Snarge: the gruesome result of our holiday flights and why it’s important to study it

Snarge: the gruesome result of our holiday flights and why it’s important to study it

Jul 21, 2021 News by Gary Searing

It’s just as unpleasant as the name sounds: ending up as goo smeared around the inside of an aircraft’s engine is a very real risk for birds.

Bird strikes can also be a problem for other frequent fliers, namely us. 

You might think that in a bird-versus-aeroplane-engine situation, an enormous machine would surely win, but bird strikes can, at worst, cause catastrophic damage to an aircraft.

Aeroplane engines are designed to withstand ingesting birds up to a certain size without failing. But to improve safety, aviation companies need to know what has been hitting their engines.

The problem with bird strikes

Our desire to fly has long been hampered by bird strikes. The first instance was recorded in 1905 when Wilbur Wright hit a flock of birds during a test flight, and the first fatal incident occurred in 1912 when pilot Calbraith Perry Rodgers collided with gulls, causing his plane to plummet into the ocean.

Birds have been implicated in numerous incidents involving aircraft over the years. These types of collisions can result in loss of human lives and cost the global aviation industry over $1 billion each year.

Improving engines to make them more resistant to bird strikes increases our safety in the skies. Understanding what does and doesn’t cause engine failure is crucial for improving safety, and knowing the species involved and its size is an important part of that.

It takes an expert eye to confidently determine this when all that’s left is half a battered feather or other assorted debris. That’s where the Museum’s experts come in.

Species identification is one of the many services provided by the Museum. This area of work ranges from answering queries about local nature finds to business consultation, including identifying pests and investigating food contamination.

Since 2014, experts at the Museum have been working with Rolls-Royce to identify birds from the grisly remnants left behind. This gunk is also known as snarge.

Snarge samples

When a bird and aeroplane collide, particularly if the animal was ingested by an engine, there generally won’t be much of the bird left. When such collisions occur, engineers at airports around the world collect or swab the remains and send these samples to the Museum.

While envelopes of bird remains probably aren’t the strangest thing to have graced the Museum’s post room, it is likely that many people would be anywhere from bemused to horrified to receive a bird foot in a bag. But for Sequencing Assistant Claire Griffin, it’s all part of the job.

Claire says, ‘The guys in the post room know the material is coming and that it’s something that they need to pass on very quickly.’

‘First the samples go into quarantine. We make sure that they are frozen to kill off any nasty bugs before I open the bag and see what surprises are waiting for us inside and the state of its preservation.’

The reality of working with material that may have spent a considerable amount of time inside a hot and greasy engine is that the quality of the samples often isn’t great for forensic DNA work.

‘What we get sent varies quite a lot. It might consist of feather fragments, bird debris or identifiable bits of bird,’ adds Claire. Frequently, the sample is just a wet wipe that has been mopped around the engine blades.

Feather remains are sent to Hein Van Grouw, Senior Curator of Birds at the Natural History Museum at Tring.

Tring is home to thousands of bird skins. These can be used like a reference library, allowing Hein to compare feathers in the collection with those from bird strikes to try and find a match – though it is not as straightforward as it sounds.

Hein explains, ‘Most of the time, the feathers we get have been all mangled or chopped into tiny bits by the engine. Getting a whole feather is really rare’.

‘I’m lucky that I have a couple of thousand bird skins I can compare with. But if you don’t know what you are looking at or get a hunch of what it could be, there is no point opening all the drawers to see if you can find a match. You can’t go through 10,000 different species.’

Identifying the type of feather, its size, its curvature or any distinguishing colours and utilising any information about locality provided with the sample can help narrow down the search.

Once he finds a potential match, Hein then compares his findings with the results of the DNA work carried out on the samples in the Museum’s molecular laboratories.

DNA analysis

To access the DNA contained in a snarge sample, Claire starts by breaking down the cells in a process called lysis. Next, millions of copies of targeted genes are produced through a process called PCR amplification so that their DNA can be sequenced.

The resulting data is sent to Dr Andrea Waeschenbach, who compares it to the DNA sequences available on the reference database GenBank.

Rather than looking at just one gene, Claire produces DNA sequences for four genes. This means Andrea can broaden her search, maximising the chances of finding confident matches with the DNA in the database.

Andrea explains, ‘I then take the matches that the database has found and make a family tree from those data. This makes it much easier to tease out the correct species, rather than just looking at the percentage matches with the reference data.’

By combining the results of feather identification and the DNA analysis, Claire, Andrea and Hein can, in most cases, confidently confirm the identity of a bird strike victim.

DNA analysis has at times led to some unexpected results, however.

‘We had a funny one once. From the sample, we got matches for a turkey, a gull and a cow,’ says Andrea. ‘We suspect that we may have sequenced the stomach contents of the gull.

‘It was a bit confusing – maybe it’d had some Turkey Twizzlers and beef burgers for lunch.’ 

When and where do bird strikes happen?

Rolls-Royce engines are installed in commercial and military aircrafts that travel around the world, meaning that the bird strikes the Museum analyses will occur almost anywhere.

Supplementary information sent with the sample about where the strike occurred can help, however, as snarge is most often discovered during engine overhaul, the precise location of the strike isn’t often known.

Almost any bird could be ingested by an engine, even those that are not known for soaring at great heights. Most bird strikes occur during take-off and landing, rather than when an aeroplane is at cruising altitudes where birds are rarely seen.

Ibis, vultures and even a white stork have been identified from samples sent to the Museum, though the team more commonly sees results for gulls, feral pigeons and swifts. The most frequently seen of all are songbirds, with birds of prey in a close second place.

While bird strikes happen relatively frequently, they are not thought to have a significant impact on bird populations overall.

Preventing bird strikes

Work to limit the damage caused by bird strikes typically comes out of a desire to reduce risk for aircraft crew and passengers. This can be done through increasing engine safety in the event of a strike and reducing the chances of a collision happening in the first place.

If there are fewer birds near a runway, it lowers the chances of a strike, so airfields usually try to make their large expanses of land as unattractive to birds as possible. 

Hein explains, ‘A lot of airfields have people who identify what is there. So, for example, if there are lapwings, they really like short grass. So the airfield might try growing the grass a little longer so the birds go somewhere else.’

Some airfields will also use drones and sounds blasted through loudspeakers to scare birds away from the area. In 2012 Gloucestershire Airport reportedly had some success blaring Tina Turner’s ‘Simply the Best’ and other hits across the runway.

It has been a quiet year and a half for the global aviation industry with the fallout from the COVID-19 pandemic significantly reducing travel. But the world is slowly reopening, and with a return of holiday and business travel, there will undoubtedly be a parallel rise in samples of bird strike victims winging their way to the Museum’s post room once again.

So, as you are herded through security, queuing in the jetway or taxiing to the runway, perhaps spare a thought for what the impact of your flight might be – literally. 

By Emily Osterloff for Natural History Museum