Thursday, January 24, 2013

RV crash cause determined: Bird avoidance

The National Transportation has released a probable cause of an accident that killed Randolph Flores of Palominas, Arizona in Jordan, UT in September 2011.

According to the NTSB, the RV-6A pilot was probably trying to avoid birds when he entered the traffic pattern to land.

While approaching the airport at the conclusion of a 4-hour flight, the pilot announced his intention over the common traffic frequency to join the traffic pattern. A short time later, an undiscernible distress transmission was made over the frequency. A few seconds later, the airplane was observed spiraling to the ground. Global positioning system data recovered from the airplane revealed that it was traveling at an appropriate airspeed for entry into the downwind leg of the traffic pattern with a sufficient margin above the stall speed to maintain flight. It then made an abrupt left turn, resulting in a spiral dive, which progressed into a spin. A postaccident examination of the airframe and engine revealed no evidence of mechanical malfunctions or failures that would have precluded normal operation.

No evidence of a bird-strike was found, and review of radar data did not reveal the presence of other aircraft in the vicinity just prior to the accident. However, radar data did reveal that the airplane passed through a cluster of primary targets (with no altitude information) at the time of the accident. Such primary targets could potentially be radar system anomalies, thermal air currents, or bird reflections. According to a bird mitigation specialist, large birds, or flocks of smaller birds, are often present at that time of year, and such birds typically fly circling patterns in thermal air currents at traffic pattern altitudes.

The pilot’s abrupt maneuver during the approach was consistent with an avoidance maneuver. The maneuver, which was calculated to be a 65-degree angle of bank to the left, most likely placed the airplane into an accelerated stall condition, which developed into a spin.

The airplane was loaded toward its aft center of gravity limit, which could have increased its pitch sensitivity, thereby exacerbating the turn. A successful recovery from an unintentional stall-spin at pattern altitude is extremely unlikely.

This crash was particularly noteworthy at the time because it almost hit an elementary school.

This is a good time to consider stalls and load factor:

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