Unknown Icing Conditions

A 473-hour instrument-rated private pilot, alone in a fully fueled Cirrus SR22-G2, took off from Reno, Nevada, early on a Sunday evening in February 2005. The sun had set half an hour earlier; the moon, above a layer of clouds, was a mere sliver. He had filed an IFR flight plan via Truckee and Sacramento with a destination of Oakland, California, 155 nm to the southwest. The cruising altitude was to be 12,000 feet. Reno, where the field elevation is 4,415 feet, was reporting an overcast at 5,500 feet, light winds, and a surface temperature of 6º Celsius with a 7º temperature-dew point spread. The pilot's weather briefing, obtained from an FSS briefer an hour before the flight, put the freezing level at 6,000 feet and reported no precipitation in the area. No pilot reports were available for the route. When he filed his flight plan, the pilot told the briefer that he might request 14,000 once airborne.

The pilot contacted Oakland ARTCC as he climbed through 10,600 feet on the way to 14,000. A couple of minutes later he requested a clearance to climb to 16,000, in order to "see if I can get above these clouds."

After cruising for a few minutes at 16,000 feet, the pilot transmitted, "I guess this isn't going to work, I'm still in the clouds. Any chance of lower?" The Center controller told him to stand by. A minute or so later, the pilot told the controller that he could get above the clouds if he could climb another 200 or 300 feet. "Do you want to go up or down?" the controller asked. Up, the pilot responded, "so I could build up some airspeed, if that's okay." He received a block altitude between 16,000 and 17,000 - meaning that he was free to cruise anywhere in that range.

Radar showed the airplane climbing. It reached a maximum altitude of 16,700 feet before beginning a gradual descent.

"I'm coming down," the pilot reported. There was an unintelligible transmission, and then, "I'm icing up." The controller asked him to say it again. "I'm icing up. I'm coming down," the pilot said.

It was his final transmission. The last radar return, a few seconds earlier, reported an altitude of 15,700 feet.

The wreckage of the Cirrus was recovered from the base of a rocky face in the Donner Pass. The engine was heavily damaged, but there was no indication of mechanical failure prior to impact. The parachute recovery system had been deployed, but had separated from the airplane and come to earth 4,000 feet away from the main wreckage. According to the manufacturer of the system, the failures of the risers and suspension lines were consistent with deployment above the placarded maximum speed of 133 knots, which is also the maneuvering speed of the airplane.

The airplane was equipped with the "icing protection system" that oozes an antifreeze solution from the porous leading edges of the wings and horizontal tail and from a slinger at the propeller. The company's promotional materials paint an optimistic picture of the capabilities of the system, which it calls "an effective way to help you make an early retreat from unexpected encounters with airframe icing." The solution, it explains, "mixes with the supercooled water in the clouds, and the mixture flows off the aircraft without freezing." If "trace" amounts of ice have already formed, "the antifreeze solution will chemically loosen the bond between the ice and airframe, allowing the aerodynamic forces to carry the ice away."

Nevertheless, flying the SR22 into known icing conditions is prohibited, as the airplane's documentation repeatedly warns. The FAA-approved language of the Pilot Operating Handbook presents a somewhat less than optimistic view of the system's abilities, noting that "no determination has been made as to the capability of this system to remove or prevent ice accumulation ... . At the first indication of icing, the most expeditious and safest course of exiting the icing conditions should be taken."

Investigators concluded that the airplane had probably encountered icing from supercooled large water droplets near the cloud tops. In fact, a retrospective analysis of weather conditions, in light of the most recent research, showed a "clear likelihood" of severe icing conditions; but the aviation icing prediction algorithms used by the National Weather Service had determined that there was only a low probability of icing in the area, and so did not trigger an icing forecast.

Severe icing occurs when liquid water is carried upward through clouds at a fairly rapid rate - in this case, possibly lifted by a westerly flow against the rising foothills of the Sierra Nevada - but does not crystallize even though its temperature drops below the freezing point. Water in this condition is said to be "supercooled." When it strikes the surface of an airplane it freezes instantly, producing a rapid buildup. Areas of icing of this type are often small, with the result that one airplane does not experience icing while another, a little later or a short distance away, does. It is noteworthy-and this has been an article of flying lore since long before anyone had dreamed of supercooled large water droplets-that the heaviest icing often seems to occur near the tops of clouds.

The National Transportation Safety Board mentioned the inaccurate icing forecast as a factor in this accident, but identified the pilot's loss of control as the primary probable cause.

The ATC transcript, although sketchy, provides some clues as to why the loss of control may have occurred. It's not certain why the pilot, after filing a cruising altitude of 12,000 feet, amended it to 14,000 and then to 16,000. Most likely he believed that he would be on top at 14,000. It's evident, in any case, that he hoped to cruise above the clouds rather than in them.

Once at 16,000 feet, he evidently found himself picking up so much ice that it was slowing down the airplane, since he made reference to wanting to get clear of the clouds in order to regain speed. The ice removal system apparently could not keep up. His first impulse, to descend, made good sense; it was the "most expeditious and safest" course of action, since it not only provided a retreat from the rapid icing but also would have taken the unturbocharged airplane to lower altitude, where its performance would have been better. But then he glimpsed sky above him. Knowing that the cloud tops would probably be lower farther west - they tend to pile up against the western side of the mountains - he made the fateful decision to climb rather than descend.

His airspeed was already low. Ice accumulating on the wings reduces their ability to lift, and at the same time makes the airplane heavier and increases its drag. The wings may collect ice asymmetrically. The naturally aspirated engine was putting out only a little more than half its sea level power. The end result was that the airplane would no longer climb. Presumably, during these critical seconds, the ice kept building. Then the airplane went out of control. It seems probable that in his effort to wring a little climb out of the airplane, the pilot stalled it. It may have entered a spin; if so, there was little chance of recovery. To regain control of a spinning airplane in darkness and instrument conditions is nearly impossible. On the other hand, a spin, even in an ice-laden airplane, might not be consistent with the indicated airspeed of more than 133 knots required to tear the parachute free. That some sort of loss of control took place, however, is obvious.

The weather report the pilot had received had encouraged him to believe that icing would not be a problem. There is a general expectation, furthermore, that icing will be confined to a band of temperatures close to the freezing point, and that once one has climbed above that band there will be no unfrozen water to contend with. In mountains and in convective conditions, however, where there may be a certain amount of vertical air movement, this rule of thumb may not hold.

With hindsight we can see that the pilot's decision to continue climbing exposed him to more rather than less icing. The crash occurred at the highest point in the pass between Reno and Sacramento; if he had descended while continuing westward, he would soon have been over lower terrain. He might have been able to drop below the overcast without danger. Instead, he chose to climb; and it was fully 10,000 feet above the freezing level that icing got the upper hand.

This article is based solely on the National Transportation Safety Board's report of the accident and is intended to bring the issues raised to the attention of our readers. It is not intended to judge or to reach any definitive conclusions about the ability or capacity of any person, living or dead, or any aircraft or accessory.