Earlier this week the British Airline Pilots Association (Balpa) called for an investigation into the consequences of a drone colliding with a civilian aircraft. Concerns over such an incident come on the back of 23 near-misses in the UK during a six-month period last year, 12 of which were given an ‘A’ class rating where there was judged to be “a serious risk of collision”.
Of particular concern is the possibility of a UAV getting sucked into a jet engine, as well as the potential for their lithium batteries to catch fire on impact. In light of this, The Engineer spoke to Ian Horsfall, head of Cranfield University’s Impact and Armour Group, about the likely results of a collision.
The widespread assumption is that a drone strike on an engine would be significantly worse than a bird strike, which themselves can be a considerable danger. According to Horsfall however, that assumption does not necessarily hold true.
“The controlling factor is how big a drone it is,” he said. “If it’s a small drone then it’s unlikely to be any worse than the equivalent size bird.”
“I think the standard test for aero engines is typically a 4lb/2kg bird or bird simulator. From an engine point of view, there’s no reason why a drone would be any worse than a bird of equivalent mass. We know for instance that jet engines can take goose impacts, which are getting up towards 10kg. And although it’ll definitely take the engine out, it won’t necessarily result in anything more serious than that.”
While losing an engine is far from ideal, planes can usually survive when it happens and pilots are well-trained in how to react. What’s more worrying is if the damage is not contained to the engine and other parts of the aircraft are compromised. Instinctively, one assumes that a UAV made of metal and plastic has the potential to cause more damage than a bird made of flesh. But according to Horsfall this is something of a red herring, and the size of the object is much more important than the material.
“The engines are built to contain an ejection of a fan blade usually, which is a fairly heavy lump of material,” he explained.
What’s more, the lithium batteries that power the majority of drones are not quite the threat that many people believe, at least in terms of their potential to cause additional engine damage.
“One of our students is doing some work looking at what would happen if (a drone) was ingested into an engine, particularly from the point of view of the battery,” said Horsfall. “The battery is the largest mass in a multi-copter, although it’s not necessarily very hard, but of course has the potential to burst into flames as well.”
Horsfall’s team performed some trials where they simulated a battery hitting a fan blade, and found that nothing happened instantaneously as some would predict. Instead, it takes “a few seconds or tens of seconds” for the battery or its remnants to catch fire, but that fire is then persistent.
“What that actually means is that if you get it ingested into the engine, it’s probably not something you need to worry about because it will get chopped up into little pieces and thrown out the back of the engine long before anything more energetic happens to it,” he explained.
“But were it to hit the radome or the leading edge…you’ve potentially got a fire source in there now as well. If you look at the picture from bird strikes, it’s not unusual for the bird to end up embedded in the aircraft, particularly if it hits the softer areas like the radome. So there is potentially a fire risk from a UAV on top of the impact risk, and it may well be that rather than worrying about them getting sucked into an engine and the consequences of that, what we need to worry about is the consequences of them hitting elsewhere on the airframe.”
The majority of the incidents leading to Balpa’s call to arms are attributed to drone hobbyists and amateurs, oblivious to potential dangers and reckless in their operation. For the most part, these drones are small and lightweight, and pose a very real but limited threat right now. In the future, however, UAVs are likely to grow in size and number, and the danger to civilian aircraft will increase accordingly. Acting now is key to minimising that risk, according to Horsfall.
“I suspect the threat is not the multi-copters that we have this year,” he said, “but the multi-copters that we have next year or the year after, and it would be nice to get the legislation ahead of the technology rather than behind it.”
Having been on the maintenance end of many birdstrikes against aircraft in the past I would suggest the best case is a drone going through the engine.
While birds are obviously not so heavy (and neither are drones) the energy involved in the ‘strike’ is phenomenal, mainly due to the speed of the aircraft (200 – 600 kts). Even a small bird has big impact.
If the bird / drone goes cleanly down an engine intake, while it is not good for the engine the energy is absorbed gradually through each of the compressor stages and is actually relatively benign in effect.
