AIN Blog: FAA’s New Rule on Ice Bridging Is Not the Best It Could Be

 - August 31, 2011, 7:54 AM

The FAA has finally put a regulatory nail in the coffin of ice bridging with a new rule requiring Part 121 airline pilots to activate deicing systems at the first indication of ice accumulation. Yet at the same time, the FAA has completely ignored a huge opportunity to improve safety for Part 91 and 135 operators that fly in icing conditions.

The new rule will become law on October 21 and apply only to Part 121 aircraft that weigh less than 60,000 pounds. The rule won’t apply to the heavier Bombardier Dash 8-Q400, one of which, operated by Colgan Air, stalled in flight following an autopilot level-off and subsequently crashed near Buffalo, N.Y., on Feb. 12, 2009. The aircraft had accumulated some ice during the approach.

For years, the FAA, NTSB and icing researchers at NASA have been trying to persuade pilots that ice bridging is a myth. The concept of ice bridging stems from a belief that if pilots turn on deicer boots too soon when flying in icing conditions, then ice will form a hollow shell on the leading edge of the wing, outside the space where the boot is inflating. This so-called bridge of ice will just stay there, accumulating more ice buildup while the boot ineffectually inflates and deflates inside the hollow shell formed by the bridge.

The ice-bridging phenomenon can be prevented, according to believers, by delaying activation of deicer boots. Waiting until the ice builds to about one-quarter or one-half inch thick enables the boots to break the ice off cleanly, preventing the buildup of a bridge and helping the boots do their job more effectively. Interestingly, some aircraft manufacturers, notably BAE, Cessna and Hawker Beechcraft, support ice bridging beliefs by requiring pilots to delay activation of boots until one quarter or one half inch of ice has accumulated.

I have interviewed pilots who insist that ice bridging is real and that they have experienced icing that built a solid bridge over their airplane’s deicer boots. There are also some interesting literary references to ice bridging (see below).

Nevertheless, in the discussion of the new rule, the FAA claims, “No one has reported ice bridging nor has it been seen during testing on modern deicing boots.” And researchers have tried to study ice bridging and say they have never found evidence that it exists, either in the real world or in icing wind-tunnel testing.

So, the FAA, NTSB and NASA insist that modern deice boots are not subject to ice bridging and that pilots should begin inflating boots immediately when any amount of ice starts accumulating on the aircraft. The reasoning is that even small amounts of ice can dramatically affect the aerodynamics of a wing and cause more severe problems than the amount of intercycle ice that normally sticks to boots when they are actuated at the first sign of icing.

OK. Let’s accept that the FAA, NTSB and NASA are correct: regardless whether ice bridging occurs with modern deicing boots or not, the best way to handle ice is to turn the boots on as soon as one flies into icing conditions. So why, I can’t help wondering, did the FAA limit this new rule to only certain aircraft?

While the NTSB didn’t blame icing for the Colgan crash, the Dash 8-Q400 did accumulate some ice before the accident. More important, smaller aircraft that are legal to fly in known icing conditions are at even more risk of aerodynamic degradation caused by ice accumulation. The NTSB issued Safety Alert 014 in December 2008 warning pilots that, “As little as one quarter inch of leading-edge ice can increase the stall speed 25 to 40 knots.” And regarding ice bridging, the NTSB added: “The Safety Board has no known cases where ice bridging has caused an incident or accident, and has investigated numerous incidents and accidents involving a delayed activation of deice boots” [italics added by author].

The NTSB’s long-open recommendation to the FAA regarding when pilots should activate deicing systems suggested that this advice apply to any boot-equipped aircraft, not just large ones: “Require manufacturers and operators of pneumatic deice boot-equipped airplanes to revise the guidance contained in their manuals and training programs to emphasize that leading edge deice boots should be activated as soon as the airplane enters icing conditions.”

The FAA doesn’t agree, however, and spelled out its conclusions in the discussion of the new Part 121 regulations: “We considered including Parts 91 and 135 operations during deliberations of the [working group] and during drafting of the NPRM. We determined, however, that the increased flexibility afforded by unscheduled operations (the types of operations governed by Parts 91 and 135), coupled with appropriate direction on when pilots should activate the ice protection systems (usually at the first sign of icing or in conditions of visible moisture and specific temperatures), provides an adequate level of safety for ice protection system activation. Pilots flying scheduled operations, on the other hand, may not have the flexibility to avoid flying into weather that would otherwise be avoided. This rule ensures that Part 121 operators of applicable airplanes will be directed to operate the ice-protection systems appropriately.”

The FAA also apparently fails to acknowledge the fact that some manufacturers still believe that ice bridging occurs on some aircraft they build and they continue to advise their pilots to wait to activate boots until a quarter or half inch of ice accumulates. The FAA tried to force these manufacturers to change their flight manuals years ago with a series of Airworthiness Directives, but the manufacturers objected and were able to persuade the FAA not to publish the directives.

So, while pilots flying certain Part 121 aircraft will soon have to activate deicing systems at the first sign of ice or when an ice-detection system alerts impending icing conditions, pilots flying airplanes under Part 91 and 135 remain subject to a confusing state of affairs, where some manufacturers advise waiting for ice to build while others tell pilots to turn on the boots right away. Cessna’s POH for the Citation 560 adds even more confusion, because while it tells pilots to wait for ice to build before boot actuation in cruise flight, Cessna advises turning boots on at the first sign of ice accumulation while on approach.

Ironically, the NTSB’s own research emphasizes the potential benefit of applying this new rule also to Part 91 and 135 operations. In a study of icing-related accidents between 1982 and 2000, the NTSB found that 819 people were killed in 583 airframe-icing accidents during that period. The majority of the 583 accidents, 80.6 percent, involved Part 91 operations. Part 135 accounted for 17.6 percent and Part 121 only 1.7 percent.
Clearly, the FAA dropped the ball on this rulemaking. The agency had a great opportunity to improve safety substantially, not by trying to persuade pilots that ice bridging doesn’t exist, but by requiring aircraft of any size operated under Parts 91, 135 and 121 to actuate boots at the first sign of ice accumulation.

All of the experts agree that this is the safest way to fly. And it would be far safer to teach pilots from day one to activate boots right away, especially if they will someday end up flying boot-equipped aircraft under Part 121.

Literature on ice bridging, excerpted from:
Robert N. Buck, Weather Flying (nonfiction)
Ernest K. Gann, Fate is the Hunter (autobiographical)

Buck spent a lot of time hunting for ice in a B-17 after World War II and doesn’t hold a high regard for pneumatic deicers. He does discuss ice bridging as follows: “The boots pulsate up and down and, in theory, break off the ice. They don’t break it off as it forms. The boot has to wait until there is a coating of ice before it can do an efficient job. It is therefore advisable to allow ice to build up at least one quarter of an inch thick before turning on the boots. They then expand and break the ice, and the windstream blows it off and away. When they have done this job, it’s time to turn them off and wait until another coating of ice has formed and then turn them on and break it off again.”

In Fate is the Hunter, Gann is copiloting a DC-2 during an ice encounter: “What the hell is wrong with those fancy deicer boots? They are not performing the task for which they are intended. Come! Function!

“I glance furtively out of the window at my side. The blinking red light indicator shows the deicers are in operation, but outside there is visible proof that they are lying down on the job. The leading edge of the wing is now one long, unbroken bar of ice. And it is clear ice, rumpled as if there were rocks beneath.

“Yes, the boots are working. But they are expanding and contracting beneath the sheath of ice and consequently useless! The ice has accumulated too fast for them.”