Pilot Debrief

CESSNA 152 near Hesperia, CA — 2009-08-16

Final reportWPR09FA398
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Date
2009-08-16
Location
Hesperia, CA, USA
Airport
L26
Aircraft
CESSNA 152
Registration
N67361
Category
Airplane
Highest injury
Fatal
Fatalities
2
Phase of flight
Approach

Probable cause

The pilot's failure to maintain adequate airspeed while maneuvering in the traffic pattern, which resulted in an aerodynamic stall/spin. Also causal was the failure of the left magneto due to improper assembly of the drive gear during installation on the engine, and the improper internal timing of the right magneto due to inadequate maintenance that reduced the ability of the magneto to produce an adequate spark, resulting in a partial loss of engine power.

NTSB narrative

The accident flight was to be a local area personal flight. After fueling the airplane, the pilot and passenger departed. A ground witness who is a pilot and former Federal Aviation Administration controller was in an automobile located near the destination airport and observed the airplane make a high and fast approach to runway 21. Unable to land, the pilot executed a go-around. The airplane returned for a second approach to runway 21, which was again flown high and fast and terminated in another go-around. The witness described the airplane's climb and left crosswind turn at the departure end of the runway with a pitch attitude of 45 degrees nose up and the airplane's left bank angle about 60 degrees. The witness stated that he lost sight of the airplane and came upon the crash site next to the road on which he was traveling. The airplane was oriented in a northerly direction. The witness reported that the wind, which he estimated at 5 knots, was also coming from a northerly direction. The accident airplane was found on near level soil about 1/4-mile southeast of the departure end of runway 21. The airplane was in a nose-down attitude, with the engine buried in the sandy soil to an estimated 1-foot depth at about a 45 degree angle. Crush deformation and ground scars were consistent with a stall/spin loss of control. The pilot's route of flight between the departure airport and the destination could not be determined; however, a more or less direct flight would be about 30 minutes. Except for the magnetos, examination of the wreckage disclosed no evidence of a preimpact mechanical malfunction or failure in the airframe or core engine components. Both magnetos had been displaced from their respective mounting pads. When the left magneto was removed, investigators noted that the castellated nut, washer, and cotter pin that secure the magneto drive gear and impulse coupler were missing. The respective threads on the shaft were undamaged, indicating that the nut was not mechanically forced off of the shaft but that it backed off during operation. The nut and associated washer were found in the oil sump, but the cotter pin was not located. The magneto drive gear that attaches to the subject magneto shaft, which is an engine part and must be installed onto the magneto when it is installed on the engine, exhibited no visible witness marks on the inside bore. Witness marks would have been present if the cotter pin had been in place and worked its way out of the castellated nut and shaft during operation of the engine. The rear of the engine case within the accessory section, which corresponds to the location of the magneto drive gear, exhibited a rotational burnishing signature consistent with having prolonged contact with the subject gear while the engine was operating. In addition to these observations, metal particles were found in the oil filter, all of which indicate that the airplane was operated with the improperly assembled drive gear for a sustained period of time. The left magneto sustained impact damage that precluded functional testing. The right magneto “E-gap” was found to be excessively out of the manufacturer’s specifications during the examination; however, the magneto produced a weak spark during hand rotation of the drive. The internal magneto timing was 34 degrees ± 1.5 degrees. According to the magneto manufacturer, ± 5 degrees is the normal allowable range for internal timing. With the timing noted during the examination, the right magneto would produce a spark, but it would be very weak and negatively affect the power production of the engine. The maintenance history documents multiple instances of magneto maintenance, removal and replacement, and timing adjustments--particularly to the left magneto--during the 2-year period between the engine’s overhaul and installation in August 2007 and the accident. In general, the historical review found that the magneto timing had to be adjusted at roughly 50-hour intervals, which is indicative of the operator’s maintenance personnel chasing the internal magneto timing issues by timing the magneto to the engine instead of resolving the internal malfunction. An August 8, 2009, entry in the flight department rental log sheet for the accident airplane lists a pilot report of an rpm drop of 225 when the left magneto was tested during the runup procedure before a flight. According to the Cessna Pilot's Operating Handbook for this airplane, the maximum allowed rpm drop on either magneto is 125. Review of the aircraft and maintenance department records found no corresponding maintenance record in response to the pilot-reported discrepancy. It is likely that the internal timing issues found with both magnetos resulted in weak ignition spark outputs that would negatively affect the power production of the engine but not necessarily produce a condition, such as prolonged roughness, that would alert the pilot to a potentially serious problem. Since the pilot flew about 30 minutes to the vicinity of the airport, the castellated nut securing the drive gear to the magneto shaft in the left magneto probably loosened sufficiently to free the gear from engagement on the shaft. Once the gear was liberated, the left magneto would not have functioned, leaving the weak right magneto as the only source of ignition spark, which would have presented itself as a serious partial loss of power. During the pilot’s ensuing attempts to land at the airport, he inadvertently stalled the airplane while maneuvering in the pattern.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Pilot could have aborted the approach earlier to avoid terrain impact.

NTSB coding

Evidence available

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  • 65 docket documents
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