In 1987, when I took a DC-10 Maintenance class, the instructor showed a film of how they test aircraft windshields (wind-screens) for bird strikes: they launched grocery-bought chickens at the wind-screen using a ‘chicken cannon’ (real thing), which launches the long-expired bird at the test wind-screen at 200 miles per hour (MPH). Since wind-screens are engineered to be heated and reinforced internally to absorb and deflect a bird strike, this is the only way to test the product for integrity. The bird strikes the wind-screen, spatters at the high rate of speed and deflects away from the cockpit. The pilots and aircraft are protected.
A wind-screen designer for high-speed locomotives used the same cannon to test locomotive wind-screens; they are designed to be just as strong. Using the chicken cannon, the tester launched the chicken at the locomotive wind-screen; the test chicken exploded through the wind-screen, created a gaping hole through the heavy metal control compartment back wall and lodged in the aft structure. Needless to say, the test was a failure; the locomotive wind-screen manufacturer asked the aircraft wind-screen manufacturer for hints as to make their product better.
The aircraft wind-screen manufacturer replied … and I quote, “Next time you test the cannon, defrost the chicken.”
These are real methods and events; I didn’t make them up; I saw the videos. I talk about them to emphasize the destructive difference between a bird at ambient temperature, whose structural strength matches your Thanksgiving turkey about to be carved; versus the frozen chicken with the pliability of a rock or brick.
I noticed in the aviation news this week that a surge in Drone activity has resulted in the Federal Aviation Administration (FAA) prompting ‘emergency’ action in regards to Unmanned Aerial Vehicle (UAV) use; an average of 250 safety incidents per month involving near misses between UAVs and aircraft of all kinds, e.g. airliners, private aircraft, helicopters, etc., have been reported. In addition, the National Transportation Safety Board (NTSB) is investigating a Staten Island, NY, incident involving an Army UH-60 helicopter striking a drone.
Since the professional UAV industry is working tirelessly to acquire aviation industry credibility, let me just say this: Professional UAV industry, you are your own worst enemy. By allowing UAV lobbyists to confound the media by using adjectives like ‘baloney’ to describe UAV strike dangers; by allowing lawyers with no aviation background to try bullying the FAA; by allowing amateurs to cause FAA emergency actions with their irresponsible behavior; the FAA, the NTSB, Air Traffic Control, Airlines, the Airline Pilots Association (ALPA) and other mainstream aviation organizations will never recognize you as safe to use their airspace. They have built and occupied that airspace for over seventy years and you need to start playing by their rules.
But, since UAV lobbyists have been misrepresenting the safety implications of a drone or UAV strike, I will defer back to my chicken cannon story to make a point about strength. I feel it is necessary to make the case for why a UAV hitting an aircraft – any aircraft – is more dangerous than people realize.
And, by the way, the average UAV is represented by the frozen chicken.
Let’s start with the helicopter, particularly the UH-60 in the NTSB accident investigation. The Sikorsky UH-60 is popularly known as the Black Hawk, as in the 2001 movie, Black Hawk Down. It is indicative of the design of most helicopters – from the Bell 47 used in the M*A*S*H series to the Eurocopter AS350 employed by police and traffic news stations everywhere – in that it has a Main Rotor for vertical lift and horizontal maneuvering, while the Tail Rotor controls torque created by the main rotor.
Both Rotors’ blades are airfoils, meaning they do not have Leading Edges that are as sharp as a Ninja sword blade; they are rounded to create lift, exactly like an aircraft’s wing. The blades cannot cut through concrete or metal, like so many action movies portray. Instead, when ANYTHING hits the Rotor blade’s leading edge, the blade is forced backwards against the direction of rotation; the blade is also structurally compromised or destroyed by the impact.
The Main Rotor turns at 258 rotations per minute (RPM); this means that the tip of the #1 Main Rotor blade passes the same spot in space 4.3 times per second … that’s 4.3 times PER SECOND. In this case, both the ambient temperature or the frozen chicken would destroy either fragile Rotor Assembly while it is operating. After the Rotor blade is destroyed from the strike, the Rotor is now out-of-balance; the helicopter is uncontrollable, it experiences incredible vibrational torque forces and the helicopter drops straight down on whatever … or, whoever … is below it, with absolutely no warning. That is the reality of a UAV strike against a helicopter … any helicopter. These are not my opinions; these are facts supported by engineering data and accident investigations. If the UH-60 in the NTSB investigation were struck in either Rotor, all occupants of the helicopter would have been killed … period!
The effects of a UAV strike on a propeller aircraft are similar, except the propeller rotates to provide forward motion; the propeller blades are small airfoils that, like a wing, provide a negative pressure (lift) in front of the propeller that pulls the aircraft forward. Again, the propeller is rotating at great speeds; when it hits the UAV, the catastrophic results will put the engine propeller shaft out of balance causing extreme torque forces on the aircraft and, in the case of a single engine aircraft, will turn the plane into a glider with enough forward momentum to bring the plane and its occupants to the crash site.
To an airliner, there are multiple dangers. Any jet airliner approaching an airport travels between 140 to 200 MPH, according to what stage of the landing phase they are in. Studies have been done that shows UAVs are impossible for pilots to see, because:
- The airliner is traveling too fast to sight a UAV, especially at night. The UAV is almost invisible to the naked eye, even when one is looking for it;
- The landing phase of flight is very busy. Pilots are lowering flaps, talking with air traffic, lowering the gear, monitoring instruments, e.g. airspeed, altitude; they are too busy to look out the window for UAVs that shouldn’t be there.
Let’s look at the dangers of impact. First, as demonstrated by the chicken cannon, the UAV can – and will – penetrate the wind-screen; the pilots will either be seriously injured or killed causing all passengers to be killed as a result.
As mentioned in an earlier article, a gull tore through the radome of a B727, went through the metal bulkhead behind it and knocked the Captain unconscious when it exploded in his lap; I know this because I helped repair the damage. The B727 was moving at about 140 MPH and all three pilots never saw the bird or the flock it belonged to.
And, what about the engines. Since US Airways, flight 1549, the ‘Miracle On The Hudson’, stands as the landmark example of why bird strikes are survivable, let’s look a bit closer. An airliner’s engine Fan rotates at around 3600 RPM. Think about that … each Fan blade hits the same point in the engine inlet sixty times per second … SIXTY TIMES PER SECOND. Can you imagine the kinetic energy that is generated?
I spoke about the consistency of an ambient temperature chicken – or in US Air 1549’s case, a Canadian Goose – is that of a cooked turkey. A bird’s bones are hollow; like with the cannon, the carcass splatters against an unforgiving object. However, in the case of the B727, the carcass of a smaller bird penetrated metal and landed in the cockpit; so, with enough force, even a bird can cause catastrophic damage to metal at 140 MPH. The flock of Canadian Geese did catastrophic damage to both of US Air 1549’s engines. Imagine a solid metal-and-plastic object, like a UAV, striking the engines’ blades that are spinning at 60 rotations per second. Result: DISASTER. And, US Air 1549 was over water.
The argument about UAVs in the national airspace has to be one of facts, not opinions or sarcasm. These are dangerous forces, dangerous results and dangerous amateurs. When even a bird as simple as a chicken can cause catastrophic damage, perhaps we need to be more selective as to how we approach greater threats to safety and lives.