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Icing - Hope is not a strategy

by Michael Lenz
Reprinted with permission from FAA Aviation News

In this article airframe icing is explored. This article is part of a continuing series of pilot reports of weather encounters as collected and analyzed by the Aviation Safety Reporting System (ASRS) staff (See FAA Aviation News - September/October 2007). Icing accidents were also used from the National Transportation Safety Board (NTSB) database.

The title of this article says it all. Hope is not a strategy, when icing is involved. Encounters with icing are often dangerous, especially to small general aviation aircraft that may not have the equipment to mitigate the icing encounter.

My research begins with 100 ASRS reports of weather encounters of all types. Sixteen flights encountered icing resulting in heading and altitude deviations, control problems, and declaration of emergencies. These 16 pilots, flying single-pilot operations, encountered icing, yet only three were informed by weather briefings of potential icing prior to their flights. Nine of these flights were operating under instrument flight rules (IFR). The icing encounters resulted in the following consequences:

  • Icing forced five pilots to deviate from assigned altitudes/airways without waiting for ATC clearance.
  • The icing encounters resulted in three declarations of emergency (one included a visual flight rules (VFR) encounter into instrument meteorological conditions (IMC)
  • Three additional reporters encountered IMC and icing, while operating under VFR.
  • Two reporters diverted to an alternate airport and landed
  • One reporter encountered control problems from severe icing and diverted
  • One reporter landed below mini-mums to avoid additional ice accumulation
  • One reporter stated that the icing condition was distracting and caused a deviation from the instrument landing system (ILS) heading
  • Many of the pilots conveyed their surprise at how quickly ice could accumulate on an aircraft. One pilot observed, "The mist came from nowhere'iced my wings and propeller in a matter of seconds.

Profiles of Icing Encounters

One ASRS study reporter reflected that an early diversion to an alternate airport might have alleviated this potential emergency situation because of icing:

"Upon arriving at the destination airport, the pilot set up for the ILS'After crossing the initial approach fix (IAF), looked up and realized'windshield was covered with ice, and glanced at mirror and leading edges, and realized that they were covered with bumpy, spiky mixed ice'Committed to landing'didn't want to take the chance of accumulating more ice, perhaps to the point that the plane couldn't fly, especially with a potential engine problem. I flew to the right of the localizer in hopes of seeing the runway out my left window."

During the icing encounters, pilots cited some problems with the availability of air traffic control (ATC) services, including delays in obtaining ATC clearances (three reports), ATC not providing a pop-up IFR clearance (one report), and being too low for radar coverage (one report). One example of rapid ice accumulation, coupled with attempts to contact ATC, occurred when a Mooney M20C pilot accumulated un-forecast rime icing at 7,000 feet,

"'descended to 5,000 feet and described icing that continued to accumulate rapidly'Several attempts to reach either controller on two radios went unanswered'I called in the blind twice that I was descending to 3,000 feet [from 5,000 feet]."

The pilot finally reached ATC, while descending through 4,100 feet, and obtained clearance to descend. In hindsight, the pilot thought a block altitude request would have helped comply with the altitude assignment. Another pilot was faced with a dilemma when the Center was busy on another frequency.

The Accidents Tells a Similar Story But with Some Key Differences.

A review of icing accidents, since 2000, shows 55 reports ranging from hard landings with iced-over windshields and non-injury runway over runs to fatal, loss of control accidents. The vast majority occurred to piston singles and light twins.

If there's a difference among the accident pilots and those reporting icing encounters to ASRS, it's in the preflight information. Only three of the 16 ASRS reports of ice encounters indicated information regarding potential icing in the preflight briefing. In contrast, almost all of the accident pilots received a preflight weather briefing and they almost always included a potential for icing.

Once en route, only eight of the accident pilots received a pilot weather report (PIREP) on icing conditions. Those eight pilots were all involved in injury accidents to some extent. Of the remaining 47 accidents, 11 did not involve injuries. Perhaps the message here is that once a PIREP of icing conditions is received in flight, prompt action is needed to avoid the hazard. It's hard to say, because we don't know about the benign events, that a PIREP message resulted in an alternate course of action with no incident.

Six of the accidents involved at least a partial power loss. Some of these involved icing that affected the engine induction system. It may be easy to overlook things like engine alternate induction air, while attempting to find an ice-free altitude, but nothing leads to a rapid altitude loss like an engine failure, compounded by an iced over aircraft. It's important to know the airplane flight manual (AFM) procedures for use of alternate induction air, when ice may be present.

