Thick Ice is Best

A recent article in this column dealt with the danger of traveling across ice at certain critical speeds where stresses build up and drastically increase the likelihood of breaking through. The reason is that an object moving at the critical speed stays abreast of a wave it creates in the ice below, and in so doing causes the stress in the ice to grow to a limit that can exceed the strength of the ice.

I cited an example wherein a 10-ton truck moving at 20 mph across foot-thick ice floating on 20 feet of water could stress the ice as much as a stopped truck weighing 100 to 150 tons. Reader Phil Johnson of Fairbanks, who is knowledgeable about operations on ice and particularly about the take-off and landing of heavy aircraft on ice, points out that the example I quote is not relevant to thick ice over deep water.

Mr. Johnson rightly notes that the stress created by an object moving across ice varies in a complex way that depends upon the salt content of the ice, the ice thickness and the depth of water below. Whereas the critical speed on foot-thick ice over 20 feet of water is about 20 mph, the critical speed on ice 3 feet thick atop 100 feet of water is near 40 mph. More importantly, the stress buildup in thick ice when a vehicle moves at critical speed across it is comparatively much less than over thin ice. The buildup in certain favorable circumstances is less than twice that caused by a stopped vehicle. This is why it is possible to operate heavy aircraft weighing in excess of 100 tons from runways on sea or lake ice only 4 or more feet thick.

Mr. Johnson gives a hint or two on ice safety. He notes that if one can successfully accelerate through the critical speed--as when an aircraft takes off--the stress in the ice rapidly drops off at great speed. Further he says, "if you can drive onto the ice above the critical speed...you are in good shape unless you want to stop."