How does an ash cloud stop jet engines?

[robertbartsch]

Can someone explain this please? Is this a question of the engine not being able to burn fuel from a lack of oxygen, a case of damaging internal parts or something else?

There was a case in the early eighties where 4 engines of a 474 stopped but they pilots were able to restart all 4. ...just wondering

Thx.

[Capt Claret]

Lack of O2 predominantly, damage can/does result. I think the 747 you refer to was only able to start 3 engines after its encounter.

[MFgeo]

There are 3 major ways in which volcanic ash particles damage jet engines:

1. Many of the ash particles are volcanic glass that melts at temperatures around 600-800 degrees Celsius, well below the combustion temperatures in modern jet engines. The melted material cools rapidly while traversing the turbine chamber, and end up deposited on the turbine blades, disrupting gas flow through the high-pressure turbine stages. This, by itself can lead to engine stoppage. This behavior of volcanic ash is distinctly different than conventional sand particles, which are mostly quartz and melt at around 1600 degrees Celsius.

2. Turbine blades, especially in the first stage would melt if not for a continuous flow of cooling air through an array of tiny holes distributed along the blade. The ash particles are small enough to enter the plenums and plug these holes, which can cause blade failures, or leave the blades operational but with greatly reduced service life, requiring expensive inspection and replacement activities well before normal service intervals.

3. The ash particles have extremely sharp edges, with hardness values similar to, in some cases higher than, the metals in the compressor and turbine blades. These particles passing through the engine cause rapid wear on the leading edges of blades, shortening service life, and in extreme cases disrupting air flow sufficiently to stop the engine. The ash particles are much more abrasive than conventional sand because most sand particles are highly rounded by the very geologic processes that produce sand.

In addition, the ash cloud may contain substantial amounts of gasses such as CO2 and/or SO2 which may reduce the O2 levels to below those needed for the engine to sustain combustion. However, the gasses disperse to sufficiently low concentrations so as not to interfere with combustion much closer to the source than do the ash particles.

[onetrack]

Here's the short record of the BA flight that ran into the Mt Galunggung volcanic ash cloud in Indonesia on 24th June 1982. All four engines were restarted, but one was shut down as a precautionary measure due to surging.
It's not mentioned in this abbreviated report, that severe ash particle-blasting of the cockpit glass rendered forward vision practically impossible, thus increasing crew stress levels.
The crew were unaware of the eruption due to the eruption information not being relayed to them, and they didn't see the ash cloud due to darkness.
There's no doubt, the lessons learnt from the BA incident are being applied today, with a great deal of safety margin in mind.

ASN Aircraft accident Boeing 747-236B G-BDXH Jakarta

[LeadSled]

Folks,
The BA Captain's interview with an Australian TV Channel was unforgettable.

In describing the final approach, with virtually nil forward visibility, he said (and it went out live) "It was like negotiating you way up a badger's a-----hole".

Tootle pip!!

[Starbear]

It was that BA incident, I believe, that produced what I consider one of the best lines in aviation that I have ever read/heard:

Despite any QRH or SOPs, "There were hands moving around the flightdeck like butterflies wings"

By the way, excellent descriptive answer MFgeo.

[brooksjg]

Thanks for the heads-up on the main problems caused by ash inside turbines.
It's still not clear to me what the RELATIVE risks are.
For example:

- anyone know what maxima are specified by engine suppliers for particles in the airflow (ie. number and size - I'd assumed there'd be datasheet items for a number of different combinations of size, characteristics such as melting point, abrasiveness, ..., and number per cubic metre of atmosphere. However, no-one so far (?) has identified such info.)

- Clearly, if there's any risk of significant build-up of melted and recongealed particles downstream from the combusters (on fixed OR moving surfaces), then the decision to fly or not should be risk-based, considering the particles per cube likely to be encountered on the flight, taking account of the different areas of ash that will be encountered, and therefore how much build-up could occur, depending on specific turbine characteristics.

- what about turbine cooling?Although it's not practical to filter ALL the air going through a large bypass turbine, what about the air that goes into the plenums feeding the turbine cooling system? Are there any filters already? Is it feasible to fit any (centrifugal type?)? (Please excuse my lack of turbine internals knowledge.)

If sensible maxima for volcanic ash particle density along routes is specified (anywhere, already?), and turbine cooling issues can be resolved, then what OTHER risks from low densities of ash also exist? (AirCon / pressurisation, instrumentation (pitots, etc.),...?

[TheChitterneFlyer]

brooksjg,

I'm not sure if you're aware of the monumental question content that you're asking.

