The Vulcan XH558 & General Aviation Thread

[Mike Bull]

Silica bags in '558's intakes-

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[Renegadenemo]

Just some late night pondering - how can a silica gel bag wreck an engine? Seeing as they're stuffed a few inches from the 1st stage compressor, so far as I can see in the pic, and might be left there by accident (I tell complaining customers that the cockup occurred because our factory is staffed by humans) are they not made of paper with gel crystals small as sand grains so they can harmlessly blow out the other end if the engine gets a mouthful of them?
Or am I missing something? Stuart?

[kneeslider]

My thoughts too Bill. I would have thought that any jet would be more than man enough to chomp up a couple of bags of silica gel.

Still, a change in procedure, and having each bag attached to a piece of string wit a 'Remove Before Flight' label hung over the end of the intake would be a no cost, low aggro permanent solution, but then we all live and learn.

I remember reading about Buccanneer operations off carriers in the '70s. At the end of an operation, the flight deck crew would empty a can of WD40 into the intakes, in an effort to staunch the dreaded corrosion.

Flight saftey is all about thoroughness and making sure that mistakes made in the past don't happen again, not pointing fingers.

[mtskull]

how can a silica gel bag wreck an engine? ...are they not made of paper with gel crystals small as sand grains so they can harmlessly blow out the other end if the engine gets a mouthful of them?

Probably because a bag filled with very small crystals the size of sand grains behaves under impact in a very similar way to a sandbag.....

[Renegadenemo]

Probably because a bag filled with very small crystals the size of sand grains behaves under impact in a very similar way to a sandbag.....

I would agree if they were thrown in there at takeoff power and the bags were strong enough to keep the mass of crystals together long enough but they seem to be hung on the engine inlet and must surely have been slurped off there very early on. I wonder if they've jumped onto the static inlet vanes and partially occluded it so the engine would run satisfactorily at low power settings but has surged once the demands overtook what could flow past them and, if so, why would you put something up there sufficiently robust to do that or not have an equally robust process to make sure they were removed every time without fail? I don't know... where's Stuart when we need him?

[Stuart Baker]

Bill,

Sorry. Some of us have to work for a living and can't spend all day on bulletin boards

The bags used in the intake are not like those you get in the box with a new camera - more like the size of one of those neck cushions you see for sale a motorway services. However, as I've said before I'm not one to speculate on the reason why one or more was left behind before departure. The enquiry is in progress and I'm sure we'll get a report.

Personally I think that the Vulcan team have done a very brave thing in releasing as much information as they have as the internet is now unfortunately littered with vitriolic comments. I have met many of those who keep this beautiful aircraft flying and worked with them on a number of issues. They are nothing less than absolutely professional and are extremely conscientious. One look at the photgraphs of the hanger will show you (http://www.flickr.com/photos/vulcanphot ... otostream/)

As to the sequence of events, I'm sure that those with the data will provide the complete story in the fullness of time. However, from the statement on the Vulcan website it sounds like first engine failure was surge leading to compressor blade failure rather than the other way round. Whether the compressor blades were damaged by the bag going through the engine first will not be clear until the engine has been stripped down. Who knows if the silica may have melted in the combustion chamber and solidified on turbine blades affecting the aerodynamics and causing the surge... This is much like the concern about flying through volcanic ash (if you've never read this from BA's in house magazine it is well worth a look: http://www.ericmoody.com/)

S

[mtskull]

how can a silica gel bag wreck an engine? ...are they not made of paper with gel crystals small as sand grains so they can harmlessly blow out the other end if the engine gets a mouthful of them?

Another possibility could be that the bags themselves didn't do any damage to the compressor blades but once the bags had split, the silica would be unlikely to "harmlessly blow out the other end". That would be more likely to happen with a modern high bypass ratio engine but the Olympus is a pure turbojet so all of that silica would have to pass through the hot section before encountering an obstacle in the form of a nice hot HP turbine. Experience with volcanic ash ingestion would suggest that the silica would fuse on to the turbine as a glassy substance and drastically reduce its efficiency.
Possibly the ensuing loss of power to the HP compressor stage could have resulted in a surge, which, even leaving aside the possibility of blade separation, would have chucked any remaining silica forwards out of the intake, to be ingested by the adjacent engine.

[mtskull]

Whoops, must have made that post about a second after Stuart succinctly covered all of the same points..

[Piston Broke]

This is much like the concern about flying through volcanic ash

Watch this on discovery not long back and had the same thought just before Stuart came on

[Renegadenemo]

They are nothing less than absolutely professional and are extremely conscientious. One look at the photgraphs of the hanger will show you (http://www.flickr.com/photos/vulcanphot ... otostream/)

I have to say that this is something on which everyone ought to agree and losing their engines is a real tragedy. I hope they get the old girl back in one piece and back in the air soon-as.

I also remember when the BA 747 lost its engines in the volcanic ash cloud and later reading about how a glass-like deposit was left on the hot components only to break off again as the engines cooled allowing them to be restarted - sort of. Is this what happens to silica gel? We have lots of it in the workshop - I feel an interesting experiment coming on.