Speed Kills: Kinetic energy and helicopter bird protection

The velocity-squared element in the kinetic energy equation helps explain recent revelations by Sikorsky that aftermarket cast acrylic windscreens available for its S-76 series twin-engine helicopters require a lower maximum speed compared to the original equipment manufacturer (OEM) alternatives.

Sikorsky issued an "all-operators" note on 17 February addressing windscreens in the aftermath of a fatal Petroleum Helicopters S-76C++ in Louisiana on 4 January. The gist of the letter is that the cast windscreens can handle less impact kinetic energy than the glass or stretched acrylic versions of the component, meaning operators using the cast parts have to fly significantly slower to obtain the same level of protection. That's because energy increases with velocity-squared...

Investigators examining the crash have found traces of a hawk on the pilot's windscreen and other parts of the helicopter, raising the spectre that the collision between the bird and the windscreen at 138kt caused the helicopter's power to decrease (revealed by the flight data recorder) and cabin noise to increase (revealed by the cockpit voice recorder), precipitating the subsequent dive and crash into swampland below.

The kinetic energy (KE) of a 1kg (2.2lb) bird, about the size of a hawk, hitting a helicopter flying at 138kt is 2,520 Joules. That number is not especially significant until you consider the following...

PHI equips its S-76 fleet with an FAA-approved cast acrylic windscreen certified and sold by Aeronautical Accessories, an affiliate of Bell Helicopter.

As delivered by Sikorsky, the S-76 has a laminated glass windscreen built and tested to withstand a 1kg (2.2lb) bird strike at speeds up to the "never exceed" (Vne) speed of 155kt, according to Vaughan Askue, S-76 sales support engineer for Sikorsky.

A 1kg bird hitting a helicopter traveling at 155kt has 3,179 Joules of KE.

The FAA did not require the bird strike test when the S-76 model was first type certificated in 1978, though Sikorsky went ahead and designed and tested to the more stringent 1kg at Vne UK Civil Aviation Authority rules, which the FAA adopted for new build helicopters starting in 2004 (FAR Part 29 Amendment 47).

Sikorsky later developed a stretched acrylic windscreen, also built to handle a 1kg bird at 155kt, that it says has the equivalent protection as glass, but does not require an AC generator to power the anti-fog elements buried in the laminates of the glass, which would in theory allow operators to remove the component and save weight.

However the acrylic version needs a hot air vent similar to automobile defoggers, cancelling out the weight savings gained by moving to the stretched acrylic windscreen. Askue doesn't know of any operators who have ordered the stretched acrylic replacement.

Glass windshields remain popular however -- Askue says the company has logged 5 million flight hours with no windscreen breaches.

In one case, an operator in Florida hit a turkey vulture at 155kt cruise speed. Though the windscreen was "starred" Askue says "nothing penetrated" into the cockpit. The jolt of the hit however did knock the throttles (see location in picture below from Ed at Picasa) out of the detent position, requiring pilot attention.

Later, Aeronautical Accessories developed and earned FAA certification for a cast acrylic aftermarket windscreen that was lighter than the stretched acyrlic, a plus from a cost-benefit standpoint in that the helicopter could carry that much more payload.

The penalty, we are learning, is that it is not as strong as the others, in part because there was no requirement that it should be.

"Available test data for this material indicates a lower level of impact tolerance as compared to glass or stretched grade acrylic material," says Sikorsky in the all-operators note, or more to the point, the cast acrylic windscreen can offer the equivalent protect if speeds are kept to 109kt or less.

From a KE perspective, that means the cast acrylic can handle 1,572 Joules (1 kg traveling at 109kt). 

Hitting a 1kg hawk at 138kt would have generated 2,520 Joules, 60% more than the windscreen could handle, if you believe Sikorsky's numbers.  

The strength difference is inherent in the build process of the two acrylics. Sikorsky notes:

...Acrylic windshields are commercially manufactured for aviation applications in several grades. Cast grade material is cast to the final product thickness and thermally formed in a mould to the final contour. Stretched grade material is initially cast in a thicker section and subsequently reduced in thickness via mechanically stretching the material while heated.This stretching process improves the mechanical properties of the resulting product, in particular, the impact tolerance and fracture behaviour...