Flaps and Tailplane Stalls

Two of the accidents occurred as the flaps were deployed. This brings up the subject of tailplane icing or Ice-Contaminated Tailplane Stall (ICTS). [Note: The following information is from Advisory Circular AC 91-74, Pilot Guide Flight in Icing Conditions. This is available at <http://rgl.faa.gov>.] Since the tailplane is ordinarily thinner than the wing, it is a more efficient collector of ice. On most aircraft the tailplane is not visible to the pilot, who therefore cannot observe how well it has been cleared of ice by any deicing system. Therefore, it is important that the pilot be alert to the possibility of tailplane stall, particularly on approach and landing.

Most aircraft have a nose-down pitching moment from the wings because the center of gravity (CG) is ahead of the center of pressure. It is the role of the tailplane to counteract this moment by providing "downward" lift. (See Figure 1).

The result of this configuration is that actions, which move the wing away from stall, such as deployment of flaps or increasing speed, may increase the negative angle of attack (AOA) of the tail. The initial deployment of the flaps should be only partial. Vibration or buffeting that follow deployment is much more likely to be because of incipient tailplane stall. The reason is that, after deploying the flaps, the wing will be at a less positive angle and, therefore, farther from stall, while the tailplane will be at a more negative angle and closer to stall. (See Figure 2).

As the pilot prepares for the deployment of the flaps after or during flight in icing, he/she should carefully assess the behavior of the aircraft for any buffet or other signs of wing stall. The initial deployment of the flaps should be only partial. Vibration or buffeting that follow deployment is much more likely to be due to incipient tailplane stall than wing stall, if there was no vibration buffet before deployment. The reason is that after deploying the flaps, the wing will be at a less positive angle, and so farther from stall, while the tailplane will be at a more negative angle, and so closer to stall. There are few known incidents of ICTS in cruise (when flaps would not ordinarily be deployed). However, when the flaps are deployed, tailplane ice, which previously had little effect other than a minor contribution to drag, can now put the tailplane at or dangerously close to stall.

One of the ASRS reporters encountered ice and tail vibration in a Cessna Caravan at 10,200 feet. The pilot began a climb:

"'But aircraft unable to climb above 10,200 feet. Tail begins vibration and aircraft pitches up. Autopilot is disengaged and pitches aircraft down in 1,500 fpm descent [emphasis added]. Pilot descent to 6,500, get[s] a bit more [ice] to slide off and leading edge of mains begins to clear with boot action and better temperatures. Tail still vibrating. Airspeed improves and ice steadily comes off'"

Legalities of Ice - The Jury Is Out

In June 2006, the FAA's legal council issued an interpretation on the definition of "Known Icing." This admittedly set off a controversy and resulted in the Aircraft Owners and Pilots Association (AOPA) requesting that the interpretation be rescinded. Excerpts from the FAA's response and its effort to seek public comment, offer insight into the complex administrative law surrounding safe operations when icing conditions are presented. It also points out the meteorological and aerodynamic conditions that affect airframe icing. [The complete text can be found on the Web at http://www.gpoaccess.gov/fr/index.html in the April 3, 2007, Federal Register (Volume 72, Number 63)].

"While various FAA regulations contain limitations on flight in known icing conditions, the regulatory provision that most commonly affects general aviation operators in this respect applies the term only indirectly. Title 14 Code of Federal Regulations (14 CFR) section 91.9, Civil aircraft flight manual, marking, and placard requirements, precludes pilots from operating contrary to the operating limitations in their aircraft's approved AFM. The operating limitations identify whether the aircraft is equipped to operate in known icing conditions and may prohibit or restrict such flights for many general aviation aircraft. Title 14 CFR section 91.103, Preflight action, requires pilots to become familiar with all available information concerning their flights before undertaking them.

Permutations on the type, combination, and strength of meteorological elements that signify or negate the presence of known icing conditions are too numerous to describe exhaustively in this letter. Any assessment of known icing conditions is necessarily fact-specific. However, the NTSB's decision making reflects the common understanding that the formation of structural ice requires two elements: visible moisture and an aircraft surface temperature at or below zero degrees Celsius. Even in the presence of these elements, there are many variables that influence whether ice will actually form on and adhere to an aircraft. The size of the water droplets, the shape of the airfoil, or the speed of the aircraft, among other factors, can make a critical difference in the initiation and growth of structural ice.