The science of turbine cooling, alone, requires a firm understanding of the complexity of jet engine technology. The cooling air is derived from many different sources, all of which would be contaminated by the air entering the intake. As previously explained by MFgeo the volcanic particles are extremely hard; not to mention miniscule, which have abrasive qualities that are extreme. Due to the massive volume of air required by a jet engine it's not possible to filter the air entering the air intake; especially such fine particles similar to those of talcum powder!

The dammage to the compressor stage prior to the combustion process is (alone) allready contributing to a huge loss of engine efficiency. The fuel spray nozzles would very quickly become clogged, further increasing the likelyhood of flameout.

As for defining the particle count per cubic metre... where would you accurately gain such data? Risk another aeroplane incident by sending it into an ash cloud to gain such data!

Ask the crew of the BA Jakarta incident if they would again risk flying within such conditions and they would give you a resounding thumbs down. They were extremely lucky to get away with a successful landing. Were it not for the Flight Engineer continuing to make relight attempts they would have ended-up in the water; the Captain had already made the decision to ditch and the cabin crew were making those preparations to do so.

Engine technolgy has come a long way since that particular incident; however, the powers of nature are all encompassing... don't mess with volcano's!

TCF

[Otto Throttle]

Brooksig

A fine idea I'm sure, provided that you could gain accurate data on the constantly changing situation inside a volcanic ash cloud on a metre by metre basis, and that you could then find a suicidal pilot daft enough to take such data at face value and go blasting off through it with no way of monitoring whether or not the actual conditions they experience en-route match the predicted conditions.

Me, well I'll just stay down here thanks.

[sycamore]

For further reading,try the Mil.Forum,then `Finnish Hornet thread..

[lomapaseo]

- anyone know what maxima are specified by engine suppliers for particles in the airflow (ie. number and size - I'd assumed there'd be datasheet items for a number of different combinations of size, characteristics such as melting point, abrasiveness, ..., and number per cubic metre of atmosphere. However, no-one so far (?) has identified such info.)

It is not possible to establish the "how much" of all the various bits of non-air that come into a gas turbine. Instead the design and regulations attempt to set a standard of a "high jumpers bar" This is based on learned experienece across multiple models of engines over 40-50 years of accumulated operational risk. Thus you get the regulations for so many birds of a given size, so much water from a rain storm, so much time in a defined icing environment. Meet the standard and you are relatively safe. By relatively I don't mean 100% but only that the engine for the specific risk encounter is not contributing appreciably to the overall risk of anything (in the whole aircraft) going bad in a given flight.

The flight risk is taken up mostly by an individual flight encounter and typically does not expect accumulated risk over multiple flights-encounters for abnormal operation. (see examples below) It therfore presumes that such abnormalities are limited in time extent and mitigated by maintainence actions between flights. This is the presumption for birds, dirt, sand, dust, gravel, stones, rumway concrete dust, etc. etc.

For commercial operations the risks of encounters are well defined by experience and thus the abnormal effects of ingestions are expected to be recognized and attended by operational controls in the air for the specific flight, retarding but not necessarily shutting down all engines, etc.

Based on documented experience, the greatest risk in a single-flight for volcanic ash ingestion has been the operating temperature in cruise causing the stuff to melt and plate out on the stationary vanes immediately behind the burner and just in front of the first turbine stage. This effect significantly affects the engine stall line of operation and results in what is commonly known by the pilots as "silent stall" where the engine no longer responds to the throttle, spools down and the EGT goes up. The specific engine than has to be shutdown (turn off the fuel) and restarted (hopefully it can be automated by quik restart). The idea is to ensure that all engines don't get into this in the same seconds of time so you should retard the throttles (lower the temperatures) ASAP on all your engines.

There is little to no commercial experience of other engine "ash" effects, e.g. blade errosion, clogging of cooling holes, oil contamination causing multiple engine power loss in a single flight encounter with volcanic ash. Of course if you continhue to dispatch these deteriorated engines over multiple flights you are going to have problems.

[Swedish Steve]

This volcanic ash is really a form of sand?
In the Middle East, there is sand in the air all the time. When the QR A319 arrived in ARN, I was always able to scoop out an egg cup full of sand from the back of the engine. The ME carriers must ingest sand all the time. So if this is the same sort of sand as volcanic ash, how much is in the atmosphere in the Gulf, and how much in the atmosphere over Europe?
Are we talking the same sort of concentration, or is the density much greater?
If the numbers are similar, I don't see a problem.
perhaps Rolls Royce knows..

[Intruder]

High content of SiO2 -- Silicon Dioxide, which is the grit component of high-grade sandpaper. It is NOT regular "sand"!

