CEN16LA076

The pilot's failure to maintain the glidepath and appropriate airspeed during landing in icing conditions, which resulted in an aerodynamic stall and a hard landing.

The Part 135 passenger flight was cleared for an instrument landing system (ILS) approach to the destination airport in marginal VFR conditions. Radar data showed the airplane intercept the final approach course, cross over the course, then re-intercept it several times as the airplane approached the airport. Throughout the approach, the airplane flew level at an altitude about 2,300 ft above ground level (agl) until about 2 miles from the runway threshold, when it began to descend. The airplane crossed the runway threshold about 650 ft agl and subsequently landed hard on the runway, resulting in substantial damage. The pilot stated that the hard landing was the result of an aerodynamic stall at a higher airspeed than normal due to ice accumulation on the wings. Two line workers observed ice on the leading edges of both wings after the airplane landed. The operator's training manual stated that an approach that becomes unstabilized below 1,000 ft above airport elevation in instrument meteorological conditions or 500 ft above airport elevation in visual conditions required an immediate missed approach or go-around. Icing conditions were widespread along the airplane's route of flight. These conditions had been forecast, and numerous pilot reports confirmed the presence of icing in the area. The extent to which the pilot had familiarized himself with the current and forecast weather conditions before the flight could not be determined. Although the airplane was approved for flight in icing conditions and equipped with deicing boots, the investigation could not determine if or when the pilot cycled the boots. The manufacturer's flight manual warned that, even after cycling the boots, ice accumulation could remain on protected and unprotected surfaces, resulting in significant increases in stall speed. The manual also advised that ice could affect the aural stall warning system, and that pilots should maintain a "comfortable margin" of airspeed above normal stall speed. The aggressive flare that was likely required to arrest the airplane's high rate of descent on final approach, combined with the airplane's higher stall speed due to ice accumulation, likely resulted in an aerodynamic stall and subsequent hard landing.

