Accident Overview
On March 10, 2009, at 0740 central daylight time, a Cessna 402B (registration N402BP) operated as a cargo flight under 14 Code of Federal Regulations Part 135 and instrument flight rules was substantially damaged during a hard landing on runway 31 at Aberdeen Regional Airport (KABR), Aberdeen, South Dakota. The commercial pilot, the sole occupant, was not injured. Instrument meteorological conditions prevailed at the time. The flight had departed from Joe Foss Field Airport (KFSD), Sioux Falls, South Dakota, at 0639.
Flight and Icing Encounter
According to the pilot, the airplane encountered moderate icing conditions during initial climb to 8,000 feet mean sea level (msl). He requested and was cleared to climb to 12,000 feet msl due to persistent icing. The deice boots, pitot tube heat, stall vane heat, propeller deice, and windshield hot plate were used throughout the flight. The wing deice boots functioned normally during climb. At 12,000 feet msl, the airplane remained above cloud tops for about 20 minutes without accruing additional ice.
During descent into KABR, the unprotected areas of the wings and windshield were still contaminated with ice and continued to accrue ice while being vectored for the instrument landing system approach to runway 31. The deice boots were cycled several times. The pilot saw the runway approach lights about 3/4 mile from the threshold. The runway was partially obscured by blowing snow due to a strong crosswind. Difficulty aligning with the runway occurred because the windshield was obscured by ice except for a narrow section protected by the hot plate. The airplane crossed the runway threshold at 120 knots indicated airspeed. The pilot reported a sensation of high sink rate immediately prior to touchdown, followed by the hard landing.
Post-Accident Observations
After the flight, 1 to 1-1/2 inches of ice accumulation was observed on unprotected areas of the airplane. Protected areas of the wing, tail, windshield, and propeller blades were free of appreciable ice contamination. The right wing primary structure, including the main wing spar and engine nacelle assembly, was substantially damaged.
Weather and Awareness
At 0746, the KABR automated surface observing system reported: wind 360 degrees true at 22 knots gusting to 30 knots; visibility 1 mile with light snow and mist; few clouds at 600 feet, broken ceiling at 1,400 feet, overcast at 2,300 feet; temperature -16°C; dew point -18°C; altimeter 30.08 inches. A peak wind from 010 degrees true at 30 knots was recorded at 0737. Active advisories included instrument flight rules conditions (ceilings below 1,000 feet and/or visibility below 3 miles) and moderate icing below 15,000 feet with a freezing level at the surface. The pilot stated he reviewed weather via a commercial computerized service before departure and was fully aware of the icing advisory and pilot reports, expecting light-to-moderate icing en route.
Regulatory and Equipment Information
The accident aircraft, a 1973 Cessna 402B (serial number 402B0353), was certified under Civil Aviation Regulations Part 3. Advisory Circular 91-74A noted that prior to 1973, small airplanes were approved for flight in icing if equipped with a minimum suite of ice protection, but that deicing equipment on non-air-carrier airplanes could not be expected to cope with heavy or prolonged moderate icing. In 1973, 14 CFR Part 23 was amended to require analysis and testing for safe operation in icing, but the 1973 model was manufactured under its previously issued type certificate without such testing. The 1973 Cessna 402B Owner's Manual stated that deicing systems do not provide adequate protection for the entire airplane and that known icing conditions should be avoided.
In 1996, the airplane's factory-installed alcohol windshield deice system was replaced with an electrically heated windshield plate on the left side only; the right windshield was left unprotected. The FAA Form 337 noted the installation did not allow flight into known icing conditions. After the accident, the Federal Aviation Administration evaluated compliance with 14 CFR Part 135.227(c) and determined that the airplane did not meet regulatory requirements because the right windshield lacked deicing or anti-icing equipment.
On March 24, 1997, Cessna issued mandatory service bulletin MEB97-4 calling for installation of a placard stating "Flight In Known Icing Is Prohibited" for affected aircraft, including the accident airplane. The service bulletin noted that affected airplanes may appear certified for known icing but are not. The operator contended that compliance was not required as it was not an airworthiness directive, and that the airplane was equipped with required deice/anti-ice systems with guidance in the company's Operation Manual and training modules.