountains ahead and climbed to 4572 metres. But no sooner did they reach that height than the St Elmo’s fire started again. Moody was convinced that this phenomenon was somehow connected to the engine failure but decided to descend to a level where the fires went out. As they came in to land, the crew also found that they were unable to see the landing lights properly and asked for them to be turned fully up. Slowly it dawned on the crew that the front windscreen was almost opaque. They landed partly by squinting through of the outer edge of the left hand front window, the only area that was still transparent. The passengers loudly applauded the final smooth touchdown.

Dusty answer

Two days later, the crew had confirmation of what had caused the incident, but Barry the Flight Engineer already knew. As they had waited for the passenger steps to be rolled out, he had noticed that his hands and uniform were covered in fine, grey dust. In fact they had flown into a cloud of volcanic ash, emanating from Mt Galunggung, 110 miles east of Jakarta. The plume only became visible to the weather satellite photographs of the time some time later.

This was what had blasted the windscreen to opacity and taken the paint off the plane's leading edges, but it was the engines that were the worst affected part, as they each processed their 1.67 million cubic metres of air per minute. In that volume of air, dust clouds do not have to be very thick for tonnes of the stuff to lodge in the hot moving parts. What is more, apart from wearing them away, the silicate mineral particles melt and fuse as they come into contact with hot components.

If you have ever owned a domestic anthracite furnace, you will know that it is often necessary – perhaps after a period of particularly cold weather when the fire has been allowed to burn high - to rake out clinker, a solid rocky mass of coal-ash (the unburned silicate mineral portion of the coal) which has fused at high temperature into a lump that will not pass through into the cinderbox, clogging the fire. Steel and coke furnaces also have to be regularly cleared of such clinker.

The first effect that the dust had on the Rolls Royce engines of BA009 was to change the shape of the blades by abrasion and so impair the efficiency of compression. Because it had been the first to be shut down, engine no. 4 was the least damaged in this respect. Ash filtered into and blocked the aircraft's pitot tubes – forward-pointing instruments that use outside air pressure to work out the aircraft's speed – so explaining why the crew received conflicting airspeed readings. Environmental control system ducts were worn away from the inside by the abrasive dust, explaining why the floor vents had appeared to be smoking. And in the engines themselves, the swirl vanes surrounding the fuel nozzles, which turn the fuel into a mist before burning, were also clogged. This meant that although the nozzle was able to pass fuel at the design flow-rate, the clogged vanes inhibited its atomisation, making re-starting of the engines more difficult than usual.

Apart from the obvious reason why having the engines of a jet airliner cut out all at once is undesirable, and even assuming the pilots do the right thing and land the plane safely, the repair bill is also not a trivial consideration for airlines. Seven years later, on 14 December 1989, Redoubt Volcano in Alaska erupted. The next day, a KLM 747 with 231 people on board entered its ash plume at 7620 metres. After an experience very similar to that of BA009, involving a steep dive of over three kilometres, the crew landed the aircraft safely at Anchorage on two engines. But the repairs to the plane – including a paint job and replacing all four engines cost $80 million.

What to do

Obviously the best advice to any pilot about flying into ash plumes is "don't". Hence today’s disruption.

* Dr Ted Nield is the Editor of Geoscientist and GeoscientistOnline