Ice Bridging-Myth or Reality?

[GravityHater]

I thought that had everything to do with holding in severe icing conditions with SLD of a size that no aircraft is certified for.

aggravated by flap extension and use of autopilot.
Oh, and the report blames the French. Really!
http://www.ntsb.gov/ntsb/brief.asp?ev_id=20001206X02420&key=1

 

[mckpickle]

I don't think Roselawn had anything to do with Ice Bridging. They were holding at less than 185 knots. (The 72 gave a overspeed warning at 185 for flaps 15) They were holding in conditions worse than ever recorded with regards to inflight icing. Anyone who's flown a 72 know that the pitch attitude at 170 is pretty high, even with flaps 15. (I say 170 because I think that was the max holding airspeed at 8000msl when the crash occured). The bottom line here is they never should have been in those conditions. With that amount of accretion the a/c was way beyond it's capabilities.

I'm not sure about the systems on the 120 or the 1900 but the ATR, (which BTW had more ice testing than any a/c in history) you just set the boots to on and let them cycle. I've never seen the boots blow under the ice. Even in severe icing the boots blew it all off and didn't do it to often to let it build underneath. The Comair crash that everyone is refering to resulted in manditory ice detectors being placed on the 120. Before the crew had to see the ice and turn it on themselves. If ice was accumulating but the crew didn't see it than they wouldn't blow it off. I'm not saying bridging can't happen but with the boots we're using today it pretty darn unlikely if you use it as prescribed.

I'll still take the ice over the thunderstorms:nuts:

 

[r1830]

As a First Officer flying both the Twin Otter and Dash 8 in Alaska I have seen some icing. 99.9% of the time the de-ice boots have worked great, but I have had ice bridging occur once. In most icing test the boots are in great condition. I think a major factor in ice bridging is the condition of the de-ice system. Fly an aircraft that has a couple of patches on the boots, has not been ice-xed in a while and dirty boots the ice seems to stick to it.

We were flying a 200 Series Twin Otter(less power than the more prominent 300 Series) from Bethel to Anchorage over the Alaska Range. It was late spring, early summer and the leading edge was covered with a combination of old deice fluid, dirt and bugs. The MEA between Sparrevohn (SQA) and Anchorage is 12000 feet and a MOCA of 10,300 feet. The 200 series Otter is usually temp limited or at the stops at altitudes above 10,000 resulting in torque settings around 37 or 38 pounds of torque. The outside air temperature was around -20 Celsius. We got into some light rime ice just past SQA at 13000 feet and cycled the boots (to early?) with ice estimated at around 1/8 to 1/4 inch. The boots inflated but most of the ice did not shed. We decided to let it build up a little more ant try it again. This time with a little more than a ½ inch of ice we cycled the boots with the same result. Climbing was not an option so we descended to 11,000 feet and continued to cycle the boots with out any real improvement. Although there seemed to be less icing at that altitude. We were in the ice for about fifteen to twenty minutes before we could descend to 6000. By that time we had accumulated about 1 1/2 inches of ice and slowed considerably. Once in the clear and with a head of steam in the descent we started shedding the ice as we descended through about 7,000 and the temperature was still below freezing. The boots seemed to be working fine after we exited the icing condition and got into warmer temperatures. I don’t know why we couldn’t shed the ice. I think part of it may have been the condition of the boots.

The incident above happened before we changed our Ops Specs in accordance with the FAA’s new recommendation. In both the Twin Otter and the Dash 8 our ops specs now have us activate the boots upon entering icing conditions. We leave them on until at least one complete cycle has been performed after exiting icing. I have not seen “ice bridging” occur since implementing these procedures. But I have seen more residual ice on the boot after the first cycle or two. But the end result was the boots were able to keep shedding most of the ice and not allowing it to build up. I must admit though I am not totally convinced that this procedure will prevent ice bridging.

On another note, climbing out of Kodiak last week we got into some freezing rain at -15 Celsius. That is the coldest temperature I have ever seen freezing rain. The cloud didn’t look threatening and the weather radar barely showed some green.

 

[CFIse]

We were flying a 200 Series Twin Otter(less power than the more prominent 300 Series) from Bethel to Anchorage over the Alaska Range. It was late spring, early summer and the leading edge was covered with a combination of old deice fluid, dirt and bugs.

What you described was boots with the inability to shed ice - but that's not ice bridging. Ice bridging is when the boots "push" the ice out so that it forms a shape around the boots and continues to build. Subsequent inflations of the boot occur in the space between the wing and the ice - the ice has "bridged" the boots.

