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On August 22, 2010, about 1819 Pacific daylight time, a Robinson R44 Clipper, N7186Z, made a forced landing in the water at San Diego, California. Point Zero Corporation was operating the helicopter under the provisions of 14 Code of Federal Regulations (CFR) Part 91. The private pilot sustained serious injuries. The helicopter sustained substantial damage from impact forces. The local personal flight was departing from a yacht. Visual meteorological conditions prevailed, and no flight plan had been filed.
The pilot reported that he filled the fuel tank, and then at 1515 departed from Montgomery Field (MYF), San Diego, to his yacht at the Convention Center Marina. About 1800, he prepared for the return trip to MYF. He completed magneto and deicing checks. He planned to fly south over the Coronado Bridge, and contacted the Navy’s North Island air traffic control tower.
The pilot stated that after the helicopter lifted off the yacht, he increased airspeed to 60 knots. As he approached the bridge, the engine lost power. He was above the bridge, but the helicopter would not climb. He heard a screeching sound in his headset. The air speed decreased, the low rotor revolutions per minute horn sounded, and the manifold pressure had increased. He pulled up on the collective, but nothing happened. He began an autorotation, and started turning into the wind. The helicopter was falling rapidly, and he was unable to get headed completely into the wind prior to touchdown. About 100 feet above ground level (agl), he deployed the floats. He pulled the nose up into a flare, and could not tell if the tail boom hit the water first. He did remember releasing the tab on the carburetor heat.
All of the plexiglass shattered, and the pilot went under water. He released the seat belt as he tried to evacuate, and realized that his back was badly hurt. He was sitting on the floating wreckage when first responders arrived within 2 minutes.
A witness was at a restaurant, and seated at a table overlooking the water. She stated that a sputtering sound drew her attention to the helicopter. It banked to her left, and started going down rapidly. She lost sight of it as it went below the tree line. She said that she did not hear the helicopter “warm up” prior to taking off from the yacht.
A Federal Aviation Administration (FAA) inspector examined the helicopter after it was retrieved from the water. He observed that the carburetor heat tab had been released; the carburetor heat knob, which is located in the center console, was in the OFF (down) position.
The closest official weather observation station was San Diego (KSAN), which was 4 nautical miles (nm) northwest of the accident site. The elevation of the weather observation station was 17 feet mean sea level (msl). An aviation routine weather report (METAR) for KSAN was issued at 1751 PDT. It stated: wind from 310 degrees at 7 knots; visibility 10 miles; sky 20,000 feet scattered; temperature 24/75 degrees Celsius/Fahrenheit; dew point 14/57 degrees Celsius/Fahrenheit; altimeter 29.87 inches of mercury. The relative humidity was 55 percent.
TESTS AND RESEARCH
Investigators from the National Transportation Safety Board, Textron Lycoming, and Robinson Helicopters examined the wreckage at the Robinson factory in Torrance, California, on August 25, 2010. Detailed examination notes are in the public docket.
Investigators established control continuity for the rotor head; there were some disconnects for the tail rotor, but all the fracture surfaces were jagged and angular. They drained about 8.25 gallons of a blue fluid that smelled like aviation gasoline from the main fuel tank, and 0.75 gallons of a clear fluid was at the bottom of the drainage container. They drained about 3.5 gallons of a blue fluid that smelled like aviation gasoline from the auxiliary fuel tank, and about 1.5 gallons of clear fluid was at the bottom of the drainage container.
The crankshaft and the accessory gears rotated freely when investigators rotated the cooling fan. Investigators obtained thumb compression on all cylinders; the valves moved freely with the same amount of lift. All of the spark plugs looked the same; there was no mechanical damage. There was some slight rotational scoring on the engine cooling fan and upper drive belt sheave. They dried the magnetos out, and observed spark at each post on both of them when manually rotated. The carburetor heat cable was stretched, so they couldn’t get a reading from it. None of the light bulb elements appeared to be stretched.
Investigators observed light blue fluid with the odor of aviation gasoline in numerous components of the fuel system when removed during the examination.
Section 4 of the R44 Pilot Operating Handbook (POH) describes normal procedures for use of carburetor heat. It directs the pilot to apply carburetor heat when conditions conducive to carburetor ice are known or suspected to exist. These conditions include high humidity or operating near water. This section contains a caution that the pilot may be unaware of carburetor ice formation as the governor will automatically increase throttle and maintain constant manifold pressure and rpm.
The accident R44 was equipped with a carburetor heat assist device. This device correlated application of carburetor heat with changes in the collective setting to reduce pilot work load. Lowering the collective mechanically added heat and raising collective reduced heat. The system included a latch at the control knob to lock the carburetor heat off when not required. The POH recommended that the control knob be unlatched (to activate carburetor heat) whenever the outside air temperature (OAT) was between 80 degrees and 25 degrees Fahrenheit, and the difference between dew point and OAT was less than 20 degrees. It continued that the pilot should adjust the carburetor heat as necessary following any change in power.
The pilot stated that he released the latch, and the FAA observed the latch in the released position at the accident site. However, the inspector observed the carburetor heat knob in the OFF (down) position.
Robinson Safety Notice SN-25
Robinson issued SN-25 in 1986 and issued a revision in 1999 to inform pilots about carburetor ice. It noted that, during takeoff, airplanes take off at wide open throttle settings, whereas helicopters take off only using power required, which makes them vulnerable to carburetor ice. This is especially the case when the engine and induction system are still cold. It recommended full carburetor heat during engine warm-up, and as required during hover and takeoff to keep the CAT gage out of the yellow arc.
Special Airworthiness Information Bulletin (SAIB) CE-09-35
The Federal Aviation Administration (FAA) Aircraft Certification Service published Special Airworthiness Information Bulletin (SAIB) CE-09-35 on June 30, 2009, to inform pilots of the potential hazards associated with carburetor icing.
The SAIB noted that carburetor icing doesn’t just occur in freezing conditions; it can occur at temperatures well above freezing temperatures when there is visible moisture or high humidity. It states that icing can occur in the carburetor at temperatures above freezing. Because vaporization of fuel, combined with the expansion of air as it flows through the carburetor (the venturi effect) causes sudden cooling, a significant amount of ice can build up within a fraction of a second. The SAIB contains a graph that illustrates the probability of carburetor icing for various temperature and relative humidity conditions.
The conditions encountered in this accident (ambient temperature 75 degrees / dew point 57 degrees Fahrenheit, 55 percent humidity), were in the area of serious icing for glide power and icing for glide and cruise power.