Bird Strike Windshield Test Temperature

[DavidDDH]

I am preparing a Test Plan for a Bird Strike test on a GA aircraft windshield (14 CFR 23.775(h)(1), Commuter Category). Does anyone have any data or past experience on appropriate temperatures to conduct the test?

Thanks, David

 

[IRstuff]

Just make sure you thaw the chicken...

There's another thread about bird strikes here somewhere...

TTFN

FAQ731-376

 

[PHovnanian]

Here's a suggestion for a tough test:

Cold soak the window at the minimum ambient expected during a flight. Then place the window into a maximum ambient environment (ground level) and fire the chicken gun.

The thermal stress together with that from the bird impact will likely be the worst the window will see.

 

[tbuelna]

DavidDDH,

FAR 23.775.h.1 is not specific about temperature. I would first consult your FAA DER, and if they cannot give you further guidance I would test at whatever (reasonable) temperature you see fit. The windshield material will likely be somewhere between the cabin and outside temperatures during flight, so that seems like a good place to start with your test plan.

There are also some pretty good commercially-available analysis tools now for bird strike. So you may want to do some analysis beforehand to supplement your test work.

Good luck.

 

[IRstuff]

http://www.atsb.gov.au/publications/2003/pdf/Hazard_aircraft_by_birds.pdf

which was cited in thread2-192021 shows that 85% of bird strikes resulting in damage occurred either on take-off/initial climb, or approach, or landing. The test temperature ought to be set accordingly.

TTFN

FAQ731-376

 

[GBor]

Birds don't generally fly high enough for a significant change in ambient temperature.

 

[FieldTeam]

Another consideration: is the window heated?
Window heat is provided for anti-fogging, anti-icing and bird protection.
A pilots window would have the conductive coating closer to the outer pane of the assembly for anti-icing and bird protection. A side/eyebrow window would have the coating closer to the inner pane for anti-fogging.
The reason for heating (for bird strike) would be to prevent the window from becoming brittle during cold soak, allowing it to retain it's flexible characteristics.

 

[rb1957]

the critical birdstrike is usually below 8,000 ft

 

[Aerodesign]

For simulations we always assume the same conditions as a test - this is invariably done at RT (20 deg C).

 

[cedarbluffranch]

Will the window be opened or closed (rolled down) during the test?

 

[DavidDDH]

Cedar
Finally a comment worth responding to. That is not currently in the plan, but it definitely has merit. I can say with great confidence that the window will not fail if the window is rolled down. The only problem at that point is having an adequate early warning system to give the pilot time to "duck"

The final discussions with the authorities will be on Tuesday. There have been a lot of ok guess, but nobody's got it correct yet. I share more after the final negotiations.

David

 

[FieldTeam]

As a cross reference, AC 25-21 (certification of transport airplane structures) shows a four pound bird at a velocity equal to the value of Vc at sea level (no temperature).
Your requirement is for a 2 pound bird at a velocity equal to maximum approach flap speed (no altitude or temp).
Since no temperature is provided, the profile for a standard day temperature at sea level (29.92inHg) is 59DegF.

 

[DavidDDH]

I promised you all the answer to the question.

1) Nobody every knowingly shot a frozen bird at a windshield or engine.
2) I am the DER and I did consult with the FAA.
3) Yes, the majority of bird strikes occurr below 10,000 ft, however only moments prior to reaching 10K the aircraft was cruising at 41K and temperature in the -50°C range.
4) Windshield heat does help. Acrylics typically have higher static strength at cold temperature, however the data I was able to acquire indicated improved impact strength with increasing temperature. The problem is as soon as you test with windshield heat on, that becomes an AFM limitation. My pilots buddies threatened my with severe pain.

The 14 CFR Part 23.775(h) requires the windshield to withstand an impact from a 2 lb bird when the velocity is equal to the maximum approach flap speed. Para. 23.775(b) ...factors...include (3) The effects of temperature and temperature gradients.
I made do consideration of temperature and temperature gradients including all of the suggestions from the above commenters and proposed a temperature of 0°C. After due consideration the authorities told me the answer was -10°F (-23°C).

To achieve this temperature, I built a small cooling chamber (closed cell PE Foam) around the exterior of the windshield. Thermocouples were placed on the exterior and interior of the windshield. The cooling chamber was packed with Dry Ice. In about 1-2 hours the temperature of the windshield was -25 to -40°F measured on the inside of the windshield. This temperature allowed amply time to uncover the windshield and evacuate the impact chamber.

BTW The tests were successful and the high speed videos are awesome.

All the best, David

 

[rb1957]

"however only moments prior to reaching 10K the aircraft was cruising at 41K and temperature in the -50°C range." ... it's mote since you've gotten the "right" answer (the only one that matters) from the FAA ... but that's some unconventional flightpath ... since that inide surface is always at RT, i'd be surprised if the outer ply is thermally isolated from the cabin environment

 

[DavidDDH]

RB
Later this winter when the temperatures get really cold. Take of one of your gloves off and touch the window next to you; then come back and tell me its RT. The plastic is fairly slow to change its temperature, but after an hour + cold soaking at altitude the windshield is quite cold.

I have actual measured data on the inside surface of the windshield at 41k ft with a temperature of -20°C in the middle of summer on a sunny day. OAT was -45 to -50 C

A typical descent rate is about 2000 ft per min so the time delta from 41k to 10k is 15 minutes. Take an ice cube out of the freezer and see how long it takes to thaw.

 

[IRstuff]

There is a difference between an icecube moving at 0 mph in still air vs. a plane's windshield moving at 250 kt. The heat transfer rate is more than 100W/m^2-K.

TTFN

FAQ731-376