The FAA Instrument Procedures Handbook, in part, stated: Many reasons exist for executing a missed approach. The primary reasons, of course, are that the required flight visibility prescribed in the IAP [instrument approach procedure] being used does not exist when natural vision is used under 14 CFR Part 91, section 91.175(c), the required enhanced flight visibility is less than that prescribed in the IAP when an EFVS [enhanced flight vision systems] is used under 14 CFR Part 91, section 91.175 (l), or the required visual references for the runway cannot be seen upon arrival at the DA [decision altitude], DH [decision height], or MAP [missed approach point]. In addition, according to 14 CFR Part 91, the aircraft must continuously be in a position from which a descent to a landing on the intended runway can be made at a normal rate of descent using normal maneuvers, and for operations conducted under Part 121 or 135, unless that descent rate allows touchdown to occur within the TDZ [touchdown zone] of the runway of intended landing. A National Transportation Safety Board's (NTSB) meteorologist gathered weather data from multiple sources and produced a Group Chairman's Weather Study Report. His report, in part, stated that the National Weather Service (NWS) Surface Analysis Chart for 0900 depicted a cold front stretching from southern Minnesota eastward into central Michigan. A surface low-pressure center with a pressure of 1020-hectopascals (hPa) was located in northwestern Lower Michigan with a stationary front stretching eastward from the low pressure center into southern Canada. The station models around the accident site depicted air temperatures in the low 30's to mid 20's Fahrenheit (F), with temperature-dew point spreads of 5 degrees F or less, a west to southwest wind between 5 and 10 knots, cloudy skies, light snow reported to the west of the accident site, and freezing rain reported to the northeast of the accident site. The NWS Storm Prediction Center Constant Pressure Charts for 0600, in part, depicted low-level troughs to the west-southwest of the accident site around the accident time with temperatures below freezing. It showed that the entire atmosphere from the surface through about 30,000 feet above mean sea level (msl) was below freezing indicating that any precipitation that would fall would likely be in the form of snow. At 0845, the recorded weather at GYY was: Wind 250 degrees at 7 knots, visibility 10 miles, overcast clouds at 1,300 feet, temperature 0 degrees C, dew point -4 degrees C, and altimeter setting 30.20 inches of mercury. Additionally, observations from GYY and from an airport near GYY indicated that marginal VFR conditions, which are ceilings between 1,000 and 3,000 feet above ground level (agl) and visibility between 3 to 5 miles, were present around accident airport subsequent to the accident flight. The 0600 sounding taken at Lincoln, Illinois, indicated a moist vertical environment from the surface through 5,500 feet msl, then a dry layer between 5,500 and 7,000 feet, then relatively moist conditions again from 7,000 feet through 14,000 feet. The sounding indicated icing (clear, rime, and mixed) between 2,000 and 4,000 feet msl and between 8,000 feet and 12,000 feet msl. The sounding was also close to saturation between -4° C and -11° C (between 2,000 and 5,500 feet msl and between 8,000 and 12,000 feet msl) which is considered a temperature range supportive of the growth of super cooled liquid water droplets. The Geostationary Operational Environmental Satellite-13 visible imagery from 0830 and 0845 indicated a general west to east movement of the clouds over the accident site at the accident time with cloud cover between SPI and the accident site. The imagery indicated a general uniform cloud top height near the accident site at the accident time, with the highest cloud tops located across Wisconsin. At 0845, the approximate cloud-top heights over the accident site were 14,000 feet. Weather surveillance radar charts depicted that the accident flight flew through base reflectivity values correspond to very light to light precipitation returns. There were no recorded lightning strikes near the accident site at the accident time. Dual-Polarization weather radar data depicted conditions consistent with the accident airplane encountering hydrometeors (precipitation) having a larger surface area in the horizontal dimension rather than the vertical dimension. The hydrometeors were shaped more like pancakes, rather than spherical or tear dropped shaped as the hydrometeors fell. The hydrometeors had characteristics are similar to the freezing drizzle and supercooled liquid water. The data depicted that accident airplane encounter icing conditions on descent into the destination airport and the worst of the icing conditions were likely between 4,590 feet and 7,550 feet msl. There was a record of multiple pilot reports (PIREPs) before and after the accident that indicated the presence of icing in clouds. However, there is no record of a report from the pilot of the accident airplane indicating the ice that the airplane had accumulated during accident flight. No Significant Meteorological Information advisory was valid for the accident site at the accident time. No Center Weather Service Unit Advisory or Meteorological Impact Statement were valid for the accident site at the accident time. Airmen's Meteorological Information (AIRMET) Zulu, Tango, and Sierra, issued at 0245 and valid at the accident time, were the AIRMETs valid for the accident site at the accident time. In addition, these AIRMETs were valid for the entire accident flight track. AIRMET Zulu forecasted moderate icing conditions below 16,000 feet, AIRMET Tango forecasted moderate turbulence between 15,000 feet and Flight Level 350, and AIRMET Sierra forecasted IFR conditions for ceilings below 1,000 feet and/or visibilities below 3 miles due to precipitation and mist The Area Forecast, issued at 0445 and valid at the accident time, forecasted an overcast ceiling at 2,500 feet msl with tops to 12,000 feet. Scattered light snow showers were not forecast until 1000. The GYY terminal forecast expected a wind from 250 degrees at 4 knots, 3 miles visibility, light snow, and an overcast ceiling at 1,200 feet agl around the time of the accident. The investigation could not verify a preflight briefing for the accident flight. The accident pilot could have reviewed weather information via a kiosk without the weather information reviewed being archived. Current Icing Potential (CIP) and Forecast Icing Potential (FIP) advisories indicated that the Supercooled Large Droplet (SLD) values were either zero or very low at both 0800 and 0900, at all flight levels below 10,000 feet msl near the accident site. FIP indicated a greater than 50 percent probability of icing at 2,000 feet at 0900 at the accident site with decreasing probabilities by 4,000 feet msl to 40 percent and below. Probabilities remained below 40 percent between 4,000 and 8,000 feet msl and this was likely due to the drier air there as indicated by the Lincoln, Illinois, sounding. Probabilities increase back up to near 40 percent and above for the accident site area by 8,000 feet msl. The most severe icing conditions of moderate were located in the accident site area at and below 2,000 feet msl and above 8,000 feet msl according to the 2-hour FIP forecast for 0900. Similar values are seen in the 1-hour FIP forecast for both 0800 and 0900. The CIP icing probabilities and icing severity values are very similar to the 1 and 2-hour FIP values for both 0800 and 0900 at all altitudes from the surface to 10,000 feet msl. However, CIP values of ice severity do remain a little higher at 4,000 feet msl and are the FIP values for the accident area. The weather radar information from a site near Romeoville, Illinois, does indicate moisture between 4,000 and 8,00