If however the bird / drone hits any other part of the airframe the picture is completely different (http://tinyurl.com/hfn8eu2 for examples). Usually what is seen from the outside is only the start of it until the bird / drone has lost all of its energy. I have seen 20mm thick aluminium plate fractured from a seagull strike that had already passed through 3 skin / frame layers.
As the article rightly mentions the bigger hazard is from the potential fire risk of the battery in an enclosed well ventilated (as its punched a hole in) space and nearby combustibles. Even more so where older aircraft with Polyimide (Kapton) cabling which is prone to carbon arc tracking if damaged and a short circuit occurs.
What is not mentioned however is while there is risk of uncontrolled flight in restricted areas this is actually an illegal activity ( https://www.caa.co.uk/Commercial-industry/Aircraft/Unmanned-aircraft/Small-unmanned-aircraft/ )
While BALPAs concerns are well founded, for both their members and the public safety perhaps more effort should be spent on reinforcing awareness of the rules (as above) and possible registration of all sales, if only to highlight awareness of the regulations. I have noted particularly the increase in news reports of people being injured by crashing or ‘lost control’ drones, which to me is more of a risk than a ‘Dronestrike’ when I’m on a commercial flight.
Unfortunately people forget that while they may be in control most of the time these devices have the capability to fly high and far in event of loss of control which with unregulated / open frequencies in use and popular protocols (Bluetooth / WIFI) for control has quite high probability.
This video is a classic case ( https://www.youtube.com/watch?v=rddoGaGbPXc ). The flight is quite tight to a well observed landmark but within the confines of a very busy military airfield airspace, indeed the airfield can be seen in the near background. As I know this area I can also say this particular flight is only a few thousand feet off the centreline of the approach for the main runway. Also seen recently was a commercial organisation who were showcasing their drone capability to several thousand feet (by way of video) within the same airspace (unfortunately I can’t find that link)
In all while we will not educate everyone, certainly educating the mass of people may be enough. And in similar vein to the idiots targeting aircraft with LASERs you will never get rid of the rouge element. Perhaps its time for jamming technology for the main operating frequencies in critical areas! But then our phones would suffer too. Maybe regulate to a specific frequency? Who knows what the future will hold here.
GI makes several aspects of this concerning matter clear: I hesitate to suggest it, but isn’t there a case for the electronic equivalent of the ‘bright flares’ ejected from military aircraft when close to the ground (to deter homing missiles) near civil airports? Many years ago I assisted a client in processing a new material -pre-oxidised carbon fibre: one of the potential end uses was in protective covers for airline seating (to restrict flame spread to the foam) Every airline accepted the potential safety aspects, but baulked at the cost. Post the 1986? Manchester Accident they could not get their hands on the technology (and lighting on the floor) fast enough: because they were required by FAA and CAA regulations to install such. QED
I’m not a pilot but have had connection with fast jets for many years. I feel the issue is not the ‘drone strike’ and can the plane survive, but:- 1) the distraction to the cockpit crew at a critical stage of take off or landing ( drone are not usually found 2000m plus) 2) the cost of repair and rectification which could amount to several £m’s. Restricted zone should be enforced at the end of runways and any flying over the perimeter fence. The zone volume should be commensurate with the full instrument approach.
I have worked with drone / UAV / RPA development and the major issue is that the general public is not only unaware of the risks and consequences, but doesn’t have a clue on restrictions, such as zones, height and distance.
I believe that it is needed to advocate safe flight and educate people, but as written before, this doesn’t eliminate the “rouge element”. Hence it is necessary to implement safety parameters on the control software / aircraft firmware, such as height limitation, maximum distance from take-off point (AKA Geofencing) and, above all, integrate no-flight zone on maps. The first two can be hard-coded into the aircraft firmware and constantly checked during flight, while the last would not permit take-off at or a route that passed close to a no-flight zone. Several serious manufactures are already adopting those measures.
As for the batteries, their discharge rates are achieving and surpassing 100C, thus the volatility of their chemistry is considerable. While they do not pose a enormous threat to large commercial aircraft, a smaller jets are in greater risk.