With modern boots bridging is a myth - I'm not old enough to know about older boots - maybe they could bridge.

 

[r1830]

CFIse

What you described was boots with the inability to shed ice - but that's not ice bridging. Ice bridging is when the boots "push" the ice out so that it forms a shape around the boots and continues to build. Subsequent inflations of the boot occur in the space between the wing and the ice - the ice has "bridged" the boots.

Sorry, I didn't explain the second activation of the boots correctly. Let me expand on that a little.

R1830

The boots inflated but most of the ice did not shed. We decided to let it build up a little more ant try it again. This time with a little more than a ½ inch of ice we cycled the boots with the same result.

At the second activation of the boots some inflation was noticable but lacked in cleaning the wing. We left the boots in auto/fast and after a couple of cycles boot inflation could not be seen. This may or may not have been ice bridging. We could not see if there was an air pocket between the ice and the boot. But, the end result was the same, we could not shed the ice till we were in a descent descending into warmer air in the clear. The boots were inflating after the wing was clear of ice.

 

[Victor Meldrew]

A big factor in shedding the ice is boot condition. Dirty or rough boots will allow the ice to adhere better (like roughing down a surface before you paint it). Properly treated and protected boots will allow the ice to shed when the boots blow.

Another factor is speed. Airflow also helps force the broken ice off the surfaces. The only booted a/c I flew regulary in ice was the C208B, let it's speed drop to below 120 KIAS with ice and then blow the boots you'd see most of it stay and you were then prime for bridging.

 

[Tim47SIP]

Icing tests!

I don't think Roselawn had anything to do with Ice Bridging.


OK, I should have been more specific. The bridging occurred during the icing tests done at Edwards AFB after the accident. If you remember, the French blamed the accident on the crews failure to get out of icing, the delay in using the de icing system, and holding with flaps 15. The NTSB didn't buy the explanations and decided to try and determine the amount of icing that adhered to the aircraft. What they found was that icing formed past the effective range of the boots and as the ice built up it moved forward. This formation was well outside the effective range of the forward part of the boots. The NTSB believes that the tail stalled and then the violent roll due to the angle of decent. Ice bridging may not have formed on the Roselawn aircraft but it did form on the right wing (test wing) of the Edwards aircraft. These tests are what spurred the re engineering of the de icing system as well as the new procedures put into place after the accident. ATR was livid and insisted that the aircraft met the FAA's requirements for icing. The aircraft had never been certified in heavy icing. There were several ATR's that crashed in heavy icing prior to the Roselawn crash which moved the NTSB towards the icing tests at Edwards. You can look up the reports on the NTSB site which also has the petitions from the French government to change the cause of the the accident from what the NTSB determined. There are also some very good icing pictures on their site.

 

[mckpickle]

I don't think Roselawn had anything to do with Ice Bridging.


OK, I should have been more specific. The bridging occurred during the icing tests done at Edwards AFB after the accident. If you remember, the French blamed the accident on the crews failure to get out of icing, the delay in using the de icing system, and holding with flaps 15. The NTSB didn't buy the explanations and decided to try and determine the amount of icing that adhered to the aircraft. What they found was that icing formed past the effective range of the boots and as the ice built up it moved forward. This formation was well outside the effective range of the forward part of the boots. The NTSB believes that the tail stalled and then the violent roll due to the angle of decent. Ice bridging may not have formed on the Roselawn aircraft but it did form on the right wing (test wing) of the Edwards aircraft. These tests are what spurred the re engineering of the de icing system as well as the new procedures put into place after the accident. ATR was livid and insisted that the aircraft met the FAA's requirements for icing. The aircraft had never been certified in heavy icing. There were several ATR's that crashed in heavy icing prior to the Roselawn crash which moved the NTSB towards the icing tests at Edwards. You can look up the reports on the NTSB site which also has the petitions from the French government to change the cause of the the accident from what the NTSB determined. There are also some very good icing pictures on their site.

What other ATR crashed due to Icing? Also, the tail did not stall due to the ice. When the pilots retracted the flaps the A/C flipped over. The ailerons had effectively stalled out due to the blocking of airflow in front of the outboard wind section. When the flaps came up the autopilot could no longer hold the a/c because the ailerons where "fluttering". To my knowlege the only other ATR incident occured when one experienced a uncommanded roll on arrival in EWR. The crew held onto the a/c and landed. The incident in Roselawn did have issues with the SCDD (super cooled drizzle droplets), and the tests proved that no a/c with boots could have survived. You have to question the crew holding in those conditions for that amount of time.