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Both Jennifer and Lenore grabbed the mixture and pulled. The engine went from a deafening roar to silence. They looked at each other. “What happened?” asked Jennifer. “I don’t know. Let’s check the engine,” Lenore said.
Ten minutes later, they had removed the cowling from the Cessna 152. A quick engine check gave them the answer. The throttle rod-end was not connected to the carburetor arm—no bolt, no nut, just air between the rod-end and the arm. Jennifer looked at Lenore. “What if this had happened in flight?”
“What I want to know,” Lenore said, “is how this happened at all. The annual inspection was signed off yesterday.”
The previous day, the annual inspection had been signed off after a lengthy inspection by a local facility. Several mechanics had been involved in the inspection, including the owner/student who had installed a headliner. The mechanic with the Inspection Authorization (IA) who signed off the annual was supervising several annuals, so most of the maintenance was performed by other mechanics.
After the inspection, the engine had been run-up according to the usual post-inspection procedures. The student and instructor had flown the airplane for a half-hour familiarization flight. The next day’s engine start resulted in a runaway engine with the apparent cause due to the lack of the throttle rod-end hardware being safetied.
Risk Management
Chapter 9
9-2
Figure 9-1. Types of risk.
T
ypes of RiskThe sum of identified and unidentified risks.Risk which has been determined through various analysis techniques. The first task of system safety is to identify, within practical limitations, all possible risks. Risk not yet identified. Some unidentified risks are subsequently identified when a mishap occurs. Some risk is never known.Risk which cannot be tolerated by the managing activity. It is a subset of identified risk that must be eliminated or controlled.Acceptable risk is the part of identified risk that is allowed to persist without further engineering or management action. Making this decision is a difficult yet necessary responsibility of the managing activity. This decision is made with full knowledge that it is the user who is exposed to this risk.Residual risk is the risk left over after system safety efforts have been fully employed. It is not necessarily the same as acceptable risk. Residual risk is the sum of acceptable risk and unidentified risk. This is the total risk passed on to the user.Total RiskIdentified RiskUnidentified RiskUnacceptable RiskAcceptable RiskResidual Risk
Three deficient areas in this annual inspection were identified by a round-table discussion group of aircraft and powerplant (A&P) mechanics and the student. These areas were:
• Lack of responsibility
• Checklist misuse
• Complacency
Lack of responsibility—no one took responsibility for the entire inspection. The chances of something being overlooked increase with an increase in the number of mechanics involved in an inspection. The responsible person is removed from the actual procedure. The student remembers hearing the IA ask one of the engine mechanics about the throttle. However, the question was vague, the answer was vague, and the rod-end was not safetied.
Checklist misuse—all checklists have a line item regarding inspection of the engine controls for rigging and safety. Perhaps the throttle rod-end had been disconnected for maintenance after the IA had signed off the control inspection. In that case, a discrepancy should have been entered onto the discrepancy sheet stating “reconnect and safety throttle rod-end.”
Complacency—an insidious and hard-to-identify attitude. Each of the mechanics involved in the incident thought someone else had inspected the throttle rod-end. The IA signed off the annual inspection because he had either asked the mechanics about the items on the checklist or in his frequent visits to the airplane had inspected the various items himself and decided that was good enough. Complacency crippled the mechanics’ quality of work by removing any thoughts of double-checking each other’s work.
While a definite answer to the question of what happened remains a matter of speculation, professional mechanics should heed warning signs of potential problems. The combination of a lengthy inspection, numerous technicians, an overworked supervisor, a poor checklist, and vague communication should raise a red flag of caution. Although the ultimate responsibility for the safety of any flight rests with the pilot in command (PIC), it is not unreasonable for the PIC to assume that mechanics also take their responsibilities seriously.
This scenario underscores the need for safety risk management at all levels of aviation. Safety risk management, a formal system of hazard identification and analysis, is essential in keeping risk at acceptable levels. Part of this process is selecting the appropriate controls to mitigate the risk of the identified hazard. The primary objective of risk management is accident prevention, which is achieved by proactively identifying, assessing, and eliminating or controlling safety-related hazards to acceptable levels.
　
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