[Tourist]

A question for those who may have technical knowledge rather than just guesses.
Most of our military helicopters have centrifugal separators on the air intakes of the engines. Will these work against the ash particles, and if so, is there any risk to any other areas such as rotor blades?
Do we need to ground rotary also?

[brooksjg]

SiO2 IS 'sand' as in deserts AND (a major component of) the dust coming out of the volcano, as well as being the 'sand' in some types of sandpaper. Key differences are that the grit in sandpaper is 'new' and deliberately left with sharp edges. Volcanic dust is also newly formed. Desert sand particles have mostly been around long enough to have had their edges smoothed off by contact with other particles. Given that all sorts of different 'rock' get melted into the magma under a volcano, who knows what the particles will comprise, anyway. Suffice it that to some extent geography determines what will come out of each one. Doesn't really matter... they're sharp-edged and relatively low melting-point, and therefore bad for engines, airframes and anything else they hit at high speed.

BUT that doesn't address the key issue. The industry and Europe generally simply cannot afford to remain grounded indefinitely, until there is NO detectable ash still floating around, anywhere. At some point, someone (ICAO?) is going to have to develop some cojones and come up with comprehensive and time-critical RISK-BASED advice, not just a simple 100% ban that serves mostly to cover their own backsides.

The key questions are:

- how much ash per cubic metre of intake air, and for how long, will a given turbine accept without any significant risk of ANY type of failure, due to dust solidifying in the turbine hot section OR ANY OTHER REASON. There's must be a 'no fly' ash density that could be calculated from that. The answer seems likely NOT to be zero particles per cube.

- then the area of ash-cloud declared off-limits can be calculated, taking into account the source volcano and (especially) the time the various zones of the ash cloud have been airborne. (Other factors: humidity, temperature, probability of rising air masses, ... . Only physics and metereology!). The key answer is the sink-rates of particles of various sizes and the actual threat from each size.

Once a safety-floor has been set, then operators can get their own advice about probable exposure on specific routes, and calculate the ECONOMICS of flying them, taking into account increased maintenance costs.

[lomapaseo]

Most of our military helicopters have centrifugal separators on the air intakes of the engines. Will these work against the ash particles, and if so, is there any risk to any other areas such as rotor blades?
Do we need to ground rotary also?

Those centrifugal separators work adequately for hovering over sand for short periods of time.

However even they only separate out a percent of the ground sand, so too long hovering will still be a problem.

Volcanic ash is made up of a lot more stuff than sand. The problem is that the lighter stuff which is dust of a flour consistency does not have enough enertia to be separated from the air molecules and thus will still turn corners in the separator and enter the engine. OK at least the big stuff causing errosion of the innards will be separated but you still have to contend with the fine stuff that might melt or more likely cause blocking of cooling holes in the turbine.

Whether the stuff melts and plates out on the critical stationary parts of the turbine is a function of the operating temperature in the engine. If you are low enough < 800C (statistically) you will probably have little plating out and immediate power loss but still have to contend with the clogging of cooling holes in the vanes and blades.

The clogging doesn't typically cause a problem in a single flight but over several flights expect to get burning of the blades and vanes and a serious loss of efficency-power.

The flour like particles (small enough to melt) are the stuff that floats at high altitudes and slowly comes down to the ground in those visible blackouts. So you might be able to operate at lower altitudes in helicopters with due care of stirring stuff up off the ground.

[Heathrow Harry]

Abrasion Damage

When the BA plane was in Jakarta it was joined a few days later by an SIA Airbus that flew into the same cloud (obviously hadn't read the papers)

Both were parked at Jakarta Halim for a while and had obvious and severe abrasion damage to nose, windows, leading edges etc

[brooksjg]

An ex-RN Sea King pilot tells me that it was SOP to flush turbines to get rid of salt deposits after flights at sea.
Park on deck with turbine at idle and spray clean (non-saline) water from a hose into the intakes using a circular 'rose' on the end of a long stick.

OK - far from ideal but as a means of cleaning the turbine apparently just as effective as flying through torrential rain. Which bit(s) got cleaned is unclear but apparently power went up and temperatures went down as a result.

[gas path]

OK - far from ideal but as a means of cleaning the turbine apparently just as effective as flying through torrential rain. Which bit(s) got cleaned is unclear but apparently power went up and temperatures went down as a result.

The water washing basically cleans the crap off the compressor section. (In the case of the RN Helicopters it is more to do with keeping corrosion in check from the salt air). This will improve the EGT margin and reduce the fuel burn. We typically see a 9-10deg improvement. The improvement in SFC equates to several million £ a year across the fleet.