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 under control. I trust you are not in too much distress.
A Boeing 747 without engine power has aerodynamic qualities only marginally superior to a brick. From its cruising altitude of 36,000 feet, the aircraft could glide about 170 kilometres, just enough to reach the airport at Jakarta provided they could get over the 12,000 foot mountains first. Landing without power would have been a huge challenge for the pilots, but it’s something that has been achieved successfully in other incidents.
Just before they reached the mountains and had to decide if they were going to get over them, the crew managed to restart the engines and were able to climb gently, and eventually landed safely at Jakarta, although one engine failed again on the way, and the cockpit windows had been rendered opaque by sandblasting.
Of course, what had happened is that the 747 had flown through a volcanic plume of dust, from Mount Galunggung in Indonesia. The engines had ingested the dust, which had melted and jammed everything. Dust had also been forced backwards down the fuel system’s vent pipes and had contaminated the fuel, although there hadn’t been time for this to cause further problems before the landing. When the stopped engines cooled enough, chunks of the solidified volcanic glass broke off inside the engines and were blown out by the airflow, and remarkably, the engines were resilient enough to be restarted.
Jet engines run so hot that the turbine blades have to be made out of ceramic materials, which can resist the heat but are fairly brittle. It was probably very lucky that lumps of broken glass rattling through the engine didn’t break any. In the event, all four engines were too damaged for further service, but the repaired plane continued flying passengers until 2009.
The volcanic ash hadn’t been recognised as a hazard at the time, unlike today with big areas of airspace over Europe being closed to flights. The airlines don’t like it because they’re losing large amounts of money, and want the flight restrictions eased. Several airlines, including British Airways, have flown test flights and said “look, we didn’t crash: everything was fine”.
Well, yes, but that doesn’t prove anything. We already know that aircraft are fine if they don’t fly through the ash. I’m not hearing about these test airliners being taken for a complete strip-down. If they were, and the engineers could say that there was evidence of ash ingestion and the damage was insignificant, that would be reassuring. If there’s no evidence of ash, then the plane didn’t fly through a dangerous region and it’s not surprising that everything is normal.
So far, I’ve only heard of one aviation incident involving the Icelandic ash plume. Five F-18 fighters from the Finnish air force accidentally flew into the ash last Thursday causing damage to three of them and of those, one had an engine completely destroyed. Now, that’s evidence.
So the real question is whether the weather forecasters and vulcanologists can tell where the ash is to a level of accuracy that will allow airliners to avoid it. That doesn’t appear to be the case at the minute. I’d keep everything grounded until we can be sure.