Likewise, a variety of sources provide meteorological information that relates to forecast and actual conditions that are conducive to inflight icing. Pilots should carefully evaluate all of the available meteorological information relevant to the proposed flight, including applicable surface observations, temperatures aloft, terminal and area forecasts, AIRMETs, SIG-METs, and pilot reports. As new technology becomes available, pilots should incorporate use of that technology into their decision-making process.

The ultimate decision of whether, when, and where to make the flight rests with the pilot. A pilot also must continue to reevaluate changing weather conditions [emphasis added]. If the composite information indicates to a reasonable and prudent pilot that he or she will encounter visible moisture at freezing or near freezing temperatures and that ice will adhere to the aircraft along the proposed route and altitude of flight, then known icing conditions likely exist. If the AFM prohibits flight in known icing conditions and the pilot operates in such conditions, FAA could take enforcement action."

A Hero Emerges

As in all weather flying, the golden rule is to "Leave yourself an out." One of the ASRS reporters did just that. This pilot encountered supercooled, large droplets in the descent after already accumulating ice en route. This made continued flight unsustainable. This "out" may not have been as golden as it should have been, but, in the pilot's words:

"My 'out' was planned in advance, warmer temps in the 2,000 feet just above ground. This planning of an absolute 'out' must be practiced, particularly in non-turbojet aircraft flying below flight levels, in winter between Idaho, Utah, Montana, and Wyoming. In other words, had I not had the 'out' that I ended up using in this case, I would not have launched that day. If surface temps were two to three degrees colder, I would not have launched."

Additional Information

All of the participating pilots were asked:

1. Why do you think the incident occurred?

2. In retrospect, is there anything you would have done differently?

3. What would you recommend that others do to avoid a similar occurrence?
Some selected responses were:

From a Cessna 210 Centurion pilot:

1. The weather unexpectedly got worse than forecasts near the end of my flight, and I expected it to be temporary. Once I was committed to the approach, I didn't want to go around and divert to my alternate with the ice buildup and the hot Exhaust Gas Temperature (EGT) on one cylinder.

2. Probably not, as I thought the low visibility was due to a short-term "squall line" and would pass quickly, and the ice was not expected.

3. Divert to the alternate if forecast or current conditions are suddenly below minimums, don't get past the point of no return as I did, with (as it turned out) false optimism that the condition would be momentary.

From a Mooney M-20 pilot:

1. Non-forecast icing conditions. Known poor radio coverage area. Lack of foresight by controller and myself to use a block altitude.

2. As per above requested a block altitude.

3. Same. If changing altitude in icing conditions, have a plan for lost radio coverage (next frequency, block altitude). I did have next frequency.

From a PA-34-200T Turbo Seneca II pilot, who was on a visual flight rules (VFR) flight at 10,500 feet and lost control of the aircraft and recovered at 3,000 feet. The icing occurred over a period of about five minutes before he lost control:

1. Mist came from nowhere. Iced my wings and propeller in a matter of seconds. I've never seen this kind of weather before.

2. No, I saved my life and wife and kids. I did not panic and regained control of the airplane. Continued my flight to destination.

3. I don't know

And finally, this instrument-rated C-172 pilot encountered a myriad of issues while conducting this flight: disorientation, struggles with icing, getting below radar coverage, time pressures, over confidence in flying experience, and over reliance from having two pilots on board. Here's his narrative:

"We departed VFR in order to get off the ground faster - the ceiling started to come down rapidly. I instructed my friend to contact ARTCC (air route traffic control center) for a pop-up IFR clearance - ice was starting to accumulate at a great rate at this point and I had to add increasingly more power to keep us at a normal cruise speed at this point. I had to add full power to maintain my assigned altitude. I asked my passenger to declare an emergency as it was clear we had picked up over a quarter of an inch of ice and could not identify the localizer to find the airport.

I was confused at this point and became completely disoriented with regards to our position. I knew from experience that zzz airport is between two ridges and I was fairly confident we were still between the two . ARTCC advised us that we were below their radar coverage and told to contact them when we got on the ground. Within 500 feet my passenger spotted the airport 90 degrees to our right. I made the turn and landed without incident.

If we were in the clouds 10 minutes longer, both my passenger and myself would have perished from the amount of ice on the aircraf. What I did not anticipate was how rapidly the ice formed, and how few options we had for airports along the route - the terrain to be very rugged and unforgiving. My passenger and I made the decision to take the flight to make it to work on Monday.

I feel very good about my flying during the incident, but my lack of situational awareness was staggering. I'm at a fairly dangerous time in my flying career where I have enough experience to be confident, but not enough to really know better."

Michael Lenz is a program analyst in Flight Standards Service's General Aviation and Commercial Division.