Probable Cause #24: Ice Box

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This article originally appeared in Aviation Safety, Nov. 2004.

Probable Cause

There are two weather phenomena that the average personal airplane just doesn't handle well: thunderstorms and airframe icing. No one in their right mind -- except researchers -- would intentionally fly an airplane into the teeth of a mature thunderstorm; avoiding that potential is why we have airborne radar and in-cockpit displays of NexRad images. Just as dangerous -- and even more insidious -- is airframe icing. Almost before a pilot is aware of it, ice can build on an airframe and dramatically affect its performance by adding weight and drag while interfering with lift production. This kind of "double-whammy" can be merely inconvenient or it can be catastrophic.

Much has been written about airframe ice: how it's forecast, where it can be found, the legalities of flying in it, what its dangers are, how to avoid it, how to get out of it and how to land an airplane coated with it. This time of year, much more will be written and, just as predictably, a few more airplanes will be damaged or destroyed by pilots who either didn't heed the warnings or who believed they don't apply. Like so many aspects of aviation, all the warnings in the world won't amount to a sample cup's worth of avgas if they're ignored.

Once an airframe becomes "iced-up," a pilot and his passengers may have a limited number of options, depending on a host of variables. Choosing a spot for an emergency landing may be the best choice; deciding to shoot a for-real instrument approach in icing conditions while already dealing with a substantial accumulation is pretty far down our list of good choices.

On November 17, 2002, at about 1440 Eastern time, a Cessna 182 owned by its pilot was destroyed when it struck terrain during a missed approach after missing the VOR-A approach procedure to its home base, Harry Clever Field (PHD), in New Philadelphia, Ohio. The 1200-hour Instrument-rated Private pilot and his wife -- the only persons aboard -- were fatally injured. The accident flight originated from Bloomington, Ill., and was conducted under IFR since widespread instrument conditions prevailed along the route.

Bad Choices

Cessna 182

Before takeoff, the pilot obtained two weather briefings. Both briefings, conducted almost two hours apart, included information on the widespread instrument conditions and a forecast of occasional moderate mixed (rime and clear) icing in clouds or precipitation at or below 10,000 feet. The second briefing included pilot reports of icing conditions along the intended route. The NTSB report on the accident doesn't provide many more details about the briefing or about the pilot's planning. For example, we don't know if he queried the briefer about freezing levels, whether the icing conditions could be circumnavigated or whether they could be avoided entirely by waiting until later in the day for them to either dissipate or move away from his intended route.

Shortly after the second briefing, the Skylane departed Bloomington and its pilot reported to a controller that he was just in the tops of the clouds and "picking up a little bit of ice." The pilot requested and received clearance to climb to 11,000 feet. Presumably, the flight was on top of the instrument and icing conditions at this altitude, although the NTSB report makes no mention of any reports on conditions during this phase of the flight. This means we don't know if the ice picked up during the departure was still present as the Skylane neared its destination.

At 1405 local time the pilot was cleared to descend to 5000 feet and then to 3000 feet. The airplane was radar-vectored for the VOR-A Approach to Harry Clever Field.

At 1429, the pilot reported, "We're picking up moderate ice here, we got a pretty good load on it." When asked if he needed to change altitude and climb back to 4000 feet. The pilot replied, "... I don't think it will make much difference, I'm not sure I could go up anyway." When asked about the type of icing, the pilot confirmed that he was picking up rime ice. At 1431, when queried about the icing conditions, the pilot reported, "Probably got about half to three inches of accumulation on the strut ..." The pilot was then cleared for the VOR-A approach and told the controller that he would remain on the approach control frequency "as he felt he probably would not get into New Philadelphia," according to the NTSB.

At 1435, the controller told the pilot he was over the final approach fix, and asked if he was still picking up ice. The pilot replied, "... It's not worse than it was before, but we got it on all over." At 1438:09, the pilot declared a missed approach and requested radar vectors to the nearby Akron-Canton (OH) Regional Airport. Shortly after the pilot acknowledged the clearance to Akron-Canton, ATC lost contact.

Aftermath

Wing Ice

The airplane came to rest about 30 degrees nose-down in an open field with low saplings and high brush. A trooper from the Ohio State Highway Patrol who was one of the first to reach the accident site reported observing ice accumulations on the top of the vertical fin in front of the anti-collision light. The trooper described the ice as milky in appearance with a rough surface. No major anomalies with the airframe or the engine were noted in the NTSB report. A smell of aviation gasoline was noted at the site.

Recorded weather at PHD about 30 minutes before the Skylane's approach began included visibility of six miles in mist, an overcast ceiling at 700 feet and a temperature of 1 degree Celsius. About 30 minutes after the approach began, the weather had not changed appreciably but did include light rain. At about the time of the accident, the airport manager later reported the weather included a ceiling of about 500 feet overcast, with visibility varying between mile and two miles. Witnesses at the accident site reported drizzle and fog plus "occasional sprits of snow and light rain."

The NTSB report does not contain any criticism of the manner in which the pilot flew the VOR-A approach. The minimum descent altitude (MDA) for the approach was 1620 feet MSL, 726 feet AGL. The radar data shows the pilot crossed BARTE intersection at 3900 feet -- a bit high -- and descended to 1600 feet -- basically the MDA -- by the time the airplane was about two miles north of the airport. At this point, a climb was initiated and the airplane turned to the left, climbing to 1800 feet. Radar contact was lost shortly thereafter. The accident site was 083 degrees and 0.4 nm from the last radar contact.

The NTSB determined the "pilot's improper decision to disregard the weather briefing, and to conduct an intentional flight into known icing conditions, with an airplane not equipped for that flight, and his failure to maintain airspeed" was the probable cause of this accident. The NTSB didn't add that pressing on to his destination and attempting an approach he suspected he would miss demonstrated a lack of understanding of the performance hit his Skylane would take while carrying a load of ice. His decision also demonstrated a degree of rigidity: There were numerous options along his route of flight -- including airports offering ILS procedures and a greater likelihood of a successful approach. In the end, trying to shoot a missed approach with an iced-up airplane put the pilot into a box from which he could not escape.


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